Spark {{site.SPARK_VERSION}} is built and distributed to work with Scala {{site.SCALA_BINARY_VERSION}} by default. (Spark can be built to work with other versions of Scala, too.) To write applications in Scala, you will need to use a compatible Scala version (e.g. {{site.SCALA_BINARY_VERSION}}.X).

To write a Spark application, you need to add a Maven dependency on Spark. Spark is available through Maven Central at:

groupId = org.apache.spark
artifactId = spark-core_{{site.SCALA_BINARY_VERSION}}
version = {{site.SPARK_VERSION}}

In addition, if you wish to access an HDFS cluster, you need to add a dependency on hadoop-client for your version of HDFS.

groupId = org.apache.hadoop
artifactId = hadoop-client
version = <your-hdfs-version>

Finally, you need to import some Spark classes into your program. Add the following lines:

{% highlight scala %} import org.apache.spark.SparkContext import org.apache.spark.SparkConf {% endhighlight %}

(Before Spark 1.3.0, you need to explicitly import org.apache.spark.SparkContext._ to enable essential implicit conversions.)

Spark {{site.SPARK_VERSION}} supports lambda expressions for concisely writing functions, otherwise you can use the classes in the org.apache.spark.api.java.function package.

Note that support for Java 7 was removed in Spark 2.2.0.

To write a Spark application in Java, you need to add a dependency on Spark. Spark is available through Maven Central at:

groupId = org.apache.spark
artifactId = spark-core_{{site.SCALA_BINARY_VERSION}}
version = {{site.SPARK_VERSION}}

In addition, if you wish to access an HDFS cluster, you need to add a dependency on hadoop-client for your version of HDFS.

groupId = org.apache.hadoop
artifactId = hadoop-client
version = <your-hdfs-version>

Finally, you need to import some Spark classes into your program. Add the following lines:

{% highlight java %} import org.apache.spark.api.java.JavaSparkContext; import org.apache.spark.api.java.JavaRDD; import org.apache.spark.SparkConf; {% endhighlight %}

Spark {{site.SPARK_VERSION}} works with Python 3.6+. It can use the standard CPython interpreter, so C libraries like NumPy can be used. It also works with PyPy 2.3+.

Python 2, 3.4 and 3.5 supports were removed in Spark 3.1.0.

Spark applications in Python can either be run with the bin/spark-submit script which includes Spark at runtime, or by including it in your setup.py as:

{% highlight python %} install_requires=[ 'pyspark=={site.SPARK_VERSION}' ] {% endhighlight %}

To run Spark applications in Python without pip installing PySpark, use the bin/spark-submit script located in the Spark directory. This script will load Spark's Java/Scala libraries and allow you to submit applications to a cluster. You can also use bin/pyspark to launch an interactive Python shell.

If you wish to access HDFS data, you need to use a build of PySpark linking to your version of HDFS. Prebuilt packages are also available on the Spark homepage for common HDFS versions.

Finally, you need to import some Spark classes into your program. Add the following line:

{% highlight python %} from pyspark import SparkContext, SparkConf {% endhighlight %}

PySpark requires the same minor version of Python in both driver and workers. It uses the default python version in PATH, you can specify which version of Python you want to use by PYSPARK_PYTHON, for example:

{% highlight bash %} $ PYSPARK_PYTHON=python3.8 bin/pyspark $ PYSPARK_PYTHON=/path-to-your-pypy/pypy bin/spark-submit examples/src/main/python/pi.py {% endhighlight %}

The first thing a Spark program must do is to create a SparkContext object, which tells Spark how to access a cluster. To create a SparkContext you first need to build a SparkConf object that contains information about your application.

Only one SparkContext should be active per JVM. You must stop() the active SparkContext before creating a new one.

{% highlight scala %} val conf = new SparkConf().setAppName(appName).setMaster(master) new SparkContext(conf) {% endhighlight %}

The first thing a Spark program must do is to create a JavaSparkContext object, which tells Spark how to access a cluster. To create a SparkContext you first need to build a SparkConf object that contains information about your application.

{% highlight java %} SparkConf conf = new SparkConf().setAppName(appName).setMaster(master); JavaSparkContext sc = new JavaSparkContext(conf); {% endhighlight %}

The first thing a Spark program must do is to create a SparkContext object, which tells Spark how to access a cluster. To create a SparkContext you first need to build a SparkConf object that contains information about your application.

