A custom runtime allows you to easily deploy and run web applications written in any language. The App Engine flexible environment provides the scaling, monitoring, and load balancing infrastructure, so you can focus on building your application.
To create a custom runtime you need:
app.yamlfile that describes your application's runtime configuration.
Dockerfilethat configures the runtime environment. In many cases, this can be just one line specifying a base image.
- To ensure your application is listening on port 8080 and has request handlers that respond to lifecycle events, such as start, stop, and health check requests.
Create an app.yaml file
You must use an
app.yaml configuration file with the following settings:
runtime: custom env: flex
For information about configuring other flexible environment settings for your app, see Configuring your App with app.yaml.
Create a Dockerfile
The Dockerfile is always named
Dockerfile and should be placed in the same
directory with the corresponding
app.yaml file. This remains true if you are
customizing a Java runtime and using
gcloud commands, however other tooling
might have different requirements. For example, Cloud SDK-based Java tools
such as the Maven, Gradle, Eclipse, and IntelliJ plugins require that the
Dockerfile is in the
src/main/docker/Dockerfile directory and that the
app.yaml file is in the
Comprehensive documentation is available on the Docker website. You need to do this whether you use your own base image or one of Google's base images. For simplicity, this page shows an example using a Google-provided base image.
Configuring the Dockerfile
The first command in a Dockerfile is usually a
FROM command specifying a base
FROM command for each of the Google-provided base images is shown in the
following table, along with the github project used to build the image:
||APIs require Go libraries.|
|Java (Open JDK 8 only)
||Dockerfiles for Java.|
|Java (Open JDK 8 + Eclipse Jetty 9.3)
||Dockerfiles for Java.|
|Python 2.7 & 3.6
Using App Engine service APIsTo access the App Engine service APIs from a Go app that is running the
golangbase image, you must import the App Engine Go libraries into your source code.
Using custom Java runtimes
To customize a Java runtime and add additional directives, you must create a Dockerfile.
Dockerfile for Java 8 / Jetty 9:
FROM gcr.io/google-appengine/jetty ADD test-webapp-1.0-SNAPSHOT.war $JETTY_BASE/webapps/root.war WORKDIR $JETTY_BASE RUN java -jar $JETTY_HOME/start.jar --approve-all-licenses --add-to-startd=jmx,stats,hawtio \ && chown -R jetty:jetty $JETTY_BASE
test-webapp-1.0-SNAPSHOT.war is the name of the built WAR file in your
target/ (maven) or
build/staged-app/ (gradle) directory.
In contrast, don't need a Dockerfile to deploy your app to App Engine, if you choose to use an uncustomized base image of one of the Google-provided Java runtimes. For more information, see Java 8 runtime or Java 8 / Jetty 9 runtime.
For a complete list of all the Google-provided images, see the google-appengine project.
Developing code for custom runtimes
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.
When shutting down an instance, App Engine flexible environment normally sends a
SIGTERM) signal to the app container. Your application does not need to
respond to this event, but your application can use it 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'
In rare cases, outages can prevents 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
By default, an application running in the flexible environment is periodically sent health check requests.
If you create a custom runtime your app must handle health check requests or disable them, otherwise, the app won't come up properly.
A health check is an HTTP request to the URL
/_ah/health. A healthy
application should respond with status code 200.
At its simplest, an application can check that its serving stack is up and running; it might also perform more sophisticated checks, such as ensuring any background processes are running.
To disable health checking, add this line to your app's configuration file:
Using Google Cloud Platform services
Applications running in the flexible environment can access the Google Cloud Client Libraries or any third-party services via standard APIs.
With custom runtimes, client libraries can use the Application Default Credentials to authenticate with and call Google APIs.
Alternatively, you can authenticate your application directly with access tokens using the Compute Engine Metadata API. You can then use these access tokens in API requests, such as requests against Google Cloud Storage and Google Cloud Datastore services and resources.
When a request is sent to your application running in App Engine, request and response details are logged automatically, and can be viewed in the Google Cloud Platform Console Stackdriver Logs Viewer.
When your application handles a request, it can also
write its own logging messages to
stderr. These files are
automatically collected and can be viewed in the Logs Viewer. Only the most
recent entries to
stderr are retained, in order
to limit their size.
The request and application logs for your app are collected by a Stackdriver Logging agent and are kept for a maximum of 90 days, up to a maximum size of 1GB. If you want to store your logs for a longer period or store a larger size than 1GB, 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|
||text||Information logged when setup fails. If your application fails to run, check this log.|
||text||Information from the Docker container publishing data to Cloud Monitoring.|
||text||Information logged on shutdown.|
||text||Standard output from your app.|
||text||Standard error from your container.|
||text||The VM syslog, outside of the Docker container.|