Routing Requests to Modules

HTTP requests from users can reach the appropriate module/version/instance in two ways: A request with a URL that ends at the domain level can be routed according to App Engine's default address routing rules. Alternatively, you can include a dispatch file that routes specific URL patterns according to your own rules.

If you test your app using the development server the available routing and dispatch features are slightly different. To programmatically create URLs that work with both production and development servers, use the get_hostname method. See routing in the development server to learn more.

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 "instance", "version", "module", and "app-id" represent application and module attributes that you have defined yourself.

These address forms are guaranteed to reach their target (if it exists). They will never be intercepted and rerouted by a pattern in the dispatch file:

Sends the request to the named module, version, and instance.

Sends the request to an available instance of the named module and version.

These address forms have a default routing behavior. Note that the default routing is overridden if there is a matching pattern in the dispatch file:

Sends the request to an available instance of the default version of the named module.

Sends the request to an available instance of the given version of the default module.

Sends the request to an available instance of the default version of the default module.

Default module

The default module is defined by explicitly giving a module the name "default," or by not including the name parameter in the module's config file. Requests that specify no module or an invalid module are routed to the default module. You can designate a default version for a module, when appropriate, in the Google Developers Console versions tab.

Soft routing

If a request matches the app-id.appspot portion of the hostname, but includes a module, version, or instance name that does not exist, then the request is routed to the default version of the default module. Soft routing does not apply to custom domains; requests to them will return a 404 if the hostname is invalid.

Restricting access to a module

All modules are public by default. If you want to restrict access to a module, add the “login: admin” parameter to its handlers.

Routing with a dispatch file

For certain URLs (described above), you can create a dispatch file that overrides the routing rules. This lets you send incoming requests to a specific module based on the path or hostname in the URL. For example, say that you want to route mobile requests like http://simple-sample.appspot.com/mobile/ to a mobile frontend, route worker requests like http://simple-sample.appspot.com/work/ to a static backend, and serve all static content from the default module.

To do this you can create a custom routing with a dispatch.yaml file. The file can be placed in your project directory at the top level, or in the directory that defines the default module.

  # Default module serves the typical web resources and all static resources.
  - url: "*/favicon.ico"
    module: default

  # Default module serves simple hostname request.
  - url: "simple-sample.appspot.com/"
    module: default

  # Send all mobile traffic to the mobile frontend.
  - url: "*/mobile/*"
    module: mobile-frontend

  # Send all work to the one static backend. 
  - url: "*/work/*"
    module: static-backend

The dispatch file can contain up to 10 routing rules. When specifying the URL string, neither the hostname nor the path can be longer than 100 characters.

As you can see, dispatch.yaml includes support for glob characters (however, yaml syntax requires that you include such expressions in quotes to denote they are strings). Glob characters can be used only before the hostname and at the end of the path. If you prefer general routing rules that match many possible requests, you could specify the following:

# Send any path that begins with “simple-sample.appspot.com/mobile” to the mobile-frontend module.
- url: "simple-sample.appspot.com/mobile*"
  module: mobile-frontend

# Send any domain/sub-domain with a path that starts with “work” to the static backend module. 
- url: "*/work*"
  module: static-backend

You can also write expressions that are more strict:

# Matches the path "/fun", but not "/fun2" or "/fun/other"
- url: "*/fun"
  module: mobile-frontend

# Matches the hostname 'customer1.myapp.com', but not '1.customer1.myapp.com. 
- url: "customer1.myapp.com/*"
  module: static-backend

Uploading the dispatch file

To upload the dispatch file by itself, use the appcfg update_dispatch command, and specify the directory that contains the file. Be sure that all the modules mentioned in the file have already been uploaded before using this command.

# cd to the project directory or the default module directory that contains dispatch.yaml
appcfg.py update_dispatch .

Note that the dispatch file is uploaded automatically when you update a module in this case:

  • You use the command appcfg.py update <dir> pointing at a module's directory
  • There is a module configuration file named app.yaml in the directory
  • The dispatch file is also in the directory

Routing in the development server

Discovering instance addresses

The development server creates all manual scaling instances at startup. Instances for automatic and basic scaling modules are managed dynamically. The server assigns a port to each module, and clients can depend on the server to load-balance and select an instance automatically. The port assignments for addressing each module appear in the server's log message stream. Here are the ports for an app that defines three modules (the scaling type of each module is not relevant):

INFO Starting module "default" running at: http://localhost:8084
INFO Starting module "module1" running at: http://localhost:8082
INFO Starting module "module2" running at: http://localhost:8083

When you use a module's address (for example http://localhost:8082/), the server will select (or create) an instance of the module and send the request to that instance.

The server assigns unique ports to each instance of a module. To discover these ports you need to use the admin server. There is a unique port for the admin server, which appears in the message log:

INFO Starting admin server at: http://localhost:8000

This address takes you to the admin server console. From there you can click on Instances to see the dynamic state of your app's instances:

dev_appserver admin console

A separate entry will appear for each manual and basic instance. The instance numbers are links with unique port addresses for each instance. You can hover over a link to see the port assigned to that instance, or click on the link to send a request directly to that instance.

Dispatch files

If your app uses a dispatch.yaml file, the log messages stream will include a dispatcher port:

INFO Starting dispatcher running at: http://localhost:8080

Requests to this port will be routed according to the rules in the dispatch file. The server does not support dispatch.yaml file rules that include hostnames (for example, url: "customer1.myapp.com/*"). Rules with relative path patterns (url: "*/fun") will work, so you can use URLs like http://localhost/fun/mobile to reach instances. The server will report an error in the log stream if you try to start an application with a dispatch.yaml file that contains host-based rules.