This repository is the result of an experiment demonstrating how to build an n-tier application stack in multiple docker containers on a single CoreOS VM. The application is a simple node/express web app that tracks hits in a redis database.

• This setup was tested on my Ubuntu 12.04 machine.
• VirtualBox
• Vagrant

$ git clone [email protected]:danielkennedy/docker-n-tier.git
$ cd docker-n-tier/
$ vagrant up
$ vagrant ssh

Note: You could do this several ways, but for simplicity's sake, I recommend simply cloning this repo again on the guest machine.

$ git clone [email protected]:danielkennedy/docker-n-tier.git
$ cd docker-n-tier/redis
$ docker build -t mysandbox/redis .
$ docker run -name redis -p 6379:6379 -d mysandbox/redis
$ cd ../nodejs
$ docker build -t mysandbox/nodejs .
$ docker run -t -i -p 3000:3000 -link redis:db -name nodeapp -d mysandbox/nodejs

On your host machine, open a browser and connect to http://localhost:8080/. If everything worked to this point, you should see a "hit count" of 1. Refresh the page, and you'll see it increment.

$ redis-cli -h localhost -p 16379
localhost:16379> get requests
"2"

Several details had to come together to make this demo work. What follows are a handful of tips/tricks that will explain the lessons learned from this exercise.

The most important piece, other than the box image (which was provided by CoreOS maintainers), is the networking component. For my example, I needed to be sure that I could reach the node application from my host machine. Beyond that, I also wanted to be able to query the redis database directly from my host system for debugging/convenience purposes.

config.vm.network "forwarded_port", guest: 3000, host: 8080
config.vm.network "forwarded_port", guest: 6379, host: 16379

Linking the docker containers to allow the node app to communicate with the redis database was the trickiest part of this whole setup. For a more detailed explanation of how this works, see this great article.

Finally, the node app needs to be able to find the redis database in order to make this work. The application is a generated express app with a very slightly modified index.js route:

var redis = require('redis');
var port = process.env.DB_PORT_6379_TCP_PORT || 6379;
var hostname = process.env.DB_PORT_6379_TCP_ADDR || 'localhost';
var client = redis.createClient(port, hostname);

In order to be a complete solution, I'll need to continue with:

• Orchestrating the boot sequence with systemd, to start the containers with each boot
• Introduce service discovery with etc, to allow more flexibility than environment variables