{% highlight python %} conf = SparkConf().setAppName(appName).setMaster(master) sc = SparkContext(conf=conf) {% endhighlight %}

In the Spark shell, a special interpreter-aware SparkContext is already created for you, in the variable called sc. Making your own SparkContext will not work. You can set which master the context connects to using the --master argument, and you can add JARs to the classpath by passing a comma-separated list to the --jars argument. You can also add dependencies (e.g. Spark Packages) to your shell session by supplying a comma-separated list of Maven coordinates to the --packages argument. Any additional repositories where dependencies might exist (e.g. Sonatype) can be passed to the --repositories argument. For example, to run bin/spark-shell on exactly four cores, use:

{% highlight bash %} $ ./bin/spark-shell --master local[4] {% endhighlight %}

Or, to also add code.jar to its classpath, use:

{% highlight bash %} $ ./bin/spark-shell --master local[4] --jars code.jar {% endhighlight %}

To include a dependency using Maven coordinates:

{% highlight bash %} $ ./bin/spark-shell --master local[4] --packages "org.example:example:0.1" {% endhighlight %}

For a complete list of options, run spark-shell --help. Behind the scenes, spark-shell invokes the more general spark-submit script.

In the PySpark shell, a special interpreter-aware SparkContext is already created for you, in the variable called sc. Making your own SparkContext will not work. You can set which master the context connects to using the --master argument, and you can add Python .zip, .egg or .py files to the runtime path by passing a comma-separated list to --py-files. You can also add dependencies (e.g. Spark Packages) to your shell session by supplying a comma-separated list of Maven coordinates to the --packages argument. Any additional repositories where dependencies might exist (e.g. Sonatype) can be passed to the --repositories argument. Any Python dependencies a Spark package has (listed in the requirements.txt of that package) must be manually installed using pip when necessary. For example, to run bin/pyspark on exactly four cores, use:

{% highlight bash %} $ ./bin/pyspark --master local[4] {% endhighlight %}

Or, to also add code.py to the search path (in order to later be able to import code), use:

{% highlight bash %} $ ./bin/pyspark --master local[4] --py-files code.py {% endhighlight %}

For a complete list of options, run pyspark --help. Behind the scenes, pyspark invokes the more general spark-submit script.

It is also possible to launch the PySpark shell in IPython, the enhanced Python interpreter. PySpark works with IPython 1.0.0 and later. To use IPython, set the PYSPARK_DRIVER_PYTHON variable to ipython when running bin/pyspark:

{% highlight bash %} $ PYSPARK_DRIVER_PYTHON=ipython ./bin/pyspark {% endhighlight %}

To use the Jupyter notebook (previously known as the IPython notebook),

{% highlight bash %} $ PYSPARK_DRIVER_PYTHON=jupyter PYSPARK_DRIVER_PYTHON_OPTS=notebook ./bin/pyspark {% endhighlight %}

You can customize the ipython or jupyter commands by setting PYSPARK_DRIVER_PYTHON_OPTS.

After the Jupyter Notebook server is launched, you can create a new notebook from the "Files" tab. Inside the notebook, you can input the command %pylab inline as part of your notebook before you start to try Spark from the Jupyter notebook.

Parallelized collections are created by calling SparkContext's parallelize method on an existing collection in your driver program (a Scala Seq). The elements of the collection are copied to form a distributed dataset that can be operated on in parallel. For example, here is how to create a parallelized collection holding the numbers 1 to 5:

{% highlight scala %} val data = Array(1, 2, 3, 4, 5) val distData = sc.parallelize(data) {% endhighlight %}

Once created, the distributed dataset (distData) can be operated on in parallel. For example, we might call distData.reduce((a, b) => a + b) to add up the elements of the array. We describe operations on distributed datasets later on.

Parallelized collections are created by calling JavaSparkContext's parallelize method on an existing Collection in your driver program. The elements of the collection are copied to form a distributed dataset that can be operated on in parallel. For example, here is how to create a parallelized collection holding the numbers 1 to 5:

{% highlight java %} List data = Arrays.asList(1, 2, 3, 4, 5); JavaRDD distData = sc.parallelize(data); {% endhighlight %}

Once created, the distributed dataset (distData) can be operated on in parallel. For example, we might call distData.reduce((a, b) -> a + b) to add up the elements of the list. We describe operations on distributed datasets later on.

Parallelized collections are created by calling SparkContext's parallelize method on an existing iterable or collection in your driver program. The elements of the collection are copied to form a distributed dataset that can be operated on in parallel. For example, here is how to create a parallelized collection holding the numbers 1 to 5:

{% highlight python %} data = [1, 2, 3, 4, 5] distData = sc.parallelize(data) {% endhighlight %}

Once created, the distributed dataset (distData) can be operated on in parallel. For example, we can call distData.reduce(lambda a, b: a + b) to add up the elements of the list. We describe operations on distributed datasets later on.

