The Compose Specification

Status of this document

This document specifies the Compose file format used to define multi-containers applications. Distribution of this document is unlimited.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.

Requirements and optional attributes

The Compose specification includes properties designed to target a local OCI container runtime, exposing Linux kernel specific configuration options, but also some Windows container specific properties, as well as cloud platform features related to resource placement on a cluster, replicated application distribution and scalability.

We acknowledge that no Compose implementation is expected to support all attributes, and that support for some properties is Platform dependent and can only be confirmed at runtime. The definition of a versioned schema to control the supported properties in a Compose file, established by the docker-compose tool where the Compose file format was designed, doesn't offer any guarantee to the end-user attributes will be actually implemented.

The specification defines the expected configuration syntax and behaviour, but - until noted - supporting any of those is OPTIONAL.

A Compose implementation to parse a Compose file using unsupported attributes SHOULD warn user. We recommend implementors to support those running modes:

default: warn user about unsupported attributes, but ignore them
strict: warn user about unsupported attributes and reject the compose file
loose: ignore unsupported attributes AND unknown attributes (that were not defined by the spec by the time implementation was created)

The Compose application model

The Compose specification allows one to define a platform-agnostic container based application. Such an application is designed as a set of containers which have to both run together with adequate shared resources and communication channels.

Computing components of an application are defined as Services. A Service is an abstract concept implemented on platforms by running the same container image (and configuration) one or more times.

Services communicate with each other through Networks. In this specification, a Network is a platform capability abstraction to establish an IP route between containers within services connected together. Low-level, platform-specific networking options are grouped into the Network definition and MAY be partially implemented on some platforms.

Services store and share persistent data into Volumes. The specification describes such a persistent data as a high-level filesystem mount with global options. Actual platform-specific implementation details are grouped into the Volumes definition and MAY be partially implemented on some platforms.

Some services require configuration data that is dependent on the runtime or platform. For this, the specification defines a dedicated concept: Configs. From a Service container point of view, Configs are comparable to Volumes, in that they are files mounted into the container. But the actual definition involves distinct platform resources and services, which are abstracted by this type.

A Secret is a specific flavour of configuration data for sensitive data that SHOULD NOT be exposed without security considerations. Secrets are made available to services as files mounted into their containers, but the platform-specific resources to provide sensitive data are specific enough to deserve a distinct concept and definition within the Compose specification.

Distinction within Volumes, Configs and Secret allows implementations to offer a comparable abstraction at service level, but cover the specific configuration of adequate platform resources for well identified data usages.

A Project is an individual deployment of an application specification on a platform. A project's name is used to group resources together and isolate them from other applications or other installation of the same Compose specified application with distinct parameters. A Compose implementation creating resources on a platform MUST prefix resource names by project and set the label com.docker.compose.project.

Illustrative example

The following example illustrates Compose specification concepts with a concrete example application. The example is non-normative.

Consider an application split into a frontend web application and a backend service.

The frontend is configured at runtime with an HTTP configuration file managed by infrastructure, providing an external domain name, and an HTTPS server certificate injected by the platform's secured secret store.

The backend stores data in a persistent volume.

Both services communicate with each other on an isolated back-tier network, while frontend is also connected to a front-tier network and exposes port 443 for external usage.

(External user) --> 443 [frontend network]
                            |
                  +--------------------+
                  |  frontend service  |...ro...<HTTP configuration>
                  |      "webapp"      |...ro...<server certificate> #secured
                  +--------------------+
                            |
                        [backend network]
                            |
                  +--------------------+
                  |  backend service   |  r+w   ___________________
                  |     "database"     |=======( persistent volume )
                  +--------------------+        \_________________/

The example application is composed of the following parts:

2 services, backed by Docker images: webapp and database
1 secret (HTTPS certificate), injected into the frontend
1 configuration (HTTP), injected into the frontend
1 persistent volume, attached to the backend
2 networks

services:
  frontend:
    image: awesome/webapp
    ports:
      - "443:8043"
    networks:
      - front-tier
      - back-tier
    configs:
      - httpd-config
    secrets:
      - server-certificate

  backend:
    image: awesome/database
    volumes:
      - db-data:/etc/data
    networks:
      - back-tier

volumes:
  db-data:
    driver: flocker
    driver_opts:
      size: "10GiB"

configs:
  httpd-config:
    external: true

secrets:
  server-certificate:
    external: true

networks:
  # The presence of these objects is sufficient to define them
  front-tier: {}
  back-tier: {}

This example illustrates the distinction between volumes, configs and secrets. While all of them are all exposed to service containers as mounted files or directories, only a volume can be configured for read+write access. Secrets and configs are read-only. The volume configuration allows you to select a volume driver and pass driver options to tweak volume management according to the actual infrastructure. Configs and Secrets rely on platform services, and are declared external as they are not managed as part of the application lifecycle: the Compose implementation will use a platform-specific lookup mechanism to retrieve runtime values.

Compose file

The Compose file is a YAML file defining version (DEPRECATED), services (REQUIRED), networks, volumes, configs and secrets. The default path for a Compose file is compose.yaml (preferred) or compose.yml in working directory. Compose implementations SHOULD also support docker-compose.yaml and docker-compose.yml for backward compatibility. If both files exist, Compose implementations MUST prefer canonical compose.yaml one.

Multiple Compose files can be combined together to define the application model. The combination of YAML files MUST be implemented by appending/overriding YAML elements based on Compose file order set by the user. Simple attributes and maps get overridden by the highest order Compose file, lists get merged by appending. Relative paths MUST be resolved based on the first Compose file's parent folder, whenever complimentary files being merged are hosted in other folders.

As some Compose file elements can both be expressed as single strings or complex objects, merges MUST apply to the expanded form.

Profiles

Profiles allow to adjust the Compose application model for various usages and environments. A Compose implementation SHOULD allow the user to define a set of active profiles. The exact mechanism is implementation specific and MAY include command line flags, environment variables, etc.

The Services top-level element supports a profiles attribute to define a list of named profiles. Services without a profiles attribute set MUST always be enabled. A service MUST be ignored by the Compose implementation when none of the listed profiles match the active ones, unless the service is explicitly targeted by a command. In that case its profiles MUST be added to the set of active profiles. All other top-level elements are not affected by profiles and are always active.

References to other services (by links, extends or shared resource syntax service:xxx) MUST not automatically enable a component that would otherwise have been ignored by active profiles. Instead the Compose implementation MUST return an error.

Illustrative example

services:
  foo:
    image: foo
  bar:
    image: bar
    profiles:
      - test
  baz:
    image: baz
    depends_on:
      - bar
    profiles:
      - test
  zot:
    image: zot
    depends_on:
      - bar
    profiles:
      - debug

Compose application model parsed with no profile enabled only contains the foo service.
If profile test is enabled, model contains the services bar and baz which are enabled by the test profile and service foo which is always enabled.
If profile debug is enabled, model contains both foo and zot services, but not bar and baz and as such the model is invalid regarding the depends_on constraint of zot.
If profiles debug and test are enabled, model contains all services: foo, bar, baz and zot.
If Compose implementation is executed with bar as explicit service to run, it and the test profile will be active even if test profile is not enabled by the user.
If Compose implementation is executed with baz as explicit service to run, the service baz and the profile test will be active and bar will be pulled in by the depends_on constraint.
If Compose implementation is executed with zot as explicit service to run, again the model will be invalid regarding the depends_on constraint of zot since zot and bar have no common profiles listed.
If Compose implementation is executed with zot as explicit service to run and profile test enabled, profile debug is automatically enabled and service bar is pulled in as a dependency starting both services zot and bar.

