XML Schema Part 0: Primer is a non-normative document
intended to provide an easily readable description of the
XML Schema facilities, and is oriented towards quickly
understanding how to create schemas using the XML Schema
language. XML
Schema Part 1: Structures and XML
Schema Part 2: Datatypes provide the complete normative
description of the XML Schema language. This primer
describes the language features through numerous examples
which are complemented by extensive references to the
normative texts.
Status of this Document
This section describes the status of this document at the
time of its publication. Other documents may supersede this document.
A list of current W3C publications and the latest
revision of this technical report can be found in the W3C technical reports index at
http://www.w3.org/TR/.
This is a W3C
Recommendation, the first part of the Second Edition of XML
Schema. This document has been reviewed by W3C Members and
other interested parties and has been endorsed by the Director as a
W3C Recommendation. It is a stable document and may be used as
reference material.
W3C's role in making the Recommendation is to draw attention
to the specification and to promote its widespread deployment. This
enhances the functionality and interoperability of the Web.
This document has been produced by the W3C XML Schema Working Group
as part of the W3C XML
Activity. The goals of the XML Schema language are discussed in
the XML Schema
Requirements document. The authors of this document are the
members of the XML Schema Working Group. Different parts of this
specification have different editors.
This second edition is not a new version,
it merely incorporates the changes dictated by the corrections to
errors found in the first
edition as agreed by the XML Schema Working Group, as a
convenience to readers. A separate list of all such corrections is
available at http://www.w3.org/2001/05/xmlschema-errata.
This document, XML Schema Part 0: Primer, provides an
easily approachable description of the XML Schema
definition language, and should be used alongside the
formal descriptions of the language contained in Parts
1 and 2
of the XML Schema specification. The intended audience of
this document includes application developers whose
programs read and write schema documents, and schema
authors who need to know about the features of the
language, especially features that provide functionality
above and beyond what is provided by DTDs. The text assumes
that you have a basic understanding of XML 1.0 and Namespaces in XML.
Each major section of the primer introduces new features of
the language, and describes those features in the context
of concrete examples.
Basic Concepts: The Purchase Order (§2) covers the basic mechanisms of
XML Schema. It describes how to declare the elements and
attributes that appear in XML documents, the distinctions
between simple and complex types, defining complex types,
the use of simple types for element and attribute values,
schema annotation, a simple mechanism for re-using element
and attribute definitions, and nil values.
Advanced Concepts I: Namespaces, Schemas & Qualification (§3), the first advanced section in
the primer, explains the basics of how namespaces are used
in XML and schema documents. This section is important for
understanding many of the topics that appear in the other
advanced sections.
Advanced Concepts II: The International Purchase Order (§4), the second advanced section
in the primer, describes mechanisms for deriving types from
existing types, and for controlling these derivations. The
section also describes mechanisms for merging together
fragments of a schema from multiple sources, and for
element substitution.
Advanced Concepts III: The Quarterly Report (§5) covers more
advanced features, including a mechanism for specifying
uniqueness among attributes and elements, a mechanism for
using types across namespaces, a mechanism for extending
types based on namespaces, and a description of how
documents are checked for conformance.
In addition to the sections just described, the primer
contains a number of appendices that
provide detailed reference information on simple types and
a regular expression language.
The primer is a non-normative document, which means that
it does not provide a definitive (from the W3C's point of
view) specification of the XML Schema language. The
examples and other explanatory material in this document
are provided to help you understand XML Schema, but they
may not always provide definitive answers. In such cases,
you will need to refer to the XML Schema specification, and
to help you do this, we provide many links pointing to the
relevant parts of the specification. More specifically, XML
Schema items mentioned in the primer text are linked to an
index [Index (§E)] of element names and attributes,
and a summary table of
datatypes, both in the primer. The table and the index
contain links to the relevant sections of XML Schema parts
1 and 2.
2 Basic Concepts: The Purchase Order
The purpose of a schema is to define a class of XML
documents, and so the term "instance document" is often
used to describe an XML document that conforms to a
particular schema. In fact, neither instances nor schemas
need to exist as documents per se -- they may
exist as streams of bytes sent between applications, as
fields in a database record, or as collections of XML
Infoset "Information Items" -- but to simplify the primer,
we have chosen to always refer to instances and schemas as
if they are documents and files.
Let us start by considering an instance document in a file
called po.xml. It
describes a purchase order generated by a home products
ordering and billing application:
Example
The Purchase Order, po.xml
<?xml version="1.0"?>
<purchaseOrder orderDate="1999-10-20">
<shipTo country="US">
<name>Alice Smith</name>
<street>123 Maple Street</street>
<city>Mill Valley</city>
<state>CA</state>
<zip>90952</zip>
</shipTo>
<billTo country="US">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<city>Old Town</city>
<state>PA</state>
<zip>95819</zip>
</billTo>
<comment>Hurry, my lawn is going wild<!/comment>
<items>
<item partNum="872-AA">
<productName>Lawnmower</productName>
<quantity>1</quantity>
<USPrice>148.95</USPrice>
<comment>Confirm this is electric</comment>
</item>
<item partNum="926-AA">
<productName>Baby Monitor</productName>
<quantity>1</quantity>
<USPrice>39.98</USPrice>
<shipDate>1999-05-21</shipDate>
</item>
</items>
</purchaseOrder>
The purchase order consists of a main element,
purchaseOrder, and the subelements
shipTo, billTo, comment,
and items. These subelements (except
comment) in turn contain other subelements, and so
on, until a subelement such as USPrice
contains a number rather than any subelements. Elements
that contain subelements or carry attributes are said to
have complex types, whereas elements that contain numbers
(and strings, and dates, etc.) but do not contain any
subelements are said to have simple types. Some elements
have attributes; attributes always have simple types.
The complex types in the instance document, and some of
the simple types, are defined in the schema for purchase
orders. The other simple types are defined as part of XML
Schema's repertoire of built-in simple types.
Before going on to examine the purchase order schema, we
digress briefly to mention the association between the
instance document and the purchase order schema. As you can
see by inspecting the instance document, the purchase order
schema is not mentioned. An instance is not actually
required to reference a schema, and although many will, we
have chosen to keep this first section simple, and to
assume that any processor of the instance document can
obtain the purchase order schema without any information
from the instance document. In later sections, we will
introduce explicit mechanisms for associating instances and
schemas.
2.1 The Purchase Order Schema
The purchase order schema is contained in the file po.xsd:
The purchase order schema consists of a schema element and a variety
of subelements, most notably element, complexType, and simpleType which
determine the appearance of elements and their content in
instance documents.
Each of the elements in the schema has
a prefix xsd: which is associated with the XML
Schema namespace through the declaration,
xmlns:xsd="http://www.w3.org/2001/XMLSchema",
that appears in the
schema element. The prefix xsd: is
used by convention to denote the XML Schema namespace,
although any prefix can be used. The same prefix, and hence
the same association, also appears on the names of built-in
simple types, e.g. xsd:string. The purpose of the association
is to identify the elements and simple types as belonging
to the vocabulary of the XML Schema language rather than
the vocabulary of the schema author. For the sake of
clarity in the text, we just mention the names of elements
and simple types (e.g.
simpleType), and omit the prefix.
2.2 Complex Type Definitions, Element & Attribute Declarations
In XML Schema, there is a basic difference between complex
types which allow elements in their content and may carry
attributes, and simple types which cannot have element
content and cannot carry attributes. There is also a major
distinction between definitions which create new types
(both simple and complex), and declarations which enable
elements and attributes with specific names and types (both
simple and complex) to appear in document instances. In
this section, we focus on defining complex types and
declaring the elements and attributes that appear within
them.
New complex types are defined using
the
complexType element and such definitions
typically contain a set of element declarations, element
references, and attribute declarations. The declarations
are not themselves types, but rather an association between
a name and the constraints which govern the appearance of that
name in documents governed by the associated schema.
Elements are declared using the element element, and
attributes are declared using the attribute element. For
example, USAddress is defined as a complex
type, and within the definition of USAddress
we see five element declarations and one attribute
declaration:
The consequence of this definition is
that any element appearing in an instance whose type is
declared to be USAddress (e.g.
shipTo in
po.xml) must consist of five elements and one
attribute. These elements must be called name,
street, city, state
and zip as specified by the values of the
declarations' name attributes, and the
elements must appear in the same sequence (order) in which
they are declared. The first four of these elements will
each contain a string, and the fifth will contain a
number. The element whose type is declared to be
USAddress may appear with an attribute called
country which must contain the string
US.
The USAddress definition
contains only declarations involving the simple types: string,
decimal and
NMTOKEN. In contrast, the
PurchaseOrderType definition contains element
declarations involving complex types, e.g.
USAddress, although note that both declarations use
the same type
attribute to identify the type, regardless of whether the
type is simple or complex.
In defining PurchaseOrderType, two of the
element declarations, for shipTo and
billTo, associate different element names with the
same complex type, namely USAddress. The
consequence of this definition is that any element
appearing in an instance document (e.g.
po.xml) whose type is declared to be
PurchaseOrderType must consist of elements named
shipTo and billTo, each
containing the five subelements (name,
street, city, state and
zip) that were declared as part of
USAddress. The shipTo and
billTo elements may also carry the
country attribute that was declared as part of
USAddress.
The PurchaseOrderType definition contains an
orderDate attribute declaration which, like
the country attribute declaration, identifies
a simple type. In fact, all attribute declarations must
reference simple types because, unlike element
declarations, attributes cannot contain other elements or
other attributes.
The element declarations we have
described so far have each associated a name with an
existing type definition. Sometimes it is preferable to use
an existing element rather than declare a new element, for
example:
Example
<xsd:element ref="comment" minOccurs="0"/>
This declaration references an existing element,
comment, that was declared elsewhere in the purchase
order schema. In general, the value of the ref attribute must reference a
global element, i.e. one that has been declared under
schema rather
than as part of a complex type definition. The consequence
of this declaration is that an element called
comment may appear in an instance document, and its
content must be consistent with that element's type, in
this case, string.
2.2.1 Occurrence Constraints
The comment element is
optional within PurchaseOrderType because the
value of the
minOccurs attribute in its declaration is 0. In
general, an element is required to appear when the value of
minOccurs
is 1 or more. The maximum number of times an element may
appear is determined by the value of a maxOccurs attribute in
its declaration. This value may be a positive integer such
as 41, or the term unbounded to indicate there
is no maximum number of occurrences. The default value for
both the
minOccurs and the maxOccurs attributes is
1. Thus, when an element such as comment is
declared without a
maxOccurs attribute, the element may not occur
more than once. Be sure that if you specify a value for
only the
minOccurs attribute, it is less than or equal to
the default value of
maxOccurs, i.e. it is 0 or 1. Similarly, if you
specify a value for only the maxOccurs attribute, it
must be greater than or equal to the default value of
minOccurs,
i.e. 1 or more. If both attributes are omitted, the element
must appear exactly once.
Attributes may appear once or not at
all, but no other number of times, and so the
syntax for specifying
occurrences of attributes is different than the syntax for
elements. In particular, attributes can be declared with a
use attribute
to indicate whether the attribute is
required (see for example, the partNum
attribute declaration in
po.xsd), optional, or even
prohibited.
Default values of both attributes and elements are declared using the
default attribute, although this attribute has a slightly
different consequence in each case. When an attribute is declared with a
default value, the value of the attribute is whatever value
appears as the attribute's value in an instance document; if
the attribute does not appear in the instance document, the schema
processor provides the attribute with a value equal to that of the
default attribute.
Note that default values for attributes only make sense if the
attributes themselves are optional, and so it is an error to specify
both a default value and anything other than a value of
optional for use.
