Merge Compose files

Docker Compose lets you merge and override a set of Compose files together to create a composite Compose file.

By default, Compose reads two files, a compose.yml and an optional compose.override.yml file. By convention, the compose.yml contains your base configuration. The override file can contain configuration overrides for existing services or entirely new services.

If a service is defined in both files, Compose merges the configurations using the rules described below and in the Compose Specification.

How to merge multiple Compose files

To use multiple override files, or an override file with a different name, you can either use the pre-defined COMPOSE_FILE environment variable, or use the -f option to specify the list of files.

Compose merges files in the order they're specified on the command line. Subsequent files may merge, override, or add to their predecessors.

For example:

$ docker compose -f compose.yml -f compose.admin.yml run backup_db

The compose.yml file might specify a webapp service.

webapp:
  image: examples/web
  ports:
    - "8000:8000"
  volumes:
    - "/data"

The compose.admin.yml may also specify this same service:

webapp:
  environment:
    - DEBUG=1

Any matching fields override the previous file. New values, add to the webapp service configuration:

webapp:
  image: examples/web
  ports:
    - "8000:8000"
  volumes:
    - "/data"
  environment:
    - DEBUG=1
    - ANOTHER_VARIABLE=value

Important

When you use multiple Compose files, you must make sure all paths in the files are relative to the base Compose file (the first Compose file specified with -f). This is required because override files need not be valid Compose files. Override files can contain small fragments of configuration. Tracking which fragment of a service is relative to which path is difficult and confusing, so to keep paths easier to understand, all paths must be defined relative to the base file.

Additional information

• Using -f is optional. If not provided, Compose searches the working directory and its parent directories for a compose.yml and a compose.override.yml file. You must supply at least the compose.yml file. If both files exist on the same directory level, Compose combines them into a single configuration.

• When you use multiple Compose files, all paths in the files are relative to the first configuration file specified with -f. You can use the --project-directory option to override this base path.

• You can use a -f with - (dash) as the filename to read the configuration from stdin. For example:

  $ docker compose -f - <<EOF
    webapp:
      image: examples/web
      ports:
       - "8000:8000"
      volumes:
       - "/data"
      environment:
       - DEBUG=1
    EOF
  

  When stdin is used, all paths in the configuration are relative to the current working directory.

• You can use the -f flag to specify a path to a Compose file that is not located in the current directory, either from the command line or by setting up a COMPOSE_FILE environment variable in your shell or in an environment file.

  For example, if you are running the Compose Rails sample, and have a compose.yml file in a directory called sandbox/rails. You can use a command like docker compose pull to get the postgres image for the db service from anywhere by using the -f flag as follows: docker compose -f ~/sandbox/rails/compose.yml pull db

  Here's the full example:

  $ docker compose -f ~/sandbox/rails/compose.yml pull db
  Pulling db (postgres:latest)...
  latest: Pulling from library/postgres
  ef0380f84d05: Pull complete
  50cf91dc1db8: Pull complete
  d3add4cd115c: Pull complete
  467830d8a616: Pull complete
  089b9db7dc57: Pull complete
  6fba0a36935c: Pull complete
  81ef0e73c953: Pull complete
  338a6c4894dc: Pull complete
  15853f32f67c: Pull complete
  044c83d92898: Pull complete
  17301519f133: Pull complete
  dcca70822752: Pull complete
  cecf11b8ccf3: Pull complete
  Digest: sha256:1364924c753d5ff7e2260cd34dc4ba05ebd40ee8193391220be0f9901d4e1651
  Status: Downloaded newer image for postgres:latest
  

Merging rules

Compose copies configurations from the original service over to the local one. If a configuration option is defined in both the original service and the local service, the local value replaces or extends the original value.

For single-value options like image, command or mem_limit, the new value replaces the old value.

original service:

services:
  myservice:
    # ...
    command: python app.py

local service:

services:
  myservice:
    # ...
    command: python otherapp.py

result:

services:
  myservice:
    # ...
    command: python otherapp.py

For the multi-value options ports, expose, external_links, dns, dns_search, and tmpfs, Compose concatenates both sets of values:

original service:

services:
  myservice:
    # ...
    expose:
      - "3000"

local service:

services:
  myservice:
    # ...
    expose:
      - "4000"
      - "5000"

result:

services:
  myservice:
    # ...
    expose:
      - "3000"
      - "4000"
      - "5000"

In the case of environment, labels, volumes, and devices, Compose "merges" entries together with locally defined values taking precedence. For environment and labels, the environment variable or label name determines which value is used:

original service:

services:
  myservice:
    # ...
    environment:
      - FOO=original
      - BAR=original

local service:

services:
  myservice:
    # ...
    environment:
      - BAR=local
      - BAZ=local

result:

services:
  myservice:
    # ...
    environment:
      - FOO=original
      - BAR=local
      - BAZ=local

Entries for volumes and devices are merged using the mount path in the container:

original service:

services:
  myservice:
    # ...
    volumes:
      - ./original:/foo
      - ./original:/bar

local service:

services:
  myservice:
    # ...
    volumes:
      - ./local:/bar
      - ./local:/baz

result:

services:
  myservice:
    # ...
    volumes:
      - ./original:/foo
      - ./local:/bar
      - ./local:/baz

For more merging rules, see Merge and override in the Compose Specification.

Example

A common use case for multiple files is changing a development Compose app for a production-like environment (which may be production, staging or CI). To support these differences, you can split your Compose configuration into a few different files:

Start with a base file that defines the canonical configuration for the services.

compose.yml

services:
  web:
    image: example/my_web_app:latest
    depends_on:
      - db
      - cache

  db:
    image: postgres:latest

  cache:
    image: redis:latest

In this example the development configuration exposes some ports to the host, mounts our code as a volume, and builds the web image.

compose.override.yml

services:
  web:
    build: .
    volumes:
      - '.:/code'
    ports:
      - 8883:80
    environment:
      DEBUG: 'true'

  db:
    command: '-d'
    ports:
     - 5432:5432

  cache:
    ports:
      - 6379:6379

When you run docker compose up it reads the overrides automatically.

To use this Compose app in a production environment, another override file is created, which might be stored in a different git repo or managed by a different team.

compose.prod.yml

services:
  web:
    ports:
      - 80:80
    environment:
      PRODUCTION: 'true'

  cache:
    environment:
      TTL: '500'

To deploy with this production Compose file you can run

$ docker compose -f compose.yml -f compose.prod.yml up -d

This deploys all three services using the configuration in compose.yml and compose.prod.yml but not the dev configuration in compose.override.yml.

For more information, see Using Compose in production.

Limitations

Docker Compose supports relative paths for the many resources to be included in the application model: build context for service images, location of file defining environment variables, path to a local directory used in a bind-mounted volume. With such a constraint, code organization in a monorepo can become hard as a natural choice would be to have dedicated folders per team or component, but then the Compose files relative paths become irrelevant.

Reference information

• Merge rules