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The Compose Specification

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• ToC {:toc}

Status of this document

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

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. It is also designed for 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 that attributes will be actually implemented.

The specification defines the expected configuration syntax and behavior. Unless noted, supporting any of these is optional.

A Compose implementation to parse a Compose file using unsupported attributes should warn users. We recommend the following implementors to support those running modes:

• Default: warn the user about unsupported attributes, but ignore them
• Strict: warn the 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)

From this point onwards, references made to 'Compose' can be interpreted as 'a Compose implementation'.

The Compose application model

The Compose Specification lets you 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 the Compose 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 configs concept. 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 flavor 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.

Note

With volumes, configs and secrets you can have a simple declaration at the top-level and then add more platform-specific information at the service level.

A project is an individual deployment of an application specification on a platform. A project's name, set with the top-level name attribute, is used to group resources together and isolate them from other applications or other installation of the same Compose specified application with distinct parameters. If you are creating resources on a platform, you must prefix resource names by project and set the label com.docker.compose.project.

Compose offers a way for users to set a custom project name and override this name, so that the same compose.yaml file can be deployed twice on the same infrastructure, without changes, by just passing a distinct name.

Project names must contain only lowercase letters, decimal digits, dashes, and underscores, and must begin with a lowercase letter or decimal digit.

Illustrative example

The following example illustrates the Compose Specification concepts outlined above. 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 the frontend is also connected to a front-tier network and exposes port 443 for external usage.

    %%{ init: { 'flowchart': { 'curve': 'linear' } } }%%
    flowchart LR
    subgraph A[INFRASTRUCTURE]
    direction TB
    subgraph TOP[" "]
        subgraph B1[Frontend Service]
            fs["`**webapp**`"]
        end
        style B1 fill:#ccd6e8, stroke-width:0px
        subgraph B2[Backend Service]
            bs["`**database**`"]
        end
        style B2 fill:#ccd6e8, stroke-width:0px
        
    end
    style TOP fill:transparent, stroke-width:2px, stroke:#62affb, stroke-dasharray: 5 5
        key[ro= read only\nr+w = read write]
        style key fill:transparent, stroke-width:0px,text-align: left, size: 94px
        
        direction TB
        id2(Server\nCertificate)
        id1(HTTP\nConfiguration)
        id1 & id2 -.-|ro| B1
        style id1 stroke:#000,stroke-width:1px,stroke-dasharray: 10
        style id2 stroke:#000,stroke-width:1px,stroke-dasharray: 10
        B2 ==r+w==> id3[(Persistent\nVolume)]
    end
    style A fill:#eeeeee, stroke-width:0px
    direction LR
    id4[External\nUser] ---id5(((443)))--->|Frontend\nNetwork| B1
    style id4 stroke:#000,stroke-width:2px
    B1 --Backend\nNetwork--> B2

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: example/webapp
    ports:
      - "443:8043"
    networks:
      - front-tier
      - back-tier
    configs:
      - httpd-config
    secrets:
      - server-certificate

  backend:
    image: example/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. Compose uses a platform-specific lookup mechanism to retrieve runtime values.

Compose file

The Compose file is a YAML file defining:

• Version (Optional)
• Services (Required)
• Networks
• Volumes
• Configs
• Secrets

The default path for a Compose file is compose.yaml (preferred) or compose.yml that is placed in the working directory. Compose also supports docker-compose.yaml and docker-compose.yml for backwards compatibility of earlier versions. If both files exist, Compose prefers the canonical compose.yaml.

You can use fragments and extensions to keep your Compose file efficient and easy to maintain.

Multiple Compose files can be merged together to define the application model. The combination of YAML files are implemented by appending or overriding YAML elements based on the Compose file order you set. Simple attributes and maps get overridden by the highest order Compose file, lists get merged by appending. Relative paths are 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 apply to the expanded form.

If you want to reuse other Compose files, or factor out parts of you application model into separate Compose files, you can also use include. This is useful if your Compose application is dependent on another application which is managed by a different team, or needs to be shared with others.

Version and name top-level elements

Version top-level element (obsolete)

The top-level version property is defined by the Compose Specification for backward compatibility. It is only informative you'll receive a warning message that it is obsolete if used.

Compose doesn't use version to select an exact schema to validate the Compose file, but prefers the most recent schema when it's implemented.

Compose validates whether it 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, you'll receive a warning message. Compose offers options to ignore unknown fields (as defined by "loose" mode).

Name top-level element

The top-level name property is defined by the Specification as the project name to be used if you don't set one explicitly. Compose offers a way for you to override this name, and sets a default project name to be used if the top-level name element is not set.

Whenever a project name is defined by top-level name or by some custom mechanism, it is exposed for interpolation and environment variable resolution as COMPOSE_PROJECT_NAME

services:
  foo:
    image: busybox
    environment:
      - COMPOSE_PROJECT_NAME
    command: echo "I'm running ${COMPOSE_PROJECT_NAME}"

Services top-level element

A service is an abstract definition of a computing resource within an application which can be scaled or replaced independently from other components. Services are backed by a set of containers, run by the platform according to replication requirements and placement constraints. As services are backed by containers, they 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 top-level 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 service container.

Each service may also include a build section, which defines how to create the Docker image for the service. Compose supports building docker images using this service definition. If not used, the build section is ignored and the Compose file is still considered valid. Build support is an optional aspect of the Compose Specification, and is described in detail in the Compose Build Specification documentation.

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 in the Compose Deploy Specification documentation. If not implemented the deploy section is ignored and the Compose file is still considered valid.

attach

When attach is defined and set to false Compose does not collect service logs, until you explicitly request it to.