The schema processor treats defaulted elements slightly differently.
When an element is declared with a default value, the value of the
element is whatever value appears as the element's content in the
instance document; if the element appears without any content, the
schema processor provides the element with a value equal to that of the
default attribute.
However, if the element does not appear in the instance document, the schema
processor does not provide the element at all. In summary, the
differences between element and attribute defaults can be stated as:
Default attribute values apply when attributes are missing, and default
element values apply when elements are empty.
The fixed attribute is used in both attribute and element
declarations to ensure that the attributes and elements are
set to particular values. For example, po.xsd
contains a declaration for the country
attribute, which is declared with a fixed value US. This
declaration means that the appearance of a country
attribute in an instance document is optional (the default value of use is optional),
although if the attribute does appear, its value must be US,
and if the attribute does not appear, the schema processor will provide
a country attribute with the value US. Note
that the concepts of a fixed value and a default value are mutually
exclusive, and so it is an error for a declaration to contain both
fixed and default attributes.
The values of the attributes used in element and attribute
declarations to constrain their occurrences are summarized in
Table 1.
Table 1. Occurrence Constraints for Elements and Attributes
element/attribute must appear once, it may have any value
(1, 1) 37, -
required, 37, -
element/attribute must appear once, its value must be 37
(2, unbounded) 37, -
n/a
element must appear twice or more, its value must be
37; in general,
minOccurs and
maxOccurs values may be positive integers, and
maxOccurs value
may also be "unbounded"
(0, 1) -, -
optional, -, -
element/attribute may appear once, it may have any value
(0, 1) 37, -
n/a
element may appear once, if it does not
appear it is not provided; if it does appear and it is empty, its
value is 37; if it does appear and it is not empty,
its value must be 37
n/a
optional, 37, -
attribute may appear once, if it does appear
its value must be 37, if it does not appear its value is 37
(0, 1) -, 37
n/a
element may appear once; if it does not
appear it is not provided; if it does
appear and it is empty, its value is 37; otherwise its value is that
given
n/a
optional, -, 37
attribute may appear once; if it does not
appear its value is 37, otherwise its value is that given
(0, 2) -, 37
n/a
element may appear once, twice, or not at all; if the
element does not appear it is not provided; if it does
appear and it is empty, its value is 37; otherwise its
value is that given; in general, minOccurs and maxOccurs values may be
positive integers, and maxOccurs value may also
be "unbounded"
(0, 0) -, -
prohibited, -, -
element/attribute must not appear
Note that neither
minOccurs,
maxOccurs, nor use
may appear in the declarations of global elements and
attributes.
2.2.2 Global Elements & Attributes
Global elements, and global attributes, are created by
declarations that appear as the children of the schema element. Once
declared, a global element or a global attribute can be
referenced in one or more declarations using the ref attribute as described
above. A declaration that references a global element
enables the referenced element to appear in the instance
document in the context of the referencing declaration. So, for
example, the comment element appears in po.xml at the same level as
the shipTo, billTo and
items elements because the declaration that
references comment appears in the complex type
definition at the same level as the declarations of the
other three elements.
The declaration of a global element also enables the
element to appear at the top-level of an instance document.
Hence purchaseOrder, which is declared as a
global element in
po.xsd, can appear as the top-level element in
po.xml. Note that this
rationale will also allow a comment element to
appear as the top-level element in a document like
po.xml.
There are a number of caveats concerning the use of
global elements and attributes. One caveat is that global
declarations cannot contain references; global declarations
must identify simple and complex types directly. Put
concretely, global declarations cannot contain the ref attribute, they must
use the type
attribute (or, as we describe shortly, be followed by an anonymous type definition). A
second caveat is that cardinality constraints cannot be
placed on global declarations, although they can be placed
on local declarations that reference global declarations.
In other words, global declarations cannot contain the
attributes minOccurs, maxOccurs, or use.
2.2.3 Naming Conflicts
We have now described how to define new complex types
(e.g. PurchaseOrderType), declare elements
(e.g. purchaseOrder) and declare attributes
(e.g. orderDate). These activities generally
involve naming, and so the question naturally arises: What
happens if we give two things the same name? The answer
depends upon the two things in question, although in
general the more similar are the two things, the more
likely there will be a conflict.
Here are some examples to illustrate when same names cause
problems. If the two things are both types, say we define a
complex type called USStates and a simple type called
USStates, there is a conflict. If the two things are a type
and an element or attribute, say we define a complex type
called USAddress and we declare an element called USAddress,
there is no conflict. If the two things are elements within
different types (i.e. not global elements), say we declare
one element called name as part of the USAddress type and a
second element called name as part of the Item type, there
is no conflict. (Such elements are sometimes called local
element declarations.) Finally, if the two things are both
types and you define one and XML Schema has defined the
other, say you define a simple type called decimal, there
is no conflict. The reason for the apparent contradiction
in the last example is that the two types belong to
different namespaces. We explore the use of namespaces
in schema in a later section.
The purchase order schema declares several elements and
attributes that have simple types. Some of these simple
types, such as string
and decimal, are built
in to XML Schema, while others are derived from the
built-in's. For example, the partNum attribute
has a type called SKU (Stock Keeping Unit)
that is derived from
string. Both built-in simple types and their
derivations can be used in all element and attribute
declarations. Table 2 lists
all the simple types built in to XML Schema, along with
examples of the different types.
XML 1.0 NMTOKENS attribute type, i.e. a whitespace
separated list of NMTOKEN's, see (1)
Notes: (1) To retain compatibility
between XML Schema and XML 1.0 DTDs, the simple types
ID, IDREF, IDREFS, ENTITY, ENTITIES, NOTATION,
NMTOKEN, NMTOKENS should only be used in attributes.
(2) A value of this type can be represented by more
than one lexical format, e.g. 100 and 1.0E2 are both
valid float formats representing "one hundred".
However, rules have been established for this type
that define a canonical lexical format, see
XML Schema Part 2. (3) Newline, tab and
carriage-return characters in a normalizedString type are
converted to space characters before schema
processing. (4) As normalizedString, and adjacent space
characters are collapsed to a single space character,
and leading and trailing spaces are removed. (5) The "g"
prefix signals time periods in the Gregorian calendar.
New simple types are defined by
deriving them from existing simple types (built-in's and
derived). In particular, we can derive a new simple type by
restricting an existing simple type, in other words, the
legal range of values for the new type are a subset of the
existing type's range of values. We use the simpleType element to
define and name the new simple type. We use the restriction element
to indicate the existing (base) type, and to identify the
"facets" that constrain the range of values. A complete
list of facets is provided in
Appendix B.
Suppose we wish to create a new type
of integer called myInteger whose range of
values is between 10000 and 99999 (inclusive). We base our
definition on the built-in simple type integer, whose range of values also
includes integers less than 10000 and greater than 99999.
To define myInteger, we restrict the range of
the integer base type
by employing two facets called minInclusive and maxInclusive:
The example shows one particular combination of a base
type and two facets used to define myInteger,
but a look at the list of built-in simple types and their
facets (Appendix B) should
suggest other viable combinations.
The purchase order schema contains
another, more elaborate, example of a simple type
definition. A new simple type called SKU is
derived (by restriction) from the simple type string. Furthermore, we
constrain the values of SKU using a facet
called pattern
in conjunction with the regular expression
"\d{3}-[A-Z]{2}" that is read "three digits
followed by a hyphen followed by two upper-case ASCII
letters":
This regular expression language is described more fully
in Appendix D.
XML Schema defines twelve facets
which are listed in Appendix
B. Among these, the enumeration facet is
particularly useful and it can be used to constrain the
values of almost every simple type, except the boolean type. The enumeration facet limits
a simple type to a set of distinct values. For example, we
can use the
enumeration facet to define a new simple type
called USState, derived from string, whose value must be one of the
standard US state abbreviations:
Example
Using the Enumeration Facet
<xsd:simpleType name="USState">
<xsd:restriction base="xsd:string">
<xsd:enumeration value="AK"/>
<xsd:enumeration value="AL"/>
<xsd:enumeration value="AR"/>
<!-- and so on ... -->
</xsd:restriction>
</xsd:simpleType>
USState would be a good replacement for the
string type currently
used in the state element declaration. By
making this replacement, the legal values of a
state element, i.e. the state
subelements of billTo and
shipTo, would be limited to one of AK,
AL, AR, etc. Note that the
enumeration values specified for a particular type must be
unique.
2.3.1 List Types
XML Schema has the concept of a list type, in addition to
the so-called atomic types that constitute most of the
types listed in Table 2.
(Atomic types, list types, and the union types described in
the next section are collectively called simple types.) The
value of an atomic type is indivisible from XML Schema's
perspective. For example, the
NMTOKEN value US is indivisible in
the sense that no part of US, such as the
character "S", has any meaning by itself. In contrast, list
types are comprised of sequences of atomic types and
consequently the parts of a sequence (the "atoms")
themselves are meaningful. For example, NMTOKENS is a list type, and an
element of this type would be a white-space delimited list
of NMTOKEN's, such as
"US UK FR". XML Schema has three built-in list types, they
are NMTOKENS, IDREFS, and ENTITIES.
In addition to using the built-in
list types, you can create new list types by derivation
from existing atomic types. (You cannot create list types
from existing list types, nor from complex types.) For
example, to create a list of myInteger's:
Several facets can be applied to list
types: length,
minLength,
maxLength, pattern,
and
enumeration. For example, to define a list of
exactly six US states (SixUSStates), we first
define a new list type called USStateList from
USState, and then we derive
SixUSStates by restricting USStateList
to only six items:
Elements whose type is SixUSStates must have
six items, and each of the six items must be one of the
(atomic) values of the enumerated type
USState, for example:
Example
<sixStates>PA NY CA NY LA AK</sixStates>
Note that it is possible to derive a list type from the
atomic type string.
However, a string may
contain white space, and white space delimits the items in
a list type, so you should be careful using
list types whose base type is
string. For example, suppose we have defined a
list type with a
length facet equal to 3, and base type string, then the following 3 item
list is legal:
Example
Asie Europe Afrique
But the following 3 "item" list is illegal:
Example
Asie Europe Amérique Latine
Even though "Amérique Latine" may exist as a single
string outside of the list, when it is included in the
list, the whitespace between Amérique and Latine
effectively creates a fourth item, and so the latter
example will not conform to the 3-item list type.
2.3.2 Union Types
Atomic types and list types enable an
element or an attribute value to be one or more instances
of one atomic type. In contrast, a union type enables an
element or attribute value to be one or more instances of
one type drawn from the union of multiple atomic and list
types. To illustrate, we create a union type for
representing American states as singleton letter
abbreviations or lists of numeric codes. The
zipUnion union type is built from one atomic type and
one list type:
Schemas can be constructed by defining sets of named types
such as PurchaseOrderType and then declaring
elements such as purchaseOrder that reference
the types using the
type= construction. This style of schema
construction is straightforward but it can be unwieldy,
especially if you define many types that are referenced
only once and contain very few constraints. In these cases,
a type can be more succinctly defined as an anonymous type
which saves the overhead of having to be named and
explicitly referenced.
The definition of the type Items in po.xsd contains two element
declarations that use anonymous types (item
and quantity). In general, you can identify
anonymous types by the lack of a type= in an element (or
attribute) declaration, and by the presence of an un-named
(simple or complex) type definition:
In the case of the item element, it has an
anonymous complex type consisting of the elements
productName, quantity,
USPrice, comment, and
shipDate, and an attribute called
partNum. In the case of the quantity
element, it has an anonymous simple type derived from
positiveInteger whose value
ranges between 1 and 99.
