How Requests are Routed

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 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]
  • 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 0 up 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:

    https://[INSTANCE_ID]-dot-[VERSION_ID]-dot-[SERVICE_ID]-dot-[MY_PROJECT_ID].appspot.com
    http://[INSTANCE_ID].[VERSION_ID].[SERVICE_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:

  1. 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
    
  2. 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 the example.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
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