The default service configuration is attach: true.

build

build specifies the build configuration for creating a container image from source, as defined in the Compose Build Specification.

blkio_config

blkio_config defines a set of configuration options to set block IO limits for a 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: Defines 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: Defines 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 a 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: Defines 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, a service container's relative CPU weight versus other containers.

cpu_period

cpu_period configures CPU CFS (Completely Fair Scheduler) period when a platform is based on Linux kernel.

cpu_quota

cpu_quota configures CPU CFS (Completely Fair Scheduler) quota when a platform is based on Linux kernel.

cpu_rt_runtime

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

 cpu_rt_period: '1400us'
 cpu_rt_period: 11000`

cpus

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

When both are set, cpus must be consistent with the cpus attribute in the Deploy Specification

cpuset

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

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

cgroup specifies the cgroup namespace to join. When unset, it is the container runtime's decision to select which cgroup namespace to use, if supported.

• host: Runs the container in the Container runtime cgroup namespace.
• private: Runs the container in its own private cgroup namespace.

cgroup_parent

cgroup_parent specifies an optional parent cgroup for the container.

cgroup_parent: m-executor-abcd

command

command overrides the default command declared by the container image, for example by Dockerfile's CMD.

command: bundle exec thin -p 3000

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

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

If the value is null, the default command from the image is used.

If the value is [] (empty list) or '' (empty string), the default command declared by the image is ignored, i.e. overridden to be empty.

configs

Configs allow services to adapt their behaviour without the need to rebuild a Docker image. Services can only access configs when explicitly granted by the configs attribute. Two different syntax variants are supported.

Compose reports an error if config doesn't exist on the platform or isn't defined in the configs top-level element in the 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.

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

Short syntax

The short syntax variant only specifies the config name. This grants the container access to the config and mounts it as files into a service’s container’s filesystem. The location of the mount point within the container defaults to /<config_name> in Linux containers, and C:\<config-name> in Windows containers.

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 fails.

services:
  redis:
    image: redis:latest
    configs:
      - my_config
      - my_other_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

container_name

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

container_name: my-web-container

Compose does not scale a service beyond one container if the Compose file specifies a container_name. Attempting to do so results in an error.

container_name follows 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.

If you have services that use Windows containers, you can use file: and registry: protocols for credential_spec. Compose also supports 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 creates services in dependency order. In the following example, db and redis are created before web.

• Compose removes 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 guarantees dependency services have been started before starting a dependent service. Compose waits 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.

• restart: When set to true Compose restarts this service after it updates the dependency service. This applies to an explicit restart controlled by a Compose operation, and excludes automated restart by the container runtime after the container dies.

• condition: Sets the condition under which dependency is considered satisfied

  • service_started: 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.

• required: When set to false Compose only warns you when the dependency service isn't started or available. If it's not defined the default value of required is true.

Service dependencies cause the following behaviors:

• Compose creates services in dependency order. In the following example, db and redis are created before web.

• Compose waits 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 removes services in dependency order. In the following example, web is removed before db and redis.

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

Compose guarantees dependency services are started before starting a dependent service. Compose guarantees dependency services marked with service_healthy are "healthy" before starting a dependent service.

deploy

deploy specifies the configuration for the deployment and lifecycle of services, as defined in the Compose Deploy Specification.

develop

develop specifies the development configuration for maintaining a container in sync with source, as defined in the Development Section.

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 in the form of HOST_PATH:CONTAINER_PATH[:CGROUP_PERMISSIONS].

devices:
  - "/dev/ttyUSB0:/dev/ttyUSB0"
  - "/dev/sda:/dev/xvda:rwm"

dns

dns defines custom DNS servers to set on the container network interface configuration. It 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_search defines custom DNS search domains to set on container network interface configuration. It 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. It must be a valid RFC 1123 hostname.

entrypoint

entrypoint declares the default entrypoint for the service container. This overrides the ENTRYPOINT instruction from the service's Dockerfile.

If entrypoint is non-null, Compose ignores any default command from the image, for example the CMD instruction in the Dockerfile.

See also command to set or override the default command to be executed by the entrypoint process.

In its short form, the value can be defined as a string:

entrypoint: /code/entrypoint.sh

Alternatively, the value can also be a list, in a manner similar to the 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

If the value is null, the default entrypoint from the image is used.

If the value is [] (empty list) or '' (empty string), the default entrypoint declared by the image is ignored, i.e. overridden to be empty.

env_file

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

env_file: .env

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

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

Relative path are resolved from the Compose file's parent folder. As absolute paths prevent the Compose file from being portable, Compose warns you when such a path is used to set env_file.

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

List elements can also be declared as a mapping, which then lets you set an additional attributes

required

required attribute defaults to true. When required is set to false and the .env file is missing, Compose silently ignores the entry.

env_file:
  - path: ./default.env
    required: true # default
  - path: ./override.env
    required: false

format

format attribute lets you to use an alternative file formats for env_file. When not set, env_file is parsed according to Compose rules as described in next section.

raw format lets you use an env_file with key=value items, but without any attempt from Compose to parse the value for interpolation. This let you pass values as-is including quotes and $ signs.

env_file:
  - path: ./default.env
    format: raw

Env_file format

Each line in an .env file must be in VAR[=[VAL]] format. The following syntax rules apply:

• Lines beginning with # are processed as comments and ignored.
• Blank lines are ignored.
• Unquoted and double-quoted (") values have Interpolation applied.
• Each line represents a key-value pair. Values can optionally be quoted.
  • VAR=VAL -> VAL
  • VAR="VAL" -> VAL
  • VAR='VAL' -> VAL
• Inline comments for unquoted values must be preceded with a space.
  • VAR=VAL # comment -> VAL
  • VAR=VAL# not a comment -> VAL# not a comment
• Inline comments for quoted values must follow the closing quote.
  • VAR="VAL # not a comment" -> VAL # not a comment
  • VAR="VAL" # comment -> VAL
• Single-quoted (') values are used literally.
  • VAR='$OTHER' -> $OTHER
  • VAR='${OTHER}' -> ${OTHER}
• Quotes can be escaped with \.
  • VAR='Let\'s go!' -> Let's go!
  • VAR="{\"hello\": \"json\"}" -> {"hello": "json"}
• Common shell escape sequences including \n, \r, \t, and \\ are supported in double-quoted values.
  • VAR="some\tvalue" -> some value
  • VAR='some\tvalue' -> some\tvalue
  • VAR=some\tvalue -> some\tvalue

VAL may be omitted, in such cases the variable value is an 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, should be enclosed in quotes to ensure they are not converted to True or False by the YAML parser.