The purchase order schema has many examples of elements
containing other elements (e.g. items),
elements having attributes and containing other elements
(e.g. shipTo), and elements containing only a
simple type of value (e.g. USPrice). However,
we have not seen an element having attributes but
containing only a simple type of value, nor have we seen an
element that contains other elements mixed with character
content, nor have we seen an element that has no content at
all. In this section we'll examine these variations in the
content models of elements.
2.5.1 Complex Types from Simple Types
Let us first consider how to declare an element that has
an attribute and contains a simple value. In an instance
document, such an element might appear as:
The purchase order schema declares a USPrice
element that is a starting point:
Example
<xsd:element name="USPrice" type="decimal"/>
Now, how do we add an attribute to
this element? As we have said before, simple types cannot
have attributes, and
decimal is a simple type. Therefore, we must
define a complex type to carry the attribute declaration.
We also want the content to be simple type decimal. So our original question
becomes: How do we define a complex type that is based on
the simple type
decimal? The answer is to derive a new
complex type from the simple type
decimal:
We use the
complexType element to start the definition of a
new (anonymous) type. To indicate that the content model of
the new type contains only character data and no elements,
we use a
simpleContent element. Finally, we derive the
new type by extending the simple
decimal type. The extension consists of adding a
currency attribute using a standard attribute
declaration. (We cover type derivation in detail in Advanced Concepts II: The International Purchase Order (§4).) The
internationalPrice element declared in this way will
appear in an instance as shown in the example at the
beginning of this section.
2.5.2 Mixed Content
The construction of the purchase order schema may be
characterized as elements containing subelements, and the
deepest subelements contain character data. XML Schema also
provides for the construction of schemas where character
data can appear alongside subelements, and character data
is not confined to the deepest subelements.
To illustrate, consider the following snippet from a
customer letter that uses some of the same elements as the
purchase order:
Example
Snippet of Customer Letter
<letterBody>
<salutation>Dear Mr.<name>Robert Smith</name>.</salutation>
Your order of <quantity>1</quantity> <productName>Baby
Monitor</productName> shipped from our warehouse on
<shipDate>1999-05-21</shipDate>. ....
</letterBody>
Notice the text appearing between elements and their child
elements. Specifically, text appears between the elements
salutation, quantity,
productName and shipDate which are all
children of letterBody, and text appears
around the element name
which is the child of a child of
letterBody. The following snippet of a schema
declares letterBody:
The elements appearing in the customer letter are
declared, and their types are defined using the element and complexType element
constructions we have seen before. To enable character data
to appear between the child-elements of
letterBody, the
mixed attribute on the type definition is set to
true.
Note that the mixed model in XML Schema
differs fundamentally from the
mixed model in XML 1.0. Under the XML Schema
mixed model, the order and number of child elements
appearing in an instance must agree with the order and
number of child elements specified in the model. In
contrast, under the XML 1.0 mixed model, the order and
number of child elements appearing in an instance cannot be
constrained. In summary, XML Schema provides full validation of
mixed models in contrast to the partial schema validation
provided by XML 1.0.
2.5.3 Empty Content
Now suppose that we want the
internationalPrice element to convey both the unit
of currency and the price as attribute values rather than
as separate attribute and content values. For example:
Such an element has no content at
all; its content model is empty. To define a type whose
content is empty, we essentially define a type that allows
only elements in its content, but we do not actually
declare any elements and so the type's content model is
empty:
In this example, we define an (anonymous) type having
complexContent, i.e. only elements. The
complexContent element signals that we intend to
restrict or extend the content model of a complex type, and the
restriction of anyType declares
two attributes but does not introduce any element content
(see Deriving Complex Types by Restriction (§4.4) for more
details on restriction). The
internationalPrice element declared in this way may
legitimately appear in an instance as shown in the example
above.
The preceding syntax for an empty-content element is
relatively verbose, and it is possible to declare the
internationalPrice element more compactly:
This compact syntax works because a complex type defined
without any simpleContent or
complexContent is interpreted as shorthand for
complex content that restricts anyType.
2.5.4 anyType
The anyType represents an abstraction called
the
ur-type which is the base type from which all
simple and complex types are derived. An
anyType type does not constrain its content in any
way. It is possible to use anyType like other
types, for example:
Example
<xsd:element name="anything" type="xsd:anyType"/>
The content of the element declared in this way is
unconstrained, so the element value may be 423.46, but it
may be any other sequence of characters as well, or indeed
a mixture of characters and elements. In fact,
anyType is the default type when none is specified,
so the above could also be written as follows:
Example
<xsd:element name="anything"/>
If unconstrained element content is needed, for example in
the case of elements containing prose which requires embedded
markup to support internationalization, then the default
declaration or a slightly restricted form of it may be suitable.
The text type described in Any Element, Any Attribute (§5.5)
is an example of such a type that is suitable for such
purposes.
2.6 Annotations
XML Schema provides three elements
for annotating schemas for the benefit of both human
readers and applications. In the purchase order schema, we
put a basic schema description and copyright information
inside the
documentation element, which is the recommended
location for human readable material. We recommend you use
the xml:lang attribute with any
documentation
elements to indicate the language of the information. Alternatively,
you may indicate the language of all information in a schema by placing
an xml:lang attribute on the schema element.
The appinfo element, which we did not use in the
purchase order schema, can be used to provide information
for tools, stylesheets and other applications. An
interesting example using appinfo is a
schema
that describes
the simple types in XML Schema Part 2: Datatypes.
Information describing this schema, e.g. which facets are
applicable to particular simple types, is represented
inside appinfo
elements, and this information was used by an application to automatically
generate text for the XML Schema Part 2 document.
Both documentation and
appinfo appear
as subelements of
annotation, which may itself appear at the
beginning of most schema constructions. To illustrate, the
following example shows annotation elements
appearing at the beginning of an element declaration and a
complex type definition:
Example
Annotations in Element Declaration & Complex Type Definition
<xsd:element name="internationalPrice">
<xsd:annotation>
<xsd:documentation xml:lang="en">
element declared with anonymous type
</xsd:documentation>
</xsd:annotation>
<xsd:complexType>
<xsd:annotation>
<xsd:documentation xml:lang="en">
empty anonymous type with 2 attributes
</xsd:documentation>
</xsd:annotation>
<xsd:complexContent>
<xsd:restriction base="xsd:anyType">
<xsd:attribute name="currency" type="xsd:string"/>
<xsd:attribute name="value" type="xsd:decimal"/>
</xsd:restriction>
</xsd:complexContent>
</xsd:complexType>
</xsd:element>
The
annotation element may also appear at the
beginning of other schema constructions such as those
indicated by the elements
schema,
simpleType, and attribute.
2.7 Building Content Models
The definitions of complex types in the purchase order
schema all declare sequences of elements that must appear
in the instance document. The occurrence of individual
elements declared in the so-called content models of these
types may be optional, as indicated by a 0 value for the
attribute
minOccurs (e.g. in comment), or be
otherwise constrained depending upon the values of minOccurs and maxOccurs. XML
Schema also provides constraints that apply to groups of
elements appearing in a content model. These constraints
mirror those available in XML 1.0 plus some additional
constraints. Note that the constraints do not apply to
attributes.
XML Schema enables groups of elements to be defined and
named, so that the elements can be used to build up the
content models of complex types (thus mimicking common
usage of parameter entities in XML 1.0). Un-named groups of
elements can also be defined, and along with elements in
named groups, they can be constrained to appear in the same
order (sequence) as they are declared. Alternatively, they
can be constrained so that only one of the elements may
appear in an instance.
To illustrate, we introduce two groups
into the PurchaseOrderType definition from the
purchase order schema so that purchase orders may contain
either separate shipping and billing addresses, or a single
address for those cases in which the shippee and billee are
co-located:
The choice
group element allows only one of its children to appear in
an instance. One child is an inner group element that references
the named group shipAndBill consisting of the
element sequence shipTo, billTo,
and the second child is a singleUSAddress.
Hence, in an instance document, the
purchaseOrder element must contain either a
shipTo element followed by a billTo
element or a singleUSAddress element. The
choice group is
followed by the comment and items
element declarations, and both the choice group and the element
declarations are children of a sequence group. The effect
of these various groups is that the address element(s) must
be followed by comment and items
elements in that order.
There exists a third option for
constraining elements in a group: All the elements in the
group may appear once or not at all, and they may appear in
any order. The all
group (which provides a simplified version of the SGML
&-Connector) is limited to the top-level of any content
model. Moreover, the group's children must all be
individual elements (no groups), and no element in the
content model may appear more than once, i.e. the
permissible values of
minOccurs and maxOccurs are 0 and 1.
For example, to allow the child elements of
purchaseOrder to appear in any order, we could
redefine PurchaseOrderType as:
By this definition, a comment element may
optionally appear within purchaseOrder, and it
may appear before or after any shipTo,
billTo and items elements, but it can
appear only once. Moreover, the stipulations of an all group do not allow us to
declare an element such as comment outside the
group as a means of enabling it to appear more than once.
XML Schema stipulates that an
all group must appear as the sole child at the
top of a content model. In other words, the following is
illegal:
Finally, named and un-named groups that appear in content
models (represented by
group and
choice,
sequence,
all respectively) may carry minOccurs and maxOccurs
attributes. By combining and nesting the various groups
provided by XML Schema, and by setting the values of minOccurs and
maxOccurs,
it is possible to represent any content model expressible
with an XML 1.0 DTD. Furthermore, the all group provides additional
expressive power.
2.8 Attribute Groups
Suppose we want to provide more information about each
item in a purchase order, for example, each item's weight
and preferred shipping method. We can accomplish this by
adding weightKg and shipBy
attribute declarations to the item element's
(anonymous) type definition:
Alternatively, we can create a named
attribute group containing all the desired attributes of an
item element, and reference this group by name
in the item element declaration:
Using an attribute group in this way can improve the
readability of schemas, and facilitates updating schemas
because an attribute group can be defined and edited in one
place and referenced in multiple definitions and
declarations. These characteristics of attribute groups
make them similar to parameter entities in XML 1.0. Note
that an attribute group may contain other attribute groups.
Note also that both attribute declarations and attribute
group references must appear at the end of complex type
definitions.
2.9 Nil Values
One of the purchase order items listed in po.xml, the Lawnmower,
does not have a shipDate element. Within the
context of our scenario, the schema author may have
intended such absences to indicate items not
yet shipped. But in general, the absence of an element does
not have any particular meaning: It may indicate that the
information is unknown, or not applicable, or the element
may be absent for some other reason. Sometimes it is
desirable to represent an unshipped item,
unknown information, or inapplicable information
explicitly with an element, rather than by an absent
element. For example, it may be desirable to represent a
"null" value being sent to or from a relational database
with an element that is present. Such cases can be
represented using XML Schema's nil mechanism which enables
an element to appear with or without a non-nil value.
XML Schema's nil mechanism involves
an "out of band" nil signal. In other words, there is no
actual nil value that appears as element content, instead
there is an attribute to indicate that the element content
is nil. To illustrate, we modify the
shipDate element declaration so that nils can be
signalled:
And to explicitly represent that
shipDate has a nil value in the instance
document, we set the nil
attribute (from the XML Schema
namespace for instances) to true:
Example
<shipDate xsi:nil="true"></shipDate>
The nil
attribute is defined as part of the XML Schema namespace
for instances,
http://www.w3.org/2001/XMLSchema-instance,
and so it must appear in the instance document with a
prefix (such as xsi:) associated with that namespace.
(As with the xsd: prefix, the
xsi: prefix is used by convention only.) Note that
the nil mechanism applies only to element values, and not
to attribute values. An element with xsi:nil="true" may not
have any element content but it may still carry attributes.