Version top-level element

Top-level version property is defined by the specification for backward compatibility but is only informative.

A Compose implementation SHOULD NOT use this version to select an exact schema to validate the Compose file, but prefer the most recent schema at the time it has been designed.

Compose implementations SHOULD validate they can fully parse the Compose file. If some fields are unknown, typically because the Compose file was written with fields defined by a newer version of the specification, Compose implementations SHOULD warn the user. Compose implementations MAY offer options to ignore unknown fields (as defined by "loose" mode).

Services top-level element

A Service is an abstract definition of a computing resource within an application which can be scaled/replaced independently from other components. Services are backed by a set of containers, run by the platform according to replication requirements and placement constraints. Being backed by containers, Services are defined by a Docker image and set of runtime arguments. All containers within a service are identically created with these arguments.

A Compose file MUST declare a services root element as a map whose keys are string representations of service names, and whose values are service definitions. A service definition contains the configuration that is applied to each container started for that service.

Each service MAY also include a Build section, which defines how to create the Docker image for the service. Compose implementations MAY support building docker images using this service definition. If not implemented the Build section SHOULD be ignored and the Compose file MUST still be considered valid.

Build support is an OPTIONAL aspect of the Compose specification, and is described in detail here

Each Service defines runtime constraints and requirements to run its containers. The deploy section groups these constraints and allows the platform to adjust the deployment strategy to best match containers' needs with available resources.

Deploy support is an OPTIONAL aspect of the Compose specification, and is described in detail here. If not implemented the Deploy section SHOULD be ignored and the Compose file MUST still be considered valid.

deploy

deploy specifies the configuration for the deployment and lifecycle of services, as defined here.

blkio_config

blkio_config defines a set of configuration options to set block IO limits for this service.

services:
  foo:
    image: busybox
    blkio_config:
       weight: 300
       weight_device:
         - path: /dev/sda
           weight: 400
       device_read_bps:
         - path: /dev/sdb
           rate: '12mb'
       device_read_iops:
         - path: /dev/sdb
           rate: 120
       device_write_bps:
         - path: /dev/sdb
           rate: '1024k'
       device_write_iops:
         - path: /dev/sdb
           rate: 30

device_read_bps, device_write_bps

Set a limit in bytes per second for read / write operations on a given device. Each item in the list MUST have two keys:

path: defining the symbolic path to the affected device.
rate: either as an integer value representing the number of bytes or as a string expressing a byte value.

device_read_iops, device_write_iops

Set a limit in operations per second for read / write operations on a given device. Each item in the list MUST have two keys:

path: defining the symbolic path to the affected device.
rate: as an integer value representing the permitted number of operations per second.

weight

Modify the proportion of bandwidth allocated to this service relative to other services. Takes an integer value between 10 and 1000, with 500 being the default.

weight_device

Fine-tune bandwidth allocation by device. Each item in the list must have two keys:

path: defining the symbolic path to the affected device.
weight: an integer value between 10 and 1000.

cpu_count

cpu_count defines the number of usable CPUs for service container.

cpu_percent

cpu_percent defines the usable percentage of the available CPUs.

cpu_shares

cpu_shares defines (as integer value) service container relative CPU weight versus other containers.

cpu_period

cpu_period allow Compose implementations to configure CPU CFS (Completely Fair Scheduler) period when platform is based on Linux kernel.

cpu_quota

cpu_quota allow Compose implementations to configure CPU CFS (Completely Fair Scheduler) quota when platform is based on Linux kernel.

cpu_rt_runtime

cpu_rt_runtime configures CPU allocation parameters for platform with support for realtime scheduler. Can be either an integer value using microseconds as unit or a duration.

 cpu_rt_runtime: '400ms'
 cpu_rt_runtime: 95000`

cpu_rt_period

cpu_rt_period configures CPU allocation parameters for platform with support for realtime scheduler. Can be either an integer value using microseconds as unit or a duration.

 cpu_rt_period: '1400us'
 cpu_rt_period: 11000`

cpus

DEPRECATED: use deploy.reservations.cpus

cpus define the number of (potentially virtual) CPUs to allocate to service containers. This is a fractional number. 0.000 means no limit.

cpuset

cpuset defines the explicit CPUs in which to allow execution. Can be a range 0-3 or a list 0,1

build

build specifies the build configuration for creating container image from source, as defined here.

cap_add

cap_add specifies additional container capabilities as strings.

cap_add:
  - ALL

cap_drop

cap_drop specifies container capabilities to drop as strings.

cap_drop:
  - NET_ADMIN
  - SYS_ADMIN

cgroup_parent

cgroup_parent specifies an OPTIONAL parent cgroup for the container.

cgroup_parent: m-executor-abcd

command

command overrides the the default command declared by the container image (i.e. by Dockerfile's CMD).

command: bundle exec thin -p 3000

The command can also be a list, in a manner similar to Dockerfile:

command: [ "bundle", "exec", "thin", "-p", "3000" ]

configs

configs grant access to configs on a per-service basis using the per-service configs configuration. Two different syntax variants are supported.

Compose implementations MUST report an error if config doesn't exist on platform or isn't defined in the configs section of this Compose file.

There are two syntaxes defined for configs. To remain compliant to this specification, an implementation MUST support both syntaxes. Implementations MUST allow use of both short and long syntaxes within the same document.

Short syntax

The short syntax variant only specifies the config name. This grants the container access to the config and mounts it at /<config_name> within the container. The source name and destination mount point are both set to the config name.

The following example uses the short syntax to grant the redis service access to the my_config and my_other_config configs. The value of my_config is set to the contents of the file ./my_config.txt, and my_other_config is defined as an external resource, which means that it has already been defined in the platform. If the external config does not exist, the deployment MUST fail.

services:
  redis:
    image: redis:latest
    configs:
      - my_config
configs:
  my_config:
    file: ./my_config.txt
  my_other_config:
    external: true

Long syntax

The long syntax provides more granularity in how the config is created within the service's task containers.

source: The name of the config as it exists in the platform.
target: The path and name of the file to be mounted in the service's task containers. Defaults to /<source> if not specified.
uid and gid: The numeric UID or GID that owns the mounted config file within the service's task containers. Default value when not specified is USER running container.
mode: The permissions for the file that is mounted within the service's task containers, in octal notation. Default value is world-readable (0444). Writable bit MUST be ignored. The executable bit can be set.

The following example sets the name of my_config to redis_config within the container, sets the mode to 0440 (group-readable) and sets the user and group to 103. The redis service does not have access to the my_other_config config.

services:
  redis:
    image: redis:latest
    configs:
      - source: my_config
        target: /redis_config
        uid: "103"
        gid: "103"
        mode: 0440
configs:
  my_config:
    external: true
  my_other_config:
    external: true

You can grant a service access to multiple configs, and you can mix long and short syntax.

container_name

container_name is a string that specifies a custom container name, rather than a generated default name.

container_name: my-web-container

Compose implementation MUST NOT scale a service beyond one container if the Compose file specifies a container_name. Attempting to do so MUST result in an error.

If present, container_name SHOULD follow the regex format of [a-zA-Z0-9][a-zA-Z0-9_.-]+

credential_spec

credential_spec configures the credential spec for a managed service account.

Compose implementations that support services using Windows containers MUST support file: and registry: protocols for credential_spec. Compose implementations MAY also support additional protocols for custom use-cases.

