This class is a general overview of the Docker Compose tool. The main goal is to explain the purpose, the benefits and how to use it.
Contents
Compose is a tool for defining and running multiples docker containers that needs to interact between each other. It's a very popular tool for development as well as for CI workflows.
The Compose tool is used by creating a file called docker-compose.yaml, where all the containers will be defined. Once you have created this file, you can bring all containers running by executing the command docker-compose up.
If you still don't have Docker Compose installed on your machine, you can follow the steps here to have it installed.
Because the Compose tool makes easy to run multiple containers in the same machine, it helps a lot during the application development phase, since you can make changes to an application running on docker, and can easily test the new code integration with the existent environment.
For example, you can have a Compose file defining a database service (like MySQL) and the application being developed. Whenever new code needs to be tested, Compose can be used used to run both services locally and the feedback from the code being developed can be gathered really quickly.
Another common usage of the Compose is during some integration tests of your CI pipeline.
By using docker-compose, you can start all your containers in your CI tool and run a set of tests to validate that all the components that your application is built from are working correctly together. This is a very common and important step of any mature CI pipeline.
The docker-compose.yaml file is where all the definition that your docker-compose command use goes. By being a yaml file, its structure is easily undestandable and well known for most Developers.
The compose file starts with the definition of the version to be used. The latest is version 3.
After the version, you can state the services that compose will run, which is a container image with any additional configuration, like environment variables, volume mounts, etc.
The example below is also here
version: '3'
services:
web:
build: .
ports:
- "5000:5000"
redis:
image: "redis:alpine"This file describe two services to be started. The web one, which requires a build based on this Dockerfile stored in the same folder. The second service is called redis and is based fom the redis:alpine image.
This is just the basics around the docker-compose.yaml file, but a detailed documentation is available here.
We will not go into a lot of details around the web and the redis images here, but it's a simple webserver that will count the number of hits.
By having the docker-compose.yaml file in place, we can just run the following command to build/start the containers in detach mode: docker-compose up -d.
By invoking this command, the docker-compose will create a new docker network to be shared between all the containers and will make the name of each service resolvable through DNS, so every container can communicate with the others by using their service name.
You can check the status of each container by running docker-compose ps and to stop the containers, just run docker-compose down in the same folder.
You can also run a single service defined in docker-compose.yaml by using the command docker-compose run.
Example: docker-compose run --rm redis
A list of all options of the docker-compose can be found here.