This page gives you an overview of the advanced traffic management capabilities available for global external Application Load Balancers. This page is for the global external Application Load Balancer only. These load balancers are always global and always in Premium Tier. If you use a load balancer in a different mode, see one of the following pages:
- 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:
Use case examples
Traffic management addresses many use cases. This section provides a few high-level examples.
Traffic steering: header-based routing
Traffic steering allows you to 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.Don't configure session affinity if you're using weighted traffic splitting. If you do, the weighted traffic splitting configuration takes precedence.
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
Service Extensions callouts let you inject custom logic into the load balancing data path. These extensions let you instruct supported Application Load Balancers to make gRPC calls to user-managed applications or services during data processing.
For more information, see Service Extensions overview.
Traffic management componentsAt a high level, load balancers provide traffic management by leveraging 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:
- 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 backendsIn global external Application Load Balancers, the backend for your traffic is determined by using a two-phased approach:
- The load balancer selects a backend service or backend bucket based on rules defined in a global URL map.
- The backend service selects a backend instance based on policies defined in a global 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
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
service can point to a backend service or a backend
bucket. This example shows backend services.
$ gcloud compute url-maps describe lb-map
defaultService: global/backendServices/web-backend-service hostRules: - hosts: - '*' pathMatcher: pathmap name: lb-map pathMatchers: - defaultService: global/backendServices/web-backend-service name: pathmap pathRules: - paths: - /video - /video/* service: global/backendServices/video-backend-service
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.
service can point to a backend service or a backend
bucket. This example shows backend services.
$ gcloud compute url-maps describe lb-map
defaultService: global/backendServices/service-a hostRules: - hosts: - '*' pathMatcher: matcher1 name: lb-map pathMatchers: - defaultService: global/backendServices/service-a name: matcher1 routeRules: - matchRules: - prefixMatch: '' routeAction: weightedBackendServices: - backendService: global/backendServices/service-a weight: 95 - backendService: global/backendServices/service-b weight: 5
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
defaultServicein the global URL map API documentation.
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 (
- A default service (
defaultService), which is the default backend service or backend bucket that is used when no other backend services or backend buckets match.
pathMatchers.routeRulesin the global URL map API documentation.
Path rules (
pathRules) specify one or more URL paths, such as
Path rules are generally intended for the type of simple host and path-based
routing described previously.
pathRulesin the global URL map API documentation.
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 (
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 enables you to, for example, send HTTP(S) requests to a set of backends based on the presence of a custom-defined HTTP header.Note: Match options and semantics differ depending on the request portion that you match. For more information, see
matchRulesin the global URL map API documentation.
If you have multiple route rules, the load balancer executes them in priority
order (based on the
priority field), which allows you to 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:
- Looks for the first match rule that matches the request.
- Stops looking at any other match rules.
- Applies the actions in the corresponding route actions.
Route rules have several components, as described in the following table.
|Route rule component (
||A number from 0 through 2,147,483,647 (that is, (2^31)-1) assigned to a
route rule within a given path matcher.
The priority determines the order of route rule evaluation. The priority of a rule decreases as its number increases so that a rule with priority
Priority numbers can have gaps. You cannot create more than one rule with the same priority.
||An optional description of up to 1,024 characters.|
||The full or partial URL of the backend service resource to which traffic is directed if this rule is matched.|
|Match rules (
||One or more rules that are evaluated against the request. These
|Route action (
||Allows you to specify what actions to take when the match rule criteria are met. These actions include traffic splitting, URL rewrites, retry and mirroring, fault injection, and CORS policies.|
|Redirect action (
||You can configure an action to respond with an HTTP redirect when the match rule criteria are met. This field cannot be used in conjunction with a route action.|
|Header action (
||You can configure request and
response header transformation rules when the criteria within
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
example.net. In the request, the hostname comes from the
Hostheader, as shown in this example curl command, where
10.1.2.9is 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).
pathMatchers.routeRules.matchRulesin the global URL map API documentation.
Route actions are specific actions to take when a route rule matches the attributes of a request.
|Route action (
||Returns a configurable 3xx
response code. It also sets the
|URL rewrites (
||Rewrites the hostname portion of the URL, the path portion of the
URL, or both, before sending a request to the selected backend service.
For the path portion rewrites, you can use wildcards in
|Header transformations (
||Adds or removes request headers before sending a request to the backend service. Can also add or remove response headers after receiving a response from the backend service.|
|Traffic mirroring (
In addition to forwarding the request to the selected backend service,
sends an identical request to the configured mirror backend service on a
fire and forget basis. The load balancer doesn't wait for a response
from the backend to which it sends the mirrored request.
Traffic mirroring is only supported in the following cases:
|Weighted traffic splitting (
||Allows traffic for a matched rule to be distributed to multiple backend
services, proportional to a user-defined weight assigned to the individual
This capability is useful for configuring staged deployments or A/B testing. For example, the route action could be configured such that 99% of the traffic is sent to a service that's running a stable version of an application, while 1% of the traffic is sent to a separate service running a newer version of that application.
Configures 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.
Retry policies aren't supported with internet NEG backends.
||Specifies the timeout for the selected route. Timeout is computed from the time that the request is fully processed up until the time that the response is fully processed. Timeout includes all retries.|
|Fault injection (
||Introduces 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.|
|Delay injection (
||Introduces delays for a user-defined portion of requests before sending the request to the selected backend service.|
|Abort injection (
||Responds directly to a fraction of requests with user-defined HTTP status codes instead of forwarding those requests to the backend service.|
|Security policies (
||Cross-origin resource sharing (CORS) policies handle settings for enforcing CORS requests.|
You can specify one of the following route actions:
- Route traffic to a single service (
- Split traffic between multiple services (
weightedBackendServices weight:x, where x must be from 0 to 1000).
- Redirect URLs (
In addition, you can combine any one of the previously mentioned route actions with one or more of the following route actions:
- Mirror traffic (
- Rewrite URL host/path (
- Retry failed requests (
- Set timeout (
- Introduce faults to a percentage of the traffic (
- Add CORS policy (
- Manipulate request/response headers (
If you need to redirect HTTP traffic to HTTPS, you can create two forwarding rules with a common IP address.For a complete example, see Set up HTTP-to-HTTPS redirect for global external Application Load Balancers.
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.
|Traffic policy (
|Load balancing locality policy (
For a backend service or bucket, traffic distribution is based on a load balancing mode and a load balancing locality policy.
The balancing mode determines the fraction of traffic that
should be sent to each backend (bucket, instance group, or
For the balancing modes supported, see Balancing mode.
For the load balancing policy algorithms supported, see
|Session affinity (
Includes HTTP cookie-based affinity, HTTP header-based affinity, client IP address affinity, and generated cookie affinity. Session affinity provides a best-effort attempt to send requests from a particular client to the same backend for as long as the backend is healthy and has capacity.
Session affinity settings are fulfilled only if the load balancing
locality policy is set to
For more information about session affinity, see
|Outlier detection (
A set of policies that 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.
For more information about outlier detection, see
- To configure traffic management for global external Application Load Balancer, see Setting up traffic management for global external Application Load Balancers.