The credential_spec must be in the format file://<filename> or registry://<value-name>.

credential_spec:
  file: my-credential-spec.json

When using registry:, the credential spec is read from the Windows registry on the daemon's host. A registry value with the given name must be located in:

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Virtualization\Containers\CredentialSpecs

The following example loads the credential spec from a value named my-credential-spec in the registry:

credential_spec:
  registry: my-credential-spec

Example gMSA configuration

When configuring a gMSA credential spec for a service, you only need to specify a credential spec with config, as shown in the following example:

services:
  myservice:
    image: myimage:latest
    credential_spec:
      config: my_credential_spec

configs:
  my_credentials_spec:
    file: ./my-credential-spec.json|

depends_on

depends_on expresses startup and shutdown dependencies between services.

Short syntax

The short syntax variant only specifies service names of the dependencies. Service dependencies cause the following behaviors:

Compose implementations MUST create services in dependency order. In the following example, db and redis are created before web.
Compose implementations MUST remove services in dependency order. In the following example, web is removed before db and redis.

Simple example:

services:
  web:
    build: .
    depends_on:
      - db
      - redis
  redis:
    image: redis
  db:
    image: postgres

Compose implementations MUST guarantee dependency services have been started before starting a dependent service. Compose implementations MAY wait for dependency services to be "ready" before starting a dependent service.

Long syntax

The long form syntax enables the configuration of additional fields that can't be expressed in the short form.

condition: condition under which dependency is considered satisfied
- service_started: is an equivalent of the short syntax described above
- service_healthy: specifies that a dependency is expected to be "healthy" (as indicated by healthcheck) before starting a dependent service.
- service_completed_successfully: specifies that a dependency is expected to run to successful completion before starting a dependent service.

Service dependencies cause the following behaviors:

Compose implementations MUST create services in dependency order. In the following example, db and redis are created before web.
Compose implementations MUST wait for healthchecks to pass on dependencies marked with service_healthy. In the following example, db is expected to be "healthy" before web is created.
Compose implementations MUST remove services in dependency order. In the following example, web is removed before db and redis.

Simple example:

services:
  web:
    build: .
    depends_on:
      db:
        condition: service_healthy
      redis:
        condition: service_started
  redis:
    image: redis
  db:
    image: postgres

Compose implementations MUST guarantee dependency services have been started before starting a dependent service. Compose implementations MUST guarantee dependency services marked with service_healthy are "healthy" before starting a dependent service.

device_cgroup_rules

device_cgroup_rules defines a list of device cgroup rules for this container. The format is the same format the Linux kernel specifies in the Control Groups Device Whitelist Controller.

device_cgroup_rules:
  - 'c 1:3 mr'
  - 'a 7:* rmw'

devices

devices defines a list of device mappings for created containers.

devices:
  - "/dev/ttyUSB0:/dev/ttyUSB0"

dns

dns defines custom DNS servers to set on the container network interface configuration. Can be a single value or a list.

dns: 8.8.8.8

dns:
  - 8.8.8.8
  - 9.9.9.9

dns_opt

dns_opt list custom DNS options to be passed to the container’s DNS resolver (/etc/resolv.conf file on Linux).

dns_opt:
  - use-vc
  - no-tld-query

dns_search

dns defines custom DNS search domains to set on container network interface configuration. Can be a single value or a list.

dns_search: example.com

dns_search:
  - dc1.example.com
  - dc2.example.com

domainname

domainname declares a custom domain name to use for the service container. MUST be a valid RFC 1123 hostname.

entrypoint

entrypoint overrides the default entrypoint for the Docker image (i.e. ENTRYPOINT set by Dockerfile). Compose implementations MUST clear out any default command on the Docker image - both ENTRYPOINT and CMD instruction in the Dockerfile - when entrypoint is configured by a Compose file. If command is also set, it is used as parameter to entrypoint as a replacement for Docker image's CMD

entrypoint: /code/entrypoint.sh

The entrypoint can also be a list, in a manner similar to Dockerfile:

entrypoint:
  - php
  - -d
  - zend_extension=/usr/local/lib/php/extensions/no-debug-non-zts-20100525/xdebug.so
  - -d
  - memory_limit=-1
  - vendor/bin/phpunit

env_file

env_file adds environment variables to the container based on file content.

env_file: .env

env_file can also be a list. The files in the list MUST be processed from the top down. For the same variable specified in two env files, the value from the last file in the list MUST stand.

env_file:
  - ./a.env
  - ./b.env

Relative path MUST be resolved from the Compose file's parent folder. As absolute paths prevent the Compose file from being portable, Compose implementations SHOULD warn users when such a path is used to set env_file.

Environment variables declared in the environment section MUST override these values – this holds true even if those values are empty or undefined.

Env_file format

Each line in an env file MUST be in VAR[=[VAL]] format. Lines beginning with # MUST be ignored. Blank lines MUST also be ignored.

The value of VAL is used as a raw string and not modified at all. If the value is surrounded by quotes (as is often the case for shell variables), the quotes MUST be included in the value passed to containers created by the Compose implementation.

VAL MAY be omitted, in such cases the variable value is empty string. =VAL MAY be omitted, in such cases the variable is unset.

# Set Rails/Rack environment
RACK_ENV=development
VAR="quoted"

environment

environment defines environment variables set in the container. environment can use either an array or a map. Any boolean values; true, false, yes, no, MUST be enclosed in quotes to ensure they are not converted to True or False by the YAML parser.

Environment variables MAY be declared by a single key (no value to equals sign). In such a case Compose implementations SHOULD rely on some user interaction to resolve the value. If they do not, the variable is unset and will be removed from the service container environment.

Map syntax:

environment:
  RACK_ENV: development
  SHOW: "true"
  USER_INPUT:

Array syntax:

environment:
  - RACK_ENV=development
  - SHOW=true
  - USER_INPUT

When both env_file and environment are set for a service, values set by environment have precedence.

expose

expose defines the ports that Compose implementations MUST expose from container. These ports MUST be accessible to linked services and SHOULD NOT be published to the host machine. Only the internal container ports can be specified.

expose:
  - "3000"
  - "8000"

extends

Extend another service, in the current file or another, optionally overriding configuration. You can use extends on any service together with other configuration keys. The extends value MUST be a mapping defined with a required service and an optional file key.

extends:
  file: common.yml
  service: webapp

If supported Compose implementations MUST process extends in the following way:

service defines the name of the service being referenced as a base, for example web or database.
file is the location of a Compose configuration file defining that service.

Restrictions

The following restrictions apply to the service being referenced:

Services that have dependencies on other services cannot be used as a base. Therefore, any key that introduces a dependency on another service is incompatible with extends. The non-exhaustive list of such keys is: links, volumes_from, container mode (in ipc, pid, network_mode and net), service mode (in ipc, pid and network_mode), depends_on.
Services cannot have circular references with extends

Compose implementations MUST return an error in all of these cases.

Finding referenced service

file value can be:

Not present. This indicates that another service within the same Compose file is being referenced.
File path, which can be either:
- Relative path. This path is considered as relative to the location of the main Compose file.
- Absolute path.

Service denoted by service MUST be present in the identified referenced Compose file. Compose implementations MUST return an error if:

Service denoted by service was not found
Compose file denoted by file was not found

Merging service definitions

Two service definitions (main one in the current Compose file and referenced one specified by extends) MUST be merged in the following way:

Mappings: keys in mappings of main service definition override keys in mappings of referenced service definition. Keys that aren't overridden are included as is.
Sequences: items are combined together into an new sequence. Order of elements is preserved with the referenced items coming first and main items after.
Scalars: keys in main service definition take precedence over keys in the referenced one.

Mappings

The following keys should be treated as mappings: build.args, build.labels, build.extra_hosts, deploy.labels, deploy.update_config, deploy.rollback_config, deploy.restart_policy, deploy.resources.limits, environment, healthcheck, labels, logging.options, sysctls, storage_opt, extra_hosts, ulimits.