Spark can create distributed datasets from any storage source supported by Hadoop, including your local file system, HDFS, Cassandra, HBase, Amazon S3, etc. Spark supports text files, SequenceFiles, and any other Hadoop InputFormat.

Text file RDDs can be created using SparkContext's textFile method. This method takes a URI for the file (either a local path on the machine, or a hdfs://, s3a://, etc URI) and reads it as a collection of lines. Here is an example invocation:

{% highlight scala %} scala> val distFile = sc.textFile("data.txt") distFile: org.apache.spark.rdd.RDD[String] = data.txt MapPartitionsRDD[10] at textFile at :26 {% endhighlight %}

Once created, distFile can be acted on by dataset operations. For example, we can add up the sizes of all the lines using the map and reduce operations as follows: distFile.map(s => s.length).reduce((a, b) => a + b).

Some notes on reading files with Spark:

• If using a path on the local filesystem, the file must also be accessible at the same path on worker nodes. Either copy the file to all workers or use a network-mounted shared file system.

• All of Spark's file-based input methods, including textFile, support running on directories, compressed files, and wildcards as well. For example, you can use textFile("/my/directory"), textFile("/my/directory/*.txt"), and textFile("/my/directory/*.gz"). When multiple files are read, the order of the partitions depends on the order the files are returned from the filesystem. It may or may not, for example, follow the lexicographic ordering of the files by path. Within a partition, elements are ordered according to their order in the underlying file.

• The textFile method also takes an optional second argument for controlling the number of partitions of the file. By default, Spark creates one partition for each block of the file (blocks being 128MB by default in HDFS), but you can also ask for a higher number of partitions by passing a larger value. Note that you cannot have fewer partitions than blocks.

Apart from text files, Spark's Scala API also supports several other data formats:

• SparkContext.wholeTextFiles lets you read a directory containing multiple small text files, and returns each of them as (filename, content) pairs. This is in contrast with textFile, which would return one record per line in each file. Partitioning is determined by data locality which, in some cases, may result in too few partitions. For those cases, wholeTextFiles provides an optional second argument for controlling the minimal number of partitions.

• For SequenceFiles, use SparkContext's sequenceFile[K, V] method where K and V are the types of key and values in the file. These should be subclasses of Hadoop's Writable interface, like IntWritable and Text. In addition, Spark allows you to specify native types for a few common Writables; for example, sequenceFile[Int, String] will automatically read IntWritables and Texts.

• For other Hadoop InputFormats, you can use the SparkContext.hadoopRDD method, which takes an arbitrary JobConf and input format class, key class and value class. Set these the same way you would for a Hadoop job with your input source. You can also use SparkContext.newAPIHadoopRDD for InputFormats based on the "new" MapReduce API (org.apache.hadoop.mapreduce).

• RDD.saveAsObjectFile and SparkContext.objectFile support saving an RDD in a simple format consisting of serialized Java objects. While this is not as efficient as specialized formats like Avro, it offers an easy way to save any RDD.

Spark can create distributed datasets from any storage source supported by Hadoop, including your local file system, HDFS, Cassandra, HBase, Amazon S3, etc. Spark supports text files, SequenceFiles, and any other Hadoop InputFormat.

Text file RDDs can be created using SparkContext's textFile method. This method takes a URI for the file (either a local path on the machine, or a hdfs://, s3a://, etc URI) and reads it as a collection of lines. Here is an example invocation:

{% highlight java %} JavaRDD distFile = sc.textFile("data.txt"); {% endhighlight %}

Once created, distFile can be acted on by dataset operations. For example, we can add up the sizes of all the lines using the map and reduce operations as follows: distFile.map(s -> s.length()).reduce((a, b) -> a + b).

Some notes on reading files with Spark:

• If using a path on the local filesystem, the file must also be accessible at the same path on worker nodes. Either copy the file to all workers or use a network-mounted shared file system.

• All of Spark's file-based input methods, including textFile, support running on directories, compressed files, and wildcards as well. For example, you can use textFile("/my/directory"), textFile("/my/directory/*.txt"), and textFile("/my/directory/*.gz").

• The textFile method also takes an optional second argument for controlling the number of partitions of the file. By default, Spark creates one partition for each block of the file (blocks being 128MB by default in HDFS), but you can also ask for a higher number of partitions by passing a larger value. Note that you cannot have fewer partitions than blocks.

