This page describes how HTTP requests from users reach the appropriate version of a service. Requests can be routed two ways:
- App Engine's default routing rules apply to requests with a URL that ends at the domain level.
- Alternatively, you can use a dispatch file that routes specific URL patterns according to your own rules.
If you test your app using the local development
available routing and dispatch features are slightly different. To
programmatically create URLs that work with both production and development
servers, use the
See routing in the development server to learn more.
Requests and domains
App Engine determines that an incoming request is intended for your app
by using the domain name of the request. A request whose domain name is
http://[YOUR_PROJECT_ID].appspot.com is routed to the app whose ID is
[YOUR_PROJECT_ID]. Every app gets an
appspot.com domain name for free.
appspot.com domains also support subdomains of the form
[SUBDOMAIN] can be any string allowed in one part of a domain name,
. character. Requests that are sent to any subdomain in this way
are routed to your app.
You can set up a custom top-level domain using G Suite and then assign subdomains to various apps, such as Google Mail or Sites. You can also associate an App Engine app with a subdomain. For more information about mapping a custom domain to your app, see Securing Custom Domains with SSL.
Requests for these URLs all go to the version of your app that you
configured to receive traffic. Each version of your app also has its own
URL, so you can deploy and test a new version before configuring that version to
receive traffic. The version-specific URL uses the ID of a specific version in
addition to the
appspot.com domain name, for example:
You can also use subdomains with the version-specific URL:
See Routing via URL for more information and examples.
The domain name used for the request is included in the request data that is
passed to your app. Therefore, you can use the request data to control how your
app responds based on the domain name in the request. For example, if you want
to redirect to an official domain, you can code your app to check the
request header and then respond accordingly based on the domain name.
Routing via URL
You can target an HTTP request with varying degrees of specificity. In the
appspot.com can be replaced with your app's
custom domain if you have one. The URL substrings
represent the resource IDs of your app.
Tip: You can use the following tools to retrieve the IDs of your app's resources:
gcloud app instances list
command to list the resource IDs within a specific GCP project.
To programmatically retrieve resource IDs, see the
list methods in the
The following URL patterns have a default routing behavior. Note that the default routing is overridden if there is a matching pattern that you have defined in your dispatch file:
- Sends the request to an available instance of the
Requests are received by any version that is configured for traffic in the
- Sends the request to an available instance of the
- Sends a request to an available instance of a specific service:
Requests are received by any version that is configured for traffic in the targeted service. If the service that you are targeting does not exist, the request gets soft routed.
- Sends a request to an available instance of a specific version in the
When a service is not targeted, requests are sent to the
If a request matches the
[YOUR_PROJECT_ID].appspot.com portion of the
hostname, but includes a service, version, or instance name that does not exist,
then the request is routed to the
default service. Soft routing does not apply
to custom domains; requests to them will return a HTTP
404 status code if the
hostname is invalid.
The following URL patterns are guaranteed to reach their target, if they exist. These requests are never intercepted and rerouted by the patterns that you have defined in your dispatch file:
- Sends the request to an available instance of a specific service and version:
If you are using manually-scaled services, you can target and send a request to a instance by including the instance ID. The instance ID is an integer in the range from
0up to the total number of instances that are running, and can be specified as follows:
- Sends a request to a specific service and version within a specific instance:
default service is created when you deploy the initial version of your
app to App Engine. Requests that specify no service or
an invalid service are routed, to the
default service. Those requests are then
handled by the versions that you have configured to receive traffic within the
default service. You can see which versions are configured for traffic in the
of the GCP Console.
Restricting access to a service
All services are public by default. If you want to restrict access to a service,
<role-name>admin</role-name> element to its
To help demonstrate the URL patterns, assume an example GCP project
requestsProject exists and includes an app that is running two
services and versions. The example app's
default service includes version
vFrontend, and the second service
service2, includes version
To target specific services and versions, you can use the following URL patterns:
To target the version in the
defaultservice using HTTPS, you can use:
To target the
vBackendversion using a custom domain without HTTPS, you can use:
requestsProject.appspot.comis mapped to the
Routing with a dispatch file
For URLs that use the patterns described earlier, you can create a dispatch file to override App Engine's routing rules and define your own custom routing rules. With a dispatch file, you can send incoming requests to a specific service based on the path or host name in the request URL.
For details about creating a dispatch file, see the
Creating a dispatch file
The dispatch file should be placed
in the same directory used for the other configuration files, such as
You can define up to 20 routing rules in the dispatch file and each rule
consists of both the
For example, you can create a dispatch file to route mobile requests like
http://simple-sample.appspot.com/mobile/ to a mobile frontend, and route worker
http://simple-sample.appspot.com/work/ to a static backend:
dispatch: # Send all mobile traffic to the mobile frontend. - url: "*/mobile/*" service: mobile-frontend # Send all work to the one static backend. - url: "*/work/*" service: static-backend
For details about how to define your dispatch.yaml, see the
Deploying the dispatch file
dispatch.yaml file can reside anywhere in your source code directory.
To deploy the dispatch configuration file without otherwise altering the currently serving version, use one of the following commands in the directory containing your dispatch file, depending on your environment:
gcloud app deploy dispatch.yaml
mvn appengine:deployDispatch dispatch.yaml
gradle appengineDeployDispatch dispatch.yaml
Routing in the development server
Discovering instance addresses
The local development server creates all instances at startup. Note that at this time basic scaling instances are not supported on the local development server. Every instance that is created is assigned its own port. The port assignments appear in the server's log message stream. Web clients can communicate with a particular instance by targeting its port. Only one instance (and port) is created for automatic scaled services. It looks like this in the server log (note that services were previously called modules):
INFO: Module instance service-auto is running at http://localhost:37251/
A unique port is assigned to each instance of a manual scaled service:
INFO: Module instance service-manual instance 0 is running at http://localhost:43190/ INFO: Module instance service-manual instance 1 is running at http://localhost:52642/
In addition, each manual scaled service is assigned one extra port so clients can access the service without specifying a specific instance. Requests to this port are automatically routed to one of the configured instances:
INFO: Module instance service-manual is running at http://localhost:12361/
The following table shows how these services can be called in the development server and in the App Engine environment:
|Service||Instance||Development server address||App Engine address|