Set up traffic management for regional external Application Load Balancers

This document shows examples of using traffic management for some specific use cases. Many other use cases are possible.

The document contains examples for the following load balancers:

Regional external Application Load Balancer
Regional internal Application Load Balancer
Cross-region internal Application Load Balancer

Regional external Application Load Balancer versus regional internal Application Load Balancer. For the regional load balancers' traffic management configuration, the regional URL map API and the regional backend service API documentation provides a full list of fields, including semantics regarding relationships, restrictions, and cardinality.

The only difference between these two load balancers is the load balancing scheme, as follows:

Regional external Application Load Balancers use EXTERNAL_MANAGED.
Regional internal Application Load Balancers use INTERNAL_MANAGED.

Regional internal Application Load Balancer versus cross-region internal Application Load Balancer. For the traffic management configuration:

Regional internal Application Load Balancers use the regional URL map API , and the regional backend service API documentation provides a full list of fields, including semantics regarding relationships, restrictions, and cardinality.
Cross-region internal Application Load Balancers use global URL map API , and global backend service API documentation provides a full list of fields, including semantics regarding relationships, restrictions, and cardinality.

Note: For cross-region internal Application Load Balancers, you can use the examples provided in this document by replacing regions/REGION/backendServices with global/backendServices.

In addition to the advanced routing features described on this page, supported Application Load Balancers integrate with Service Extensions to let you insert custom logic into the load balancing data path.

Before you begin

Make sure that you understand how traffic management works. For more information, read Traffic management overview for regional external Application Load Balancers.

Configure traffic management

Within your chosen configuration environment, you set up traffic management by using YAML configurations. A URL map and a backend service each has its own YAML file. Depending on your desired functionality, you need to write either a URL map YAML file, a backend service YAML file, or both.

For help writing these YAML files, you can use the examples on this page and the Cloud Load Balancing API documentation.

The Google Cloud console is not supported.

For regional internal Application Load Balancers and regional external Application Load Balancers, the regional URL map API and the regional backend service API documentation provides a full list of fields, including semantics regarding relationships, restrictions, and cardinality.

Map traffic to a single service

Send all traffic to a single service. Make sure to replace the placeholders.

    defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
    hostRules:
    - hosts:
      - '*'
      pathMatcher: matcher1
    name: URL_MAP_NAME
    pathMatchers:
    - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
      name: matcher1
      routeRules:
        - matchRules:
            - prefixMatch: /PREFIX
          priority: 1
          routeAction:
            weightedBackendServices:
              - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
                weight: 100

Split traffic between two services

Split traffic between two or among multiple services. Make sure to replace the placeholders.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   name: URL_MAP_NAME
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
     - matchRules:
       - prefixMatch: /PREFIX
       priority: 2
       routeAction:
         weightedBackendServices:
         - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
           weight: 95
         - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_2
           weight: 5

Configure a URL redirect

The following example returns a configurable 3xx response code. The example also sets the Location response header with the appropriate URI, replacing the host and path as specified in the redirect action.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: URL_MAP_NAME
   hostRules:
   - hosts:
     - "HOST TO REDIRECT FROM" # Use * for all hosts
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     defaultUrlRedirect:
       hostRedirect: "HOST TO REDIRECT TO" # Omit to keep the requested host
       pathRedirect: "PATH TO REDIRECT TO" # Omit to keep the requested path
       redirectResponseCode: MOVED_PERMANENTLY_DEFAULT
       stripQuery: True

Mirror traffic

In addition to forwarding the request to the selected backend service, you can send an identical request to the configured mirror backend service on a fire and forget basis. This means that the load balancer doesn't wait for a response from the backend to which it sends the mirrored request. Request mirroring is useful for testing a new version of a backend service. You can also use it to debug production errors on a debug version of your backend service, rather than on the production version.

