Share Compose configurations between files and projectsEstimated reading time: 8 minutes
Compose supports two methods of sharing common configuration:
- Extending an entire Compose file by using multiple Compose files
- Extending individual services with the
extendsfield (for Compose file versions up to 2.1)
Multiple Compose files
Using multiple Compose files enables you to customize a Compose application for different environments or different workflows.
Understanding multiple Compose files
By default, Compose reads two files, a
docker-compose.yml and an optional
docker-compose.override.yml file. By convention, the
contains your base configuration. The override file, as its name implies, can
contain configuration overrides for existing services or entirely new
If a service is defined in both files, Compose merges the configurations using the rules described in Adding and overriding configuration.
To use multiple override files, or an override file with a different name, you
can use the
-f option to specify the list of files. Compose merges files in
the order they’re specified on the command line. See the
docker-compose command reference for more information
When you use multiple configuration files, you must make sure all paths in the
files are relative to the base Compose file (the first Compose file specified
-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.
Example use case
In this section, there are two common use cases for multiple Compose files: changing a Compose app for different environments, and running administrative tasks against a Compose app.
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.
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.
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.
Now, it would be nice to use this Compose app in a production environment. So, create another override file (which might be stored in a different git repo or managed by a different team).
web: ports: - 80:80 environment: PRODUCTION: 'true' cache: environment: TTL: '500'
To deploy with this production Compose file you can run
docker-compose -f docker-compose.yml -f docker-compose.prod.yml up -d
This deploys all three services using the configuration in
docker-compose.prod.yml (but not the
dev configuration in
See production for more information about Compose in production.
Another common use case is running adhoc or administrative tasks against one or more services in a Compose app. This example demonstrates running a database backup.
Start with a docker-compose.yml.
web: image: example/my_web_app:latest depends_on: - db db: image: postgres:latest
In a docker-compose.admin.yml add a new service to run the database export or backup.
dbadmin: build: database_admin/ depends_on: - db
To start a normal environment run
docker-compose up -d. To run a database
backup, include the
docker-compose.admin.yml as well.
docker-compose -f docker-compose.yml -f docker-compose.admin.yml \ run dbadmin db-backup
extendskeyword is supported in earlier Compose file formats up to Compose file version 2.1 (see extends in v1 and extends in v2), but is not supported in Compose version 3.x. See the Version 3 summary of keys added and removed, along with information on how to upgrade. See moby/moby#31101 to follow the discussion thread on the possibility of adding support for
extendsin some form in future versions.
extends keyword enables the sharing of common configurations
among different files, or even different projects entirely. Extending services
is useful if you have several services that reuse a common set of configuration
extends you can define a common set of service options in one
place and refer to it from anywhere.
Keep in mind that
depends_on are never shared between
extends. These exceptions exist to avoid implicit
dependencies; you always define
volumes_from locally. This ensures
dependencies between services are clearly visible when reading the current file.
Defining these locally also ensures that changes to the referenced file don’t
Understand the extends configuration
When defining any service in
docker-compose.yml, you can declare that you are
extending another service like this:
web: extends: file: common-services.yml service: webapp
This instructs Compose to re-use the configuration for the
defined in the
common-services.yml file. Suppose that
looks like this:
webapp: build: . ports: - "8000:8000" volumes: - "/data"
In this case, you get exactly the same result as if you wrote
docker-compose.yml with the same
values defined directly under
You can go further and define (or re-define) configuration locally in
web: extends: file: common-services.yml service: webapp environment: - DEBUG=1 cpu_shares: 5 important_web: extends: web cpu_shares: 10
You can also write other services and link your
web service to them:
web: extends: file: common-services.yml service: webapp environment: - DEBUG=1 cpu_shares: 5 depends_on: - db db: image: postgres
Example use case
Extending an individual service is useful when you have multiple services that have a common configuration. The example below is a Compose app with two services: a web application and a queue worker. Both services use the same codebase and share many configuration options.
In a common.yml we define the common configuration:
app: build: . environment: CONFIG_FILE_PATH: /code/config API_KEY: xxxyyy cpu_shares: 5
In a docker-compose.yml we define the concrete services which use the common configuration:
webapp: extends: file: common.yml service: app command: /code/run_web_app ports: - 8080:8080 depends_on: - queue - db queue_worker: extends: file: common.yml service: app command: /code/run_worker depends_on: - queue
Adding and overriding configuration
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
mem_limit, the new value
replaces the old value.
# original service command: python app.py # local service command: python otherapp.py # result command: python otherapp.py
imagein Compose file version 1
In the case of
image, when using version 1 of the Compose file format, using one option in the local service causes Compose to discard the other option if it was defined in the original service.
For example, if the original service defines
image: webappand the local service defines
build: .then the resulting service has a
build: .and no
This is because
imagecannot be used together in a version 1 file.
For the multi-value options
tmpfs, Compose concatenates both sets of values:
# original service expose: - "3000" # local service expose: - "4000" - "5000" # result expose: - "3000" - "4000" - "5000"
In the case of
“merges” entries together with locally-defined values taking precedence. For
labels, the environment variable or label name determines
which value is used:
# original service environment: - FOO=original - BAR=original # local service environment: - BAR=local - BAZ=local # result environment: - FOO=original - BAR=local - BAZ=local
devices are merged using the mount path in the
# original service volumes: - ./original:/foo - ./original:/bar # local service volumes: - ./local:/bar - ./local:/baz # result volumes: - ./original:/foo - ./local:/bar - ./local:/baz
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