Traffic management overview for internal Application Load Balancers

• Traffic steering. Intelligently route traffic based on HTTP(S) parameters (for example, host, path, headers, and other request parameters).
• Traffic actions. Perform request-based and response-based actions (for example, redirects and header transformations).
• Traffic policies. Fine-tune load balancing behavior (for example, advanced load balancing algorithms).

You can set up these features by using URL maps and backend services. For more information, see the following topics:

• URL map overview
• Using URL maps
• Understanding backend services

Use case examples

Traffic management addresses many use cases. This section provides a few high-level examples.

Traffic steering: header-based routing

Traffic steering lets you direct traffic to service instances based on HTTP parameters such as request headers. For example, if a user's device is a mobile device with user-agent:Mobile in the request header, traffic steering can send that traffic to service instances designated to handle mobile traffic, and send traffic that doesn't have user-agent:Mobile to instances designated to handle traffic from other devices.

Traffic actions: weight-based traffic splitting

Deploying a new version of an existing production service generally incurs some risk. Even if your tests pass in staging, you probably don't want to subject 100% of your users to the new version immediately. With traffic management, you can define percentage-based traffic splits across multiple backend services.

For example, you can send 95% of the traffic to the previous version of your service and 5% to the new version of your service. After you've validated that the new production version works as expected, you can gradually shift the percentages until 100% of the traffic reaches the new version of your service. Traffic splitting is typically used for deploying new versions, A/B testing, service migration, and similar processes.

Traffic policies: request mirroring

Your organization might have specific compliance requirements mandating that all traffic be mirrored to an additional service that can, for example, record the request details in a database for later replay.

Extensibility with Service Extensions

The integration with Service Extensions lets you insert custom logic into the load balancing data path of supported Application Load Balancers.

For more information, see Service Extensions overview.

Traffic management components

For cross-region internal Application Load Balancers, traffic management uses the global URL maps and global backend services resources.

You can set up traffic steering and traffic actions by using URL maps. Google Cloud resources that are associated with URL maps include the following:

• Route rule
• Rule match
• Rule action

You can set up traffic policies by using backend services. Google Cloud resources that are associated with backend services include the following:

• Circuit breakers
• Locality load balancer policy
• Consistent hash load balancer settings
• Outlier detection

The following diagram shows the resources that are used to implement each feature.

Routing requests to backends

• The load balancer selects a backend service with backends. The backends can be Compute Engine virtual machine (VM) instances in an unmanaged instance group, Compute Engine VMs in a managed instance group (MIG), or containers by means of a Google Kubernetes Engine (GKE) node in a network endpoint group (NEG). The load balancer chooses a backend service based on rules defined in a regional URL map.
• The backend service selects a backend instance based on policies defined in a regional backend service.

When you configure routing, you can choose between the following modes:

• Simple host and path rule
• Advanced host, path, and route rule

The two modes are mutually exclusively. Each URL map can contain only one mode or the other mode.

Simple host and path rule

In a simple host and path rule, URL maps work as described in the URL map overview.

The following diagram shows the logical flow of a simple host and path rule.

A request is initially evaluated by using host rules. A host is the domain specified by the request. If the request host matches one of the entries in the hosts field, the associated path matcher is used.

Next, the path matcher is evaluated. Path rules are evaluated on the longest-path-matches-first basis, and you can specify path rules in any order. After the most specific match is found, the request is routed to the corresponding backend service. If the request does not match, the default backend service is used.

A typical simple host and path rule might look something like the following, where video traffic goes to video-backend-service, and all other traffic goes to web-backend-service.

gcloud compute url-maps describe lb-map

defaultService: regions/us-west1/backendServices/web-backend-service
hostRules:
- hosts:
  - '*'
  pathMatcher: pathmap
name: lb-map
pathMatchers:
- defaultService: regions/us-west1/backendServices/web-backend-service
  name: pathmap
  pathRules:
  - paths:
    - /video
    - /video/*
    service: regions/us-west1/backendServices/video-backend-service
region: regions/us-west1

Advanced host, path, and route rule

Advanced host, path, and route rules provide additional configuration options compared to simple host and path rules. These options enable more advanced traffic management patterns and also modify some of the semantics. For example, route rules have an associated priority value and are interpreted in priority order (rather than by using longest-path-matches-first semantics).