Apart from text files, Spark's Java API also supports several other data formats:

• JavaSparkContext.wholeTextFiles lets you read a directory containing multiple small text files, and returns each of them as (filename, content) pairs. This is in contrast with textFile, which would return one record per line in each file.

• For SequenceFiles, use SparkContext's sequenceFile[K, V] method where K and V are the types of key and values in the file. These should be subclasses of Hadoop's Writable interface, like IntWritable and Text.

• For other Hadoop InputFormats, you can use the JavaSparkContext.hadoopRDD method, which takes an arbitrary JobConf and input format class, key class and value class. Set these the same way you would for a Hadoop job with your input source. You can also use JavaSparkContext.newAPIHadoopRDD for InputFormats based on the "new" MapReduce API (org.apache.hadoop.mapreduce).

• JavaRDD.saveAsObjectFile and JavaSparkContext.objectFile support saving an RDD in a simple format consisting of serialized Java objects. While this is not as efficient as specialized formats like Avro, it offers an easy way to save any RDD.

PySpark can create distributed datasets from any storage source supported by Hadoop, including your local file system, HDFS, Cassandra, HBase, Amazon S3, etc. Spark supports text files, SequenceFiles, and any other Hadoop InputFormat.

Text file RDDs can be created using SparkContext's textFile method. This method takes a URI for the file (either a local path on the machine, or a hdfs://, s3a://, etc URI) and reads it as a collection of lines. Here is an example invocation:

{% highlight python %}

distFile = sc.textFile("data.txt") {% endhighlight %}

Once created, distFile can be acted on by dataset operations. For example, we can add up the sizes of all the lines using the map and reduce operations as follows: distFile.map(lambda s: len(s)).reduce(lambda a, b: a + b).

Some notes on reading files with Spark:

• If using a path on the local filesystem, the file must also be accessible at the same path on worker nodes. Either copy the file to all workers or use a network-mounted shared file system.

• All of Spark's file-based input methods, including textFile, support running on directories, compressed files, and wildcards as well. For example, you can use textFile("/my/directory"), textFile("/my/directory/*.txt"), and textFile("/my/directory/*.gz").

• The textFile method also takes an optional second argument for controlling the number of partitions of the file. By default, Spark creates one partition for each block of the file (blocks being 128MB by default in HDFS), but you can also ask for a higher number of partitions by passing a larger value. Note that you cannot have fewer partitions than blocks.

Apart from text files, Spark's Python API also supports several other data formats:

• SparkContext.wholeTextFiles lets you read a directory containing multiple small text files, and returns each of them as (filename, content) pairs. This is in contrast with textFile, which would return one record per line in each file.

• RDD.saveAsPickleFile and SparkContext.pickleFile support saving an RDD in a simple format consisting of pickled Python objects. Batching is used on pickle serialization, with default batch size 10.

• SequenceFile and Hadoop Input/Output Formats

Note this feature is currently marked Experimental and is intended for advanced users. It may be replaced in future with read/write support based on Spark SQL, in which case Spark SQL is the preferred approach.

Writable Support

PySpark SequenceFile support loads an RDD of key-value pairs within Java, converts Writables to base Java types, and pickles the resulting Java objects using Pyrolite. When saving an RDD of key-value pairs to SequenceFile, PySpark does the reverse. It unpickles Python objects into Java objects and then converts them to Writables. The following Writables are automatically converted:

Arrays are not handled out-of-the-box. Users need to specify custom ArrayWritable subtypes when reading or writing. When writing, users also need to specify custom converters that convert arrays to custom ArrayWritable subtypes. When reading, the default converter will convert custom ArrayWritable subtypes to Java Object[], which then get pickled to Python tuples. To get Python array.array for arrays of primitive types, users need to specify custom converters.

Saving and Loading SequenceFiles

Similarly to text files, SequenceFiles can be saved and loaded by specifying the path. The key and value classes can be specified, but for standard Writables this is not required.

