Ruby Runtime Environment

The Ruby runtime allows you to run your app in App Engine in a sandbox environment. This document explains the details of the Ruby runtime environment, including what headers are provided to your code and other information to successfully deploy your application on App Engine.

Specify the Ruby runtime for App Engine in the standard environment in the app.yaml file:

runtime: rubyVERSION

Where VERSION is the Ruby MAJOR and MINOR version numbers. For example, to use the latest Ruby version, Ruby 3.3 (preview), specify 33.

For other supported Ruby versions, and the corresponding Ubuntu version for your Ruby version, see the Runtime support schedule.

Ruby version

The Ruby runtime uses the latest stable release of the version that is specified in your app.yaml file. App Engine automatically updates to new patch release versions, but it will not automatically update the minor version.

For example, your application might be deployed at Ruby 2.6.0 and automatically updated to version 2.6.1 at a later deployment, but it will not be automatically updated to Ruby 2.7.


For more information on declaring and managing dependencies, see Specifying dependencies.

Application startup

The runtime starts your application using the entrypoint defined in app.yaml. The entrypoint should start a process that responds to HTTP requests on the port defined by the environment variable PORT. For example:

entrypoint: bundle exec rails server -p $PORT

Most web applications use a Rack-supported web server such as Puma, Unicorn or Thin.

You must add the server as a dependency in your application's Gemfile configuration file. The runtime will install all dependencies before your entrypoint is called.

source ""

gem "rack"
gem "puma"

An example entrypoint using puma for a Rails application:

entrypoint: bundle exec rails server Puma -p $PORT

An example entrypoint using puma for any Rack application:

entrypoint: bundle exec rackup -s Puma -p $PORT

For applications that can handle requests without a Rack server, you can just execute a ruby script:

entrypoint: bundle exec ruby app.rb

Environment variables

The following environment variables are set by the runtime:

Environment variable Description
GAE_APPLICATION The ID of your App Engine application. This ID is prefixed with 'region code~' such as 'e~' for applications deployed in Europe.
GAE_DEPLOYMENT_ID The ID of the current deployment.
GAE_ENV The App Engine environment. Set to standard.
GAE_INSTANCE The ID of the instance on which your service is currently running.
GAE_MEMORY_MB The amount of memory available to the application process, in MB.
GAE_RUNTIME The runtime specified in your app.yaml file.
GAE_SERVICE The service name specified in your app.yaml file. If no service name is specified, it is set to default.
GAE_VERSION The current version label of your service.
GOOGLE_CLOUD_PROJECT The Google Cloud project ID associated with your application.
PORT The port that receives HTTP requests.
NODE_ENV (Only available in the Node.js runtime) Set to production when your service is deployed.

You can define additional environment variables in your app.yaml file, but the above values cannot be overridden, except for NODE_ENV.

HTTPS and forwarding proxies

App Engine terminates HTTPS connections at the load balancer and forwards requests to your application. Some applications need to determine the original request IP and protocol. The user's IP address is available in the standard X-Forwarded-For header. Applications that require this information should configure their web framework to trust the proxy.


The runtime includes a writable /tmp directory, with all other directories having read-only access. Writing to /tmp takes up system memory. For more information, see the TempDir and TempFile documentation.

Metadata server

Each instance of your application can use the App Engine metadata server to query information about the instance and your project.

You can access the metadata server through the following endpoints:

  • http://metadata

Requests sent to the metadata server must include the request header Metadata-Flavor: Google. This header indicates that the request was sent with the intention of retrieving metadata values.

The following table lists the endpoints where you can make HTTP requests for specific metadata:

Metadata endpoint Description
/computeMetadata/v1/project/numeric-project-id The project number assigned to your project.
/computeMetadata/v1/project/project-id The project ID assigned to your project.
/computeMetadata/v1/instance/region The region the instance is running in.
/computeMetadata/v1/instance/service-accounts/default/email The default service account email assigned to your project.
/computeMetadata/v1/instance/service-accounts/default/ Lists all the default service accounts for your project.
/computeMetadata/v1/instance/service-accounts/default/scopes Lists all the supported scopes for the default service accounts.
/computeMetadata/v1/instance/service-accounts/default/token Returns the auth token that can be used to authenticate your application to other Google Cloud APIs.

For example, to retrieve your project ID, send a request to