Docker Compose Tutorial: Advanced Docker made simple

Docker is an in-demand, DevOps technology used to set up and deploy applications using containers. Docker’s environment streamlines the application development lifecycle, and Docker Compose, and advanced Docker tool, can be used to simplify your workflow

In this article, we will refresh your knowledge of Docker and show you how to get started with Docker Compose.

If you’re new to DevOps, check out our beginner’s guide to Docker and Kubernetes before proceeding with this tutorial.

Today, we will go over:

• Brief refresher on Docker
• Fundamentals of Docker
• Getting started with Docker Compose
• Take your Docker knowledge to the next level

Dockerfile vs Docker Compose#

Both Dockerfile and Docker Compose are tools in the Docker image ecosystem. Dockerfile is a text file that contains an image, and the commands a developer can call to assemble the image. The commands are typically simple processes like installing dependencies, copying files, and configuring settings.

Docker Compose is a tool for defining and running multi-container Docker applications. Information describing the services and networks for an application are contained within a YAML file, called docker-compose.yml.

One of the base functions of Docker Compose is to build images from Dockerfiles. However, Docker Compose is capable of orchestrating the containerization and deployment of multiple software packages. You can select which images are used for certain services, set environment-specific variables, configure network connections, and much more.

Below are the three necessary steps that begin most Docker workflows:

• Building a Dockerfile for each image you wish to add
• Use Docker Compose to assemble the images with the build command.
• Specify the path to individual Dockerfiles using the build command in conjunction with /path/to/dockerfiles

In summary, Dockerfiles define the instructions to a single image in an application, but Docker Compose is the tool that allows you to create and manipulate a multi-container application.

How to use and install Docker Compose#

Compose uses the Docker Engine, so you’ll need to have the Docker Engine installed on your device. You can run Compose on Windows, Mac, and 64-bit Linux. Installing Docker Compose is actually quite easy.

On desktop systems, such as Docker Desktop for Mac and Windows, Docker Compose is already included. No additional steps are needed. On Linux systems, you’ll need to:

1. Install the Docker Engine
2. Run the following command to download Docker Compose

sudo curl -L "https://github.com/docker/compose/releases/download/1.26.2/docker-compose-$(uname -s)-$(uname -m)" -o /usr/local/bin/docker-compose

3. Apply permissions to the binary, like so:

sudo chmod +x /usr/local/bin/docker-compose

4. Test the installation to check it worked properly

$ docker-compose --version
docker-compose version 1.26.2, build 1110ad01

Regardless of how you chose to install it, once you have Docker Compose downloaded and running properly, you can start using it with your Dockerfiles. This process requires three basic steps:

1. Define your app’s environment using a Dockerfile. This way, it can be reproduced.
2. Define the services for your app in a docker-compose.yml file. This way, they can run in an isolated environment.
3. Run docker-compose to start your app.

You can easily add Docker Compose to a pre-existing project. If you already have some Dockerfiles, add Docker Compose files by opening the Command Palette. Use the Docker: Docker Compose Files to the Workspace command, and, when promoted, choose the Dockerfiles you want to include.

You can also add Docker Compose files to your workspace when you add a Dockerfile. Similarly, open the Command Palette and use the Docker: Add Docker Files to Workspace command.

You’ll then be asked if you want to add any Docker Compose files. In both cases, Compose extension will add the docker-compose.yml file to your workspace.

Docker Compose file structure#

Now that we know how to download Docker Compose, we need to understand how Compose files work. It’s actually simpler than it seems. In short, Docker Compose files work by applying multiple commands that are declared within a single docker-compose.yml configuration file.

The basic structure of a Docker Compose YAML file looks like this:

Now, let’s look at a real-world example of a Docker Compose file and break it down step-by-step to understand all of this better. Note that all the clauses and keywords in this example are industry-standard and commonly used.

With just these, you can start a development workflow. There are some more advanced keywords that you can use in production, but for now, let’s just get started with the necessary clauses.

• version ‘3’: This denotes that we are using version 3 of Docker Compose, and Docker will provide the appropriate features. At the time of writing this article, version 3.7 is latest version of Compose.

• services: This section defines all the different containers we will create. In our example, we have two services, web and database.

• web: This is the name of our Flask app service. Docker Compose will create containers with the name we provide.

• build: This specifies the location of our Dockerfile, and . represents the directory where the docker-compose.yml file is located.