Environment variables can be declared by a single key (no value to equals sign). In this case Compose relies on you to resolve the value. If the value is not resolved, the variable is unset and is 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 (incoming) port or a range of ports that Compose exposes from the 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.

Syntax is <portnum>/[<proto>] or <startport-endport>/[<proto>] for a port range. When not explicitly set, tcp protocol is used.

expose:
  - "3000"
  - "8000"
  - "8080-8085/tcp"

Note

If the Dockerfile for the image already exposes ports, it is visible to other containers on the network even if expose is not set in your Compose file.

extends

extends lets you share common configurations among different files, or even different projects entirely. With extends you can define a common set of service options in one place and refer to it from anywhere. You can refer to another Compose file and select a service you want to also use in your own application, with the ability to override some attributes for your own needs.

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

• service: Defines the name of the service being referenced as a base, for example web or database.
• file: The location of a Compose configuration file defining that service.

Restrictions

Service being referenced by extends can have dependency declared on other resources. Typically it can have an explicit volumes declaration. extends then will not import the target volume definition in the extending compose model, it is Compose file author responsibility to define an equivalent resource for the extended service to be consistent. Compose will check a resource with referenced ID exists in the Compose model

Dependencies on other resources in an extends target can be:

• An explicit references by volumes, networks, configs, secrets, links, volumes_from or depends_on
• A reference to another service using the service:{name} syntax in namespace declaration (ipc, pid, network_mode)

Circular references with extends are not supported, Compose returns an error when one is detected.

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.

A service denoted by service must be present in the identified referenced Compose file. Compose returns an error if:

• The service denoted by service is not found.
• The Compose file denoted by file is not found.

Merging service definitions

Two service definitions, the main one in the current Compose file and the referenced one specified by extends, are merged in the following way:

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

Mappings

The following keys should be treated as mappings: annotations, 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 the main mapping cannot specify disable: true unless the referenced mapping also specifies disable: true. Compose returns 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 the 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 the referenced service definition contains extends mapping, the items under it are simply copied into the new merged definition. The 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.

annotations

annotations defines annotations for the container. annotations can use either an array or a map.

annotations:
  com.example.foo: bar

annotations:
  - com.example.foo=bar

external_links

external_links link service containers to services managed outside of your 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).

Short syntax

Short syntax uses plain strings in a list. 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"
  - "myhostv6=::1"

IPv6 addresses can be enclosed in square brackets, for example:

extra_hosts:
  - "myhostv6=[::1]"

The separator = is preferred but : can also be used. For example:

extra_hosts:
  - "somehost:162.242.195.82"
  - "myhostv6:::1"

Long syntax

Alternatively, extra_hosts can be set as a mapping between hostname(s) and IP(s)

extra_hosts:
  somehost: "162.242.195.82"
  otherhost: "50.31.209.229"
  myhostv6: "::1"

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

162.242.195.82  somehost
50.31.209.229   otherhost
::1             myhostv6

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 the service containers are "healthy". It works in the same way, and has the same default values, as the HEALTHCHECK Dockerfile instruction set by the service's Docker image. Your Compose file can override the values set in the Dockerfile.

healthcheck:
  test: ["CMD", "curl", "-f", "http://localhost"]
  interval: 1m30s
  timeout: 10s
  retries: 3
  start_period: 40s
  start_interval: 5s

interval, timeout, start_period, and start_interval are specified as durations.

test defines the command Compose runs 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 runs 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 disables the healthcheck, and is mostly useful to disable the Healthcheck Dockerfile instruction set by the service's Docker 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. It 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@sha256:0ed5d5928d4737458944eb604cc8509e245c3e19d02ad83935398bc4b991aac7
    image: library/redis
    image: docker.io/library/redis
    image: my_private.registry:5000/redis

If the image does not exist on the platform, Compose attempts to pull it based on the pull_policy. If you are also using the Compose Build Specification, there are alternative options for controlling the precedence of pull over building the image from source, however pulling the image is the default behavior.

image may be omitted from a Compose file as long as a build section is declared. If you are not using the Compose Build Specification, Compose won't work if image is missing from the Compose file.

init

init runs 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 the service container. Available values are platform specific, but Compose defines specific values which must be implemented as described if supported:

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

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

uts

uts configures the UTS namespace mode set for the service container. When unspecified it is the runtime's decision to assign a UTS namespace, if supported. Available values are:

• 'host': Results in the container using the same UTS namespace as the host.

    uts: "host"

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 creates containers with canonical labels:

• com.docker.compose.project set on all resources created by Compose 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 prefix in the Compose file results in a runtime error.

label_file

label_file can be used to load multiple labels from a file. The file uses a key-value frmat, comparable to env_file.

You can specify multiple files as a List. The files in the list are processed in top-down order. If the same label is defined in multiple files, the value from the last file in the list overrides earlier ones.

services:
  one:
    label_file: ./app.labels

  two:
    label_file: 
      - ./app.labels
      - ./additional.labels

links

Note

Availability of the links attribute is implementation specific.

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 are reachable at a hostname identical to the alias, or the service name if no alias is specified.

Links are not required to enable services to communicate. When no specific network configuration is set, any service is 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 does not override the network configuration and services not attached to a shared network are not be able to communicate. Compose doesn't warn you about a 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 sets a service container's network mode. Available values are platform specific, but Compose defines specific values which must be implemented as described if supported:

• none: Turns off all container networking.
• host: Gives the container raw access to the host's network interface.
• service:{name}: Gives the service container access to the specified service only.
• container:{name}: Gives the service container access to the specified container ().

