Spring Docker Example

This repository provides an example of how to work with Spring while running a Ruby on Rails application inside a Docker container. Support for this workflow has been added in Spring 1.7.

The aim was to be able to have Spring installed on your development host, but have everything else to do with your application running inside Docker containers.

Demo

1. Install Docker and Docker Compose
2. Build the containers: $ docker-compose build
3. Install Spring on your development host (version must match the Gemfile.lock): $ gem install spring --version=1.7.0
4. Run something: $ bin/rails c (this will bring up the Docker container which runs the spring server, and then connect to it)

How it works

The Dockerfile contains a fairly standard setup for a Rails application under Docker, although it is necessary to make sure that the UIDs on the host and inside the container are matching, to ensure that Spring's socket file can be accessed on both sides.

The docker-compose.yml file defines a standard web container which runs your web server. It also defines a spring container which runs the spring server.

In order for the spring client on the development host to communicate with the spring server inside the container, we need to set some environment variables:

• SPRING_SOCKET=tmp/spring.sock: defines a fixed location for the Spring socket file, so that it can be found both inside and outside the container (inside the container it is at /app/tmp/spring.sock and outside it is at /path/to/spring-docker-example/tmp/spring.sock)
• SPRING_SERVER_COMMAND='docker-compose up spring': defines what we need to run to bring up the spring server when it is not already running. When we first run bin/rails c, Spring will detect that there isn't already a server running, and run this command.

There are obviously various ways to set these environment variables (I like direnv) but for simplicity we're setting them in config/spring_client.rb which will be automatically loaded by Spring.

Mac OS X support

I use Linux, which means that my development machine is also my Docker host (there is no intermediate virtual machine). The Spring socket file is shared beetween the host and the container via a Dockers shared volume.

On OS X you'll be using Docker Machine or possibly the beta of Docker for Mac.

Either way, the Docker host will be a virtual machine. So the socket file will be shared with the virtual machine via a Docker shared volume, but you'll need to figure out some way to further share it from the virtual machine to your OS X host.

This should be possible but I don't know exactly how. If anybody figures out how to make this work, please submit a pull request to update this documentation.

PID namespace

The spring stop and spring status commands rely on being able to have direct access to the spring server process. We set pid: host (see docs) in docker-compose.yml to achieve this, but it will only work if your Docker host is also your development machine (i.e. if you're using Linux).

Actually running Spring commands will work regardless of the PID namespace, since this happens purely via communication over the socket.

In theory, spring stop and spring status could be modified to communicate over the socket too, thus removing this limitation. However it's not something I'm working on.