By default, the mirrored backend service receives all requests, even if the original traffic is being split between multiple weighted backend services. You can configure the mirrored backend service to receive only a percentage of the requests by using the optional mirrorPercent flag to specify the percentage of requests to be mirrored expressed as a value between 0 and 100.0. Percentage-based request mirroring is in Preview.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: 1
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           requestMirrorPolicy:
             backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_2
             mirrorPercent: 50.0

Note the following limitations when using traffic mirroring:

Traffic mirroring is supported when both backend services have managed instance groups, zonal NEGs, or hybrid NEGs backends. It is not supported for internet NEGs, serverless NEGs, and Private Service Connect backends.
Requests to the mirrored backend service do not generate any logs or metrics for Cloud Logging and Cloud Monitoring.

Rewrite the requested URL

Rewrite the host name portion of the URL, the path portion of the URL, or both, before sending a request to the selected backend service. Make sure to replace the placeholders.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           urlRewrite:
             hostRewrite: "new-host-name.com" # Omit to keep the requested host
             pathPrefixRewrite: "/new-path/" # Omit to keep the requested path

Retry a request

Configure the conditions under which the load balancer retries failed requests, how long the load balancer waits before retrying, and the maximum number of retries permitted.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           retryPolicy:
             retryConditions: 502, 504
             numRetries: 3
             perTryTimeout:
               seconds: 1
               nanos: 500000000

Specify the route timeout

Specify the timeout for the selected route. Timeout is computed from the time the request is fully processed until the response is fully processed. Timeout includes all retries. Make sure to replace the placeholders.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           timeout:
             seconds: 30
             nanos: 500000000

Configure fault injection

Introduce errors when servicing requests to simulate failures, including high latency, service overload, service failures, and network partitioning. This feature is useful for testing the resiliency of a service to simulated faults.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           faultInjectionPolicy:
             delay:
               fixedDelay:
                 seconds: 10
                 nanos: 500000000
               percentage: 25
             abort:
               httpStatus: 503
               percentage: 50

Configure CORS

Configure cross-origin resource sharing (CORS) policies to handle settings for enforcing CORS requests.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
           corsPolicy:
               allowOrigins: my-domain.com
               allowMethods: GET, POST
               allowHeaders: Authorization, Content-Type
               maxAge: 1200
               allowCredentials: True

Add and remove request and response headers

Add and remove request headers before sending a request to the backend service. Also add and remove response headers after receiving a response from the backend service.

Regional external Application Load Balancers and internal Application Load Balancers also support the use of variables in custom headers. You can specify one or more variables in the custom header value (headerValue) fields that are then translated to their corresponding per-request values. For a list of supported header values, see Create custom headers in URL maps.

   defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
   name: regional-lb-map
   region: region/REGION
   hostRules:
   - hosts:
     - '*'
     pathMatcher: matcher1
   pathMatchers:
   - defaultService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
     name: matcher1
     routeRules:
       - matchRules:
           - prefixMatch: /PREFIX
         priority: PRIORITY # 0 is highest
         routeAction:
           weightedBackendServices:
             - backendService: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
               weight: 100
               headerAction:
                 requestHeadersToAdd:
                 - headerName: header-1-name
                   headerValue: header-1-value
                   replace: True
                 requestHeadersToRemove:
                 - header-2-name
                 - header-3-name
                 responseHeadersToAdd:
                 - headerName: header-4-name
                   headerValue: header-4-value
                   replace: True
                responseHeadersToRemove:
                - header-5-name
                - header-6-name

Configure outlier detection

Specify the criteria for eviction of unhealthy backend VMs or endpoints in NEGs, along with criteria defining when a backend or endpoint is considered healthy enough to receive traffic again. Make sure to replace the placeholders.

    loadBalancingScheme: LOAD_BALANCING_SCHEME
    localityLbPolicy: RANDOM
    name: projects/PROJECT_ID/regions/REGION/backendServices/BACKEND_SERVICE_1
    outlierDetection:
      baseEjectionTime:
        nanos: 0
        seconds: '30'
      consecutiveErrors: 5
      consecutiveGatewayFailure: 3
      enforcingConsecutiveErrors: 2
      enforcingConsecutiveGatewayFailure: 100
      enforcingSuccessRate: 100
      interval:
        nanos: 0
        seconds: '1'
      maxEjectionPercent: 50
      successRateMinimumHosts: 5
      successRateRequestVolume: 100
      successRateStdevFactor: 1900
    region: region/REGION

