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Redis – docker-compose.yml dash syntax in YAML

In the docker-compose.yml files, why are certain enumerations done with a dash -, and others without?

services:
  web:           # the enumeration of [build, ports, volumes, environment] doesn't use a -
    build: .
    ports:             
      - "5000:5000"     # why the - here?  could we remove it and have ports: "5000:5000"?
    volumes:
      - .:/code         # why the - here?
    environment:
      FLASK_ENV: development        # why no - here?
  redis:
    image: "redis:alpine"

Another example:

version: '2'
services:
   db:
     image: mysql:5.7
     volumes:
       - ./mysql:/var/lib/mysql                # could we remove this - prefix?
     restart: always
     environment:
       MYSQL_ROOT_PASSWORD: somewordpress        # no - in this enumeration, why?
       MYSQL_DATABASE: wordpress
       MYSQL_USER: wordpress
       MYSQL_PASSWORD: wordpress
   wordpress:
     depends_on:
       - db                            # would it be ok without - ?
     image: wordpress:latest
     volumes:
       - ./wp:/var/www/html            # same
...
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4

Answers


  1. Chosen as BEST ANSWER
    0

    After further tests, I noticed that

    volumes: ./mysql:/var/lib/mysql

    fails with:

    ERROR: The Compose file './docker-compose.yml' is invalid because:
services.db.volumes contains an invalid type, it should be an array

    so it's either:

    volumes: 
    - ./mysql:/var/lib/mysql

    or

    volumes: ['./mysql:/var/lib/mysql']

    according to YAML Multi-Line Arrays.


  2. 0

    According to: https://docs.ansible.com/ansible/latest/reference_appendices/YAMLSyntax.html

    Dashes represent Lists.

    All members of a list are lines beginning at the same indentation level starting with a - 

    # A list of tasty fruits
- Apple
- Orange
- Strawberry
- Mango

    No Dashes Means that they are Key Value Pairs to a Dictionary.

    A dictionary is represented in a simple key: value form

    martin:
  name: Martin D'vloper
  job: Developer
  skill: Elite

    According to the Docker Compose Documentation at: https://docs.docker.com/compose/compose-file/compose-file-v2/

    environment:
  RACK_ENV: development
  SHOW: 'true'
  SESSION_SECRET:

    is the same as:

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

    Notice, that docker doesn’t care which one you use in the environment key , as long as they are consistent. Docker-compose syntax just happens to define it that way.

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  3. 0

    This comes directly from YAML syntax specification.

    • Dashes/hyphens denote (unnamed) list elements, thus in the format -<value>
    • Entries without dashes are named child elements, format <name>: <value>
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  4. 0

    The dash in YAML denotes an array of elements (values only).
    The other syntax in YAML, of key: value is a dictionary (key-value pairs).

    So, for example, ports is an array of values, with one value: "5000:5000". The : here has no significance from the YAML standpoint.

    As another example, and clarification about the environment directive in docker compose:

    The environment definition in fact supports two formats.

    When defined like this:

    environment:
  KEY: value
  KEY2: value2

    it is a YAML dictionary of key-value pairs.

    When defined like this:

    environment:
  - KEY=value
  - KEY2=value2

    it is an array of values, and once again, the = here has no significance from YAML standpoint, it is passed as is to the consumer that processes it (docker-compose binary in this case).

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