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

When set, the networks attribute is not allowed and Compose rejects any Compose file containing both attributes.

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 the service on the network. Other containers on the same network can use either the service name or an 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.

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

In the following example, service frontend is able to reach the backend service at the hostname backend or database on the back-tier network. The service monitoring is able to reach same backend service at backend or mysql on the admin network.

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

  monitoring:
    image: example/monitoring
    networks:
      - admin

  backend:
    image: example/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 a service container when joining the network.

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

services:
  frontend:
    image: example/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

mac_address

mac_address sets the MAC address used by the service container when connecting to this particular network.

driver_opts

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

services:
  app:
    networks:
      app_net:
        driver_opts:
          foo: "bar"
          baz: 1

priority

priority indicates in which order Compose connects 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 the service container.

Note Container runtimes might reject this value (ie. Docker Engine >= v25.0). In that case, you should use networks.mac_address instead.

mem_limit

mem_limit configures a limit on the amount of memory a container can allocate, set as a string expressing a byte value.

When both are set, mem_limit must be consistent with the limits.memory attribute in the Deploy Specification

mem_reservation

mem_reservation configures a reservation on the amount of memory a container can allocate, set as a string expressing a byte value.

When both are set, mem_reservation must be consistent with the reservations.memory attribute in the Deploy Specification

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.

• 0: Turns off anonymous page swapping.
• 100: Sets all anonymous pages as swappable.

The default value is platform specific.

memswap_limit

memswap_limit defines the amount of memory the container is allowed to swap to disk. This is a modifier attribute that only has meaning if memory is also set. Using swap lets the container 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 is 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.
• 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 configures 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. Value must be within [-1000,1000] range.

pid

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

pids_limit

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

pids_limit: 10

When both are set, pids_limit must be consistent with the pids attribute in the Deploy Specification

platform

platform defines the target platform the containers for the service run on. It uses the os[/arch[/variant]] syntax.

The values of os, arch, and variant must conform to the convention used by the OCI Image Spec.

Compose uses this attribute to determine which version of the image is pulled and/or on which platform the service’s build is performed.

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

ports

Exposes container ports.

Note

Port mapping must not be used with network_mode: host otherwise a runtime error occurs.

Short syntax

The short syntax is a colon-separated string to set the 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 offers support for platform-specific protocol names.

If host IP is not set, it binds to all network interfaces. Ports 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 container runtime automatically allocates any unassigned port of the host.

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

Examples:

ports:
  - "3000"
  - "3000-3005"
  - "8000:8000"
  - "9090-9091:8080-8081"
  - "49100:22"
  - "8000-9000:80"
  - "127.0.0.1:8001:8001"
  - "127.0.0.1:5000-5010:5000-5010"
  - "6060:6060/udp"

Note

If Host IP mapping is not supported by a container engine, Compose rejects the Compose file and ignores 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. It is defined as a string and can be set as a range using syntax start-end. It means the actual port is assigned a remaining available port, within the set range.
• host_ip: The Host IP mapping, unspecified means all network interfaces (0.0.0.0).
• protocol: The port protocol (tcp or udp). Defaults to tcp.
• app_protocol: The application procotol (TCP/IP level 4 / OSI level 7) this port is used for. This is optional and can be used as a hint for Compose to offer richer behavior for protocols that it understands.
• mode: host: For publishing a host port on each node, or ingress for a port to be load balanced. Defaults to ingress.
• name: A human-readable name for the port, used to document its usage within the service

ports:
  - name: web
    target: 80
    host_ip: 127.0.0.1
    published: "8080"
    protocol: tcp
    app_protocol: http
    mode: host    

  - name: web-secured
    target: 443
    host_ip: 127.0.0.1
    published: "8083-9000"
    protocol: tcp
    app_protocol: https
    mode: host

post_start

post_start defines a sequence of lifecycle hooks to run after a container has started. The exact timing of when the command is run is not guaranteed.

• command: The command to run after the container has started. This attribute is required.
• user: The user to run the command. If not set, the command is run with the same user as the main service command.
• privileged: Lets the post_start command run with privileged access.
• working_dir: The working directory in which to run the command. If not set, it is run in the same working directory as the main service command.
• environment: Sets the environment variables to run the post_start command. The command inherits the environment set for the service's main command, and this section lets you to append or override values.

services:
  test:
    post_start:
      - command: ./do_something_on_startup.sh
        user: root
        privileged: true
        environment:
          - FOO=BAR

pre_stop

pre_stop defines a sequence of lifecycle hooks to run before service termination.

Configuration is equivalent to `post_start.

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. If unassigned, the service is always started but if assigned, it is only started if the profile is activated.

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

services:
  frontend:
    image: frontend
    profiles: ["frontend"]

  phpmyadmin:
    image: phpmyadmin
    depends_on:
      - db
    profiles:
      - debug

pull_policy

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

• always: Compose always pulls the image from the registry.
• never: Compose doesn't pull the image from a registry and relies on the platform cached image. If there is no cached image, a failure is reported.
• missing: Compose pulls the image only if it's not available in the platform cache. This is the default option if you are not also using the Compose Build Specification. if_not_present is considered an alias for this value for backward compatibility.
• build: Compose builds the image. Compose rebuilds the image if it's already present.

read_only

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

restart

restart defines the policy that the platform applies on container termination.

• no: The default restart policy. It does not restart the container under any circumstances.
• always: The policy always restarts the container until its removal.
• on-failure: The policy restarts the container if the exit code indicates an error.
• unless-stopped: The policy restarts the container irrespective of the exit code but stops 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 the 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

scale specifies the default number of containers to deploy for this service. When both are set, scale must be consistent with the replicas attribute in the Deploy Specification.

