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.
These options apply only to deployed apps. When you are testing locally, the routing behavior depends on the particular runtime and development environment that you're using.
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.
The appspot.com
domains also support subdomains of the form
[SUBDOMAIN]-dot-[YOUR_PROJECT_ID].appspot.com
, where
[SUBDOMAIN]
can be any string allowed in one part of a domain name,
excluding the .
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:
http://[VERSION_ID]-dot-[YOUR_PROJECT_ID].appspot.com
.
You can also use subdomains with the version-specific URL:
http://[SUBDOMAIN]-dot-[VERSION_ID]-dot-[YOUR_PROJECT_ID].appspot.com
.
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 Host
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
following examples, appspot.com
can be replaced with your app's
custom domain if you have one. The URL substrings
[VERSION_ID]
, [SERVICE_ID]
, and [MY_PROJECT_ID]
, each
represent the resource IDs of your app.
Tip: You can use the following tools to retrieve the IDs of your app's resources:
Console
In the GCP Console, you can view the corresponding Instances, Services, and Versions pages.
gcloud
Run the
gcloud app instances list
command to list the resource IDs within a specific GCP project.
API
To programmatically retrieve resource IDs, see the list
methods in the
Admin API.
Default routing
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
default
service: https://[MY_PROJECT_ID].appspot.com http://[MY_CUSTOM_DOMAIN]
Requests are received by any version that is configured for traffic in the
default
service. - Sends the request to an available instance of the
-
- Sends a request to an available instance of a specific service:
https://[SERVICE_ID]-dot-[MY_PROJECT_ID].appspot.com http://[SERVICE_ID].[MY_CUSTOM_DOMAIN]
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
default
service:https://[VERSION_ID]-dot-[MY_PROJECT_ID].appspot.com http://[VERSION_ID].[MY_CUSTOM_DOMAIN]
When a service is not targeted, requests are sent to the
default
service.
Soft routing
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.
Targeted routing
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:
https://[VERSION_ID]-dot-[SERVICE_ID]-dot-[MY_PROJECT_ID].appspot.com http://[VERSION_ID].[SERVICE_ID].[MY_PROJECT_ID].[MY_CUSTOM_DOMAIN]
Default service
The 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
Versions page
of the GCP Console.
Example
To help demonstrate the URL patterns, assume an example GCP project
with ID 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
vBackend
.
To target specific services and versions, you can use the following URL patterns:
-
To target the version in the
default
service using HTTPS, you can use:https://vFrontend-dot-default-dot-requestsProject.appspot.com https://vFrontend-dot-requestsProject.appspot.com
-
To target the
vBackend
version using a custom domain without HTTPS, you can use:http://vBackend.service2.example.net http://vBackend.example.net
where
requestsProject.appspot.com
is mapped to theexample.net
domain.
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
dispatch.yaml
reference.
Creating a dispatch file
The dispatch file should be placed
either in the root of your project directory, or in the root
directory of your default
service.
You can define up to 20 routing rules in the dispatch file and each rule
consists of both the service
and url
elements.
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
requests like 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 dispatch.yaml
reference documentation.
Deploying the dispatch file
To deploy the dispatch configuration file, run the following command:
gcloud
gcloud app deploy dispatch.yaml