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The docker-compose.yml file

Function of docker-compose.yml

docker-compose.yml is used exclusively for local application set-up

In the Divio application architecture, the docker-compose.yml file is not used for cloud deployments, but only for configuration of the local environment. On the cloud, the deployment is taken care of by dedicated systems on our servers.

This means that entries in or changes to docker-compose.yml will not affect cloud deployments in any way.

In order to do something useful with containers, they have to be arranged - orchestrated - as part of an application, usually referred to as an 'application'.

There are multiple ways of orchestrating a Docker application, but Docker Compose is probably the most human-friendly. It's what we use for our local development environments.

To configure the orchestration, Docker Compose uses a docker-compose.yml file. It specifies what images are required, what ports they need to expose, whether they have access to the host filesystem, what commands should be run when they start up, and so on.

Services defined in docker-compose.yml

In a docker-compose.yml file, services represent the containers that will be created in the application.

When you create a new Divio application using one of our get started templates, it will include a docker-compose.yml file ready for local use, with the services already defined.

If you start with a blank application, you will need to populate the docker-compose.yml file yourself. This is a fairly straightforward process once you know what you are doing. Our how to deploy a web application guide includes steps for creating a complete docker-compose.yml file from scratch.

For a working local application, various things need to be defined in the file. In a Divio application, there will be a web service, that's built in a container using the Dockerfile. There will typically also be a db service, from a standard postgres or other database image.

Most Divio applications will use a docker-compose.yml that contains entries along these lines.

version: "3"

build: .
command: python runserver
DATABASE_URL: postgres://postgres@database_default:5432/db
- "database_default"
- "8000:80"
- ".:/app:rw"
- "./data:/data:rw"

image: postgres:13.5-alpine
SERVICE_MANAGER: "fsm-postgres"
- ".:/app:rw"
- "database-default:/var/lib/postgresql/data/"


Some applications will have additional services (such as Celery for example) defined.

Let's look at the components of the file more closely.

The application container service, web

The first definition in the file is for the web service. In order, the directives mean:

  • build: build it from the Dockerfile in the current directory
  • command: by default, when the command docker-compose run is issued, execute python runserver (this will override the CMD instruction in the Dockerfile)
  • environment: supply environment variables to the container
  • links: a link to the database container (database_default)
  • ports: map the external port 8000 to the internal port 80
  • volumes:
    • .:/app:rw maps the parent directory on the host to /app in the container, with read and write access
    • /data:/data:rw maps the data directory on the host to /data in the container, with read and write access

The volumes directive

When you execute a docker-compose command, the volumes directive in docker-compose.yml file mounts source directories or volumes from your computer at target paths inside the container. If a matching target path exists already as part of the container image, it will be overwritten by the mounted path.

For example:

- ".:/app:rw"
- "./data:/data:rw"

will mount the entire application code (at the relative path .) as the /app directory inside the container, even if there was already an /app directory there*, in read-write mode (i.e. the container can write as well as read files on the host).

Why mount data?

The entry for "./data:/data:rw" is used by the Divio CLI when running divio pull media and divio upload media to work with your media files.

This allows you to make changes to the application from your computer during the local development process, that will be picked up by the application inside Docker. These changes will be available to the application only as long as the host directory is mounted inside the container. In order to be made permanent, they need to be committed into the repository so that they will be picked up when the image and container are rebuilt.

Implications for local testing

Nearly everything in /app in the container is also present in the application repository and thus on the host machine. This means that it is safe to replace the container's /app files with those from the host.

However, any files in /app that are placed there during the build process, i.e. the execution of the Dockerfile, will not be available in the local environment. For a standard Django application, these will include:

  • the compiled pip requirements, in requirements.txt
  • collected static files, in static_collected

In most cases, this will not matter, but sometimes these files are required in local development. For example, the requirements.txt may contain useful information about dependency relationships, or the Dockerfile may have performed custom processing of static files.

In that case, the - ".:/app:rw" line can be commented out in docker-compose.yml. In this case, the container will use the files baked into the image, and will not use the local host's files.

This will allow local configuration to replicate the cloud environment even more closely.

Environment variables

Environment variables are loaded from the environment declaration or you could also specify a file:

env_file: .env

The database container service, database_default

The second definition is for the database_default service.

On the cloud, the application's database runs on one of our database clusters; locally, it runs on a Postgres instance in database_default.

The directives mean:

  • image: build the container from the postgres:13.5-alpine image
  • volumes: map the parent directory on the host to /app in the container, with read and write access
  • environment: sets various environment variables for the running container. The SERVICE_MANAGER variable provides information about the database service so that the Divio CLI can handle it correctly (fsm-postgres and fsm-mysql are currently supported).

See Expose the database's port to the host for an example of adding configuration to docker-compose.yml.

Required database service configuration

The Divio CLI expects that the database service will be called database_default (or, in some older applications, db). If the name is changed, operations such as divio app pull db will fail.

The volumes directive needs to map the container's /app directory as described above, for the same reason.

Further reading

If you are new and would like to familiarise yourself with Divio, dive into our get started section.

The how to configure Celery section describes adding additional services in Docker Compose for a more complex local set-up.