One exception that applies to healthcheck is that main mapping cannot specify disable: true unless referenced mapping also specifies disable: true. Compose implementations MUST return an error in this case.

For example, the input below:

services:
  common:
    image: busybox
    environment:
      TZ: utc
      PORT: 80
  cli:
    extends:
      service: common
    environment:
      PORT: 8080

Produces the following configuration for the cli service. The same output is produced if array syntax is used.

environment:
  PORT: 8080
  TZ: utc
image: busybox

Items under blkio_config.device_read_bps, blkio_config.device_read_iops, blkio_config.device_write_bps, blkio_config.device_write_iops, devices and volumes are also treated as mappings where key is the target path inside the container.

For example, the input below:

services:
  common:
    image: busybox
    volumes:
      - common-volume:/var/lib/backup/data:rw
  cli:
    extends:
      service: common
    volumes:
      - cli-volume:/var/lib/backup/data:ro

Produces the following configuration for the cli service. Note that mounted path now points to the new volume name and ro flag was applied.

image: busybox
volumes:
- cli-volume:/var/lib/backup/data:ro

If referenced service definition contains extends mapping, the items under it are simply copied into the new merged definition. Merging process is then kicked off again until no extends keys are remaining.

For example, the input below:

services:
  base:
    image: busybox
    user: root
  common:
    image: busybox
    extends:
      service: base
  cli:
    extends:
      service: common

Produces the following configuration for the cli service. Here, cli services gets user key from common service, which in turn gets this key from base service.

image: busybox
user: root

Sequences

The following keys should be treated as sequences: cap_add, cap_drop, configs, deploy.placement.constraints, deploy.placement.preferences, deploy.reservations.generic_resources, device_cgroup_rules, expose, external_links, ports, secrets, security_opt. Any duplicates resulting from the merge are removed so that the sequence only contains unique elements.

For example, the input below:

services:
  common:
    image: busybox
    security_opt:
      - label:role:ROLE
  cli:
    extends:
      service: common
    security_opt:
      - label:user:USER

Produces the following configuration for the cli service.

image: busybox
security_opt:
- label:role:ROLE
- label:user:USER

In case list syntax is used, the following keys should also be treated as sequences: dns, dns_search, env_file, tmpfs. Unlike sequence fields mentioned above, duplicates resulting from the merge are not removed.

Scalars

Any other allowed keys in the service definition should be treated as scalars.

external_links

external_links link service containers to services managed outside this Compose application. external_links define the name of an existing service to retrieve using the platform lookup mechanism. An alias of the form SERVICE:ALIAS can be specified.

external_links:
  - redis
  - database:mysql
  - database:postgresql

extra_hosts

extra_hosts adds hostname mappings to the container network interface configuration (/etc/hosts for Linux). Values MUST set hostname and IP address for additional hosts in the form of HOSTNAME:IP.

extra_hosts:
  - "somehost:162.242.195.82"
  - "otherhost:50.31.209.229"

Compose implementations MUST create matching entry with the IP address and hostname in the container's network configuration, which means for Linux /etc/hosts will get extra lines:

162.242.195.82  somehost
50.31.209.229   otherhost

group_add

group_add specifies additional groups (by name or number) which the user inside the container MUST be a member of.

An example of where this is useful is when multiple containers (running as different users) need to all read or write the same file on a shared volume. That file can be owned by a group shared by all the containers, and specified in group_add.

services:
  myservice:
    image: alpine
    group_add:
      - mail

Running id inside the created container MUST show that the user belongs to the mail group, which would not have been the case if group_add were not declared.

healthcheck

healthcheck declares a check that's run to determine whether or not containers for this service are "healthy". This overrides HEALTHCHECK Dockerfile instruction set by the service's Docker image.

healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 1m30s
  timeout: 10s
  retries: 3
  start_period: 40s

interval, timeout and start_period are specified as durations.

test defines the command the Compose implementation will run to check container health. It can be either a string or a list. If it's a list, the first item must be either NONE, CMD or CMD-SHELL. If it's a string, it's equivalent to specifying CMD-SHELL followed by that string.

# Hit the local web app
test: ["CMD", "curl", "-f", "http://localhost"]

Using CMD-SHELL will run the command configured as a string using the container's default shell (/bin/sh for Linux). Both forms below are equivalent:

test: ["CMD-SHELL", "curl -f http://localhost || exit 1"]

test: curl -f https://localhost || exit 1

NONE disable the healthcheck, and is mostly useful to disable Healthcheck set by image. Alternatively the healthcheck set by the image can be disabled by setting disable: true:

healthcheck:
  disable: true

hostname

hostname declares a custom host name to use for the service container. MUST be a valid RFC 1123 hostname.

image

image specifies the image to start the container from. Image MUST follow the Open Container Specification addressable image format, as [<registry>/][<project>/]<image>[:<tag>|@<digest>].

    image: redis
    image: redis:5
    image: redis@sha356:0ed5d5928d4737458944eb604cc8509e245c3e19d02ad83935398bc4b991aac7
    image: library/redis
    image: docker.io/library/redis
    image: my_private.registry:5000/redis

If the image does not exist on the platform, Compose implementations MUST attempt to pull it based on the pull_policy. Compose implementations with build support MAY offer alternative options for the end user to control precedence of pull over building the image from source, however pulling the image MUST be the default behavior.

image MAY be omitted from a Compose file as long as a build section is declared. Compose implementations without build support MUST fail when image is missing from the Compose file.

init

init run an init process (PID 1) inside the container that forwards signals and reaps processes. Set this option to true to enable this feature for the service.

services:
  web:
    image: alpine:latest
    init: true

The init binary that is used is platform specific.

ipc

ipc configures the IPC isolation mode set by service container. Available values are platform specific, but Compose specification defines specific values which MUST be implemented as described if supported:

shareable which gives the container own private IPC namespace, with a possibility to share it with other containers.
service:{name} which makes the container join another (shareable) container's IPC namespace.

    ipc: "shareable"
    ipc: "service:[service name]"

isolation

isolation specifies a container’s isolation technology. Supported values are platform-specific.

labels

labels add metadata to containers. You can use either an array or a map.

It's recommended that you use reverse-DNS notation to prevent your labels from conflicting with those used by other software.

labels:
  com.example.description: "Accounting webapp"
  com.example.department: "Finance"
  com.example.label-with-empty-value: ""

labels:
  - "com.example.description=Accounting webapp"
  - "com.example.department=Finance"
  - "com.example.label-with-empty-value"

Compose implementations MUST create containers with canonical labels:

com.docker.compose.project set on all resources created by Compose implementation to the user project name
com.docker.compose.service set on service containers with service name as defined in the Compose file

The com.docker.compose label prefix is reserved. Specifying labels with this previx in the Compose file MUST result in a runtime error.

links

links defines a network link to containers in another service. Either specify both the service name and a link alias (SERVICE:ALIAS), or just the service name.

web:
  links:
    - db
    - db:database
    - redis

Containers for the linked service MUST be reachable at a hostname identical to the alias, or the service name if no alias was specified.

Links are not required to enable services to communicate - when no specific network configuration is set, any service MUST be able to reach any other service at that service’s name on the default network. If services do declare networks they are attached to, links SHOULD NOT override the network configuration and services not attached to a shared network SHOULD NOT be able to communicate. Compose implementations MAY NOT warn the user about this configuration mismatch.

