Structuring web services in App Engine

Region ID

The REGION_ID is an abbreviated code that Google assigns based on the region you select when you create your app. The code does not correspond to a country or province, even though some region IDs may appear similar to commonly used country and province codes. For apps created after February 2020, REGION_ID.r is included in App Engine URLs. For existing apps created before this date, the region ID is optional in the URL.

Learn more about region IDs.

This guide covers how to structure the services and related resources of your App Engine app.

Directory structure

Each version of your App Engine service is defined in an app.yaml configuration file. For simple apps, the minimum requirement for deployment is to define the app.yaml file. The app.yaml file acts as a deployment descriptor and defines the scaling type and the CPU, disk, and memory resources for a specific version of a service. If you are deploying several versions of a service, you can create multiple YAML files in the same directory to represent the configuration for each of your versions.

For the Java runtime, configuration files are in the YAML format.

For each service, you can create separate directories in the root of your app when you are developing locally. If you host your app out of a version control system (VCS), for example GitHub, you can also structure your app to use separate directories in a repository, or use separate repositories for each service. Each directory or repository should represent a single service and contain that service's app.yaml file along with the associated source code.

You have the option of specifying a unique name for each of your service's app.yaml file. For example, you can name a configuration file after your service, or use unique names to represent each version of that particular service, like service1.yaml or app.flexible.yaml.

The other optional configuration files should reside in the root directory or repository of the default service of your app. These optional configuration files apply app-wide settings that are not specific to a particular service, including the dispatch.yaml, index.yaml, and cron.yaml files.


A simple app only requires the app.yaml to be added in the same location as the app's source files. For a single service app, that app will only include the default service and all the files can live in the same directory, at the root of that app:

Hierarchy graph of single YAML service

The following example demonstrates how to structure what an app with three services might look like if you are developing your app locally. The optional dispatch.yaml file has been added to that app in the root directory. Also in the root are three directories for each of the app's services. The subdirectory for service1 includes the source and configuration files for that service. Similarly, both service2 and service3 are in separate directories, which contain each service's files, although service3 includes two versions of the YAML configuration file:

Hierarchy graph of YAML services

In the following example, a single service has the optional dispatch.yaml file and two configuration files that represent different versions of that service, service1.yaml and service2.yaml:

Hierarchy graph of small YAML services

Design considerations for instance uptime

Hardware or software failures that cause early termination or frequent instance restarts can occur without warning and can take considerable time to resolve. Your application should be able to handle such failures.

Here are some good strategies for avoiding downtime due to instance restarts:

  • Reduce the amount of time it takes for your instances restart or for new ones to start.
  • For long-running computations, periodically create checkpoints so that you can resume from that state.
  • Your app should be "stateless" so that nothing is stored on the instance.
  • Use queues for performing asynchronous task execution.
  • If you configure your instances to manual scaling:
    • Use load balancing across multiple instances.
    • Configure more instances than required to handle normal traffic.
    • Write fall-back logic that uses cached results when a manual scaling instance is unavailable.

Learn more about instances at How instances are managed.

The default service

Every App Engine application includes a default service. You must deploy the initial version of your app to the default service before you can create and deploy additional services to your app.

The default service can be optionally specified in the app.yaml with the setting service: default.

Requests sent to your app using your Google Cloud project are sent to the default service, for example, To learn more about targeting your other services, see Communicating Between Services.

Optional configuration files

The following configuration files control optional features that apply to all of the services in an individual app. See the following topics for details about each of the optional features:

  • cron.yaml configures regularly scheduled tasks that operate at defined times or regular intervals.
  • dispatch.yaml overrides routing default rules by sending incoming requests to a specific service based on the path or hostname in the URL.
  • index.yaml specifies which indexes your app needs if using Datastore queries.
    • Note that this file is available for all runtimes except for the .NET runtime.

Data and file storage considerations

From App Engine, you can easily access other Google Cloud services such as Datastore, Cloud SQL, and Cloud Storage.

You also have the option to use an external or third-party database if that database is supported by your language and accessible from your App Engine instance.

For details about storing files in Google Cloud or externally, see Understanding Data and File Storage.

You can also choose how you want to serve your static content. You can serve your app's static content directly from that app in App Engine, host your static content on a Google Cloud option like Cloud Storage, or use a third-party content delivery network (CDN). For more information about serving static content, see Serving Static Files.

What's next

If you are working with multiple services and want to deploy them together, see the steps to deploy multiple services.