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 acompose.yml
and acompose.override.yml
file. You must supply at least thecompose.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 fromstdin
. 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 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:
console
$ 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.