Links also express implicit dependency between services in the same way as depends_on, so they determine the order of service startup.

logging

logging defines the logging configuration for the service.

logging:
  driver: syslog
  options:
    syslog-address: "tcp://192.168.0.42:123"

The driver name specifies a logging driver for the service's containers. The default and available values are platform specific. Driver specific options can be set with options as key-value pairs.

network_mode

network_mode set service containers network mode. Available values are platform specific, but Compose specification define specific values which MUST be implemented as described if supported:

none which disable all container networking
host which gives the container raw access to host's network interface
service:{name} which gives the containers access to the specified service only

    network_mode: "host"
    network_mode: "none"
    network_mode: "service:[service name]"

networks

networks defines the networks that service containers are attached to, referencing entries under the top-level networks key.

services:
  some-service:
    networks:
      - some-network
      - other-network

aliases

aliases declares alternative hostnames for this service on the network. Other containers on the same network can use either the service name or this alias to connect to one of the service's containers.

Since aliases are network-scoped, the same service can have different aliases on different networks.

Note: A network-wide alias can be shared by multiple containers, and even by multiple services. If it is, then exactly which container the name resolves to is not guaranteed.

The general format is shown here:

services:
  some-service:
    networks:
      some-network:
        aliases:
          - alias1
          - alias3
      other-network:
        aliases:
          - alias2

In the example below, service frontend will be able to reach the backend service at the hostname backend or database on the back-tier network, and service monitoring will be able to reach same backend service at db or mysql on the admin network.

services:
  frontend:
    image: awesome/webapp
    networks:
      - front-tier
      - back-tier

  monitoring:
    image: awesome/monitoring
    networks:
      - admin

  backend:
    image: awesome/backend
    networks:
      back-tier:
        aliases:
          - database
      admin:
        aliases:
          - mysql

networks:
  front-tier:
  back-tier:
  admin:

ipv4_address, ipv6_address

Specify a static IP address for containers for this service when joining the network.

The corresponding network configuration in the top-level networks section MUST have an ipam block with subnet configurations covering each static address.

services:
  frontend:
    image: awesome/webapp
    networks:
      front-tier:
        ipv4_address: 172.16.238.10
        ipv6_address: 2001:3984:3989::10

networks:
  front-tier:
    ipam:
      driver: default
      config:
        - subnet: "172.16.238.0/24"
        - subnet: "2001:3984:3989::/64"

link_local_ips

link_local_ips specifies a list of link-local IPs. Link-local IPs are special IPs which belong to a well known subnet and are purely managed by the operator, usually dependent on the architecture where they are deployed. Implementation is Platform specific.

Example:

services:
  app:
    image: busybox
    command: top
    networks:
      app_net:
        link_local_ips:
          - 57.123.22.11
          - 57.123.22.13
networks:
  app_net:
    driver: bridge

priority

priority indicates in which order Compose implementation SHOULD connect the service’s containers to its networks. If unspecified, the default value is 0.

In the following example, the app service connects to app_net_1 first as it has the highest priority. It then connects to app_net_3, then app_net_2, which uses the default priority value of 0.

services:
  app:
    image: busybox
    command: top
    networks:
      app_net_1:
        priority: 1000
      app_net_2:

      app_net_3:
        priority: 100
networks:
  app_net_1:
  app_net_2:
  app_net_3:

mac_address

mac_address sets a MAC address for service container.

mem_limit

DEPRECATED: use deploy.limits.memory

mem_reservation

DEPRECATED: use deploy.reservations.memory

mem_swappiness

mem_swappiness defines as a percentage (a value between 0 and 100) for the host kernel to swap out anonymous memory pages used by a container.

a value of 0 turns off anonymous page swapping.
a value of 100 sets all anonymous pages as swappable.

Default value is platform specific.

memswap_limit

memswap_limit defines the amount of memory container is allowed to swap to disk. This is a modifier attribute that only has meaning if memory is also set. Using swap allows the container to write excess memory requirements to disk when the container has exhausted all the memory that is available to it. There is a performance penalty for applications that swap memory to disk often.

If memswap_limit is set to a positive integer, then both memory and memswap_limit MUST be set. memswap_limit represents the total amount of memory and swap that can be used, and memory controls the amount used by non-swap memory. So if memory="300m" and memswap_limit="1g", the container can use 300m of memory and 700m (1g - 300m) swap.
If memswap_limit is set to 0, the setting MUST be ignored, and the value is treated as unset.
If memswap_limit is set to the same value as memory, and memory is set to a positive integer, the container does not have access to swap. See Prevent a container from using swap.
If memswap_limit is unset, and memory is set, the container can use as much swap as the memory setting, if the host container has swap memory configured. For instance, if memory="300m" and memswap_limit is not set, the container can use 600m in total of memory and swap.
If memswap_limit is explicitly set to -1, the container is allowed to use unlimited swap, up to the amount available on the host system.

oom_kill_disable

If oom_kill_disable is set Compose implementation MUST configure the platform so it won't kill the container in case of memory starvation.

oom_score_adj

oom_score_adj tunes the preference for containers to be killed by platform in case of memory starvation. Valu MUST be within [-1000,1000] range.

pid

pid sets the PID mode for container created by the Compose implementation. Supported values are platform specific.

pid_limit

pid_limit tunes a container’s PIDs limit. Set to -1 for unlimited PIDs.

pids_limit: 10

platform

platform defines the target platform containers for this service will run on, using the os[/arch[/variant]] syntax. Compose implementation MUST use this attribute when declared to determine which version of the image will be pulled and/or on which platform the service’s build will be performed.

platform: osx
platform: windows/amd64
platform: linux/arm64/v8

ports

Exposes container ports. Port mapping MUST NOT be used with network_mode: host and doing so MUST result in a runtime error.

Short syntax

The short syntax is a comma-separated string to set host IP, host port and container port in the form:

[HOST:]CONTAINER[/PROTOCOL] where:

HOST is [IP:](port | range)
CONTAINER is port | range
PROTOCOL to restrict port to specified protocol. tcp and udp values are defined by the specification, Compose implementations MAY offer support for platform-specific protocol names.

Host IP, if not set, MUST bind to all network interfaces. Port can be either a single value or a range. Host and container MUST use equivalent ranges.

Either specify both ports (HOST:CONTAINER), or just the container port. In the latter case, the Compose implementation SHOULD automatically allocate and unassigned host port.

HOST:CONTAINER SHOULD always be specified as a (quoted) string, to avoid conflicts with yaml base-60 float.

Samples:

ports:
  - "3000"
  - "3000-3005"
  - "8000:8000"
  - "9090-9091:8080-8081"
  - "49100:22"
  - "127.0.0.1:8001:8001"
  - "127.0.0.1:5000-5010:5000-5010"
  - "6060:6060/udp"

Note: Host IP mapping MAY not be supported on the platform, in such case Compose implementations SHOULD reject the Compose file and MUST inform the user they will ignore the specified host IP.

Long syntax

The long form syntax allows the configuration of additional fields that can't be expressed in the short form.

target: the container port
published: the publicly exposed port
protocol: the port protocol (tcp or udp), unspecified means any protocol
mode: host for publishing a host port on each node, or ingress for a port to be load balanced.

ports:
  - target: 80
    published: 8080
    protocol: tcp
    mode: host

privileged

privileged configures the service container to run with elevated privileges. Support and actual impacts are platform-specific.

profiles

profiles defines a list of named profiles for the service to be enabled under. When not set, service is always enabled.

If present, profiles SHOULD follow the regex format of [a-zA-Z0-9][a-zA-Z0-9_.-]+.

pull_policy

pull_policy defines the decisions Compose implementations will make when it starts to pull images. Possible values are:

always: Compose implementations SHOULD always pull the image from the registry.
never: Compose implementations SHOULD NOT pull the image from a registry and SHOULD rely on the platform cached image. If there is no cached image, a failure MUST be reported.
missing: Compose implementations SHOULD pull the image only if it's not available in the platform cache. This SHOULD be the default option for Compose implementations without build support. if_not_present SHOULD be considered an alias for this value for backward compatibility
build: Compose implementations SHOULD build the image. Compose implementations SHOULD rebuild the image if already present.

