Nov 7, 2016

How to Install and Use Docker Compose on Ubuntu

 
For complex applications with a lot of components, orchestrating all the containers to start up and shut down together can quickly become headache. Docker is a great tool, but to really take full advantage of its potential it's best if each component of your application runs in its own container.






The Docker community came up with a popular solution called Fig, which allowed you to use a single YAML file to orchestrate all your Docker containers and configurations. This became so popular that the Docker team eventually decided to make their own version based on the Fig source. They called it Docker Compose. In short, it makes dealing with the orchestration processes of Docker containers (such as starting up, shutting down, and setting up intra-container linking and volumes) really easy. 

In this article, we will install Docker and Docker Compose and this guide will also help you to understand how Docker Compose works.

Prerequisites

To follow the steps we have mention in this article, you will need the following:
  • Ubuntu 14.04 or later
  • A non-root user with sudo privileges

Installing Docker

You will have to install Docker first if you haven't already. The easiet way to install Docker is to download and install their installation script (you'll be prompted for a sudo password).
  • wget -qO- https://get.docker.com/ | sh
The above command downloads and executes a small installation script written by the Docker team.

Working with Docker is a headache if your user is not configured correctly, so add your user to the docker group with the following command.
  • sudo usermod -aG docker $(whoami)
Log out and log in from your server to activate your new groups.

Installing Docker Compose

Since you have Docker installed, let's move to the next step of installing Docker Compose. You need to install python-pip as prerequisite:
  • sudo apt-get -y install python-pip
Then you can install Docker Compose:
  • sudo pip install docker-compose

Running a Container with Docker Compose

The public Docker registry, Docker Hub, includes a simple Hello World image. Now that we have Docker Compose installed, let's test it with this really simple example.

Create a directory for our YAML file:
  • mkdir hello-world
Then change into the directory:
  • cd hello-world
Now create the YAML file using your favorite text editor (we will use nano):
  • nano docker-compose.yml
Put the following contents into the file, save the file, and exit the text editor:
docker-compose.yml
 
my-test:
  image: hello-world

The first line will be used as part of the container name. The second line specifies which image to use to create the container. The image will be downloaded from the official Docker Hub repository.

While still in the ~/hello-world directory, execute the following command to create the container:
  • docker-compose up
The output should start with the following:

Output of docker-compose up
Creating helloworld_my-test_1... Attaching to helloworld_my-test_1 my-test_1 | my-test_1 | Hello from Docker. my-test_1 | This message shows that your installation appears to be working correctly. my-test_1 |

The output then explains what Docker is doing:
  1. The Docker client contacted the Docker daemon.
  2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
  3. The Docker daemon created a new container from that image which runs the executable that produces the output you are currently reading.
  4. The Docker daemon streamed that output to the Docker client, which sent it to your terminal.
If the process doesn't exit on its own, press CTRL-C.

 

Learning Docker Compose Commands

Let's go over the commands the docker-compose tool supports.

The docker-compose command works on a per-directory basis. You can have multiple groups of Docker containers running on one machine — just make one directory for each container and one docker-compose.yml file for each container inside its directory. 

So far we've been running docker-compose up on our own and using CTRL-C to shut it down. This allows debug messages to be displayed in the terminal window. This isn't ideal though, when running in production you'll want to have docker-compose act more like a service. One simple way to do this is to just add the -d option when you up your session:
  • docker-compose up -d
docker-compose will now fork to the background.

To show your group of Docker containers (both stopped and currently running), use the following command:
  • docker-compose ps
For example, the following shows that the helloworld_my-test_1 container is stopped:

Output of `docker-compose ps`
Name Command State Ports ----------------------------------------------- helloworld_my-test_1 /hello Exit 0
A running container will show the Up state:
Output of `docker-compose ps`
Name Command State Ports --------------------------------------------------------------- nginx_nginx_1 nginx -g daemon off; Up 443/tcp, 80/tcp

To stop all running Docker containers for an application group, issue the following command in the same directory as the docker-compose.yml file used to start the Docker group:
  • docker-compose stop
Note: docker-compose kill is also available if you need to shut things down more forcefully.

In some cases, Docker containers will store their old information in an internal volume. If you want to start from scratch you can use the rm command to fully delete all the containers that make up your container group:
  • docker-compose rm
If you try any of these commands from a directory other than the directory that contains a Docker container and .yml file, it will complain and not show you your containers:


Output from wrong directory Can't find a suitable configuration file in this directory or any parent. Are you in the right directory? Supported filenames: docker-compose.yml, docker-compose.yaml, fig.yml, fig.yaml


Accessing the Docker Container Filesystem

If you need to work on the command prompt inside a container, you can use the docker exec command.

The Hello World! example exits after it is run, so we need to start a container that will keep running so we can then use docker exec to access the filesystem for the container.

Create a new directory for it and change into it:
  • mkdir ~/nginx && cd $_
Create a docker-compose.yml file in our new directory:
  • nano docker-compose.yml
and paste in the following:

~/nginx/docker-compose.yml
nginx:
  image: nginx

Save the file and exit. We just need to start the Nginx container as a background process with the following command:
  • docker-compose up -d
The Nginx image will be downloaded and then the container will be started in the background.

Now we need the CONTAINER ID for the container. List of all the containers that are running:
  • docker ps
You will see something similar to the following:

Output of `docker ps` 
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 
e90e12f70418 nginx "nginx -g 'daemon off" 6 minutes ago Up 5 min


If we wanted to make a change to the filesystem inside this container, we'd take its ID (in this example e90e12f70418) and use docker exec to start a shell inside the container:
  • docker exec -it e90e12f70418 /bin/bash
The -t option opens up a terminal, and the -i option makes it interactive. The /bin/bash options opens a bash shell to the running container. Be sure to use the ID for your container.

You will see a bash prompt for the container similar to:

root@e90e12f70418:/#

From here, you can work from the command prompt. Keep in mind, however, that unless you are in a directory that is saved as part of a data volume, your changes will disappear as soon as the container is restarted. Another caveat is that most Docker images are created with very minimal Linux installs, so some of the command line utilities and tools you are used to may not be present.

 






Conclusion

We covers the basic concepts of Docker Compose and how to get it installed and running on Ubuntu.

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