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Elixir Runtime for Google Cloud Platform

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This repository contains the source for the Elixir Runtime for the Google App Engine Flexible Environment. It can also be used to run Elixir applications in Google Kubernetes Engine and other Docker-based hosting environments.

This runtime is maintained by Google, but is experimental and not covered by any SLA or deprecation policy. It may change at any time.

Elixir on Google App Engine

Google App Engine is a platform-as-a-service offering on Google Cloud Platform. It is an easy way to build scalable web and mobile backends in any language on Google's infrastructure.

You may consider deploying your Elixir application to Google App Engine if:

  • Your application is an HTTP web or mobile backend using an Elixir-based framework such as Phoenix.
  • You want to focus on application development, and allow Google's infrastructure and operations teams to handle your operations needs such as monitoring, scaling, and upgrades.

You should consider a different hosting solution such as, e.g., Google Kubernetes Engine if:

  • Your application uses Erlang's hot upgrade feature because it stores critical state in long-running processes. App Engine is designed for "stateless" apps that use a separate store such as a database for long-term state.

Elixir support is specific to the flexible environment of App Engine. The standard environment does not currently support Elixir.

Using the Elixir Runtime

The Elixir Runtime for Google Cloud Platform is an experimental runtime making it easy to run a Elixir web application in the Flexible Environment of Google App Engine. It is not a "custom runtime" in that it does not require you to use docker or provide your own Dockerfile. Instead, it is a full-featured runtime built on the same technology that powers the official language runtimes provided by Google.

To use the Elixir Runtime, you should have an Elixir project that, when run, listens on port 8080 or honors the PORT environment variable. A project that uses Phoenix will work. You will also need a Google Cloud project with billing enabled, and you must have the Google Cloud SDK installed.

(Note: some very early versions of this README directed you to set the app/use_runtime_builders configuration in gcloud. This step is no longer necessary with gcloud 175.0.0 and later, and you should now remove this config if you currently have it.)

Configuring an app with releases

Generally, we recommend that you set up releases for your application. Releases are the community's standard way to package and deploy your code, and they also help optimize the size and performance of your deployed application. Releases can be built using the mix release feature of Elixir 1.9 or later, or using the Distillery package.

If your application uses releases, the Elixir Runtime will build a release automatically, in the :prod environment, when you deploy to App Engine. You must set include_erts: true in your release configuration so the Erlang runtime is included. The release will be built in the cloud on the same OS and architecture that it will run on, so you do not need to worry about cross-compilation.

Once you have configured releases, create a file called app.yaml at the root of your application directory, with the following contents:

env: flex
runtime: gs://elixir-runtime/elixir.yaml
  release_app: my_app

Replace my_app with the name of your release.

See the App Engine documentation for more information on things you can set in the app.yaml configuration file. A variety of settings are available to control scaling, health checks, cron jobs, and so forth. There is no Elixir-specific section in the documentation, but much of the information for other languages such as Ruby will still apply.

Configuring an app without releases

The Elixir Runtime also supports deploying an application that does not build releases. If your app does not use releases, create a file called app.yaml at the root of your application directory, with the following contents:

env: flex
runtime: gs://elixir-runtime/elixir.yaml
entrypoint: mix phx.server

Set the entrypoint field to a command that launches your app in the foreground. If you do not specify an entrypoint, the Elixir Runtime will examine your app and attempt to guess an appropriate command to use. Generally, this guess will be mix phx.server for Phoenix apps, or mix run --no-halt for non-Phoenix apps. However, for best results, it is recommended that you provide an explicit entrypoint.

See the App Engine documentation for more information on things you can set in the app.yaml configuration file. A variety of settings are available to control scaling, health checks, cron jobs, and so forth. There is no Elixir-specific section in the documentation, but much of the information for other languages such as Ruby will still apply.

Deploy your application

Once the app.yaml config file is set up, you can deploy to App Engine with

gcloud app deploy

By default, the Runtime will build your project by downloading its deps, and compile with MIX_ENV=prod. For Phoenix apps that use Brunch, it will also perform a brunch build. Finally, if you are using releases, a release will be built for your application automatically; otherwise your application code will be compiled in place.

You may update your application by modifying your source and redeploying. Again, the Elixir Runtime will take care of rebuilding your application in the cloud when you deploy.