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

Services can 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 must not imply granting any service access to it. Such grant must be explicit within service specification as secrets service element.

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: example/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 container, or absolute path of the file if an alternate location is required. 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. The default value is world-readable permissions (mode 0444). The writable bit must be ignored if set. The executable bit may be set.

Note that the uid, gid, and mode attributes are implementation specific.

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's lifecycle is not directly managed by Compose.

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

security_opt

security_opt overrides the default labeling scheme for each container.

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

For further default labeling schemes you can override, see Security configuration.

shm_size

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

stdin_open

stdin_open configures a service containers to run with an allocated stdin.

stop_grace_period

stop_grace_period specifies how long Compose 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. It's 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 Compose uses to stop the service containers. If unset containers are stopped by Compose by sending SIGTERM.

stop_signal: SIGUSR1

storage_opt

storage_opt defines storage driver options for a service.

storage_opt:
  size: '1G'

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. It can be a single value or a list.

tmpfs: 
 - <path>
 - <path>:<options>

• <path>: The path inside the container where the tmpfs will be mounted.
• <options>: Comma-separated list of options for the tmpfs mount.

Available options:

• mode: Sets the file system permissions.
• uid: Sets the user ID that owns the mounted tmpfs.
• gid: Sets the group ID that owns the mounted tmpfs.

services:
  app:
    tmpfs:
      - /data:mode=755,uid=1009,gid=1009
      - /run

tty

tty configures service container to run with a TTY.

ulimits

ulimits overrides the default ulimits for a container. It's specified either as an integer for a single limit or as mapping for soft/hard limits.

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

user

user overrides the user used to run the container process. The default is set by the image (i.e. Dockerfile USER). If it's not set, then 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 define mount host paths or named volumes that are accessible by service containers. You can use volumes to define multiple types of mounts; volume, bind, tmpfs, or npipe.

If the mount is a host path and is only used by a single service, it can be declared as part of the service definition. To reuse a volume across multiple services, a named volume must be declared in the top-level volumes key.

The following 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: example/backend
    volumes:
      - type: volume
        source: db-data
        target: /data
        volume:
          nocopy: true
          subpath: sub
      - 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 colon-separated values to specify a volume mount (VOLUME:CONTAINER_PATH), or an access mode (VOLUME:CONTAINER_PATH:ACCESS_MODE).

• VOLUME: Can be either a host path on the platform hosting containers (bind mount) or a volume name.
• CONTAINER_PATH: The path in the container where the volume is mounted.
• ACCESS_MODE: A comma-separated , list of options:
  • rw: Read and write access. This is the default if none is specified.
  • ro: Read-only access.
  • z: SELinux option indicating that the bind mount host content is shared among multiple containers.
  • Z: SELinux option indicating that the bind mount host content is private and unshared for other containers.

Note

The SELinux re-labeling bind mount option is ignored on platforms without SELinux.

Note Relative host paths are only supported by Compose 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. When Compose deploys to a non-local platform it rejects 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. Either volume, bind, tmpfs, npipe, or cluster
• 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: Used to configure additional bind options:
  • propagation: The propagation mode used for the bind.
  • create_host_path: Creates a directory at the source path on host if there is nothing present. Compose does nothing if there is something present at the path. This is automatically implied by short syntax for backward compatibility with docker-compose legacy.
  • selinux: The SELinux re-labeling option z (shared) or Z (private)
• volume: Configures additional volume options:
  • nocopy: Flag to disable copying of data from a container when a volume is created.
  • subpath: Path inside a volume to mount instead of the volume root.
• tmpfs: Configures additional tmpfs options:
  • size: The size for the tmpfs mount in bytes (either numeric or as bytes unit).
  • mode: The file mode for the tmpfs mount as Unix permission bits as an octal number.
• 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. You can optionally specify read-only access ro or read-write rw. If no access level is specified, then read-write access is used.

You can also mount volumes from a container that is not managed by Compose by using the container: prefix.

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

working_dir

working_dir overrides the container's working directory which is specified by the image, for example Dockerfile's WORKDIR.

Networks top-level element

Networks are the layer that allow services to communicate with each other.

The top-level networks element lets you configure named networks that can be reused across multiple services. To use a network across multiple services, you must explicitly grant each service access by using the networks attribute within the services top-level element. The networks top-level element has additional syntax that provides more granular control.

Examples

Basic example

In the following example, at runtime, networks front-tier and back-tier are created and the frontend service is connected to front-tier and back-tier networks.

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

networks:
  front-tier:
  back-tier:

Advanced example

services:
  proxy:
    build: ./proxy
    networks:
      - frontend
  app:
    build: ./app
    networks:
      - frontend
      - backend
  db:
    image: postgres
    networks:
      - backend

networks:
  frontend:
    # Use a custom driver
    driver: custom-driver-1
  backend:
    # Use a custom driver which takes special options
    driver: custom-driver-2
    driver_opts:
      foo: "1"
      bar: "2"

The advanced example shows a Compose file which defines two custom networks. The proxy service is isolated from the db service, because they do not share a network in common. Only app can talk to both.

Attributes

driver

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

networks:
  db-data:
    driver: overlay

driver_opts

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

networks:
  db-data:
    driver_opts:
      foo: "bar"
      baz: 1

attachable

If attachable is set to true, then standalone containers should be able to 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 enables IPv6 networking. For an example, see step four of Create an IPv6 network.

external

If set to true:

• external specifies that this network’s lifecycle is maintained outside of that of the application. Compose doesn't attempt to create these networks, and returns an error if one doesn't exist.
• All other attributes apart from name are irrelevant. If Compose detects any other attribute, it rejects the Compose file as invalid.

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

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

networks:
  outside:
    external: true

ipam

ipam specifies a custom 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 a:
  • 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.

networks:
  mynet1:
    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 provides external connectivity to networks. internal, when set to true, allows you to create an externally isolated network.

labels

Add metadata to containers using labels. You can use either an array or a dictionary.