{% highlight python %}

rdd = sc.parallelize(range(1, 4)).map(lambda x: (x, "a" * x)) rdd.saveAsSequenceFile("path/to/file") sorted(sc.sequenceFile("path/to/file").collect()) [(1, u'a'), (2, u'aa'), (3, u'aaa')] {% endhighlight %}

Saving and Loading Other Hadoop Input/Output Formats

PySpark can also read any Hadoop InputFormat or write any Hadoop OutputFormat, for both 'new' and 'old' Hadoop MapReduce APIs. If required, a Hadoop configuration can be passed in as a Python dict. Here is an example using the Elasticsearch ESInputFormat:

{% highlight python %} $ ./bin/pyspark --jars /path/to/elasticsearch-hadoop.jar

conf = {"es.resource" : "index/type"} # assume Elasticsearch is running on localhost defaults rdd = sc.newAPIHadoopRDD("org.elasticsearch.hadoop.mr.EsInputFormat", "org.apache.hadoop.io.NullWritable", "org.elasticsearch.hadoop.mr.LinkedMapWritable", conf=conf) rdd.first() # the result is a MapWritable that is converted to a Python dict (u'Elasticsearch ID', {u'field1': True, u'field2': u'Some Text', u'field3': 12345}) {% endhighlight %}

Note that, if the InputFormat simply depends on a Hadoop configuration and/or input path, and the key and value classes can easily be converted according to the above table, then this approach should work well for such cases.

If you have custom serialized binary data (such as loading data from Cassandra / HBase), then you will first need to transform that data on the Scala/Java side to something which can be handled by Pyrolite's pickler. A Converter trait is provided for this. Simply extend this trait and implement your transformation code in the convert method. Remember to ensure that this class, along with any dependencies required to access your InputFormat, are packaged into your Spark job jar and included on the PySpark classpath.

See the Python examples and the Converter examples for examples of using Cassandra / HBase InputFormat and OutputFormat with custom converters.

To illustrate RDD basics, consider the simple program below:

{% highlight scala %} val lines = sc.textFile("data.txt") val lineLengths = lines.map(s => s.length) val totalLength = lineLengths.reduce((a, b) => a + b) {% endhighlight %}

The first line defines a base RDD from an external file. This dataset is not loaded in memory or otherwise acted on: lines is merely a pointer to the file. The second line defines lineLengths as the result of a map transformation. Again, lineLengths is not immediately computed, due to laziness. Finally, we run reduce, which is an action. At this point Spark breaks the computation into tasks to run on separate machines, and each machine runs both its part of the map and a local reduction, returning only its answer to the driver program.

If we also wanted to use lineLengths again later, we could add:

{% highlight scala %} lineLengths.persist() {% endhighlight %}

before the reduce, which would cause lineLengths to be saved in memory after the first time it is computed.

To illustrate RDD basics, consider the simple program below:

{% highlight java %} JavaRDD lines = sc.textFile("data.txt"); JavaRDD lineLengths = lines.map(s -> s.length()); int totalLength = lineLengths.reduce((a, b) -> a + b); {% endhighlight %}

The first line defines a base RDD from an external file. This dataset is not loaded in memory or otherwise acted on: lines is merely a pointer to the file. The second line defines lineLengths as the result of a map transformation. Again, lineLengths is not immediately computed, due to laziness. Finally, we run reduce, which is an action. At this point Spark breaks the computation into tasks to run on separate machines, and each machine runs both its part of the map and a local reduction, returning only its answer to the driver program.

If we also wanted to use lineLengths again later, we could add:

{% highlight java %} lineLengths.persist(StorageLevel.MEMORY_ONLY()); {% endhighlight %}

before the reduce, which would cause lineLengths to be saved in memory after the first time it is computed.

To illustrate RDD basics, consider the simple program below:

{% highlight python %} lines = sc.textFile("data.txt") lineLengths = lines.map(lambda s: len(s)) totalLength = lineLengths.reduce(lambda a, b: a + b) {% endhighlight %}

The first line defines a base RDD from an external file. This dataset is not loaded in memory or otherwise acted on: lines is merely a pointer to the file. The second line defines lineLengths as the result of a map transformation. Again, lineLengths is not immediately computed, due to laziness. Finally, we run reduce, which is an action. At this point Spark breaks the computation into tasks to run on separate machines, and each machine runs both its part of the map and a local reduction, returning only its answer to the driver program.

If we also wanted to use lineLengths again later, we could add:

{% highlight python %} lineLengths.persist() {% endhighlight %}

before the reduce, which would cause lineLengths to be saved in memory after the first time it is computed.