Changing the environment/config

If your application needs environment variables to be set, you can use the standard env_variables: field in the app.yaml file. For example:

  MY_VAR: value1

This will set those environment variables both at build time and at runtime.

One environment variable of particular note is MIX_ENV, which controls the "environment" your application runs in. It can affect build parameters such as compilation settings; and many frameworks, including Phoenix, use it to select a set of configuration to use.

By default, the Elixir runtime builds and runs your app in the prod environment, but you can change this by setting the MIX_ENV environment variable. For example:

  MIX_ENV: staging

This will not only set the MIX_ENV during the building and running of your application, but if you are using a Distillery release, it will also cause Distillery to build the app with that environment. (So make sure there is a corresponding clause for the environment in your rel/config.exs file.)

Specifying the Erlang and Elixir versions

The Elixir Runtime uses the asdf tool to install and manage Erlang and Elixir. By default, it will run your application on recent stable releases of those languages. However, the default versions may change at any time, and indeed may depend on the type of application. (For example, as of July 2019, the default Elixir version for most applications is 1.9.0, except if the application uses a version of Distillery prior to 2.1, in which case it defaults to 1.8.2 to avoid mix task collisions.)

If you need more stability in the language versions used, you may provide a .tool-versions file with versions for erlang and elixir. See the asdf documentation for more information on the format of the .tool-versions file.

When you deploy an Elixir application, the Elixir runtime will install the requested releases of Erlang and Elixir into your application image "just-in-time". In most cases, this is pretty quick. Asdf installs Elixir directly from precompiled binaries hosted on For Erlang, the Elixir Runtime itself provides prebuilt binaries of most recent versions of Erlang since Erlang 20, and can install any of these directly.

However, if you request an Erlang version for which we do not have a prebuilt binary, the Elixir runtime will have to build Erlang from source. This can take a good 10-20 minutes by itself, and it often causes App Engine deployment to time out. To fix this, set the following:

gcloud config set app/cloud_build_timeout 60m

This allocates 60 minutes to the "build" phase of app engine deployment, which should be more than sufficient to build both Erlang and your application. (Feel free to set it to a different value if you want.) If this gcloud configuration is not explicitly set, it defaults to 10 minutes.

Customizing application builds

The Elixir Runtime provides a standard build script that includes installation of Erlang and Elixir, fetching dependencies, compiling your application, and (for release-based applications) building the release.

There is also a space for custom build commands that are executed after compilation but before release generation. Your application might use this space for application-specific or framework-specific build steps such as building asset files or obtaining credentials.

Phoenix applications generally use Webpack or Brunch to build assets. So, by default, if the Elixir runtime detects that your app uses Phoenix and contains a brunch or webpack config file, it will automatically give you a custom build command that attempts the appropriate build.

Specifically, for Phoenix 1.4 using Webpack, this command is:

cd assets \
&& npm install \
&& node_modules/webpack/bin/webpack.js --mode production \
&& cd .. \
&& mix phx.digest

Similarly, for Phoenix 1.3 using Brunch, this command is:

cd assets \
&& npm install \
&& node_modules/brunch/bin/brunch build --production \
&& cd .. \
&& mix phx.digest

(It is slightly different for Phoenix 1.2 applications, and for Phoenix umbrella applications.)

You may also provide your own custom build commands, by setting the runtime_config: -> build: setting in your app.yaml file. The value should be an array of shell commands to be executed in order. For example:

    - mix phx.digest
    - mix do.something.else

Note that if you provide your own custom build commands, they will override any Webpack or Brunch build that the Elixir Runtime gives you by default, so if you still want to use one of those asset build systems, you will have to include a command explicitly in your config.

