Services top-level elements

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.

Examples

Simple example

The following example demonstrates how to define two simple services, set their images, map ports, and configure basic environment variables using Docker Compose.

services:
  web:
    image: nginx:latest
    ports:
      - "8080:80"

  db:
    image: postgres:13
    environment:
      POSTGRES_USER: example
      POSTGRES_DB: exampledb

Advanced example

In the following example, the proxy service uses the Nginx image, mounts a local Nginx configuration file into the container, exposes port 80 and depends on the backend service.

The backend service builds an image from the Dockerfile located in the backend directory that is set to build at stage builder.

services:
  proxy:
    image: nginx
    volumes:
      - type: bind
        source: ./proxy/nginx.conf
        target: /etc/nginx/conf.d/default.conf
        read_only: true
    ports:
      - 80:80
    depends_on:
      - backend

  backend:
    build:
      context: backend
      target: builder

For more example Compose files, explore the Awesome Compose samples.

Attributes

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

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 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: a short syntax and a long syntax.

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

With the depends_on attribute, you can control the order of service startup and shutdown. It is useful if services are closely coupled, and the startup sequence impacts the application's functionality.

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. Introduced in Docker Compose version 2.17.0.

• 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. Introduced in Docker Compose version 2.20.0.

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.

driver_opts

driver_opts specifies a list of options as key-value pairs to pass to the driver. These options are driver-dependent.

services:
  app:
    networks:
      app_net:
        driver_opts:
          com.docker.network.bridge.host_binding_ipv4: "127.0.0.1"

Consult the network drivers documentation for more information.

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

The env_file attribute is used to specify one or more files that contain environment variables to be passed to the containers.

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

List elements can also be declared as a mapping, which then lets you set an additional attribute required. This 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

required attribute is available with Docker Compose version 2.24.0 or later.

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.

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

The environment attribute 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.

When a service uses extends, it can also specify dependencies on other resources, an explicit volumes declaration for instance. However, it's important to note that extends does not automatically incorporate the target volume definition into the extending Compose file. Instead, you are responsible for ensuring that an equivalent resource exists for the service being extended to maintain consistency. Docker Compose verifies that a resource with the referenced ID is present within the Compose model.

Dependencies on other resources in an extends target can be:

• An explicit reference 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.

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. Introduced in Docker Compose version 2.24.1. 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

The healthcheck attribute 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.

For more information on HEALTHCHECK, see the Dockerfile reference.

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. Introduced in Docker Compose version 2.20.2

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.

• 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]"

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.

links

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

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

Containers for the linked service 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.

mac_address

Available with Docker Compose version 2.24.0 and later.

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 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 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.

network_mode

network_mode sets a service container's network mode.

• none: Turns off all container networking.
• host: Gives the container raw access to the host's network interface.
• service:{name}: Gives the container access to the specified container by referring to its service name.
• container:{name}: Gives the container access to the specified container by referring to its container ID.

For more information container networks, see the Docker Engine documentation.

    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

The networks attribute defines the networks that service containers are attached to, referencing entries under the networks top-level element. The networks attribute helps manage the networking aspects of containers, providing control over how services are segmented and interact within the Docker environment. This is used to specify which networks the containers for that service should connect to. This is important for defining how containers communicate with each other and externally.

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

For more information about the networks top-level element, see Networks.

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.

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.

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:

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 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

The ports is used to define the port mappings between the host machine and the containers. This is crucial for allowing external access to services running inside containers. It can be defined using short syntax for simple port mapping or long syntax, which includes additional options like protocol type and network mode.

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 protocol (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. Introduced in Docker Compose version 2.26.0.
• 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 it's 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: Specifies the command to run once the container starts. This attribute is required, and you can choose to use either the shell form or the exec form.
• 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 environment variables specifically for the post_start command. While the command inherits the environment variables defined for the service’s main command, this section lets you add new variables or override existing ones.

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

For more information, see Use lifecycle hooks.

pre_stop

pre_stop defines a sequence of lifecycle hooks to run before the container is stopped. These hooks won't run if the container stops by itself or is terminated suddenly.

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[:max-retries]: The policy restarts the container if the exit code indicates an error. Optionally, limit the number of restart retries the Docker daemon attempts.
• 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: on-failure:3
    restart: unless-stopped

You can find more detailed information on restart policies in the Restart Policies (--restart) section of the Docker run reference page.

runtime

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

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

The default is runc. To use a different runtime, see Alternative runtimes.

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

The secrets attribute grants access to sensitive data defined by the secrets top-level element on a per-service basis. Services can be granted access to multiple secrets.

Two different syntax variants are supported; the short syntax and the long syntax. Long and short syntax for secrets may be used in the same Compose file.

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

The following example sets the name of the server-certificate secret file to server.cert within the container, sets the mode to 0440 (group-readable), and sets the user and group to 103. The value of server-certificate is set to the contents of the file ./server.cert.

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

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's container to run with an allocated stdin. This is the same as running a container with the -i flag. For more information, see Keep STDIN open.

Supported values are true or false.

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: /run

tmpfs:
  - /run
  - /tmp

tty

tty configures a service's container to run with a TTY. This is the same as running a container with the -t or --tty flag. For more information, see Allocate a pseudo-TTY.

Supported values are true or false.

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"

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"

volumes

The volumes attribute 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 volumes top-level element.

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:

For more information about the volumes top-level element, see Volumes.

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. Introduced in Docker Compose version 2.14.0.
• consistency: The consistency requirements of the mount. Available values are platform specific.

Tip

Working with large repositories or monorepos, or with virtual file systems that are no longer scaling with your codebase? Compose now takes advantage of Synchronized file shares and automatically creates file shares for bind mounts. Ensure you're signed in to Docker with a paid subscription and have enabled both Access experimental features and Manage Synchronized file shares with Compose in Docker Desktop's settings.

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.