Spark's API relies heavily on passing functions in the driver program to run on the cluster. There are two recommended ways to do this:

• Anonymous function syntax, which can be used for short pieces of code.
• Static methods in a global singleton object. For example, you can define object MyFunctions and then pass MyFunctions.func1, as follows:

{% highlight scala %} object MyFunctions { def func1(s: String): String = { ... } }

myRdd.map(MyFunctions.func1) {% endhighlight %}

Note that while it is also possible to pass a reference to a method in a class instance (as opposed to a singleton object), this requires sending the object that contains that class along with the method. For example, consider:

{% highlight scala %} class MyClass { def func1(s: String): String = { ... } def doStuff(rdd: RDD[String]): RDD[String] = { rdd.map(func1) } } {% endhighlight %}

Here, if we create a new MyClass instance and call doStuff on it, the map inside there references the func1 method of that MyClass instance, so the whole object needs to be sent to the cluster. It is similar to writing rdd.map(x => this.func1(x)).

In a similar way, accessing fields of the outer object will reference the whole object:

{% highlight scala %} class MyClass { val field = "Hello" def doStuff(rdd: RDD[String]): RDD[String] = { rdd.map(x => field + x) } } {% endhighlight %}

is equivalent to writing rdd.map(x => this.field + x), which references all of this. To avoid this issue, the simplest way is to copy field into a local variable instead of accessing it externally:

{% highlight scala %} def doStuff(rdd: RDD[String]): RDD[String] = { val field_ = this.field rdd.map(x => field_ + x) } {% endhighlight %}

Spark's API relies heavily on passing functions in the driver program to run on the cluster. In Java, functions are represented by classes implementing the interfaces in the org.apache.spark.api.java.function package. There are two ways to create such functions:

• Implement the Function interfaces in your own class, either as an anonymous inner class or a named one, and pass an instance of it to Spark.
• Use lambda expressions to concisely define an implementation.

While much of this guide uses lambda syntax for conciseness, it is easy to use all the same APIs in long-form. For example, we could have written our code above as follows:

{% highlight java %} JavaRDD lines = sc.textFile("data.txt"); JavaRDD lineLengths = lines.map(new Function<String, Integer>() { public Integer call(String s) { return s.length(); } }); int totalLength = lineLengths.reduce(new Function2<Integer, Integer, Integer>() { public Integer call(Integer a, Integer b) { return a + b; } }); {% endhighlight %}

Or, if writing the functions inline is unwieldy:

{% highlight java %} class GetLength implements Function<String, Integer> { public Integer call(String s) { return s.length(); } } class Sum implements Function2<Integer, Integer, Integer> { public Integer call(Integer a, Integer b) { return a + b; } }

JavaRDD lines = sc.textFile("data.txt"); JavaRDD lineLengths = lines.map(new GetLength()); int totalLength = lineLengths.reduce(new Sum()); {% endhighlight %}

Note that anonymous inner classes in Java can also access variables in the enclosing scope as long as they are marked final. Spark will ship copies of these variables to each worker node as it does for other languages.

Spark's API relies heavily on passing functions in the driver program to run on the cluster. There are three recommended ways to do this:

• Lambda expressions, for simple functions that can be written as an expression. (Lambdas do not support multi-statement functions or statements that do not return a value.)
• Local defs inside the function calling into Spark, for longer code.
• Top-level functions in a module.

For example, to pass a longer function than can be supported using a lambda, consider the code below:

{% highlight python %} """MyScript.py""" if name == "main": def myFunc(s): words = s.split(" ") return len(words)

sc = SparkContext(...)
sc.textFile("file.txt").map(myFunc)

{% endhighlight %}

Note that while it is also possible to pass a reference to a method in a class instance (as opposed to a singleton object), this requires sending the object that contains that class along with the method. For example, consider:

{% highlight python %} class MyClass(object): def func(self, s): return s def doStuff(self, rdd): return rdd.map(self.func) {% endhighlight %}

Here, if we create a new MyClass and call doStuff on it, the map inside there references the func method of that MyClass instance, so the whole object needs to be sent to the cluster.

In a similar way, accessing fields of the outer object will reference the whole object:

{% highlight python %} class MyClass(object): def init(self): self.field = "Hello" def doStuff(self, rdd): return rdd.map(lambda s: self.field + s) {% endhighlight %}

To avoid this issue, the simplest way is to copy field into a local variable instead of accessing it externally:

{% highlight python %} def doStuff(self, rdd): field = self.field return rdd.map(lambda s: field + s) {% endhighlight %}

// Wrong: Don't do this!! rdd.foreach(x => counter += x)

println("Counter value: " + counter) {% endhighlight %}

// Wrong: Don't do this!! rdd.foreach(x -> counter += x);

println("Counter value: " + counter); {% endhighlight %}

def increment_counter(x): global counter counter += x rdd.foreach(increment_counter)

print("Counter value: ", counter) {% endhighlight %}

While most Spark operations work on RDDs containing any type of objects, a few special operations are only available on RDDs of key-value pairs. The most common ones are distributed "shuffle" operations, such as grouping or aggregating the elements by a key.