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

networks:
  mynet1:
    labels:
      com.example.description: "Financial transaction network"
      com.example.department: "Finance"
      com.example.label-with-empty-value: ""

networks:
  mynet1:
    labels:
      - "com.example.description=Financial transaction network"
      - "com.example.department=Finance"
      - "com.example.label-with-empty-value"

Compose sets com.docker.compose.project and com.docker.compose.network labels.

name

name sets a custom name for the network. The name field can be used to reference networks which contain special characters. The name is used as is and is not 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 Compose 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 container engine. Compose offers a neutral way for services to mount volumes, and configuration parameters to allocate them to infrastructure.

The top-level volumes declaration lets you configure named volumes that can be reused across multiple services. To use a volume across multiple services, you must explicitly grant each service access by using the volumes attribute within the services top-level element. The volumes attribute has additional syntax that provides more granular control.

Example

The following example shows a two-service setup where a database's data directory is shared with another service as a volume, named db-data, so that it can be periodically backed up.

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

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

volumes:
  db-data:

The db-data volume is mounted at the /var/lib/backup/data and /etc/data container paths for backup and backend respectively.

Running docker compose up creates the volume if it doesn't already exist. Otherwise, the existing volume is used and is recreated if it's manually deleted outside of Compose.

Attributes

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

driver

Specifies which volume driver should be used. Default and available values are platform specific. If the driver is not available, Compose returns an error and doesn't deploy the application.

volumes:
  db-data:
    driver: foobar

driver_opts

driver_opts specifies a list of options as key-value pairs to pass to the driver for this volume. The 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 exists on the platform and its lifecycle is managed outside of that of the application. Compose doesn't then create the volume, and returns an error if the volume doesn't exist.
• All other attributes apart from name are irrelevant. If Compose detects any other attribute, it rejects the Compose file as invalid.

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

services:
  backend:
    image: example/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.

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

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

Compose sets com.docker.compose.project and com.docker.compose.volume labels.

name

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

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

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

For example, if DATABASE_VOLUME=my_volume_001 in your .env file:

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

Running docker compose up uses the volume called my_volume_001.

It can also be used in conjunction with the external property. This means the name of the volume used to lookup the actual volume on the 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

Configs top-level element

Configs allow services to adapt their behaviour without the need to rebuild a Docker image.

Services can only access configs when explicitly granted by a configs attribute within the services top-level element.

As with volumes, configs are mounted as files into a service's container's filesystem. 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:

• Is owned by the user running the container command but can be overridden by service configuration.
• Has world-readable permissions (mode 0444), unless the service is configured to override this.

The top-level configs declaration defines or references configuration data that is granted to services in your Compose 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.
• environment: The config content is created with the value of an environment variable.
• content: The content is created with the inlined value.
• external: If set to true, external specifies that this config has already been created. Compose does not attempt to create it, and if it does not exist, an error occurs.
• name: The name of the config object in the container engine to look up. 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.

Example 1

<project_name>_http_config is created when the application is deployed, by registering the content of the httpd.conf as the configuration data.

configs:
  http_config:
    file: ./httpd.conf

Alternatively, http_config can be declared as external. Compose looks up http_config to expose the configuration data to relevant services.

configs:
  http_config:
    external: true

Example 2

<project_name>_app_config is created when the application is deployed, by registering the inlined content as the configuration data. This comes with the benefits Compose will infer variables when creating the config, which allows to adjust content according to service configuration:

configs:
  app_config:
    content: |
      debug=${DEBUG}
      spring.application.admin.enabled=${DEBUG}
      spring.application.name=${COMPOSE_PROJECT_NAME}

Example 3

External configs lookup can also use a distinct key by specifying a name.

The following example modifies the previous one to look up a config using the parameter HTTP_CONFIG_KEY. The the actual lookup key will is set at deployment time by the interpolation of variables, but exposed to containers as hard-coded ID http_config.

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

If external is set to true, all other attributes apart from name are irrelevant. If Compose detecs any other attribute, it rejects the Compose file as invalid.

Secrets top-level element

Secrets are a flavor of Configs focusing on sensitive data, with specific constraint for this usage.

Services can only access secrets when explicitly granted by a secrets attribute within the services top-level element.

The top-level secrets declaration defines or references sensitive data that is granted to the services in your Compose application. The source of the secret is either file or environment.

• file: The secret is created with the contents of the file at the specified path.
• environment: The secret is created with the value of an environment variable.
• external: If set to true, external specifies that this secret has already been created. Compose 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 isn't scoped with the project name.

Example 1

server-certificate secret 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

Example 2

token secret is created as <project_name>_token when the application is deployed, by registering the content of the OAUTH_TOKEN environment variable as a platform secret.

secrets:
  token:
    environment: "OAUTH_TOKEN"

Alternatively, server-certificate can be declared as external. Compose looks up the server-certificate secret to expose to relevant services.

secrets:
  server-certificate:
    external: true

Example 3

External secrets lookup can also use a distinct key by specifying a name.

The following example modifies the previous example to look up a secret using the name CERTIFICATE_KEY. The actual lookup key is set at deployment time by the interpolation of variables, but exposed to containers as hard-coded ID server-certificate.

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

If external is set to true, all other attributes apart from name are irrelevant. If Compose detects any other attribute, it rejects the Compose file as invalid.

Your Compose file needs to explicitly grant access to the secrets to relevant services in your application.

Fragments

With Compose, you can use built-in YAML features to make your Compose file neater and more efficient. Anchors and aliases let you create re-usable blocks. This is useful if you start to find common configurations that span multiple services. Having re-usable blocks minimizes potential mistakes.

Anchors are created using the & sign. The sign is followed by an alias name. You can use this alias with the * sign later to reference the value following the anchor. Make sure there is no space between the & and the * characters and the following alias name.

