Build custom runtimes

A custom runtime lets you use an alternate implementation of any supported flexible environment language, or to customize a Google-provided one. It also lets you to write code in any other language that can handle incoming HTTP requests (example). With a custom runtime, the flexible environment provides and manages your scaling, monitoring, and load balancing infrastructure for you, so you can focus on building your application.

To create a custom runtime you must:

Provide an app.yaml file

Your app.yaml configuration file must contain at least the following settings:

runtime: custom
env: flex

For information about what else you can set for your app, see Configuring your App with app.yaml.

Create a Dockerfile

Comprehensive documentation on creating Dockerfiles is available on the Docker website. If you are using a custom runtime, you must provide a Dockerfile whether you are providing your own base image or using one of Google's base images.

Specify a base image

The first command in a Dockerfile is usually a FROM command specifying a base image. A base image is used to create the container and build your application. You can author your own base image or select a base image from container registries like DockerHub.

Locate the Dockerfile

In general, the Dockerfile is always named Dockerfile and is placed in the same directory as the corresponding app.yaml file. In some cases, however, the tooling environment might have different requirements. For example, Cloud SDK-based Java tools such as the Maven, Gradle, Eclipse, and IntelliJ plugins require the Dockerfile to be in src/main/docker/Dockerfile and the app.yaml file to be in src/main/appengine/app.yaml. See the documentation for your tooling environment for more information.

Code structure

This section describes behavior that your code must implement whether you use a Google-provided base image or your own base image.

Listen to port 8080

The App Engine front end will route incoming requests to the appropriate module on port 8080. You must be sure that your application code is listening on 8080.

Handle lifecycle events

The flexible environment periodically sends your application certain lifecycle events.

Application shutdown

When an instance needs to be shut down, new incoming requests are routed to other instances (if any) and requests that are currently being processed are given time to complete. When shutting down an instance, flexible environment normally sends a STOP (SIGTERM) signal to the app container. Your app does not need to respond to this event, but it can use this to perform any necessary clean-up actions before the container is shut down. Under normal conditions, the system waits up to 30 seconds for the app to stop and then sends a KILL (SIGKILL) signal, immediately shutting down the instance.

In rare cases, outages can prevent App Engine from providing 30 seconds of shutdown time, which means the STOP and KILL signals might not be sent before an instance terminates. To handle this possibility, you should periodically checkpoint the state of your instance, using it primarily as an in-memory cache rather than a reliable data store.

Health check requests

You can use periodic health check requests to confirm that a VM instance has been successfully deployed, and to check that a running instance maintains a healthy status.

Build and deploy your custom runtime

After you configure your app.yaml and DOCKER file, you can build and deploy that container image to App Engine.

Alternatively, you can deploy prebuilt container images of your custom runtimes that are stored in Artifact Registry. For example, you can use Cloud Build to separately build your images and then store those images in Artifact Registry. For more information, see Push and pull images.

Integrate your application with Google Cloud

Applications running in custom runtimes can use the Google Cloud Client Libraries to access Google Cloud services. Applications in custom runtimes can also use any third-party service using standard APIs.

Authenticate with Google Cloud services

Application Default Credentials provide the simplest way to authenticate with and call Google APIs.

If your application uses Cloud Build for compiling Docker images, the cloudbuild network hosts the Application Default Credentials enabling the associated Google Cloud services to automatically find your credentials.

For more information about authentication, see Authentication at Google.


When a request is sent to your application running in App Engine, the request and response details are logged automatically. They can be viewed in the Google Cloud console Logs Explorer.

When your application handles a request, it can also write its own logging messages to stdout and stderr. These files are automatically collected and can be viewed in the Logs Explorer. Only the most recent entries to stdout and stderr are retained, in order to limit their size.

You can also write custom logs to /var/log/app_engine/custom_logs, using a file ending with .log or .json.

If you include any third-party agents in your application container, make sure you configure the agents to log to stdout and stderr, or to a custom log. This ensures that any errors produced by these agents is visible in Cloud Logging.

The request and application logs for your app are collected by a Cloud Logging agent and are kept for a maximum of 90 days, up to a maximum size of 1 GB. If you want to store your logs for a longer period or store a larger size than 1 GB, you can export your logs to Cloud Storage. You can also export your logs to BigQuery and Pub/Sub for further processing.

Other logs are also available for your use. The following are some of the logs that are configured by default:

Log name Payload type Purpose
crash.log text Information logged when setup fails. If your application fails to run, check this log.
monitoring.* text Information from the Docker container publishing data to Cloud Monitoring.
shutdown.log text Information logged on shutdown.
stdout text Standard output from your app.
stderr text Standard error from your container.
syslog text The VM syslog, outside of the Docker container.