A schema can be viewed as a
collection (vocabulary) of type definitions and element
declarations whose names belong to a particular namespace
called a target namespace. Target namespaces enable us
to distinguish between definitions and declarations from
different vocabularies. For example, target namespaces
would enable us to distinguish between the declaration for
element in the
XML Schema language vocabulary, and a declaration for
element in a hypothetical chemistry language
vocabulary. The former is part of the
http://www.w3.org/2001/XMLSchema target
namespace, and the latter is part of another target
namespace.
When we want to check that an
instance document conforms to one or more schemas (through
a process called schema validation), we need to identify
which element and attribute declarations and type
definitions in the schemas should be used to check which
elements and attributes in the instance document. The
target namespace plays an important role in the
identification process. We examine the role of the target
namespace in the next section.
The schema author also has several options that affect how
the identities of elements and attributes are represented
in instance documents. More specifically, the author can
decide whether or not the appearance of locally declared
elements and attributes in an instance must be qualified by
a namespace, using either an explicit prefix or implicitly
by default. The schema author's choice regarding
qualification of local elements and attributes has a number
of implications regarding the structures of schemas and
instance documents, and we examine some of these
implications in the following sections.
3.1 Target Namespaces & Unqualified Locals
In a new version of the purchase order schema, po1.xsd, we explicitly declare
a target namespace, and specify that both locally defined
elements and locally defined attributes must be
unqualified. The target namespace in po1.xsd is
http://www.example.com/PO1, as indicated by the
value of the
targetNamespace attribute.
Qualification of local elements and attributes can be
globally specified by a pair of attributes,
elementFormDefault and
attributeFormDefault, on the schema element, or can be
specified separately for each local declaration using the
form attribute.
All such attributes' values may each be set to
unqualified or qualified, to indicate
whether or not locally declared elements and attributes
must be unqualified.
In po1.xsd we globally
specify the qualification of elements and attributes by
setting the values of both
elementFormDefault and
attributeFormDefault to
unqualified. Strictly speaking, these settings are
unnecessary because the values are the defaults for the two
attributes; we make them here to highlight the contrast between
this case and other cases we describe later.
Example
Purchase Order Schema with Target Namespace, po1.xsd
To see how the target namespace of this schema is
populated, we examine in turn each of the type
definitions and element declarations. Starting from the end
of the schema, we first define a type called
USAddress that consists of the elements
name, street, etc. One consequence of
this type definition is that the USAddress
type is included in the schema's target namespace. We next
define a type called PurchaseOrderType that
consists of the elements shipTo,
billTo, comment, etc.
PurchaseOrderType is also included in the schema's
target namespace. Notice that the type references in the
three element declarations are prefixed, i.e.
po:USAddress, po:USAddress and
po:comment, and the prefix is associated with the
namespace http://www.example.com/PO1. This is
the same namespace as the schema's target namespace, and so
a processor of this schema will know to look within this
schema for the definition of the type
USAddress and the declaration of the element
comment. It is also possible to refer to types in
another schema with a different target namespace, hence
enabling re-use of definitions and declarations between
schemas.
At the beginning of the schema
po1.xsd, we declare the elements
purchaseOrder and comment. They are
included in the schema's target namespace. The
purchaseOrder element's type is prefixed, for the
same reason that USAddress is prefixed. In
contrast, the comment element's type, string, is not prefixed. The
po1.xsd schema
contains a default namespace declaration, and so unprefixed
types such as string
and unprefixed elements such as element and complexType are
associated with the default namespace
http://www.w3.org/2001/XMLSchema. In fact, this
is the target namespace of XML Schema itself, and so a
processor of po1.xsd
will know to look within the schema of XML Schema
-- otherwise known as the "schema for schemas" -- for the
definition of the type
string and the declaration of the element called
element.
Let us now examine how the target namespace of the schema
affects a conforming instance document:
Example
A Purchase Order with Unqualified Locals, po1.xml
<?xml version="1.0"?>
<apo:purchaseOrder xmlns:apo="http://www.example.com/PO1"
orderDate="1999-10-20">
<shipTo country="US">
<name>Alice Smith</name>
<street>123 Maple Street</street>
<!-- etc. -->
</shipTo>
<billTo country="US">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<!-- etc. -->
</billTo>
<apo:comment>Hurry, my lawn is going wild<!/apo:comment>
<!-- etc. -->
</apo:purchaseOrder>
The instance document declares one namespace,
http://www.example.com/PO1, and associates it with
the prefix apo:. This prefix is used to
qualify two elements in the document, namely
purchaseOrder and comment. The
namespace is the same as the target namespace of the schema
in po1.xsd, and so a
processor of the instance document will know to look in
that schema for the declarations of
purchaseOrder and comment. In fact,
target namespaces are so named because of the sense in
which there exists a target namespace for the elements
purchaseOrder and comment. Target
namespaces in the schema therefore control the validation
of corresponding namespaces in the instance.
The prefix apo: is applied to the global
elements purchaseOrder and
comment elements. Furthermore,
elementFormDefault and
attributeFormDefault require that the prefix is
not applied to any of the locally declared
elements such as shipTo, billTo,
name and street, and it is
not applied to any of the attributes (which were all
declared locally). The purchaseOrder and
comment are global elements because they are
declared in the context of the schema as a whole rather
than within the context of a particular type. For example,
the declaration of purchaseOrder appears as a
child of the
schema element in
po1.xsd, whereas the declaration of
shipTo appears as a child of the complexType element that
defines PurchaseOrderType.
When local elements and attributes are not required to be
qualified, an instance author may require more or less
knowledge about the details of the schema to create schema
valid instance documents. More specifically, if the author
can be sure that only the root element (such as
purchaseOrder) is global, then it is a simple matter
to qualify only the root element. Alternatively, the author
may know that all the elements are declared globally, and
so all the elements in the instance document can be
prefixed, perhaps taking advantage of a default namespace
declaration. (We examine this approach in Global vs. Local Declarations (§3.3).) On the other hand, if
there is no uniform pattern of global and local
declarations, the author will need detailed knowledge of
the schema to correctly prefix global elements and
attributes.
3.2 Qualified Locals
Elements and attributes can be independently required to
be qualified, although we start by describing the
qualification of local elements. To specify that all
locally declared elements in a schema must be qualified, we
set the value of
elementFormDefault to qualified:
And in this conforming instance document, we qualify all
the elements explicitly:
Example
A Purchase Order with Explicitly Qualified Locals
<?xml version="1.0"?>
<apo:purchaseOrder xmlns:apo="http://www.example.com/PO1"
orderDate="1999-10-20">
<apo:shipTo country="US">
<apo:name>Alice Smith</apo:name>
<apo:street>123 Maple Street</apo:street>
<!-- etc. -->
</apo:shipTo>
<apo:billTo country="US">
<apo:name>Robert Smith</apo:name>
<apo:street>8 Oak Avenue</apo:street>
<!-- etc. -->
</apo:billTo>
<apo:comment>Hurry, my lawn is going wild<!/apo:comment>
<!-- etc. -->
</apo:purchaseOrder>
Alternatively, we can replace the explicit qualification
of every element with implicit qualification provided by a
default namespace, as shown here in
po2.xml:
Example
A Purchase Order with Default Qualified Locals, po2.xml
<?xml version="1.0"?>
<purchaseOrder xmlns="http://www.example.com/PO1"
orderDate="1999-10-20">
<shipTo country="US">
<name>Alice Smith</name>
<street>123 Maple Street</street>
<!-- etc. -->
</shipTo>
<billTo country="US">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<!-- etc. -->
</billTo>
<comment>Hurry, my lawn is going wild<!/comment>
<!-- etc. -->
</purchaseOrder>
In po2.xml, all the elements in the
instance belong to the same namespace, and the namespace
statement declares a default namespace that applies to all
the elements in the instance. Hence, it is unnecessary to
explicitly prefix any of the elements. As another
illustration of using qualified elements, the schemas in Advanced Concepts III: The Quarterly Report (§5) all require qualified
elements.
Qualification of attributes is very similar to the
qualification of elements. Attributes that must be
qualified, either because they are declared globally or
because the
attributeFormDefault attribute is set to
qualified, appear prefixed in instance documents.
One example of a qualified attribute is the xsi:nil attribute that
was introduced in Nil Values (§2.9). In
fact, attributes that are required to be qualified must be
explicitly prefixed because the Namespaces in XML
specification does not provide a mechanism for defaulting
the namespaces of attributes. Attributes that are not
required to be qualified appear in instance documents
without prefixes, which is the typical case.
The qualification mechanism we have
described so far has controlled all local element and
attribute declarations within a particular target
namespace. It is also possible to control qualification on
a declaration by declaration basis using the form attribute. For example,
to require that the locally declared attribute
publicKey is qualified in instances, we declare it
in the following way:
Notice that the value of the form attribute overrides the
value of the
attributeFormDefault attribute for the
publicKey attribute only. Also, the form attribute can be applied
to an element declaration in the same manner. An instance
document that conforms to the schema is:
Example
Instance with a Qualified Attribute
<?xml version="1.0"?>
<purchaseOrder xmlns="http://www.example.com/PO1"
xmlns:po="http://www.example.com/PO1"
orderDate="1999-10-20">
<!-- etc. -->
<secure po:publicKey="GpM7">
<!-- etc. -->
</secure>
</purchaseOrder>
3.3 Global vs. Local Declarations
Another authoring style, applicable when all element names are
unique within a namespace, is to create schemas in which
all elements are global. This is similar in effect to the
use of <!ELEMENT> in a DTD. In the example below, we
have modified the original po1.xsd
such that all the elements are declared globally. Notice
that we have omitted the
elementFormDefault and
attributeFormDefault attributes in this example
to emphasize that their values are irrelevant when there
are only global element and attribute declarations.
Example
Modified version of po1.xsd
using only global element declarations
This "global" version of po1.xsd
will validate the instance document
po2.xml which, as we described previously, is also
schema valid against the "qualified" version of po1.xsd. In other words, both schema
approaches can validate the same, namespace defaulted,
document. Thus, in one respect the two schema approaches
are similar, although in another important respect the two
schema approaches are very different. Specifically, when
all elements are declared globally, it is not possible to
take advantage of local names. For example, you can only
declare one global element called "title". However, you can
locally declare one element called "title" that has a
string type, and is a subelement of "book". Within the same
schema (target namespace) you can declare a second element
also called "title" that is an enumeration of the values
"Mr Mrs Ms".
3.4 Undeclared Target Namespaces
In Basic Concepts: The Purchase Order (§2) we explained the basics of
XML Schema using a schema that did not declare a target
namespace and an instance document that did not declare a
namespace. So the question naturally arises: What is the
target namespace in these examples and how is it
referenced?
In the purchase order schema,
po.xsd, we did not declare a target namespace
for the schema, nor did we declare a prefix (like
po: above) associated with the schema's target
namespace with which we could refer to types and elements
defined and declared within the schema. The consequence of
not declaring a target namespace in a schema is that the
definitions and declarations from that schema, such as
USAddress and purchaseOrder, are
referenced without namespace qualification. In other words
there is no explicit namespace prefix applied to the
references nor is there any implicit namespace applied to
the reference by default. So for example, the
purchaseOrder element is declared using the type
reference PurchaseOrderType. In contrast, all
the XML Schema elements and types used in po.xsd are explicitly qualified with
the prefix xsd: that is associated with the
XML Schema namespace.
In cases where a schema is designed without a target namespace, it is
strongly recommended that all XML Schema elements and types are explicitly
qualified with a prefix such as xsd: that is associated
with the XML Schema namespace (as in po.xsd). The rationale for this recommendation is
that if XML Schema elements and types are associated with the XML Schema
namespace by default, i.e. without prefixes, then references to XML
Schema types may not be distinguishable from references to user-defined
types.