If pull_policy and build both presents, Compose implementations SHOULD build the image by default. Compose implementations MAY override this behavior in the toolchain.

read_only

read_only configures service container to be created with a read-only filesystem.

restart

restart defines the policy that the platform will apply on container termination.

no: The default restart policy. Does not restart a container under any circumstances.
always: The policy always restarts the container until its removal.
on-failure: The policy restarts a container if the exit code indicates an error.
unless-stopped: The policy restarts a container irrespective of the exit code but will stop restarting when the service is stopped or removed.

    restart: "no"
    restart: always
    restart: on-failure
    restart: unless-stopped

runtime

runtime specifies which runtime to use for the service’s containers.

The value of runtime is specific to implementation. For example, runtime can be the name of an implementation of OCI Runtime Spec, such as "runc".

web:
  image: busybox:latest
  command: true
  runtime: runc

scale

-DEPRECATED: use deploy/replicas_

scale specifies the default number of containers to deploy for this service.

secrets

secrets grants access to sensitive data defined by secrets on a per-service basis. Two different syntax variants are supported: the short syntax and the long syntax.

Compose implementations MUST report an error if the secret doesn't exist on the platform or isn't defined in the secrets section of this Compose file.

Short syntax

The short syntax variant only specifies the secret name. This grants the container access to the secret and mounts it as read-only to /run/secrets/<secret_name> within the container. The source name and destination mountpoint are both set to the secret name.

The following example uses the short syntax to grant the frontend service access to the server-certificate secret. The value of server-certificate is set to the contents of the file ./server.cert.

services:
  frontend:
    image: awesome/webapp
    secrets:
      - server-certificate
secrets:
  server-certificate:
    file: ./server.cert

Long syntax

The long syntax provides more granularity in how the secret is created within the service's containers.

source: The name of the secret as it exists on the platform.
target: The name of the file to be mounted in /run/secrets/ in the service's task containers. Defaults to source if not specified.
uid and gid: The numeric UID or GID that owns the file within /run/secrets/ in the service's task containers. Default value is USER running container.
mode: The permissions for the file to be mounted in /run/secrets/ in the service's task containers, in octal notation. Default value is world-readable permissions (mode 0444). The writable bit MUST be ignored if set. The executable bit MAY be set.

The following example sets the name of the server-certificate secret file to server.crt within the container, sets the mode to 0440 (group-readable) and sets the user and group to 103. The value of server-certificate secret is provided by the platform through a lookup and the secret lifecycle not directly managed by the Compose implementation.

services:
  frontend:
    image: awesome/webapp
    secrets:
      - source: server-certificate
        target: server.cert
        uid: "103"
        gid: "103"
        mode: 0440
secrets:
  server-certificate:
    external: true

Services MAY be granted access to multiple secrets. Long and short syntax for secrets MAY be used in the same Compose file. Defining a secret in the top-level secrets MUTS NOT imply granting any service access to it. Such grant must be explicit within service specification as secrets service element.

security_opt

security_opt overrides the default labeling scheme for each container.

security_opt:
  - label:user:USER
  - label:role:ROLE

shm_size

shm_size configures the size of the shared memory (/dev/shm partition on Linux) allowed by the service container. Specified as a byte value.

stdin_open

stdin_open configures service containers to run with an allocated stdin.

stop_grace_period

stop_grace_period specifies how long the Compose implementation MUST wait when attempting to stop a container if it doesn't handle SIGTERM (or whichever stop signal has been specified with stop_signal), before sending SIGKILL. Specified as a duration.

    stop_grace_period: 1s
    stop_grace_period: 1m30s

Default value is 10 seconds for the container to exit before sending SIGKILL.

stop_signal

stop_signal defines the signal that the Compose implementation MUST use to stop the service containers. If unset containers are stopped by the Compose Implementation by sending SIGTERM.

stop_signal: SIGUSR1

sysctls

sysctls defines kernel parameters to set in the container. sysctls can use either an array or a map.

sysctls:
  net.core.somaxconn: 1024
  net.ipv4.tcp_syncookies: 0

sysctls:
  - net.core.somaxconn=1024
  - net.ipv4.tcp_syncookies=0

You can only use sysctls that are namespaced in the kernel. Docker does not support changing sysctls inside a container that also modify the host system. For an overview of supported sysctls, refer to configure namespaced kernel parameters (sysctls) at runtime.

tmpfs

tmpfs mounts a temporary file system inside the container. Can be a single value or a list.

tmpfs: /run

tmpfs:
  - /run
  - /tmp

tty

tty configure service container to run with a TTY.

ulimits

ulimits overrides the default ulimits for a container. Either specifies as a single limit as an integer or soft/hard limits as a mapping.

ulimits:
  nproc: 65535
  nofile:
    soft: 20000
    hard: 40000

user

user overrides the user used to run the container process. Default is that set by image (i.e. Dockerfile USER), if not set, root.

userns_mode

userns_mode sets the user namespace for the service. Supported values are platform specific and MAY depend on platform configuration

userns_mode: "host"

volumes

volumes defines mount host paths or named volumes that MUST be accessible by service containers.

If the mount is a host path and only used by a single service, it MAY be declared as part of the service definition instead of the top-level volumes key.

To reuse a volume across multiple services, a named volume MUST be declared in the top-level volumes key.

This example shows a named volume (db-data) being used by the backend service, and a bind mount defined for a single service

services:
  backend:
    image: awesome/backend
    volumes:
      - type: volume
        source: db-data
        target: /data
        volume:
          nocopy: true
      - type: bind
        source: /var/run/postgres/postgres.sock
        target: /var/run/postgres/postgres.sock

volumes:
  db-data:

Short syntax

The short syntax uses a single string with comma-separated values to specify a volume mount (VOLUME:CONTAINER_PATH), or an access mode (VOLUME:CONTAINER:ACCESS_MODE).

VOLUME MAY be either a host path on the platform hosting containers (bind mount) or a volume name. ACCESS_MODE MAY be set as read-only by using ro or read and write by using rw (default).

Note: Relative host paths MUST only be supported by Compose implementations that deploy to a local container runtime. This is because the relative path is resolved from the Compose file’s parent directory which is only applicable in the local case. Compose Implementations deploying to a non-local platform MUST reject Compose files which use relative host paths with an error. To avoid ambiguities with named volumes, relative paths SHOULD always begin with . or ...

Long syntax

The long form syntax allows the configuration of additional fields that can't be expressed in the short form.

type: the mount type volume, bind, tmpfs or npipe
source: the source of the mount, a path on the host for a bind mount, or the name of a volume defined in the top-level volumes key. Not applicable for a tmpfs mount.
target: the path in the container where the volume is mounted
read_only: flag to set the volume as read-only
bind: configure additional bind options
- propagation: the propagation mode used for the bind
volume: configure additional volume options
- nocopy: flag to disable copying of data from a container when a volume is created
tmpfs: configure additional tmpfs options
- size: the size for the tmpfs mount in bytes
consistency: the consistency requirements of the mount. Available values are platform specific

volumes_from

volumes_from mounts all of the volumes from another service or container, optionally specifying read-only access (ro) or read-write (rw). If no access level is specified, then read-write MUST be used.