As in the earlier simple host and path rule example, you can configure advanced traffic management by using a URL map. For example, the following URL map configures routing where 95% of the traffic is routed to one backend service, and 5% of the traffic is routed to another backend service.

gcloud compute url-maps describe lb-map

defaultService: regions/us-west1/backendServices/service-a
hostRules:
- hosts:
  - '*'
  pathMatcher: matcher1
name: lb-map
pathMatchers:
- defaultService: regions/us-west1/backendServices/service-a
  name: matcher1
  routeRules:
  - matchRules:
    - prefixMatch: ''
    routeAction:
      weightedBackendServices:
      - backendService: regions/us-west1/backendServices/service-a
        weight: 95
      - backendService: regions/us-west1/backendServices/service-b
        weight: 5
region: regions/us-west1

Host rules

When a request reaches your load balancer, the request's host field is evaluated against the hostRules defined in the URL map. Each host rule consists of a list of one or more hosts and a single path matcher (pathMatcher). If no hostRules are defined, the request is routed to the defaultService.

Path matchers

After a request matches a host rule, the load balancer evaluates the path matcher corresponding to the host.

A path matcher is made up of the following:

• One or more path rules (pathRules) or route rules (routeRules).
• A default service (defaultService), which is the default backend service that is used when no other backend services match.

Path rules

Path rules (pathRules) specify one or more URL paths, such as / or /video. Path rules are generally intended for the type of simple host and path-based routing described previously.

Route rules

A route rule (routeRules) matches information in an incoming request and makes a routing decision based on the match.

Route rules can contain a variety of different match rules (matchRules) and a variety of different route actions (routeAction).

A match rule evaluates the incoming request based on the HTTP(S) request's path, headers, and query parameters. Match rules support various types of matches (for example, prefix match) as well as modifiers (for example, case insensitivity). This lets you, for example, send HTTP(S) requests to a set of backends based on the presence of a custom-defined HTTP header.

If you have multiple route rules, the load balancer executes them in priority order (based on the priority field), which lets you specify custom logic for matching, routing, and other actions.

Within a given route rule, when the first match is made, the load balancer stops evaluating the match rules, and any remaining match rules are ignored.

Google Cloud performs the following actions:

1. Looks for the first match rule that matches the request.
2. Stops looking at any other match rules.
3. Applies the actions in the corresponding route actions.

Route rules have several components, as described in the following table.

• routeRules[]
• routeRules[].priority
• routeRules[].description
• routeRules[].service
• routeRules[].matchRules[]
• routeRules[].routeAction
• routeRules[].urlRedirect
• routeRules[].headerAction

Match rules

Match rules (matchRules) match one or more attributes of a request and take actions specified in the route rule. The following list provides some examples of request attributes that can be matched by using match rules:

• Host: A hostname is the domain name portion of a URL; for example, the hostname portion of the URL http://example.net/video/hd is example.net. In the request, the hostname comes from the Host header, as shown in this example curl command, where 10.1.2.9 is the load-balanced IP address:

  curl -v http://10.1.2.9/video/hd --header 'Host: example.com'
  

• Paths follow the hostname; for example /images. The rule can specify whether the entire path or only the leading portion of the path needs to match.

• Other HTTP request parameters, such as HTTP headers, which allow cookie matching, as well as matching based on query parameters (GET variables).

Route actions

Route actions are specific actions to take when a route rule matches the attributes of a request.

You can specify one of the following route actions:

• Route traffic to a single service (service).
• Split traffic between multiple services (weightedBackendServices weight:x, where x must be from 0 to 1000).
• Redirect URLs (urlRedirect).

In addition, you can combine any one of the previously mentioned route actions with one or more of the following route actions:

• Mirror traffic (requestMirrorPolicy).
• Rewrite URL host and path (urlRewrite).
• Retry failed requests (retryPolicy).
• Set timeout (timeout).
• Introduce faults to a percentage of the traffic (faultInjectionPolicy).
• Add CORS policy (corsPolicy).
• Manipulate request or response headers (headerAction).

• urlRedirect
• urlRewrite
• headerAction
• requestMirrorPolicy
• weightedBackendServices
• retryPolicy
• timeout
• faultInjectionPolicy
• corsPolicy

HTTP-to-HTTPS redirects

If you need to redirect HTTP traffic to HTTPS, you can create two forwarding rules with a common IP address.

gcloud compute addresses create NAME \
    --region=us-west1 \
    --subnet=backend-subnet \
    --purpose=SHARED_LOADBALANCER_VIP

For a complete example, see Set up HTTP-to-HTTPS redirect for internal Application Load Balancers.

Traffic policies

By using backend service resources, you can configure traffic policies to fine-tune load balancing within an instance group or network endpoint group (NEG). These policies only take effect after a backend service has been selected by using your URL map (as described previously).

Traffic policies enable you to:

• Control the load balancing algorithm among instances within the backend service.
• Control the volume of connections to an upstream service.
• Control the eviction of unhealthy hosts from a backend service.

What's next

• To configure traffic management for internal Application Load Balancers, see Set up traffic management for internal Application Load Balancers.