Element declarations from a schema with no target
namespace validate unqualified elements in the instance
document. That is, they validate elements for which no
namespace qualification is provided by either an explicit
prefix or by default (xmlns:). So, to validate
a traditional XML 1.0 document which does not use
namespaces at all, you must provide a schema with no target
namespace. Of course, there are many XML 1.0 documents that
do not use namespaces, so there will be many schema
documents written without target namespaces; you must be
sure to give to your processor a schema document that
corresponds to the vocabulary you wish to validate.
4 Advanced Concepts II: The International Purchase Order
The purchase order schema described in Basic Concepts: The Purchase Order (§2)
was contained in a single document, and most
of the schema constructions-- such as element declarations
and type definitions-- were constructed from scratch. In
reality, schema authors will want to compose schemas from
constructions located in multiple documents, and to create new
types based on existing types. In this section, we examine
mechanisms that enable such compositions and creations.
4.1 A Schema in Multiple Documents
As schemas become larger, it is often desirable to divide
their content among several schema documents for purposes
such as ease of maintenance, access control, and
readability. For these reasons, we have taken the schema
constructs concerning addresses out of po.xsd, and put them in a new file
called address.xsd.
The modified purchase order schema file is called ipo.xsd:
Addresses for International Purchase Order schema, address.xsd
<schema targetNamespace="http://www.example.com/IPO"
xmlns="http://www.w3.org/2001/XMLSchema"
xmlns:ipo="http://www.example.com/IPO">
<annotation>
<documentation xml:lang="en">
Addresses for International Purchase order schema
Copyright 2000 Example.com. All rights reserved.
</documentation>
</annotation>
<complexType name="Address">
<sequence>
<element name="name" type="string"/>
<element name="street" type="string"/>
<element name="city" type="string"/>
</sequence>
</complexType>
<complexType name="USAddress">
<complexContent>
<extension base="ipo:Address">
<sequence>
<element name="state" type="ipo:USState"/>
<element name="zip" type="positiveInteger"/>
</sequence>
</extension>
</complexContent>
</complexType>
<complexType name="UKAddress">
<complexContent>
<extension base="ipo:Address">
<sequence>
<element name="postcode" type="ipo:UKPostcode"/>
</sequence>
<attribute name="exportCode" type="positiveInteger" fixed="1"/>
</extension>
</complexContent>
</complexType>
<!-- other Address derivations for more countries -->
<simpleType name="USState">
<restriction base="string">
<enumeration value="AK"/>
<enumeration value="AL"/>
<enumeration value="AR"/>
<!-- and so on ... -->
</restriction>
</simpleType>
<!-- simple type definition for UKPostcode -->
</schema>
The various purchase order and
address constructions are now contained in two schema
files, ipo.xsd and
address.xsd. To
include these constructions as part of the international
purchase order schema, in other words to include them in
the international purchase order's namespace, ipo.xsd contains the include element:
The effect of this
include element is to bring in the definitions
and declarations contained in
address.xsd, and make them available as part of
the international purchase order schema target namespace.
The one important caveat to using include is that the target
namespace of the included components must be the same as
the target namespace of the including schema, in this case
http://www.example.com/IPO. Bringing in
definitions and declarations using the include mechanism
effectively adds these components to the existing target
namespace. In Redefining Types & Groups (§4.5), we
describe a similar mechanism that enables you to modify
certain components when they are brought in.
In our example, we have shown only one including document
and one included document. In practice it is possible to
include more than one document using multiple include elements, and
documents can include documents that themselves include
other documents. However, nesting documents in this manner
is legal only if all the
included parts of the schema are declared with the same
target namespace.
Instance documents that conform to schema whose
definitions span multiple schema documents need only
reference the 'topmost' document and the common namespace,
and it is the responsibility of the processor to gather
together all the definitions specified in the various
included documents. In our example above, the instance
document ipo.xml (see
Using Derived Types in Instance Documents (§4.3)) references
only the common target namespace,
http://www.example.com/IPO, and (by implication) the one schema file
http://www.example.com/schemas/ipo.xsd. The
processor is responsible for obtaining the schema file
address.xsd.
In Importing Types (§5.4) we describe how
schemas can be used to validate content from more than one
namespace.
4.2 Deriving Types by Extension
To create our address constructs, we start by creating a
complex type called Address in the usual way
(see address.xsd).
The Address type contains the basic elements
of an address: a name, a street and a city. (Such a
definition will not work for all countries, but it
serves the purpose of our example.) From this starting
point we derive two new complex types that contain all the
elements of the original type plus additional elements that
are specific to addresses in the US and the UK. The
technique we use here to derive new (complex) address types
by extending an existing type is the same technique we used
in Complex Types from Simple Types (§2.5.1),
except that our base type here is a complex type whereas
our base type in the previous section was a simple type.
We define the two new complex types,
USAddress and UKAddress, using the
complexType
element. In addition, we indicate that the content models
of the new types are complex, i.e. contain elements, by
using the
complexContent element, and we indicate that we
are extending the base type Address by the
value of the
base attribute on the extension element.
When a complex type is derived by extension, its effective
content model is the content model of the base type plus
the content model specified in the type derivation.
Furthermore, the two content models are treated as two
children of a sequential group. In the case of
UKAddress, the content model of
UKAddress is the content model of
Address plus the declarations for a
postcode element and an exportCode
attribute. This is like defining the UKAddress
from scratch as follows:
In our example scenario, purchase orders are generated in
response to customer orders which may involve shipping and
billing addresses in different countries. The international
purchase order, ipo.xml
below, illustrates one such case where goods are shipped to
the UK and the bill is sent to a US address. Clearly it is
better if the schema for international purchase orders
does not have to spell out every possible combination of
international addresses for billing and shipping, and even
more so if we can add new complex types of international
address simply by creating new derivations of
Address.
XML Schema allows us to define the
billTo and shipTo elements as
Address types (see
ipo.xsd) but to use instances of international
addresses in place of instances of Address.
In other words, an instance document whose content conforms
to the UKAddress type will be valid if that
content appears within the document at a location where an
Address is expected (assuming the
UKAddress content itself is valid). To make this
feature of XML Schema work, and to identify exactly which
derived type is intended, the derived type must be
identified in the instance document. The type is identified
using the
xsi:type attribute which is part of the XML
Schema instance namespace. In the example, ipo.xml, use of the
UKAddress and USAddress derived types
is identified through the values assigned to the xsi:type attributes.
Example
An International Purchase order, ipo.xml
<?xml version="1.0"?>
<ipo:purchaseOrder
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:ipo="http://www.example.com/IPO"
orderDate="1999-12-01">
<shipTo exportCode="1" xsi:type="ipo:UKAddress">
<name>Helen Zoe</name>
<street>47 Eden Street</street>
<city>Cambridge</city>
<postcode>CB1 1JR</postcode>
</shipTo>
<billTo xsi:type="ipo:USAddress">
<name>Robert Smith</name>
<street>8 Oak Avenue</street>
<city>Old Town</city>
<state>PA</state>
<zip>95819</zip>
</billTo>
<items>
<item partNum="833-AA">
<productName>Lapis necklace</productName>
<quantity>1</quantity>
<USPrice>99.95</USPrice>
<ipo:comment>Want this for the holidays<!/ipo:comment>
<shipDate>1999-12-05</shipDate>
</item>
</items>
</ipo:purchaseOrder>
In addition to deriving new complex types by extending
content models, it is possible to derive new types by
restricting the content models of existing types.
Restriction of complex types is conceptually the same as
restriction of simple types, except that the restriction of
complex types involves a type's declarations rather than
the acceptable range of a simple type's values. A complex
type derived by restriction is very similar to its base
type, except that its declarations are more limited than
the corresponding declarations in the base type. In fact,
the values represented by the new type are a subset of the
values represented by the base type (as is the case
with restriction of simple types). In other words, an
application prepared for the values of the base type would
not be surprised by the values of the restricted type.
For example, suppose we want to update our definition of a purchase
order so that it must contain a comment;
the schema shown in ipo.xsd allows a purchaseOrder
element to appear without any child
comment
elements. To create our new
RestrictedPurchaseOrderType
type, we define the new type in the usual way, indicate
that it is derived by restriction from the base type
PurchaseOrderType
, and provide a new (more restrictive) value for
the minimum number of
comment element
occurrences. Notice that types derived by restriction must
repeat all the particle components
(element declarations, model groups, and wildcards) of the base type definition that
are to be included in the derived type. However,
attribute declarations do not need to be repeated in the derived type definition; in this example, RestrictedPurchaseOrderType will inherit the
orderDate attribute declaration from PurchaseOrderType.
Example
Deriving RestrictedPurchaseOrderType by Restriction from PurchaseOrderType
This change narrows the
allowable number of
comment elements from a minimum of 0 to a
minimum of 1. Note that all
RestrictedPurchaseOrderType
type elements will also be acceptable as
PurchaseOrderType
type elements.
To further illustrate restriction, Table 3 shows several examples of
how element and attribute declarations within type
definitions may be restricted (the table shows element
syntax although the first three examples are equally valid
attribute restrictions).
Table 3. Restriction Examples
Base
Restriction(s)
Notes
default="1"
setting a default value where none was previously
given
fixed="100"
setting a fixed value where none was previously given
In A Schema in Multiple Documents (§4.1) we
described how to include definitions and declarations
obtained from external schema files having the same target
namespace. The
include mechanism enables you to use externally
created schema components "as-is", that is, without any
modification. We have just described how to derive new
types by extension and by restriction, and the redefine mechanism we
describe here enables you to redefine simple and
complex types, groups, and attribute groups that are
obtained from external schema files. Like the include mechanism,
redefine
requires the external components to be in the same target
namespace as the redefining schema, although external
components from schemas that have no namespace can also be
redefined. In the latter cases, the redefined components
become part of the redefining schema's target namespace.
To illustrate the
redefine mechanism, we use it instead of
the include
mechanism in the International Purchase Order schema,
ipo.xsd, and we
use it to modify the definition of the complex type
Address contained in
address.xsd:
Example
Using redefine in the International Purchase Order
The redefine
element acts very much like the include element as it includes
all the declarations and definitions from the address.xsd file. The
complex type definition of Address uses the
familiar extension syntax to add a country
element to the definition of Address. However,
note that the base type is also Address.
Outside of the
redefine element, any such attempt to define a
complex type with the same name (and in the same namespace)
as the base from which it is being derived would cause an
error. But in this case, there is no error, and the
extended definition of Address becomes the
only definition of Address.
Now that Address has been redefined, the
extension applies to all schema components that make use of
Address. For example,
address.xsd contains definitions of international
address types that are derived from Address.
These derivations reflect the redefined
Address type, as shown in the following snippet:
....
<shipTo exportCode="1" xsi:type="ipo:UKAddress">
<name>Helen Zoe</name>
<street>47 Eden Street</street>
<city>Cambridge</city>
<!-- country was added to Address which is base type of UKAddress -->
<country>United Kingdom</country>
<!-- postcode was added as part of UKAddress -->
<postcode>CB1 1JR</postcode>
</shipTo>
....
Our example has been carefully constructed so that the
redefined Address type does not conflict in
any way with the types that are derived from the original
Address definition. But note that it would be
very easy to create a conflict. For example, if the
international address type derivations had extended
Address by adding a country element,
then the redefinition of Address would be
adding an element of the same name to the content model of
Address. It is illegal to have two elements of
the same name (and in the same target namespace) but
different types in a content model, and so the attempt to
redefine Address would cause an error. In
general,
redefine does not protect you from such errors,
and it should be used cautiously.