You can use more than one anchor and alias in a single Compose file.

Example 1

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

In the example above, a default-volume anchor is created based on the db-data volume. It is later reused by the alias *default-volume to define the metrics volume.

Anchor resolution takes place before variables interpolation, so variables can't be used to set anchors or aliases.

Example 2

services:
  first:
    image: my-image:latest
    environment: &env
      - CONFIG_KEY
      - EXAMPLE_KEY
      - DEMO_VAR
  second:
    image: another-image:latest
    environment: *env

If you have an anchor that you want to use in more than one service, use it in conjunction with an extension to make your Compose file easier to maintain.

Example 3

You may want to partially override values. Compose follows the rule outlined by YAML merge type.

In the following example, metrics volume specification uses alias to avoid repetition but overrides name attribute:

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

Example 4

You can also extend the anchor to add additional values.

services:
  first:
    image: my-image:latest
    environment: &env
      FOO: BAR
      ZOT: QUIX
  second:
    image: another-image:latest
    environment:
      <<: *env
      YET_ANOTHER: VARIABLE

Note

YAML merge only applies to mappings, and can't be used with sequences.

In example above, the environment variables must be declared using the FOO: BAR mapping syntax, while the sequence syntax - FOO=BAR is only valid when no fragments are involved.

Extension

As with Fragments, Extensions can be used to make your Compose file more efficient and easier to maintain. Extensions can also be used with anchors and aliases.

Use the prefix x- as a top-level element to modularize configurations that you want to reuse. Compose ignores any fields that start with x-, this is the sole exception where Compose silently ignores unrecognized fields.

They also can be used within any structure in a Compose file where user-defined keys are not expected. Compose use those to enable experimental features, the same way browsers add support for custom CSS features

Example 1

x-custom:
  foo:
    - bar
    - zot

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

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

Example 2

x-env: &env
  environment:
    - CONFIG_KEY
    - EXAMPLE_KEY
 
services:
  first:
    <<: *env
    image: my-image:latest
  second:
    <<: *env
    image: another-image:latest

In this example, the environment variables do not belong to either of the services. They’ve been lifted out completely into the x-env extension field. This defines a new node which contains the environment field. The &env YAML anchor is used so both services can reference the extension field’s value as *env.

Example 3

x-function: &function
 labels:
   function: "true"
 depends_on:
   - gateway
 networks:
   - functions
 deploy:
   placement:
     constraints:
       - 'node.platform.os == linux'
services:
 # Node.js gives OS info about the node (Host)
 nodeinfo:
   <<: *function
   image: functions/nodeinfo:latest
   environment:
     no_proxy: "gateway"
     https_proxy: $https_proxy
 # Uses `cat` to echo back response, fastest function to execute.
 echoit:
   <<: *function
   image: functions/alpine:health
   environment:
     fprocess: "cat"
     no_proxy: "gateway"
     https_proxy: $https_proxy

The nodeinfo and echoit services both include the x-function extension via the &function anchor, then set their specific image and environment.

Example 4

Using YAML merge it is also possible to use multiple extensions and share and override additional attributes for specific needs:

x-environment: &default-environment
  FOO: BAR
  ZOT: QUIX
x-keys: &keys
  KEY: VALUE
services:
  frontend:
    image: example/webapp
    environment: 
      << : [*default-environment, *keys]
      YET_ANOTHER: VARIABLE

Note

YAML merge only applies to mappings, and can't be used with sequences.

In the example above, the environment variables are declared using the FOO: BAR mapping syntax, while the sequence syntax - FOO=BAR is only valid when no fragments are involved.

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

Specifying byte values

Values 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).

    2b
    1024kb
    2048k
    300m
    1gb

Specifying durations

Values express a duration as a string in the form of {value}{unit}. The supported units are us (microseconds), ms (milliseconds), s (seconds), m (minutes) and h (hours). Values can combine multiple values 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 is supported. Default values can be defined inline using typical shell syntax:

• ${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 only if VARIABLE is unset in the environment.

Interpolation can also be nested:

• ${VARIABLE:-${FOO}}
• ${VARIABLE?$FOO}
• ${VARIABLE:-${FOO:-default}}

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

Compose will process any string following a $ sign as long as it makes it a valid variable definition: either an alphanumeric name ([_a-z][_a-z0-9]*) or a braced string starting with ${. In other circumstances, it will be preserved without attempt to interpolate a value.

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 Compose can't resolve a substituted variable and no default value is defined, it displays a warning and substitutes 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 is applied before a merge on a per-file basis.

Interpolation applies only to YAML values, not to keys. For the few places where keys are actually arbitrary user-defined strings, such as labels or environment, an alternate equal sign syntax must be used for interpolation to apply. For example:

services:
  foo:
    labels:
      "$VAR_NOT_INTERPOLATED_BY_COMPOSE": "BAR"

services:
  foo:
    labels:
      - "$VAR_INTERPOLATED_BY_COMPOSE=BAR"

Merge and override

Compose lets you define a Compose application model through multiple Compose files. When doing so, Compose follows the rules declared in this section to merge Compose files.

Mapping

A YAML mapping gets merged by adding missing entries and merging the conflicting ones.

Merging the following example YAML trees:

services:
  foo:
    key1: value1
    key2: value2

services:
  foo:
    key2: VALUE
    key3: value3

Results in a Compose application model equivalent to the YAML tree:

services:
  foo:
    key1: value1
    key2: VALUE
    key3: value3

Sequence

A YAML sequence is merged by appending values from the overriding Compose file to the previous one.

Merging the following example YAML trees:

services:
  foo:
    DNS:
      - 1.1.1.1

services:
  foo:
    DNS: 
      - 8.8.8.8

Results in a Compose application model equivalent to the YAML tree:

services:
  foo:
    DNS:
      - 1.1.1.1
      - 8.8.8.8

Exceptions

Shell commands

When merging Compose files that use the services attributes command, entrypoint and healthcheck: test, the value is overridden by the latest Compose file, and not appended.