• ports: This is used to map the container’s ports to the host machine.

• volumes: This is just like the -v option for mounting disks in Docker. In this example, we attach our code files directory to the containers’ ./code directory. This way, we won’t have to rebuild the images if changes are made.

• links: This will link one service to another. For the bridge network, we must specify which container should be accessible to which container using links.

• image: If we don’t have a Dockerfile and want to run a service using a pre-built image, we specify the image location using the image clause. Compose will fork a container from that image.

• environment: The clause allows us to set up an environment variable in the container. This is the same as the -e argument in Docker when running a container.

Congrats! Now you know a bit about Docker Compose and the necessary parts you’ll need to get started with your workflow.

Docker Compose commands#

Now that we know how to create a docker-compose file, let’s go over the most common Docker Compose commands that we can use with our files. Keep in mind that we will only be discussing the most frequently-used commands.

docker-compose: Every Compose command starts with this command. You can also use docker-compose <command> --help to provide additional information about arguments and implementation details.

$ docker-compose --help
Define and run multi-container applications with Docker.

docker-compose build: The build command builds or rebuild images in the docker-compose.yml file. This file contains all the necessary configurations for all the services that make up the application.

The job of the build command prepares images to create containers. If a service is using the pre-built image, it will skip this command. The docker-compose build command reads the Dockerfile for each service, including the instructions to build the image. The built images can then be used to create containers for each service using the docker-compose up command.

Furthermore, we use the docker-compose build command for building the images for the services in a consistent and reproducible way, making deployment in different environments easier.

$ docker-compose build
database uses an image, skipping
Building web
Step 1/11 : FROM python:3.9-rc-buster
 ---> 2e0edf7d3a8a
Step 2/11 : RUN apt-get update && apt-get install -y docker.io

docker-compose images: This command will list the images you’ve built using the current docker-compose file.

$ docker-compose images
          Container                  Repository        Tag       Image Id       Size  
--------------------------------------------------------------------------------------
7001788f31a9_docker_database_1   mysql/mysql-server   5.7      2a6c84ecfcb2   333.9 MB
docker_database_1                mysql/mysql-server   5.7      2a6c84ecfcb2   333.9 MB
docker_web_1                     <none>               <none>   d986d824dae4   953 MB

docker-compose stop: This command stops the running containers of specified services.

$ docker-compose stop
Stopping docker_web_1      ... done
Stopping docker_database_1 ... done

docker-compose run: This is similar to the docker run command. It will create containers from images built for the services mentioned in the compose file.

$ docker-compose run web
Starting 7001788f31a9_docker_database_1 ... done
 * Serving Flask app "app.py" (lazy loading)
 * Environment: development
 * Debug mode: on
 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)
 * Restarting with stat
 * Debugger is active!
 * Debugger PIN: 116-917-688

docker-compose up: This command does the work of the docker-compose build and docker-compose run commands. It builds the images if they are not located locally and starts the containers. If images are already built, it will fork the container directly.

$ docker-compose up
Creating docker_database_1 ... done
Creating docker_web_1      ... done
Attaching to docker_database_1, docker_web_1

docker-compose ps: This command list all the containers in the current docker-compose file. They can then either be running or stopped.

$ docker-compose ps
      Name                 Command             State               Ports         
---------------------------------------------------------------------------------
docker_database_1   /entrypoint.sh mysqld   Up (healthy)   3306/tcp, 33060/tcp   
docker_web_1        flask run               Up             0.0.0.0:5000->5000/tcp
 
$ docker-compose ps
      Name                 Command          State    Ports
----------------------------------------------------------
docker_database_1   /entrypoint.sh mysqld   Exit 0        
docker_web_1        flask run               Exit 0    

docker-compose down: This command is similar to the docker system prune command. However, in Compose, it stops all the services and cleans up the containers, networks, and images.

$ docker-compose down
Removing docker_web_1      ... done
Removing docker_database_1 ... done
Removing network docker_default
(django-tuts) Venkateshs-MacBook-Air:Docker venkateshachintalwar$ docker-compose images
Container   Repository   Tag   Image Id   Size
----------------------------------------------
(django-tuts) Venkateshs-MacBook-Air:Docker venkateshachintalwar$ docker-compose ps
Name   Command   State   Ports
------------------------------

Congrats! You’ve now learned most of the basic commands for Docker Compose. Why stop there? Check out the documentation of other commands and keep learning!