- 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 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
At a high level, load balancers provide traffic management by leveraging regional URL maps and regional backend services resources.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
In regional internal Application Load Balancers, the backend for your traffic is determined by using a two-phased approach:- 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.
hostRules[] and defaultService in the
regional URL map API documentation.
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.
pathMatchers[], pathMatchers[].pathRules[], and
pathMatchers[].routeRules[] in the regional URL map API
documentation.
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.
pathRules[] in the regional URL map API
documentation.
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.
Note: Match options and semantics differ depending on the request portion that you match. For more information, seematchRules[] in the regional URL map API
documentation.
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:
- 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 (API field name) |
Description |
|---|---|
Priority (priority) |
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 4 is evaluated before a rule with
priority 25. The first rule that matches the request is applied.
Priority numbers can have gaps. You cannot create more than one rule with the same priority. |
Description (description) |
An optional description of up to 1,024 characters. |
Service (service) |
The full or partial URL of the backend service resource to which traffic is directed if this rule is matched. |
Match rules (matchRules) |
One or more rules that are evaluated against the request. These
matchRules can match all or a subset of the request's HTTP
attributes, such as the path, HTTP headers, and query (GET)
parameters.Within a matchRule, all matching criteria
must be met for the routeRule's routeActions to
take effect. If a routeRule has multiple
matchRules, the routeActions of the
routeRule take effect when a request matches any of the
routeRule's matchRules.
|
Route action (routeAction) |
Lets you 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 (urlRedirect) |
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 (headerAction) |
You can configure request and
response header transformation rules when the criteria within
matchRules are met.
|
routeRules[]routeRules[].priorityrouteRules[].descriptionrouteRules[].servicerouteRules[].matchRules[]routeRules[].routeActionrouteRules[].urlRedirectrouteRules[].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/hdisexample.net. In the request, the hostname comes from theHostheader, as shown in this example curl command, where10.1.2.9is 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[].matchRules[] in the regional URL map API
documentation.
Route actions
Route actions are specific actions to take when a route rule matches the attributes of a request.
Route action (API field name) |
Description |
|---|---|
Redirects (urlRedirect) |
Returns a configurable 3xx
response code. It also sets the Location response header
with the appropriate URI, replacing the host and path as specified in
the redirect action.
|
URL rewrites (urlRewrite) |
Rewrites the hostname portion of the URL, the path portion of the URL, or both, before sending a request to the selected backend service. |
Header transformations (headerAction) |
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 (requestMirrorPolicy) |
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. Note the following limitations when using traffic mirroring:
|
Weighted traffic splitting (weightedBackendServices) |
Allows traffic for a matched rule to be distributed to multiple backend
services, proportional to a user-defined weight assigned to the individual
backend service. 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. |
Retries (retryPolicy) |
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. |
Timeout (timeout) |
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 (faultInjectionPolicy) |
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 (faultInjectionPolicy) |
Introduces delays for a user-defined portion of requests before sending the request to the selected backend service. |
Abort injection (faultInjectionPolicy) |
Responds directly to a fraction of requests with user-defined HTTP status codes instead of forwarding those requests to the backend service. |
Security policies (corsPolicy) |
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 (
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).
urlRedirecturlRewriteheaderActionrequestMirrorPolicyweightedBackendServicesretryPolicytimeoutfaultInjectionPolicycorsPolicy
HTTP-to-HTTPS redirects
If you need to redirect HTTP traffic to HTTPS, you can create two forwarding rules with a common IP address.
For two forwarding rules to share a common internal IP address, you must reserve the IP address and include the--purpose=SHARED_LOADBALANCER_VIP flag:
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.
Traffic policy (API field name) |
Description |
|---|---|
Load balancing locality policy (LocalityLbPolicy) |
For a backend service, traffic distribution is based on a load balancing mode and a load balancing locality policy. The balancing mode determines the weighting/fraction of traffic that
should be sent to each backend (instance group or
For the balancing modes supported, see Balancing mode. For the load balancing policy algorithms supported, see
|
Session affinity (consistentHash) |
Includes HTTP cookie-based affinity, HTTP header-based affinity, client IP address affinity, stateful cookie-based session 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 back is healthy and has capacity. For more information about session affinity, see
|
Outlier detection (outlierDetection) |
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
|
Circuit breaking (circuitBreakers) |
Sets upper limits on the volume of connections and requests per connection to a backend service. For more information about
circuit breaking, see |
What's next
- To configure traffic management for internal Application Load Balancers, see Set up traffic management for internal Application Load Balancers.