Merging the following example YAML trees:

services:
  foo:
    command: ["echo", "foo"]

services:
  foo:
    command: ["echo", "bar"]

Results in a Compose application model equivalent to the YAML tree:

services:
  foo:
    command: ["echo", "bar"]

Unique resources

Applies to the ports, volumes, secrets and configs services attributes. While these types are modeled in a Compose file as a sequence, they have special uniqueness requirements:

Attribute	Unique key
volumes	target
secrets	source
configs	source
ports	{ip, target, published, protocol}

When merging Compose files, Compose appends new entries that do not violate a uniqueness constraint and merge entries that share a unique key.

Merging the following example YAML trees:

services:
  foo:
    volumes:
      - foo:/work

services:
  foo:
    volumes:
      - bar:/work

Results in a Compose application model equivalent to the YAML tree:

services:
  foo:
    volumes:
      - bar:/work

Reset value

In addition to the previously described mechanism, an override Compose file can also be used to remove elements from your application model. For this purpose, the custom YAML tag !reset can be set to override a value set by the overriden Compose file. A valid value for attribute must be provided, but will be ignored and target attribute will be set with type's default value or null.

For readability, it is recommended to explicitly set the attribute value to the null (null) or empty array [] (with !reset null or !reset []) so that it is clear that resulting attribute will be cleared.

A base compose.yaml file:

services:
app:
    image: myapp
    ports:
      - "8080:80"
    environment:
      FOO: BAR             

And an compose.override.yaml file:

services:
  app:
    image: myapp
    ports: !reset []
    environment:
      FOO: !reset null

Results in:

services:
  app:
    image: myapp

Replace value

While !reset can be used to remove a declaration from a Compose file using an override file, !override allows you to fully replace an attribute, bypassing the standard merge rules. A typical example is to fully replace a resource definition, to rely on a distinct model but using the same name.

A base compose.yaml file:

services:
app:
    image: myapp
    ports:
      - "8080:80"

To remove the original port, but expose a new one, the following override file is used:

services:
  app:
    ports: !override
      - "8443:443" 

This results in:

services:
  app:
    image: myapp
    ports:
      - "8443:443" 

If !override had not been used, both 8080:80 and 8443:443 would be exposed as per the merging rules outlined above.

Include

A Compose application can declare dependency on another Compose application. This is useful if:

• You want to reuse other Compose files.
• You need to factor out parts of your application model into separate Compose files so they can be managed separately or shared with others.
• Teams need to keep a Compose file reasonably complicated for the limited amount of resources it has to declare for it's own sub-domain, within a larger deployment.

The include top-level section is used to define the dependency on another Compose application, or sub-domain. Each path listed in the include section is loaded as an individual Compose application model, with it's own project directory, in order to resolve relative paths.

Once the included Compose application is loaded, all resources definitions are copied into the current Compose application model. Compose displays a warning if resource names conflict and doesn't try to merge them. To enforce this, include is evaluated after the Compose file(s) selected to define the Compose application model have been parsed and merged, so that conflicts between Compose files are detected.

include applies recursively so an included Compose file which declares its own include section, triggers those other files to be included as well.

Any volumes, networks, or other resources pulled in from the included Compose file can be used by the current Compose application for cross-service references. For example:

include:
  - my-compose-include.yaml  #with serviceB declared
services:
  serviceA:
    build: .
    depends_on:
      - serviceB #use serviceB directly as if it was declared in this Compose file

Compose also supports the use of interpolated variables with include. It's recommended that you specify mandatory variables. For example:

include:
  -${INCLUDE_PATH:?FOO}/compose.yaml

Short syntax

The short syntax only defines paths to other Compose files. The file is loaded with the parent folder as the project directory, and an optional .env file that is loaded to define any variables' default values by interpolation. The local project's environment can override those values.

include:
  - ../commons/compose.yaml
  - ../another_domain/compose.yaml

services:
  webapp:
    depends_on:
      - included-service # defined by another_domain

In the above example, both ../commons/compose.yaml and ../another_domain/compose.yaml are loaded as individual Compose projects. Relative paths in Compose files being referred by include are resolved relative to their own Compose file path, not based on the local project's directory. Variables are interpolated using values set in the optional .env file in same folder, and is overridden by the local project's environment.

Long syntax

The long syntax offers more control over the sub-project parsing:

include:
   - path: ../commons/compose.yaml
     project_directory: ..
     env_file: ../another/.env

path

path is required and defines the location of the Compose file(s) to be parsed and included into the local Compose model. path can be set either to a string when a single Compose file is involved, or to a list of strings when multiple Compose files need to be merged together to define the Compose model to be included in the local application.

include:
   - path: 
       - ../commons/compose.yaml
       - ./commons-override.yaml

project_directory

project_directory defines a base path to resolve relative paths set in the Compose file. It defaults to the directory of the included Compose file.

env_file

env_file defines an environment file(s) to use to define default values when interpolating variables in the Compose file being parsed. It defaults to .env file in the project_directory for the Compose file being parsed.

env_file can be set either to a string or a list of strings when multiple environment files need to be merged to define a project environment.

include:
   - path: ../another/compose.yaml
     env_file:
       - ../another/.env
       - ../another/dev.env

The local project's environment has precedence over the values set by the Compose file, so that the local project can override values for customization.

Profiles

With profiles you can define a set of active profiles so your Compose application model is adjusted for various usages and environments. 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 are always enabled.

A service is ignored by Compose when none of the listed profiles match the active ones, unless the service is explicitly targeted by a command. In that case its profile is added to the set of active profiles.

Note

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) do not automatically enable a component that would otherwise have been ignored by active profiles. Instead Compose returns 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

In the above example:

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

See how you can use profiles in Docker Compose.