UPDATE: Since the time of writing this post, Docker has become much more mainstream. Some of the APIs have evolved and there is also native installation options for Mac available (instead of boot2docker). The docker tutorials are the best way to get started, https://docs.docker.com/get-started/. In addition docker-compose is useful for spinning up groups of containers as demonstrated in this post.

With the rise of new development methodologies such as Continuous Delivery, long gone are the days where a Software Engineer pushes code into the abyss and hope it comes out unscathed on the other side.  We are seeing a shift in the industry where the traditional walls between Development, Quality Assurance and Operations are slowly being broken down, these roles are merging and we are seeing a new breed of Engineer.  The buzz word “DevOps” has become prominent in the industry and as a result we are seeing project development teams that are more agile, more efficient and able to respond more quickly to change.  This shift has led to a rise of new tools and frameworks to help us automate deployment, automate testing and standardise infrastructure.

One of the tools at the forefront of this transformation is Docker, Docker is an open platform for developers and sysadmins to build, ship, and run distributed applications.  Before diving further into this practical exercise I would suggest having a read over What is Docker?

Before beginning the exercise you will need to install Docker,  I use boot2docker on MacOS, for further details on installation for your platform visit Docker Installation.  Another option is to use a cloud provider to run your docker host, Digital Ocean provide Docker ready servers running on the cloud for as little as $0.007/hour, this is an especially attractive option if you are limited by bandwidth or resources.

A few basics

Docker Image

A docker image is a read-only blue-print for a container, an example blue-print may be the Ubuntu operating system, or a CentOS one. Every container that you run in Docker will be based off a docker image.

Dockerfile

A Dockerfile contains code that tells Docker how to build a Docker image. Docker images are layered and so can be extended, this allows you to stack extra functionality on top of existing base images. A commonly used base image is ubuntu:latest which is a blue-print of the base installation of an Ubuntu distribution.

Docker Container

A docker container can be thought of as a light weight self-contained instance of a virtual machine running a linux distribution (usually with modifications), they are extremely cheap to start and stop.  Docker containers are spawned using a docker image, they should be considered as stateless/ephemeral resources.

Docker Hub

Docker Hub brings Software Engineering DRY principles to the system infrastructure world, it is a global repository platform that holds Dockerfiles and images. There are already images available that run ubuntu, redhat, mysql, rabbitmq, mongodb, nginx to name just a few.

Diving into Docker

The database container

Let’s start by creating our MySQL database container, luckily for us MySQL has already been “dockerised” and is available for us to pull via Docker Hub, the defaults are fine so there is no need to write our own Dockerfile or build any new images.  A new container can be started using the docker run command.

The first run may take some time while images are downloaded, they will be cached for subsequent runs.

docker run --name wordpress-db -e MYSQL_ROOT_PASSWORD=mysecretpassword -d mysql

The application container

docker run --name wordpress-app --link wordpress-db:mysql -d wordpress

docker ps

CONTAINER ID        IMAGE               COMMAND                CREATED              STATUS              PORTS               NAMES
c39600354fcb        wordpress:latest    "/entrypoint.sh apac   About a minute ago   Up About a minute   80/tcp              wordpress-app       
20e66802e914        mysql:latest        "/entrypoint.sh mysq   About a minute ago   Up About a minute   3306/tcp            wordpress-db        

docker exec -i -t wordpress-app bash
ping mysql
64 bytes from 172.17.0.2: icmp_seq=0 ttl=64 time=0.085 ms
64 bytes from 172.17.0.2: icmp_seq=1 ttl=64 time=0.127 ms
64 bytes from 172.17.0.2: icmp_seq=2 ttl=64 time=0.108 ms

docker logs wordpress-app

mkdir wordpress-nginx
cd wordpress-nginx
vi default.conf

server {
    listen       80;
    server_name  localhost;

    error_log /var/log/nginx/error.log warn;

    location / {
        proxy_pass http://wordpress-app:80/;
        proxy_redirect http://server_name http://wordpress-app:80/;
        proxy_set_header   Host               $host;
        proxy_set_header   X-Forwarded-For    $proxy_add_x_forwarded_for;
        proxy_set_header   X-Forwarded-Proto  http;
    }

    error_page   500 502 503 504  /50x.html;
    location = /50x.html {
        root   /usr/share/nginx/html;
    }
}

vi Dockerfile

FROM nginx
COPY default.conf /etc/nginx/conf.d/default.conf

docker build -t wordpress-nginx .
docker run -d --name=wordpress-nginx --link=wordpress-app:wordpress-app -p 80:80 wordpress-nginx
docker ps

CONTAINER ID        IMAGE                    COMMAND                CREATED             STATUS              PORTS                         NAMES
2b9f99664249        wordpress-nginx:latest   "nginx -g 'daemon of   3 seconds ago       Up 2 seconds        443/tcp, 0.0.0.0:80->80/tcp   wordpress-nginx     
c39600354fcb        wordpress:latest         "/entrypoint.sh apac   9 minutes ago       Up 3 minutes        80/tcp                        wordpress-app       
20e66802e914        mysql:latest             "/entrypoint.sh mysq   9 minutes ago       Up 4 minutes        3306/tcp                      wordpress-db