In Scala, these operations are automatically available on RDDs containing Tuple2 objects (the built-in tuples in the language, created by simply writing (a, b)). The key-value pair operations are available in the PairRDDFunctions class, which automatically wraps around an RDD of tuples.

For example, the following code uses the reduceByKey operation on key-value pairs to count how many times each line of text occurs in a file:

{% highlight scala %} val lines = sc.textFile("data.txt") val pairs = lines.map(s => (s, 1)) val counts = pairs.reduceByKey((a, b) => a + b) {% endhighlight %}

We could also use counts.sortByKey(), for example, to sort the pairs alphabetically, and finally counts.collect() to bring them back to the driver program as an array of objects.

Note: when using custom objects as the key in key-value pair operations, you must be sure that a custom equals() method is accompanied with a matching hashCode() method. For full details, see the contract outlined in the Object.hashCode() documentation.

While most Spark operations work on RDDs containing any type of objects, a few special operations are only available on RDDs of key-value pairs. The most common ones are distributed "shuffle" operations, such as grouping or aggregating the elements by a key.

In Java, key-value pairs are represented using the scala.Tuple2 class from the Scala standard library. You can simply call new Tuple2(a, b) to create a tuple, and access its fields later with tuple._1() and tuple._2().

RDDs of key-value pairs are represented by the JavaPairRDD class. You can construct JavaPairRDDs from JavaRDDs using special versions of the map operations, like mapToPair and flatMapToPair. The JavaPairRDD will have both standard RDD functions and special key-value ones.

For example, the following code uses the reduceByKey operation on key-value pairs to count how many times each line of text occurs in a file:

{% highlight scala %} JavaRDD lines = sc.textFile("data.txt"); JavaPairRDD<String, Integer> pairs = lines.mapToPair(s -> new Tuple2(s, 1)); JavaPairRDD<String, Integer> counts = pairs.reduceByKey((a, b) -> a + b); {% endhighlight %}

We could also use counts.sortByKey(), for example, to sort the pairs alphabetically, and finally counts.collect() to bring them back to the driver program as an array of objects.

Note: when using custom objects as the key in key-value pair operations, you must be sure that a custom equals() method is accompanied with a matching hashCode() method. For full details, see the contract outlined in the Object.hashCode() documentation.

While most Spark operations work on RDDs containing any type of objects, a few special operations are only available on RDDs of key-value pairs. The most common ones are distributed "shuffle" operations, such as grouping or aggregating the elements by a key.

In Python, these operations work on RDDs containing built-in Python tuples such as (1, 2). Simply create such tuples and then call your desired operation.

For example, the following code uses the reduceByKey operation on key-value pairs to count how many times each line of text occurs in a file:

{% highlight python %} lines = sc.textFile("data.txt") pairs = lines.map(lambda s: (s, 1)) counts = pairs.reduceByKey(lambda a, b: a + b) {% endhighlight %}

We could also use counts.sortByKey(), for example, to sort the pairs alphabetically, and finally counts.collect() to bring them back to the driver program as a list of objects.

{% highlight scala %} scala> val broadcastVar = sc.broadcast(Array(1, 2, 3)) broadcastVar: org.apache.spark.broadcast.Broadcast[Array[Int]] = Broadcast(0)

scala> broadcastVar.value res0: Array[Int] = Array(1, 2, 3) {% endhighlight %}

{% highlight java %} Broadcast<int[]> broadcastVar = sc.broadcast(new int[] {1, 2, 3});

broadcastVar.value(); // returns [1, 2, 3] {% endhighlight %}

{% highlight python %}

broadcastVar = sc.broadcast([1, 2, 3]) <pyspark.broadcast.Broadcast object at 0x102789f10>

broadcastVar.value [1, 2, 3] {% endhighlight %}

A numeric accumulator can be created by calling SparkContext.longAccumulator() or SparkContext.doubleAccumulator() to accumulate values of type Long or Double, respectively. Tasks running on a cluster can then add to it using the add method. However, they cannot read its value. Only the driver program can read the accumulator's value, using its value method.