Configure circuit breaking

Circuit breaking lets you set failure thresholds to prevent client requests from overloading your backends. After requests reach a limit that you've set, the load balancer stops allowing new connections or sending additional requests, giving your backends time to recover. Thus, circuit breaking prevents cascading failures by returning an error to the client rather than overloading a backend. This allows some traffic to be served while providing time for managing the overload situation, such as handling a traffic spike by increasing capacity through autoscaling.

Set upper limits on requests per connection as well as the volume of connections to a backend service. Also limit the number of pending requests and retries.

    loadBalancingScheme: LOAD_BALANCING_SCHEME # EXTERNAL_MANAGED or INTERNAL_MANAGED
    localityLbPolicy: RANDOM
    affinityCookieTtlSec: 0
    backends:
    - balancingMode: UTILIZATION
      capacityScaler: 1.0
      group: region/REGION/instanceGroups/INSTANCE_GROUP
      maxUtilization: 0.8
    circuitBreakers:
      maxConnections: 1000
      maxPendingRequests: 200
      maxRequests: 1000
      maxRequestsPerConnection: 100
      maxRetries: 3
    connectionDraining:
      drainingTimeoutSec: 0
    healthChecks:
    - region/REGION/healthChecks/HEALTH_CHECK

Set up traffic splitting: detailed steps

This example demonstrates the following steps:

Create distinct templates for different services.
Create instance groups for those templates.
Create routing rules that set up 95% / 5% traffic splitting.
Send curl commands showing that the traffic split percentages roughly match the configuration.

These instructions assume the following:

The region is us-west1.
A target proxy and forwarding rule have been created, along with a URL map named regional-lb-map.
The URL map sends all traffic to one backend service, called red-service, which is the default backend service.
You set up an alternate path that sends 5% of the traffic to blue-service and 95% of traffic to green-service.
A path matcher is used.
You are using Cloud Shell or another environment with bash installed.

Define the services

The following bash function creates a backend service, including the instance template and the managed instance group.

These instructions assume that an HTTP health check (regional-lb-basic-check) has been created. For instructions, see Set up a regional external Application Load Balancer.

function make_service() {
  local name="$1"
  local region="$2"
  local zone="$3"
  local network="$4"
  local subnet="$5"
  local subdir="$6"

  www_dir="/var/www/html/$subdir"

  (set -x; \
  gcloud compute instance-templates create "${name}-template" \
    --region="$region" \
    --network="$network" \
    --subnet="$subnet" \
    --tags=allow-ssh,load-balanced-backend \
    --image-family=debian-10 \
    --image-project=debian-cloud \
    --metadata=startup-script="#! /bin/bash
  apt-get update
  apt-get install apache2 -y
  a2ensite default-ssl
  a2enmod ssl
  sudo mkdir -p $www_dir
  /bin/hostname | sudo tee ${www_dir}index.html
  systemctl restart apache2"; \
  gcloud compute instance-groups managed create \
    "${name}-instance-group" \
    --zone="$zone" \
    --size=2 \
    --template="${name}-template"; \
  gcloud compute backend-services create "${name}-service" \
    --load-balancing-scheme=LOAD_BALANCING_SCHEME\
    --protocol=HTTP \
    --health-checks=regional-lb-basic-check \
    --health-checks-region="$region" \
    --region="$region"; \
  gcloud compute backend-services add-backend "${name}-service" \
    --balancing-mode='UTILIZATION' \
    --instance-group="${name}-instance-group" \
    --instance-group-zone="$zone" \
    --region="$region")
}

Create the services

Call the function to make three services, red, green, and blue. The red service acts as the default service for requests to /. The green and blue services are both set up on /PREFIX to handle 95% and 5% of the traffic, respectively.