String value defines another service in the Compose application model to mount volumes from. The container: prefix, if supported, allows to mount volumes from a container that is not managed by the Compose implementation.

volumes_from:
  - service_name
  - service_name:ro
  - container:container_name
  - container:container_name:rw

working_dir

working_dir overrides the container's working directory from that specified by image (i.e. Dockerfile WORKDIR).

Networks top-level element

Networks are the layer that allow services to communicate with each other. The networking model exposed to a service is limited to a simple IP connection with target services and external resources, while the Network definition allows fine-tuning the actual implementation provided by the platform.

Networks can be created by specifying the network name under a top-level networks section. Services can connect to networks by specifying the network name under the service networks subsection

In the following example, at runtime, networks front-tier and back-tier will be created and the frontend service connected to the front-tier network and the back-tier network.

services:
  frontend:
    image: awesome/webapp
    networks:
      - front-tier
      - back-tier

networks:
  front-tier:
  back-tier:

driver

driver specifies which driver should be used for this network. Compose implementations MUST return an error if the driver is not available on the platform.

driver: overlay

Default and available values are platform specific. Compose specification MUST support the following specific drivers: none and host

host use the host's networking stack
none disable networking

host or none

The syntax for using built-in networks such as host and none is different, as such networks implicitly exists outside the scope of the Compose implementation. To use them one MUST define an external network with the name host or none and an alias that the Compose implementation can use (hostnet or nonet in the following examples), then grant the service access to that network using its alias.

services:
  web:
    networks:
      hostnet: {}

networks:
  hostnet:
    external: true
    name: host

services:
  web:
    ...
    networks:
      nonet: {}

networks:
  nonet:
    external: true
    name: none

driver_opts

driver_opts specifies a list of options as key-value pairs to pass to the driver for this network. These options are driver-dependent - consult the driver's documentation for more information. Optional.

driver_opts:
  foo: "bar"
  baz: 1

attachable

If attachable is set to true, then standalone containers SHOULD be able attach to this network, in addition to services. If a standalone container attaches to the network, it can communicate with services and other standalone containers that are also attached to the network.

networks:
  mynet1:
    driver: overlay
    attachable: true

enable_ipv6

enable_ipv6 enable IPv6 networking on this network.

ipam

ipam specifies custom a IPAM configuration. This is an object with several properties, each of which is optional:

driver: Custom IPAM driver, instead of the default.
config: A list with zero or more configuration elements, each containing:
- subnet: Subnet in CIDR format that represents a network segment
- ip_range: Range of IPs from which to allocate container IPs
- gateway: IPv4 or IPv6 gateway for the master subnet
- aux_addresses: Auxiliary IPv4 or IPv6 addresses used by Network driver, as a mapping from hostname to IP
options: Driver-specific options as a key-value mapping.

A full example:

ipam:
  driver: default
  config:
    - subnet: 172.28.0.0/16
      ip_range: 172.28.5.0/24
      gateway: 172.28.5.254
      aux_addresses:
        host1: 172.28.1.5
        host2: 172.28.1.6
        host3: 172.28.1.7
  options:
    foo: bar
    baz: "0"

internal

By default, Compose implementations MUST provides external connectivity to networks. internal when set to true allow to create an externally isolated network.

labels

Add metadata to containers using Labels. Can use either an array or a dictionary.

Users SHOULD use reverse-DNS notation to prevent labels from conflicting with those used by other software.

labels:
  com.example.description: "Financial transaction network"
  com.example.department: "Finance"
  com.example.label-with-empty-value: ""

labels:
  - "com.example.description=Financial transaction network"
  - "com.example.department=Finance"
  - "com.example.label-with-empty-value"

Compose implementations MUST set com.docker.compose.project and com.docker.compose.network labels.

external

If set to true, external specifies that this network’s lifecycle is maintained outside of that of the application. Compose Implementations SHOULD NOT attempt to create these networks, and raises an error if one doesn't exist.

In the example below, proxy is the gateway to the outside world. Instead of attempting to create a network, Compose implementations SHOULD interrogate the platform for an existing network simply called outside and connect the proxy service's containers to it.

services:
  proxy:
    image: awesome/proxy
    networks:
      - outside
      - default
  app:
    image: awesome/app
    networks:
      - default

networks:
  outside:
    external: true

name

name sets a custom name for this network. The name field can be used to reference networks which contain special characters. The name is used as is and will not be scoped with the project name.

networks:
  network1:
    name: my-app-net

It can also be used in conjunction with the external property to define the platform network that the Compose implementation should retrieve, typically by using a parameter so the Compose file doesn't need to hard-code runtime specific values:

networks:
  network1:
    external: true
    name: "${NETWORK_ID}"

Volumes top-level element

Volumes are persistent data stores implemented by the platform. The Compose specification offers a neutral abstraction for services to mount volumes, and configuration parameters to allocate them on infrastructure.

The volumes section allows the configuration of named volumes that can be reused across multiple services. Here's an example of a two-service setup where a database's data directory is shared with another service as a volume so that it can be periodically backed up:

services:
  backend:
    image: awesome/database
    volumes:
      - db-data:/etc/data

  backup:
    image: backup-service
    volumes:
      - db-data:/var/lib/backup/data

volumes:
  db-data:

An entry under the top-level volumes key can be empty, in which case it uses the platform's default configuration for creating a volume. Optionally, you can configure it with the following keys:

driver

Specify which volume driver should be used for this volume. Default and available values are platform specific. If the driver is not available, the Compose implementation MUST return an error and stop application deployment.

driver: foobar

driver_opts

driver_opts specifies a list of options as key-value pairs to pass to the driver for this volume. Those options are driver-dependent.

volumes:
  example:
    driver_opts:
      type: "nfs"
      o: "addr=10.40.0.199,nolock,soft,rw"
      device: ":/docker/example"

external

If set to true, external specifies that this volume already exist on the platform and its lifecycle is managed outside of that of the application. Compose implementations MUST NOT attempt to create these volumes, and MUST return an error if they do not exist.

In the example below, instead of attempting to create a volume called {project_name}_data, Compose looks for an existing volume simply called data and mount it into the db service's containers.

services:
  backend:
    image: awesome/database
    volumes:
      - db-data:/etc/data

volumes:
  db-data:
    external: true

labels

labels are used to add metadata to volumes. You can use either an array or a dictionary.

It's recommended that you use reverse-DNS notation to prevent your labels from conflicting with those used by other software.

labels:
  com.example.description: "Database volume"
  com.example.department: "IT/Ops"
  com.example.label-with-empty-value: ""

labels:
  - "com.example.description=Database volume"
  - "com.example.department=IT/Ops"
  - "com.example.label-with-empty-value"

Compose implementation MUST set com.docker.compose.project and com.docker.compose.volmume labels.

name

name set a custom name for this volume. The name field can be used to reference volumes that contain special characters. The name is used as is and will not be scoped with the stack name.

volumes:
  data:
    name: "my-app-data"

It can also be used in conjunction with the external property. Doing so the name of the volume used to lookup for actual volume on platform is set separately from the name used to refer to it within the Compose file:

volumes:
  db-data:
    external:
      name: actual-name-of-volume

This make it possible to make this lookup name a parameter of a Compose file, so that the model ID for volume is hard-coded but the actual volume ID on platform is set at runtime during deployment:

volumes:
  db-data:
    external:
      name: ${DATABASE_VOLUME}

Configs top-level element

Configs allow services to adapt their behaviour without the need to rebuild a Docker image. Configs are comparable to Volumes from a service point of view as they are mounted into service's containers filesystem. The actual implementation detail to get configuration provided by the platform can be set from the Configuration definition.

When granted accessto a config, the config content is mounted as a file in the container. The location of the mount point within the container defaults to /<config-name> in Linux containers and C:\<config-name> in Windows containers.