Installing Debian packages

The Elixir runtime provides a minimal set of Debian packages needed to run ERTS. If your application requires additional packages, you may specify them in the runtime_config: -> packages: setting of your app.yaml file. The Elixir runtime will then make sure they get installed in the Docker image. For example:

    - libpq-dev
    - imagemagick

Inside the Elixir Runtime

The Elixir Runtime comprises three parts:

  • A series of base Docker images that are used by the runtime, and can also be used directly by applications. They are all based on a stable version of Debian (the same Debian base image used by Google's officially supported runtimes).
    • elixir-ubuntu18 contains an installation of Ubuntu 18.04 plus a few dependencies for the Erlang VM, and some common configuration for App Engine runtimes. However, it does not include an installation of Erlang or Elixir itself. This image is used as a runtime base image for release-based applications. An application release, with its embedded ERTS, is installed directly atop this image.
    • elixir-asdf extends elixir-ubuntu18 by installing asdf and the erlang and elixir plugins, but does not include any actual installations. This image is used as a base image for most other images.
    • elixir-base extends elixir-asdf by installing a default recent version of both Erlang and Elixir. It may be used as a convenient base image for applications that do not care about specific versions of the language.
    • elixir-builder extends elixir-asdf and installs additional build tools such as NodeJS and the Google Cloud SDK. It is used as a base image for builds, but still requires that asdf be used to install a version of Erlang and Elixir.
  • An elixir-generate-dockerfile image. This is the heart of an App Engine runtime. It analyzes an Elixir project and constructs a Dockerfile that can be used to build the project and prepare it for running in App Engine. When you deploy an Elixir application to App Engine, this analyzer-generator runs on your application first, and then the generated Dockerfile is used to build the Docker image that gets pushed to App Engine servers. The generated Dockerfile generally contains two stages: a build stage based on elixir-builder that performs the build, and a runtime stage based on either elixir-ubuntu18 (for release-based builds) or elixir-asdf (for non-release-based).
  • A configuration file that references a specific build of the above analyzer-generator image. This file serves as the official definition of the Elixir Runtime, and is available in Google Cloud Storage at the location gs://elixir-runtime/elixir.yaml. When you set the runtime field of your app.yaml, it points at this configuration file.

Building and Releasing

Build and release scripts are provided that support test builds as well as official releases.


You must have the gcloud sdk installed, and have access to a Google Cloud project. The CloudBuild API must be enabled in your project. You should also set up a cloud storage bucket that will hold your test runtime definition.

To perform an official build/release, you must have write access to the gcp-elixir project and the elixir-runtime storage bucket.

Test builds

To perform a test build, run the script:


By default, this will build the runtime images to Google Cloud Container Registry in your current project (which should be configured in gcloud). It will tag them with the current date and time. It will also build a runtime definition and write it to a tmp directory in this repo directory.

If you want to upload the definition to cloud storage (and thus make it available to use for App Engine deployments), you must provide the name of a cloud storage bucket to upload it to. You must of course have write access to this storage bucket.

./ -b my-storage-bucket

You can then use the build of the runtime by pointing at the uploaded definition file in cloud storage:

runtime: gs://my-storage-bucket/elixir-<datetime-tag>.yaml

You may also mark the build as a "staging" build by providing the -s flag to This will tag the built images with the staging tag, and will also write an elixir-staging.yaml definition, so you can use:

runtime: gs://my-storage-bucket/elixir-staging.yaml

There are several additional options to Pass the -h flag for more information.

Prebuilding Erlang binaries

A basic test build as configured above will likely be slow because it must compile Erlang from source while deploying. To fix this, provide a set of prebuilt Erlang binaries.

First, choose the Erlang versions that will be covered. This list should include at least the default Erlang version specified in the script. Write them, one per line, in a file in this directory called erlang-versions.txt.

Next, execute the script to build these versions of Erlang. This script uses the "elixir-asdf" base image so you must first build the runtime itself.

The runtime build script also uses the erlang-versions.txt file to decide which Erlang versions it can install using prebuilt binaries. So after you edit the version list, you must rebuild the runtime itself.

From this point, the prebuilt binaries can be revved independent of the runtime. When you update the runtime, you generally will not need to rebuild the Erlang prebuilt images, or vice versa.


Official releases of the Elixir Runtime are done in the gcp-elixir project and uploaded to the elixir-runtime storage bucket. If you have sufficient access, you can perform an official release as follows:

./ -p gcp-elixir -n runtime -b elixir-runtime -s -i
./ -p gcp-elixir -n runtime -b elixir-runtime

To update the official prebuilt Erlang binaries, do this:

./ -p gcp-elixir -n runtime -s -e <versions-to-build>
./ -p gcp-elixir -n runtime -e <versions-to-release>

Generally, you should provide versions explicitly, otherwise it will build or release ALL versions in the erlang-versions.txt file, which would take a very long time.

Contributing changes


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