make_service red us-west1 us-west1-a lb-network backend-subnet ""
make_service green us-west1 us-west1-a lb-network backend-subnet /PREFIX
make_service blue us-west1 us-west1-a lb-network backend-subnet /PREFIX

Create the URL map

gcloud

Export the existing URL map using the gcloud compute url-maps export command:

gcloud compute url-maps export regional-lb-map \
  --destination=regional-lb-map-config.yaml \
  --region=us-west1

Update the URL map file regional-lb-map-config.yaml by adding this to the end of the file:

hostRules:
- hosts:
  - '*'
  pathMatcher: matcher1
pathMatchers:
- defaultService: projects/PROJECT_ID/regions/us-west1/backendServices/red-service
  name: matcher1
  routeRules:
  - priority: 2
    matchRules:
      - prefixMatch: /PREFIX
    routeAction:
      weightedBackendServices:
        - backendService: projects/PROJECT_ID/regions/us-west1/backendServices/green-service
          weight: 95
        - backendService: projects/PROJECT_ID/regions/us-west1/backendServices/blue-service
          weight: 5

Update the URL map using the gcloud compute url-maps import command:

gcloud compute url-maps import regional-lb-map \
   --region=us-west1 \
   --source=regional-lb-map-config.yaml

Test the configuration

To test the configuration, first ensure that requests to the load balancer's IP address set up earlier are handled by the default red configuration.

Then check to make sure that requests sent to FORWARDING_RULE_IP_ADDRESS/PREFIX are split as expected.

Set up session affinity based on `HTTP_COOKIE`

Traffic control enables you to configure session affinity based on a provided cookie. To configure HTTP_COOKIE based session affinity for a backend service named red-service, follow these directions.

Use the gcloud compute backend-services export command to get the backend service configuration.

gcloud compute backend-services export red-service \
    --destination=red-service-config.yaml \
    --region=us-west1

Update the red-service-config.yaml file as follows:

sessionAffinity: 'HTTP_COOKIE'
localityLbPolicy: 'RING_HASH'
consistentHash:
 httpCookie:
  name: 'http_cookie'
  path: '/cookie_path'
  ttl:
    seconds: 100
    nanos: 500000000
 minimumRingSize: 10000

In the red-service-config.yaml file, delete the following line:
```
sessionAffinity: NONE
```

Update the backend service configuration file:

gcloud compute backend-services import red-service \
    --source=red-service-config.yaml \
    --region=us-west1

Troubleshooting

Use this information for troubleshooting when traffic is not being routed according to the route rules and traffic policies that you configured.

For information about logging and monitoring, see External HTTP(S) logging and monitoring.

Symptoms:

Increased traffic to services in rules above the rule in question.
An unexpected increase in 4xx and 5xx HTTP responses for a given route rule.

Solution: Check the order of your route rules. Route rules are interpreted in the order in which they are specified.

Route rules within a URL map are interpreted in the order in which they are specified. This is different from the way that path rules are interpreted by longest prefix match. For a path rule, internal Application Load Balancers will only select a single path rule; however, when you use route rules, more than one might apply.

When you define route rules, check to be sure that rules at the top of the list do not inadvertently route traffic that would otherwise have been routed by a subsequent route rule. The service that receives misdirected traffic would likely reject requests, and the service in your route rules would receive reduced traffic or no traffic at all.

What's next

Clean up the load balancer setup.

Set up traffic management for regional external Application Load Balancers

Before you begin

Configure traffic management

Map traffic to a single service

Split traffic between two services

Configure a URL redirect

Mirror traffic

Rewrite the requested URL

Retry a request

Specify the route timeout

Configure fault injection

Configure CORS

Add and remove request and response headers

Configure outlier detection

Configure circuit breaking

Set up traffic splitting: detailed steps

Define the services

Create the services

Create the URL map

gcloud

Test the configuration

Set up session affinity based on HTTP_COOKIE

Troubleshooting

What's next

Set up session affinity based on `HTTP_COOKIE`