4.6 Substitution Groups
XML Schema provides a mechanism,
called substitution groups, that allows elements to be
substituted for other elements. More specifically, elements
can be assigned to a special group of elements that are
said to be substitutable for a particular named element
called the head element. (Note that the head element
as well as the substitutable
elementsmust
be declared as global
elements.) To illustrate, we
declare two elements called customerComment
and shipComment and assign them to a
substitution group whose head element is
comment, and so customerComment and
shipComment can be used anyplace that we are
able to use comment. Elements in a
substitution group must have the same type as the head
element, or they can have a type that has been derived from
the head element's type. To declare these two new elements,
and to make them substitutable for the
comment element, we use the following syntax:
When these declarations are added to the international
purchase order schema, shipComment and
customerComment can be substituted for
comment in the instance document, for example:
....
<items>
<item partNum="833-AA">
<productName>Lapis necklace</productName>
<quantity>1</quantity>
<USPrice>99.95</USPrice>
<ipo:shipComment>
Use gold wrap if possible
</ipo:shipComment>
<ipo:customerComment>
Want this for the holidays!
</ipo:customerComment>
<shipDate>1999-12-05</shipDate>
</item>
</items>
....
Note that when an instance document contains element
substitutions whose types are derived from those of their
head elements, it is not necessary to identify the
derived types using the xsi:type construction that
we described in Using Derived Types in Instance Documents (§4.3).
The existence of a substitution group does not require any
of the elements in that class to be used, nor does it
preclude use of the head element. It simply provides a
mechanism for allowing elements to be used interchangeably.
4.7 Abstract Elements and Types
XML Schema provides a mechanism to
force substitution for a particular element or type. When
an element or type is declared to be "abstract", it cannot
be used in an instance document. When an element is
declared to be abstract, a member of that element's
substitution group must appear in the instance document.
When an element's corresponding type definition is declared
as abstract, all instances of that element must use xsi:type to
indicate a derived type that is not abstract.
In the substitution group example we described in Substitution Groups (§4.6), it would be useful to
specifically disallow use of the comment
element so that instances must make use of the
customerComment and shipComment
elements. To declare the comment element
abstract, we modify its original declaration in the
international purchase order schema, ipo.xsd, as follows:
With comment declared as abstract, instances
of international purchase orders are now only valid if they
contain customerComment and
shipComment elements.
Declaring an element as abstract requires the use of a
substitution group. Declaring a type as abstract simply
requires the use of a type derived from it (and identified
by the
xsi:type attribute) in the instance document.
Consider the following schema definition:
The transport element is not abstract,
therefore it can appear in instance documents. However,
because its type definition is abstract, it may never
appear in an instance document without an xsi:type attribute that
refers to a derived type. That means the following is not
schema-valid:
because it uses a non-abstract type that is substitutable
for Vehicle.
4.8 Controlling the Creation & Use of Derived Types
So far, we have been able to derive
new types and use them in instance documents without any
restraints. In reality, schema authors will sometimes want
to control derivations of particular types, and the use of
derived types in instances.
XML Schema provides a couple of mechanisms that control
the derivation of types. One of these mechanisms allows the
schema author to specify that for a particular complex
type, new types may not be derived from it, either (a) by
restriction, (b) by extension, or (c) at all. To
illustrate, suppose we want to prevent any derivation of
the Address type by restriction because we
intend for it only to be used as the base for extended
types such as USAddress and
UKAddress. To prevent any such derivations, we
slightly modify the original definition of
Address as follows:
The restriction value of
the final
attribute prevents derivations by restriction. Preventing
derivations at all, or by extension, are indicated by the
values #all and extension
respectively. Moreover, there exists an optional finalDefault attribute
on the schema
element whose value can be one of the values allowed for
the final
attribute. The effect of specifying the finalDefault attribute
is equivalent to specifying a final attribute on every type
definition and element declaration in the schema.
Another type-derivation mechanism
controls which facets can be applied in the derivation of a
new simple type. When a simple type is defined, the fixed attribute
may be applied to any of its facets to prevent a derivation
of that type from modifying the value of the fixed facets.
For example, we can define a Postcode simple
type as:
However, we cannot derive a new postal code in which we
re-apply any facet that was fixed in the base definition:
Example
Illegal Derivation from Postcode
<simpleType name="UKPostcode">
<restriction base="ipo:Postcode">
<pattern value="[A-Z]{2}\d\d[A-Z]{2}"/>
<!-- illegal attempt to modify facet fixed in base type -->
<length value="6" fixed="true"/>
</restriction>
</simpleType>
In addition to the mechanisms that
control type derivations, XML Schema provides a mechanism
that controls which derivations and substitution groups may
be used in instance documents. In Using Derived Types in Instance Documents (§4.3), we described how the
derived types, USAddress and
UKAddress, could be used by the shipTo
and billTo elements in instance documents.
These derived types can replace the content model provided
by the Address type because they are derived
from the Address type. However, replacement by
derived types can be controlled using the block attribute in a type
definition. For example, if we want to block any
derivation-by-restriction from being used in place of
Address (perhaps for the same reason we
defined Address with final="restriction"), we can
modify the original definition of Address as
follows:
Example
Preventing Derivations by Restriction of Address in
the Instance
The restriction value on
the block
attribute prevents derivations-by-restriction from
replacing Address in an instance. However, it
would not prevent UKAddress and
USAddress from replacing Address
because they were derived by extension. Preventing
replacement by derivations at all, or by
derivations-by-extension, are indicated by the values
#all and extension respectively.
As with final,
there exists an optional blockDefault attribute
on the schema
element whose value can be one of the values allowed for
the block
attribute. The effect of specifying the blockDefault attribute
is equivalent to specifying a block attribute on every type
definition and element declaration in the schema.
5 Advanced Concepts III: The Quarterly Report
The home-products ordering and billing application can
generate ad-hoc reports that summarize how many of which
types of products have been billed on a per region basis.
An example of such a report, one that covers the fourth
quarter of 1999, is shown in
4Q99.xml.
Notice that in this section we use qualified elements in
the schema, and default namespaces where possible in the
instances.
The report lists, by number and quantity, the parts billed
to various zip codes, and it provides a description of each
part mentioned. In summarizing the billing data, the
intention of the report is clear and the data is
unambiguous because a number of constraints are in effect.
For example, each zip code appears only once (uniqueness
constraint). Similarly, the description of every billed
part appears only once although parts may be billed to
several zip codes (referential constraint), see for example
part number 455-BX. In the following sections, we'll see
how to specify these constraints using XML Schema.
XML Schema enables us to indicate
that any attribute or element value must be unique within a
certain scope. To indicate that one particular attribute or
element value is unique, we use the unique element first to
"select" a set of elements, and then to identify the
attribute or element "field" relative to each selected
element that has to be unique within the scope of the set
of selected elements. In the case of our report schema,
report.xsd, the
selector
element's xpath
attribute contains an XPath expression,
r:regions/r:zip,
that selects a list of all the
zip elements in a report instance. Likewise, the
field element's
xpath attribute
contains a second XPath expression, @code,
that specifies that the code attribute values
of those elements must be unique. Note that the XPath
expressions limit the scope of what must be unique. The
report might contain another code attribute,
but its value does not have to be unique because it lies
outside the scope defined by the XPath expressions. Also
note that the XPath expressions you can use in the
xpath attribute
are limited to a subset
of the full XPath language defined in XML Path Language 1.0.
We can also indicate combinations of fields that
must be unique. Going
back to our purchase order example, suppose we want each item to have a
unique combination of part number and product name. We could achieve
such a constraint by specifying that for each item element,
the combined values of its partNum attribute
and its productName child must be unique.
To define combinations of values, we simply
use multiple field elements to identify
all the values involved:
In the 1999 quarterly report, the
description of every billed part appears only once. We
could enforce this constraint using unique, however, we also want
to ensure that every part-quantity element listed under a
zip code has a corresponding part description. We enforce
the constraint using the
key and
keyref elements. The report schema, report.xsd, shows that the
key and keyref constructions are
applied using almost the same syntax as unique. The key element
applies to the number attribute value of
part elements that are children of the
parts element. This declaration of
number as a key means that its value must be unique
and cannot be set to nil (i.e. is not nillable), and the
name that is associated with the
key, pNumKey, makes the key referenceable from
elsewhere.
To ensure that the part-quantity elements have
corresponding part descriptions, we say that the
number attribute (
<field xpath="@number"/>) of those
elements
(
<selector xpath="r:regions/r:zip/r:part"/>)
must reference the pNumKey key. This
declaration of number as a keyref does not
mean that its value must be unique, but it does mean there
must exist a pNumKey with the same value.
As you may have figured out by analogy with unique, it is possible to
define combinations of
key and
keyref values. Using this mechanism, we could go
beyond simply requiring the product numbers to be equal,
and define a combination of values that must be equal. Such
values may involve combinations of multiple value types
(string, integer,
date, etc.), provided that the order and type of
the field element
references is the same in both the key and keyref definitions.
5.3 XML Schema Constraints vs. XML 1.0 ID Attributes
XML 1.0
provides a mechanism for ensuring uniqueness using
the ID attribute and its associated attributes IDREF and
IDREFS. This mechanism is also provided in XML Schema
through the ID, IDREF, and IDREFS simple types which can be used
for declaring XML 1.0-style attributes. XML Schema also
introduces new mechanisms that are more flexible and
powerful. For example, XML Schema's mechanisms can be
applied to any element and attribute content, regardless of
its type. In contrast, ID is a type of attribute
and so it cannot be applied to attributes, elements or
their content. Furthermore, Schema enables you to specify
the scope within which uniqueness applies whereas the scope
of an ID is fixed to be the whole document. Finally, Schema
enables you to create
keys or a
keyref from combinations of element and
attribute content whereas ID has no such facility.
5.4 Importing Types
The report schema, report.xsd, makes use of the
simple type xipo:SKU that is defined in
another schema, and in another target
namespace. Recall that we used include so that the schema in
ipo.xsd could make use
of definitions and declarations from address.xsd. We cannot use include here because
it can only pull in definitions and declarations from a
schema whose target namespace is the same as the including
schema's target namespace. Hence, the include element does not
identify a namespace (although it does require a
schemaLocation). The import mechanism that we
describe in this section is an important mechanism that
enables schema components from different target namespaces
to be used together, and hence enables the schema
validation of instance content defined across multiple
namespaces.
To import the type SKU
and use it in the report schema, we identify the namespace
in which SKU is defined, and associate that
namespace with a prefix for use in the report schema.
Concretely, we use the
import element to identify SKU's
target namespace, http://www.example.com/IPO,
and we associate the namespace with the prefix
xipo using a standard namespace declaration. The
simple type SKU, defined in the namespace
http://www.example.com/IPO, may then be
referenced as xipo:SKU in any of the report
schema's definitions and declarations.
In our example, we imported one simple type from one
external namespace, and used it for declaring attributes.
XML Schema in fact permits multiple schema components to be
imported, from multiple namespaces, and they can be
referred to in both definitions and declarations. For
example in
report.xsd we could additionally reuse the
comment element declared in ipo.xsd by referencing that element
in a declaration:
Example
<element ref="xipo:comment"/>
Note however, that we cannot reuse the shipTo
element from ipo.xsd, and
the following is not legal because only global schema
components can be imported:
Example
<element ref="xipo:shipTo"/>
In ipo.xsd,
comment is declared as a global element, in other
words it is declared as an element of the schema. In contrast,
shipTo is declared locally, in other words it is an
element declared inside a complex type definition,
specifically the PurchaseOrderType type.