The code below shows an accumulator being used to add up the elements of an array:

{% highlight scala %} scala> val accum = sc.longAccumulator("My Accumulator") accum: org.apache.spark.util.LongAccumulator = LongAccumulator(id: 0, name: Some(My Accumulator), value: 0)

scala> sc.parallelize(Array(1, 2, 3, 4)).foreach(x => accum.add(x)) ... 10/09/29 18:41:08 INFO SparkContext: Tasks finished in 0.317106 s

scala> accum.value res2: Long = 10 {% endhighlight %}

While this code used the built-in support for accumulators of type Long, programmers can also create their own types by subclassing AccumulatorV2. The AccumulatorV2 abstract class has several methods which one has to override: reset for resetting the accumulator to zero, add for adding another value into the accumulator, merge for merging another same-type accumulator into this one. Other methods that must be overridden are contained in the API documentation. For example, supposing we had a MyVector class representing mathematical vectors, we could write:

{% highlight scala %} class VectorAccumulatorV2 extends AccumulatorV2[MyVector, MyVector] {

private val myVector: MyVector = MyVector.createZeroVector

def reset(): Unit = { myVector.reset() }

def add(v: MyVector): Unit = { myVector.add(v) } ... }

// Then, create an Accumulator of this type: val myVectorAcc = new VectorAccumulatorV2 // Then, register it into spark context: sc.register(myVectorAcc, "MyVectorAcc1") {% endhighlight %}

Note that, when programmers define their own type of AccumulatorV2, the resulting type can be different than that of the elements added.

A numeric accumulator can be created by calling SparkContext.longAccumulator() or SparkContext.doubleAccumulator() to accumulate values of type Long or Double, respectively. Tasks running on a cluster can then add to it using the add method. However, they cannot read its value. Only the driver program can read the accumulator's value, using its value method.

The code below shows an accumulator being used to add up the elements of an array:

{% highlight java %} LongAccumulator accum = jsc.sc().longAccumulator();

sc.parallelize(Arrays.asList(1, 2, 3, 4)).foreach(x -> accum.add(x)); // ... // 10/09/29 18:41:08 INFO SparkContext: Tasks finished in 0.317106 s

accum.value(); // returns 10 {% endhighlight %}

While this code used the built-in support for accumulators of type Long, programmers can also create their own types by subclassing AccumulatorV2. The AccumulatorV2 abstract class has several methods which one has to override: reset for resetting the accumulator to zero, add for adding another value into the accumulator, merge for merging another same-type accumulator into this one. Other methods that must be overridden are contained in the API documentation. For example, supposing we had a MyVector class representing mathematical vectors, we could write:

{% highlight java %} class VectorAccumulatorV2 implements AccumulatorV2<MyVector, MyVector> {

private MyVector myVector = MyVector.createZeroVector();

public void reset() { myVector.reset(); }

public void add(MyVector v) { myVector.add(v); } ... }

// Then, create an Accumulator of this type: VectorAccumulatorV2 myVectorAcc = new VectorAccumulatorV2(); // Then, register it into spark context: jsc.sc().register(myVectorAcc, "MyVectorAcc1"); {% endhighlight %}

Note that, when programmers define their own type of AccumulatorV2, the resulting type can be different than that of the elements added.

Warning: When a Spark task finishes, Spark will try to merge the accumulated updates in this task to an accumulator. If it fails, Spark will ignore the failure and still mark the task successful and continue to run other tasks. Hence, a buggy accumulator will not impact a Spark job, but it may not get updated correctly although a Spark job is successful.

An accumulator is created from an initial value v by calling SparkContext.accumulator(v). Tasks running on a cluster can then add to it using the add method or the += operator. However, they cannot read its value. Only the driver program can read the accumulator's value, using its value method.

The code below shows an accumulator being used to add up the elements of an array:

{% highlight python %}

accum = sc.accumulator(0) accum Accumulator<id=0, value=0>

sc.parallelize([1, 2, 3, 4]).foreach(lambda x: accum.add(x)) ... 10/09/29 18:41:08 INFO SparkContext: Tasks finished in 0.317106 s

accum.value 10 {% endhighlight %}

While this code used the built-in support for accumulators of type Int, programmers can also create their own types by subclassing AccumulatorParam. The AccumulatorParam interface has two methods: zero for providing a "zero value" for your data type, and addInPlace for adding two values together. For example, supposing we had a Vector class representing mathematical vectors, we could write:

{% highlight python %} class VectorAccumulatorParam(AccumulatorParam): def zero(self, initialValue): return Vector.zeros(initialValue.size)

def addInPlace(self, v1, v2):
    v1 += v2
    return v1

vecAccum = sc.accumulator(Vector(...), VectorAccumulatorParam()) {% endhighlight %}