By default, the config MUST be owned by the user running the container command but can be overriden by service configuration. By default, the config MUST have world-readable permissions (mode 0444), unless service is configured to override this.

Services can only access configs when explicitly granted by a configs subsection.

The top-level configs declaration defines or references configuration data that can be granted to the services in this application. The source of the config is either file or external.

file: The config is created with the contents of the file at the specified path.
external: If set to true, specifies that this config has already been created. Compose implementation does not attempt to create it, and if it does not exist, an error occurs.
name: The name of config object on Platform to lookup. This field can be used to reference configs that contain special characters. The name is used as is and will not be scoped with the project name.

In this example, http_config is created (as <project_name>_http_config)when the application is deployed, and my_second_config MUST already exists on Platform and value will be obtained by lookup.

In this example, server-http_config is created as <project_name>_http_config when the application is deployed, by registering content of the httpd.conf as configuration data.

configs:
  http_config:
    file: ./httpd.conf

Alternatively, http_config can be declared as external, doing so Compose implementation will lookup server-certificate to expose configuration data to relevant services.

configs:
  http_config:
    external: true

External configs lookup can also use a distinct key by specifying a name. The following example modifies the previous one to lookup for config using a parameter HTTP_CONFIG_KEY. Doing so the actual lookup key will be set at deployment time by interpolation of variables, but exposed to containers as hard-coded ID http_config.

configs:
  http_config:
    external: true
    name: "${HTTP_CONFIG_KEY}"

Compose file need to explicitly grant access to the configs to relevant services in the application.

Secrets top-level element

Secrets are a flavour of Configs focussing on sensitive data, with specific constraint for this usage. As the platform implementation may significally differ from Configs, dedicated Secrets section allows to configure the related resources.

The top-level secrets declaration defines or references sensitive data that can be granted to the services in this application. The source of the secret is either file or external.

file: The secret is created with the contents of the file at the specified path.
external: If set to true, specifies that this secret has already been created. Compose implementation does not attempt to create it, and if it does not exist, an error occurs.
name: The name of the secret object in Docker. This field can be used to reference secrets that contain special characters. The name is used as is and will not be scoped with the project name.

In this example, server-certificate is created as <project_name>_server-certificate when the application is deployed, by registering content of the server.cert as a platform secret.

secrets:
  server-certificate:
    file: ./server.cert

Alternatively, server-certificate can be declared as external, doing so Compose implementation will lookup server-certificate to expose secret to relevant services.

secrets:
  server-certificate:
    external: true

External secrets lookup can also use a distinct key by specifying a name. The following example modifies the previous one to look up for secret using a parameter CERTIFICATE_KEY. Doing so the actual lookup key will be set at deployment time by interpolation of variables, but exposed to containers as hard-coded ID server-certificate.

secrets:
  server-certificate:
    external: true
    name: "${CERTIFICATE_KEY}"

Compose file need to explicitly grant access to the secrets to relevant services in the application.

Fragments

It is possible to re-use configuration fragments using YAML anchors.

volumes:
  db-data: &default-volume
    driver: default
  metrics: *default-volume

In previous sample, an anchor is created as default-volume based on db-data volume specification. It is later reused by alias *default-volume to define metrics volume. Same logic can apply to any element in a Compose file. Anchor resolution MUST take place before variables interpolation, so variables can't be used to set anchors or aliases.

It is also possible to partially override values set by anchor reference using the YAML merge type. In following example, metrics volume specification uses alias to avoid repetition but override name attribute:

services:
  backend:
    image: awesome/database
    volumes:
      - db-data
      - metrics
volumes:
  db-data: &default-volume
    driver: default
    name: "data"
  metrics:
    <<: *default-volume
    name: "metrics"

Extension

Special extension fields can be of any format as long as their name starts with the x- character sequence. They can be used within any structure in a Compose file. This is the sole exception for Compose implementations to silently ignore unrecognized field.

x-custom:
  foo:
    - bar
    - zot

services:
  webapp:
    image: awesome/webapp
    x-foo: bar

The contents of such fields are unspecified by Compose specification, and can be used to enable custom features. Compose implementation to encounter an unknown extension field MUST NOT fail, but COULD warn about unknown field.

For platform extensions, it is highly recommended to prefix extension by platform/vendor name, the same way browsers add support for custom CSS features

service:
  backend:
    deploy:
      placement:
        x-aws-role: "arn:aws:iam::XXXXXXXXXXXX:role/foo"
        x-aws-region: "eu-west-3"
        x-azure-region: "france-central"

Informative Historical Notes

This section is informative. At the time of writing, the following prefixes are known to exist:

prefix	vendor/organization
docker	Docker
kubernetes	Kubernetes

Using extensions as fragments

With the support for extension fields, Compose file can be written as follows to improve readability of reused fragments:

x-logging: &default-logging
  options:
    max-size: "12m"
    max-file: "5"
  driver: json-file

services:
  frontend:
    image: awesome/webapp
    logging: *default-logging
  backend:
    image: awesome/database
    logging: *default-logging

specifying byte values

Value express a byte value as a string in {amount}{byte unit} format: The supported units are b (bytes), k or kb (kilo bytes), m or mb (mega bytes) and g or gb (giga bytes).

specifying durations

Value express a duration as a string in thte in the form of {value}{unit}. The supported units are us (microseconds), ms (milliseconds), s (seconds), m (minutes) and h (hours). Value can can combine mutiple values and using without separator.

  10ms
  40s
  1m30s
  1h5m30s20ms

Interpolation

Values in a Compose file can be set by variables, and interpolated at runtime. Compose files use a Bash-like syntax ${VARIABLE}

Both $VARIABLE and ${VARIABLE} syntax are supported. Default values can be defined inline using typical shell syntax: latest

${VARIABLE:-default} evaluates to default if VARIABLE is unset or empty in the environment.
${VARIABLE-default} evaluates to default only if VARIABLE is unset in the environment.

Similarly, the following syntax allows you to specify mandatory variables:

${VARIABLE:?err} exits with an error message containing err if VARIABLE is unset or empty in the environment.
${VARIABLE?err} exits with an error message containing err if VARIABLE is unset in the environment.

Other extended shell-style features, such as ${VARIABLE/foo/bar}, are not supported by the Compose specification.

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don't want processed by Compose.

web:
  build: .
  command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"

If the Compose implementation can't resolve a substituted variable and no default value is defined, it MUST warn the user and substitute the variable with an empty string.

As any values in a Compose file can be interpolated with variable substitution, including compact string notation for complex elements, interpolation MUST be applied before merge on a per-file-basis.

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spec.md

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The Compose Specification

Table of Contents

Status of this document

Requirements and optional attributes

The Compose application model

Illustrative example

Compose file

Profiles

Illustrative example

Version top-level element

Services top-level element

deploy

blkio_config

device_read_bps, device_write_bps

device_read_iops, device_write_iops

weight

weight_device

cpu_count

cpu_percent

cpu_shares

cpu_period

cpu_quota

cpu_rt_runtime

cpu_rt_period

cpus

cpuset

build

cap_add

cap_drop

cgroup_parent

command

configs

Short syntax

Long syntax

container_name

credential_spec

Example gMSA configuration

depends_on

Short syntax

Long syntax

device_cgroup_rules

devices

dns

dns_opt

dns_search

domainname

entrypoint

env_file

Env_file format

environment

expose

extends

Restrictions

Finding referenced service

Merging service definitions

Mappings

Sequences

Scalars

external_links

extra_hosts

group_add

healthcheck

hostname

image

init

ipc

isolation

labels

links

logging

network_mode

networks

aliases

ipv4_address, ipv6_address