Complex types can also be imported, and they can be used
as the base types for deriving new types. Only named
complex types can be imported; local, anonymously defined
types cannot. Suppose we want to include in our reports the
name of an analyst, along with contact information. We can
reuse the (globally defined) complex type
USAddress from
address.xsd, and extend it to define a new type
called Analyst in the report
schema by adding the new elements
phone and email:
Using this new type we declare an element called
analyst as part of the purchaseReport
element declaration (declarations not shown) in the report
schema. Then, the following instance document would conform
to the modified report schema:
Example
Instance Document Conforming to Report Schema with
Analyst Type
<r:purchaseReport
xmlns:r="http://www.example.com/Report"
period="P3M" periodEnding="1999-12-31">
<!-- regions and parts elements omitted -->
<r:analyst>
<name>Wendy Uhro</name>
<street>10 Corporate Towers</street>
<city>San Jose</city>
<state>CA</state>
<zip>95113</zip>
<r:phone>408-271-3366</r:phone>
<r:email>[email protected]</r:email>
</r:analyst>
</r:purchaseReport>
Note that the report now has both qualified and unqualified elements. This
is because some of the elements (name, street, city,
state and zip) are locally declared in ipo.xsd, whose
elementFormDefault is unqualified (by default). The other elements in the
example are declared in report.xsd, whose elementFormDefault is set to qualified.
When schema components are imported from multiple
namespaces, each namespace must be identified with a
separate import
element. The
import elements themselves must appear as the
first children of the
schema element. Furthermore, each namespace must
be associated with a prefix, using a standard namespace
declaration, and that prefix is used to qualify references
to any schema components belonging to that namespace.
Finally, import
elements optionally contain a schemaLocation
attribute to help locate resources associated with the
namespaces. We discuss the schemaLocation
attribute in more detail in a later section.
5.4.1 Type Libraries
As XML schemas become more widespread, schema authors will
want to create simple and complex types that can be shared
and used as building blocks for creating new
schemas. XML Schemas already provides types that play this
role, in particular, the types described in the Simple Types appendix and in an
introductory
type library.
Schema authors will undoubtedly want to create their own
libraries of types to represent currency, units of measurement,
business addresses, and so on. Each library might consist of a
schema containing one or more definitions, for example, a schema
containing a currency type:
Example
Example Currency Type in Type Library
<schema targetNamespace="http://www.example.com/Currency"
xmlns:c="http://www.example.com/Currency"
xmlns="http://www.w3.org/2001/XMLSchema">
<annotation>
<documentation xml:lang="en">
Definition of Currency type based on ISO 4217
</documentation>
</annotation>
<complexType name="Currency">
<simpleContent>
<extension base="decimal">
<attribute name="name">
<simpleType>
<restriction base="string">
<enumeration value="AED">
<annotation>
<documentation xml:lang="en">
United Arab Emirates: Dirham (1 Dirham = 100 Fils)
</documentation>
</annotation>
</enumeration>
<enumeration value="AFA">
<annotation>
<documentation xml:lang="en">
Afghanistan: Afghani (1 Afghani = 100 Puls)
</documentation>
</annotation>
</enumeration>
<enumeration value="ALL">
<annotation>
<documentation xml:lang="en">
Albania, Lek (1 Lek = 100 Qindarka)
</documentation>
</annotation>
</enumeration>
<!-- and other currencies -->
</restriction>
</simpleType>
</attribute>
</extension>
</simpleContent>
</complexType>
</schema>
An example of an element appearing in an instance and
having this type:
Example
<convertFrom name="AFA">199.37</convertFrom>
Once we have defined the currency type, we can make it
available for re-use in other schemas through the import mechanism just
described.
5.5 Any Element, Any Attribute
In previous sections we have seen several mechanisms for
extending the content models of complex types. For example,
a mixed content model can contain arbitrary character data
in addition to elements, and for example, a content model
can contain elements whose types are imported
from external namespaces. However, these mechanisms provide
very broad and very narrow controls respectively. The
purpose of this section is to describe a flexible mechanism
that enables content models to be extended by any elements
and attributes belonging to specified namespaces.
To illustrate, consider a version of the quarterly report,
4Q99html.xml, in
which we have embedded an XHTML representation of the XML
parts data. The XHTML content appears as the content of the
element htmlExample, and the default
namespace is changed on the outermost XHTML element
(table) so that all the XHTML elements belong
to the XHTML namespace,
http://www.w3.org/1999/xhtml:
Example
Quarterly Report with
XHTML, 4Q99html.xml
<purchaseReport
xmlns="http://www.example.com/Report"
period="P3M" periodEnding="1999-12-31">
<regions>
<!-- part sales listed by zip code, data from 4Q99.xml -->
</regions>
<parts>
<!-- part descriptions from 4Q99.xml -->
</parts>
<htmlExample>
<table xmlns="http://www.w3.org/1999/xhtml"
border="0" width="100%">
<tr>
<th align="left">Zip Code</th>
<th align="left">Part Number</th>
<th align="left">Quantity</th>
</tr>
<tr><td>95819</td><td> </td><td> </td></tr>
<tr><td> </td><td>872-AA</td><td>1</td></tr>
<tr><td> </td><td>926-AA</td><td>1</td></tr>
<tr><td> </td><td>833-AA</td><td>1</td></tr>
<tr><td> </td><td>455-BX</td><td>1</td></tr>
<tr><td>63143</td><td> </td><td> </td></tr>
<tr><td> </td><td>455-BX</td><td>4</td></tr>
</table>
</htmlExample>
</purchaseReport>
To permit the appearance of XHTML in
the instance document we modify the report schema by
declaring a new element htmlExample whose
content is defined by the
any element. In general, an any element specifies that any
well-formed XML is permissible in a type's content model.
In the example, we require the XML to belong to the
namespace http://www.w3.org/1999/xhtml, in
other words, it should be XHTML. The example also requires
there to be at least one element present from this
namespace, as indicated by the values of minOccurs and maxOccurs:
Example
Modification to purchaseReport Declaration to Allow
XHTML in Instance
The modification permits some
well-formed XML belonging to the namespace
http://www.w3.org/1999/xhtml to appear inside the
htmlExample element. Therefore 4Q99html.xml is permissible
because there is one element which (with its children) is
well-formed, the element appears inside the appropriate
element (htmlExample), and the instance
document asserts that the element and its content belongs
to the required namespace. However, the XHTML may not
actually be valid because nothing in 4Q99html.xml by itself can
provide that guarantee. If such a guarantee is required,
the value of the processContents
attribute should be set to strict
(the default value). In this case, an XML processor is
obliged to obtain the schema associated with the required
namespace, and validate the XHTML appearing within the
htmlExample element.
In another example, we define a text
type which is
similar to the text type defined in XML Schema's introductory
type library (see also Type Libraries (§5.4.1)), and
is suitable for internationalized human-readable text. The
text type allows an unrestricted mixture of character content
and element content from any namespace, for example Ruby annotations, along with
an optional xml:lang attribute. The lax
value of the
processContents
attribute instructs an XML processor to validate the element
content on a can-do basis: It will validate elements and attributes for
which it can obtain schema information, but it will not signal errors
for those it cannot obtain any schema information.
Namespaces may be used to permit and
forbid element content in various ways depending upon the
value of the
namespace attribute, as shown in Table 4:
Table 4. Namespace Attribute in Any
Value of Namespace Attribute
Allowable Element Content
##any
Any well-formed XML from any namespace (default)
##local
Any well-formed XML that is not qualified, i.e. not
declared to be in a namespace
##other
Any well-formed XML that is from a namespace other
than the target
namespace of the type being defined (unqualified
elements are not allowed)
"http://www.w3.org/1999/xhtml ##targetNamespace"
Any well-formed XML belonging to any namespace in the
(whitespace separated) list; ##targetNamespace is
shorthand for the target namespace of the type being
defined
In addition to the any element which enables element
content according to namespaces, there is a corresponding
anyAttribute element which enables attributes to
appear in elements. For example, we can permit any XHTML
attribute to appear as part of the htmlExample
element by adding
anyAttribute to its declaration:
Example
Modification to htmlExample Declaration to Allow XHTML
Attributes
1. In an instance document, the
attribute
xsi:schemaLocation provides hints from the
author to a processor regarding the location of schema
documents. The author warrants that these schema documents
are relevant to checking the validity of the document
content, on a namespace by namespace basis. For example, we
can indicate the location of the Report schema to a
processor of the Quarterly Report:
Example
Using schemaLocation in the Quarterly Report,
4Q99html.xml
The schemaLocation
attribute value consists of one or more pairs of URI references,
separated by white space. The first member of each pair is a namespace
name, and the second member of the pair is a hint describing where to
find an appropriate schema document for that namespace.
The presence of
these hints does not require the processor to obtain or use
the cited schema documents, and the processor is free to
use other schemas obtained by any suitable means, or to use
no schema at all.
A schema is not required to have a namespace (see Undeclared Target Namespaces (§3.4)) and so there is a
noNamespaceSchemaLocation attribute which is
used to provide hints for the locations of schema documents
that do not have target namespaces.
2. In a schema, the
include element has a required schemaLocation
attribute, and it contains a URI reference which must
identify a schema document. The effect is to compose a
final effective schema by merging the declarations and
definitions of the including and the included schemas. For
example, in Advanced Concepts II: The International Purchase Order (§4), the type
definitions of Address,
USAddress, UKAddress,
USState (along with their attribute and local
element declarations) from
address.xsd were added to the element
declarations of purchaseOrder and
comment, and the type definitions of
PurchaseOrderType, Items and
SKU (along with their attribute and local element
declarations) from
ipo.xsd to create a single schema.
3. Also in a schema, the
import element has optional namespace and schemaLocation
attributes. If present, the schemaLocation
attribute is understood in a way which parallels the
interpretation of
xsi:schemaLocation in (1). Specifically, it
provides a hint from the author to a processor regarding
the location of a schema document that the author warrants
supplies the required components for the namespace
identified by the
namespace attribute. To import components that
are not in any target namespace, the import element is used without
a namespace
attribute (and with or without a schemaLocation
attribute). References to components imported in this
manner are unqualified.
Note that the
schemaLocation is only a hint and some
processors and applications will have reasons to not use
it. For example, an XHTML editor may have a built-in XHTML
schema.
5.7 Conformance
An instance document may be processed against a schema to
verify whether the rules specified in the schema are
honored in the instance. Typically, such processing
actually does two things, (1) it checks for conformance to
the rules, a process called schema validation, and (2) it
adds supplementary information that is not immediately
present in the instance, such as types and default values,
called infoset contributions.
The author of an instance document, such as a particular
purchase order, may claim, in the instance itself, that it
conforms to the rules in a particular schema. The author
does this using the schemaLocation
attribute discussed above. But regardless of whether a
schemaLocation attribute is present, an
application is free to process the document against any
schema. For example, a purchasing application may have the
policy of always using a certain purchase order schema,
regardless of any schemaLocation
values.
Conformance checking can be thought of as proceeding in
steps, first checking that the root element of the document
instance has the right contents, then checking that each
subelement conforms to its description in a schema, and so
on until the entire document is verified. Processors are
required to report what checking has been carried out.
To check an element for conformance, the processor first
locates the declaration for the element in a schema, and
then checks that the targetNamespace
attribute in the schema matches the actual namespace URI of
the element. Alternatively, it may determine that the schema does not
have a
targetNamespace attribute and the instance
element is not namespace-qualified.
Supposing the namespaces match, the processor then
examines the type of the element, either as given by the
declaration in the schema, or by an xsi:type attribute in the
instance. If the latter, the instance type must be an
allowed substitution for the type given in the schema; what
is allowed is controlled by the block attribute in the
element declaration. At this same time, default values and
other infoset contributions are applied.
