December 17, 2018

Deploying Crystal application to GAE (Google App Engine)


In this article I will be creating a dumb and simple API and deploy it to GAE (Google App Engine). There is not so much documentation about deploying a crystal app to GAE, so it might be valuable for some developers.

For the web server I will be using Kemal. It is a simple web server for Crystal, and very similar to Sinatra in Ruby.


First you will need to install Kemal. Add kemalr/kemal to your shards.yml file.


name: app_name
version: 0.1.0

    main: src/

crystal: 0.27.0

license: MIT

    github: kemalcr/kemal

And install the dependencies.

$ shards install

Everything is setup, so let's start coding.


For this example, I will be creating one route, which is the root(/) and returning a static JSON.


require "kemal"

before_all do |env|
  env.response.content_type = "application/json"

get "/" do
  {"app": "Hello World", "version": "0.1.0"}.to_json

Build and run the app.

$ crystal build src/
$ ./main

Access http://localhost:3000, then it will display the following JSON.

  "app": "Hello World",
  "version": "0.1.0"

Not so interesting, but let's get it up on Google App Engine.


Theres are two things you have to complete before starting to deploy to GAE. You should follow google's official documentation for these.


GAE has a lot of support for many languages (Go, PHP, Java, etc...), which makes it extremely easy to deploy. Unfortunately Crystal has no support, so you have to create your own custom runtime.

To create a custom runtime you must,

  • Provide an app.yaml file that describes your application's runtime configuration to App Engine.
  • Add a Dockerfile that internally configures the runtime environment.
  • The server needs to be listening to port 8080

Provide an app.yaml

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

runtime: custom
env: flex

This needs to be stored in the root of the project.

Add a Dockerfile, and listens to port 8080.

Provide your Dockerfile to the same path as your app.yaml. And don't forget to name it Dockerfile.

FROM crystallang/crystal

RUN mkdir -p /usr/src/app

WORKDIR /usr/src/app

COPY ./ ./

RUN crystal build src/ --release


CMD ["./main", "-p", "8080"]

This Dockerfile should configure the environment for crystal, copies all the code, and build.

And most importantly run the app in port 8080, which GAE expects.

Finally Deploy it!


gcloud app deploy

After waiting for few minutes, it will be deployed, and can be accessed by


At the end it was very simple. The only complexity was configuring the Dockerfile (which was still very simple for a crystal app). This method can be applied to any kind of language without support from GAE!

Happy Coding!

Written by...
Kazushi Kawamura
Software Engineer based in Tokyo.
2019 © Kazushi Kawamura.