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Merge Compose files ​

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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.yaml and an optional compose.override.yaml file. By convention, the compose.yaml 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:

console
$ docker compose -f compose.yaml -f compose.admin.yaml run backup_db

The compose.yaml file might specify a webapp service.

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

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

yaml
webapp:
  environment:
    - DEBUG=1

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

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

Merging rules ​

  • Paths are evaluated relative to the base file. 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.

Tip

You can use docker compose config to review your merged configuration and avoid path-related issues.

  • 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:

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

      local service:

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

      result:

      yaml
      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:

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

local service:

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

result:

yaml
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:

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

local service:

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

result:

yaml
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:

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

local service:

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

result:

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

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

Additional information ​

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

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

    console
    $ 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](https://docs.docker.com/compose/how-tos/environment-variables/envvars/#compose_file) in your shell or in an environment file.

For example, if you are running the [Compose Rails sample](https://github.com/docker/awesome-compose/tree/master/official-documentation-samples/rails/README.md), and have a `compose.yaml` file in a directory called `sandbox/rails`. You can use a command like
[docker compose pull](https://docs.docker.com/reference/cli/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.yaml pull db`

Here's the full example:









```console
$ docker compose -f ~/sandbox/rails/compose.yaml pull db
Pulling db (postgres:18)...
18: 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:18

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.yaml

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

  db:
    image: postgres:18

  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.yaml

yaml
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 repository or managed by a different team.

compose.prod.yaml

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

  cache:
    environment:
      TTL: '500'

To deploy with this production Compose file you can run

console
$ docker compose -f compose.yaml -f compose.prod.yaml up -d

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

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 ​