Introduction
Docker is a great tool for automating the deployment of Linux applications inside software containers, but to take full advantage of its potential each component of an application should run in its own individual container. For complex applications with a lot of components, orchestrating all the containers to start up, communicate, and shut down together can quickly become unwieldy.
The Docker community came up with a popular solution called Fig, which allowed you to use a single YAML file to orchestrate all of your Docker containers and configurations. This became so popular that the Docker team decided to make Docker Compose based on the Fig source, which is now deprecated. Docker Compose makes it easier for users to orchestrate the processes of Docker containers, including starting up, shutting down, and setting up intra-container linking and volumes.
In this tutorial, we'll show you how to install the latest version of Docker Compose to help you manage multi-container applications on a Debian 9 server.
Prerequisites
To follow this article, you will need:
- A Debian 9 server and a non-root user with sudo privileges. This initial server setup with Debian 9 tutorial explains how to set this up.
- Docker installed with the instructions from Step 1 and Step 2 of How To Install and Use Docker on Debian 9
Note: Even though the Prerequisites give instructions for installing Docker on Debian 9, the dockercommands in this article should work on other operating systems as long as Docker is installed.
Step 1 — Installing Docker Compose
Although we can install Docker Compose from the official Debian repositories, it is several minor versions behind the latest release, so we'll install it from Docker's GitHub repository. The command below is slightly different than the one you'll find on the Releases page. By using the -o flag to specify the output file first rather than redirecting the output, this syntax avoids running into a permission denied error caused when using sudo.
We'll check the current release and, if necessary, update it in the command below:
sudo curl -L https://github.com/docker/compose/releases/download/1.22.0/docker-compose-`uname -s`-`uname -m` -o /usr/local/bin/docker-compose
Next we'll set the permissions:
sudo chmod +x /usr/local/bin/docker-compose
Then we'll verify that the installation was successful by checking the version:
docker-compose --version
This will print out the version we installed:
Outputdocker-compose version 1.22.0, build f46880fe
Now that we have Docker Compose installed, we're ready to run a "Hello World" example.
Step 2 — Running a Container with Docker Compose
The public Docker registry, Docker Hub, includes a Hello World image for demonstration and testing. It illustrates the minimal configuration required to run a container using Docker Compose: a YAML file that calls a single image. We'll create this minimal configuration to run our hello-world container.
First, we'll create a directory for the YAML file and move into it:
mkdir hello-world
cd hello-world
Then, we'll create the YAML file:
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 in the YAML file is used as part of the container name. The second line specifies which image to use to create the container. When we run the docker-compose up command, it will look for a local image by the name we specified, hello-world. With this in place, we’ll save and exit the file.
We can look manually at images on our system with the docker images command:
docker images
When there are no local images at all, only the column headings display:
OutputREPOSITORY TAG IMAGE ID CREATED SIZE
Now, while still in the ~/hello-world directory, we'll execute the following command:
docker-compose up
The first time we run the command, if there's no local image named hello-world, Docker Compose will pull it from the Docker Hub public repository:
OutputPulling my-test (hello-world:)...
latest: Pulling from library/hello-world
9db2ca6ccae0: Pull complete
Digest: sha256:4b8ff392a12ed9ea17784bd3c9a8b1fa3299cac44aca35a85c90c5e3c7afacdc
Status: Downloaded newer image for hello-world:latest
. . .
After pulling the image, docker-compose creates a container, attaches, and runs the hello program, which in turn confirms that the installation appears to be working:
Output. . .
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 |
. . .
Then it prints an explanation of what it did:
Output To generate this message, Docker took the following steps:
my-test_1 | 1\. The Docker client contacted the Docker daemon.
my-test_1 | 2\. The Docker daemon pulled the "hello-world" image from the Docker Hub.
my-test_1 | (amd64)
my-test_1 | 3\. The Docker daemon created a new container from that image which runs the
my-test_1 | executable that produces the output you are currently reading.
my-test_1 | 4\. The Docker daemon streamed that output to the Docker client, which sent it
my-test_1 | to your terminal.
Docker containers only run as long as the command is active, so once hello finished running, the container stopped. Consequently, when we look at active processes, the column headers will appear, but the hello-world container won't be listed because it's not running:
docker ps
OutputCONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
We can see the container information, which we'll need in the next step, by using the -a flag. This shows all containers, not just active ones:
docker ps -a
OutputCONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
06069fd5ca23 hello-world "/hello" 35 minutes ago Exited (0) 35 minutes ago hello-world_my-test_1
This displays the information we'll need to remove the container when we're done with it.
Step 3 — Removing the Image (Optional)
To avoid using unnecessary disk space, we'll remove the local image. To do so, we'll need to delete all the containers that reference the image using the docker rm command, followed by either the CONTAINER IDor the NAME. Below, we're using the CONTAINER ID from the docker ps -a command we just ran. Be sure to substitute the ID of your container:
docker rm 06069fd5ca23
Once all containers that reference the image have been removed, we can remove the image:
docker rmi hello-world
Conclusion
We've now installed Docker Compose, tested our installation by running a Hello World example, and removed the test image and container.
While the Hello World example confirmed our installation, the simple configuration does not show one of the main benefits of Docker Compose — being able to bring a group of Docker containers up and down all at the same time. To see the power of Docker Compose in action, you might like to check out this practical example, How To Configure a Continuous Integration Testing Environment with Docker and Docker Compose on Ubuntu 16.04.