Next the processor checks the immediate attributes and
contents of the element, comparing these against the
attributes and contents permitted by the element's type.
For example, considering a shipTo element such
as the one in The Purchase Order Schema (§2.1), the
processor checks what is permitted for an
Address, because that is the shipTo
element's type.
If the element has a simple type, the processor verifies
that the element has no attributes or contained elements,
and that its character content matches the rules for the
simple type. This sometimes involves checking the character
sequence against regular expressions or enumerations, and
sometimes it involves checking that the character sequence
represents a value in a permitted range.
If the element has a complex type, then the processor
checks that any required attributes are present and that
their values conform to the requirements of their simple
types. It also checks that all required subelements are
present, and that the sequence of subelements (and any
mixed text) matches the content model declared for the
complex type. Regarding subelements, schemas can either
require exact name matching, permit substitution by an
equivalent element or permit substitution by any element
allowed by an 'any' particle.
Unless a schema indicates otherwise (as it can for 'any'
particles) conformance checking then proceeds one level
more deeply by looking at each subelement in turn,
repeating the process described above.
A Acknowledgements
Many people have contributed ideas, material and feedback
that has improved this document. In particular, the editor
acknowledges contributions from David Beech,
Paul Biron, Don Box, Allen Brown, David Cleary, Dan
Connolly, Roger Costello, Martin Dürst, Martin Gudgin, Dave Hollander,
Joe Kesselman, John McCarthy, Andrew Layman, Eve Maler,
Ashok Malhotra, Noah Mendelsohn, Michael Sperberg-McQueen,
Henry Thompson, Misha Wolf, and Priscilla Walmsley for
validating the examples.
At the time the first edition of this
specification was published, the members of the XML Schema Working Group
were:
Jim Barnette, Defense Information Systems Agency (DISA)
Paul V. Biron, Health Level Seven
Don Box, DevelopMentor
Allen Brown, Microsoft
Lee Buck, TIBCO Extensibility
Charles E. Campbell, Informix
Wayne Carr, Intel
Peter Chen, Bootstrap Alliance and LSU
David Cleary, Progress Software
Dan Connolly, W3C (staff contact)
Ugo Corda, Xerox
Roger L. Costello, MITRE
Haavard Danielson, Progress Software
Josef Dietl, Mozquito Technologies
David Ezell, Hewlett-Packard Company
Alexander Falk, Altova GmbH
David Fallside, IBM
Dan Fox, Defense Logistics Information Service (DLIS)
Matthew Fuchs, Commerce One
Andrew Goodchild, Distributed Systems Technology Centre (DSTC Pty Ltd)
Paul Grosso, Arbortext, Inc
Martin Gudgin, DevelopMentor
Dave Hollander, Contivo, Inc (co-chair)
Mary Holstege, Invited Expert
Jane Hunter, Distributed Systems Technology Centre (DSTC Pty Ltd)
Rick Jelliffe, Academia Sinica
Simon Johnston, Rational Software
Bob Lojek, Mozquito Technologies
Ashok Malhotra, Microsoft
Lisa Martin, IBM
Noah Mendelsohn, Lotus Development Corporation
Adrian Michel, Commerce One
Alex Milowski, Invited Expert
Don Mullen, TIBCO Extensibility
Dave Peterson, Graphic Communications Association
Jonathan Robie, Software AG
Eric Sedlar, Oracle Corp.
C. M. Sperberg-McQueen, W3C (co-chair)
Bob Streich, Calico Commerce
William K. Stumbo, Xerox
Henry S. Thompson, University of Edinburgh
Mark Tucker, Health Level Seven
Asir S. Vedamuthu, webMethods, Inc
Priscilla Walmsley, XMLSolutions
Norm Walsh, Sun Microsystems
Aki Yoshida, SAP AG
Kongyi Zhou, Oracle Corp.
The XML Schema Working Group has benefited in its work from the
participation and contributions of a number of people not currently
members of the Working Group, including
in particular those named below. Affiliations given are those current at
the time of their work with the WG.
Paula Angerstein, Vignette Corporation
David Beech, Oracle Corp.
Gabe Beged-Dov, Rogue Wave Software
Greg Bumgardner, Rogue Wave Software
Dean Burson, Lotus Development Corporation
Mike Cokus, MITRE
Andrew Eisenberg, Progress Software
Rob Ellman, Calico Commerce
George Feinberg, Object Design
Charles Frankston, Microsoft
Ernesto Guerrieri, Inso
Michael Hyman, Microsoft
Renato Iannella, Distributed Systems Technology Centre (DSTC Pty Ltd)
Dianne Kennedy, Graphic Communications Association
Janet Koenig, Sun Microsystems
Setrag Khoshafian, Technology Deployment International (TDI)
Ara Kullukian, Technology Deployment International (TDI)
Andrew Layman, Microsoft
Dmitry Lenkov, Hewlett-Packard Company
John McCarthy, Lawrence Berkeley National Laboratory
Murata Makoto, Xerox
Eve Maler, Sun Microsystems
Murray Maloney, Muzmo Communication, acting for Commerce One
Chris Olds, Wall Data
Frank Olken, Lawrence Berkeley National Laboratory
Shriram Revankar, Xerox
Mark Reinhold, Sun Microsystems
John C. Schneider, MITRE
Lew Shannon, NCR
William Shea, Merrill Lynch
Ralph Swick, W3C
Tony Stewart, Rivcom
Matt Timmermans, Microstar
Jim Trezzo, Oracle Corp.
Steph Tryphonas, Microstar
The lists given above pertain to the first edition.
At the time work on this second edition was completed,
the membership of the Working Group was:
Leonid Arbouzov, Sun Microsystems
Jim Barnette, Defense Information Systems Agency (DISA)
Paul V. Biron, Health Level Seven
Allen Brown, Microsoft
Charles E. Campbell, Invited expert
Peter Chen, Invited expert
Tony Cincotta, NIST
David Ezell, National Association of Convenience Stores
Matthew Fuchs, Invited expert
Sandy Gao, IBM
Andrew Goodchild, Distributed Systems Technology Centre (DSTC Pty Ltd)
Xan Gregg, Invited expert
Mary Holstege, Mark Logic
Mario Jeckle, DaimlerChrysler
Marcel Jemio, Data Interchange Standards Association
Kohsuke Kawaguchi, Sun Microsystems
Ashok Malhotra, Invited expert
Lisa Martin, IBM
Jim Melton, Oracle Corp
Noah Mendelsohn, IBM
Dave Peterson, Invited expert
Anli Shundi, TIBCO Extensibility
C. M. Sperberg-McQueen, W3C (co-chair)
Hoylen Sue, Distributed Systems Technology Centre (DSTC Pty Ltd)
Henry S. Thompson, University of Edinburgh
Asir S. Vedamuthu, webMethods, Inc
Priscilla Walmsley, Invited expert
Kongyi Zhou, Oracle Corp.
We note with sadness the accidental death of Mario Jeckle
shortly after the completion of work on this document.
In addition to those named above, several
people served on the Working Group during the development
of this second edition:
Oriol Carbo, University of Edinburgh
Tyng-Ruey Chuang, Academia Sinica
Joey Coyle, Health Level 7
Tim Ewald, DevelopMentor
Nelson Hung, Corel
Melanie Kudela, Uniform Code Council
Matthew MacKenzie, XML Global
Cliff Schmidt, Microsoft
John Stanton, Defense Information Systems Agency
John Tebbutt, NIST
Ross Thompson, Contivo
Scott Vorthmann, TIBCO Extensibility
B Simple Types & their Facets
The legal values for each simple type can be constrained
through the application of one or more facets. Tables B1.a
and B1.b list all of XML Schema's built-in simple types
and the facets applicable to each type. The names of the
simple types and the facets are linked from the tables to
the corresponding descriptions in XML
Schema Part 2: Datatypes.
Note: (1) Although the whiteSpace facet is
applicable to this type, the only value that can be specified is collapse.
The facets listed in Table B1.b apply only to simple types
which are ordered. Not all simple types are ordered and so
B1.b does not list all of the simple types.
Note: (1) Although the fractionDigits facet is
applicable to this type, the only value that can be specified is zero.
C Using Entities
XML 1.0 provides various types of entities which are named
fragments of content that can be used in the construction
of both DTD's (parameter entities) and instance documents.
In Building Content Models (§2.7), we noted how named
groups mimic parameter entities. In this section we show
how entities can be declared in instance documents, and how
the functional equivalents of entities can be declared in
schemas.
Suppose we want to declare and use an entity in an
instance document, and that document is also constrained by
a schema. For example:
Example
Declaring and referencing an entity in an instance
document.
Here, we declare an entity called eacute as
part of an internal (DTD) subset, and we reference this
entity in the content of the city element.
Note that when this instance document is processed, the
entity will be
resolved
before schema validation takes
place. In other words, a schema processor will determine
the validity of the city element using
Montréal as the element's value.
We can achieve a similar but not identical outcome by
declaring an element in a schema, and by setting the
element's content appropriately:
And this element can be used in an instance document:
Example
Using an element instead of an entity in an instance
document.
<?xml version="1.0" ?>
<purchaseOrder xmlns="http://www.example.com/PO1"
xmlns:c="http://www.example.com/characterElements"
orderDate="1999-10-20">
<!-- etc. -->
<city>Montr<c:eacute/>al</city>
<!-- etc. -->
</purchaseOrder>
In this case, a schema processor will process two
elements, a city element, and an
eacute element for the contents of which the
processor will supply the single character
é. Note that the extra element will
complicate string matching; the two forms of the name
"Montréal" given in the two examples above will not
match each other using normal string-comparison techniques.
D Regular Expressions
XML Schema's
pattern facet uses a regular expression language
that supports
Unicode. It is fully described in
XML Schema Part 2. The language is similar to the
regular expression language used in the Perl
Programming language, although expressions are matched
against entire lexical representations rather than
user-scoped lexical representations such as line and
paragraph. For this reason, the expression language does
not contain the metacharacters ^ and $, although ^ is used
to express exception, e.g. [^0-9]x.
Table D1. Examples of Regular Expressions
Expression
Match(es)
Chapter \d
Chapter 0, Chapter 1, Chapter 2 ....
Chapter\s\d
Chapter followed by a single whitespace character
(space, tab, newline, etc.), followed by a single
digit
Chapter\s\w
Chapter followed by a single whitespace character
(space, tab, newline, etc.), followed by a word
character (XML 1.0
Letter or Digit)
Española
Española
\p{Lu}
any uppercase character, the value of \p{} (e.g.
"Lu") is defined by Unicode
\p{IsGreek}
any Greek character, the 'Is' construction may be
applied to any block name (e.g. "Greek") as defined
by Unicode
\P{IsGreek}
any non-Greek character, the 'Is' construction may be
applied to any block name (e.g. "Greek") as defined
by Unicode
any non-digit character followed by the character x
\Dx
any non-digit character followed by the character x
.x
any character followed by the character x
.*abc.*
1x2abc, abc1x2, z3456abchooray ....
ab{2}x
abbx
ab{2,4}x
abbx, abbbx, abbbbx
ab{2,}x
abbx, abbbx, abbbbx ....
(ab){2}x
ababx
E Index
E.1 XML Schema Elements
Each element name is followed by one or more links to examples
(identified by section number) in the Primer
, plus a link to a formal XML description in either the
Structures or Datatypes parts of the XML Schema
specification.
Each attribute name is
followed by one or more pairs of references. Each pair of
references consists of a link to an example in the Primer,
plus a link to a formal XML description in either the
Structures or Datatypes parts of the XML Schema
specification.
