Meaning of ampersand (&) in docker-compose.yml file

Question

I recently came across this and was wondering what &django means

version: '2'

services:
  django: &django

I can't see anything in the docs related to this.

Answer 1

These are a YAML feature called anchors, and are not particular to Docker Compose. I would suggest you have a look at below URL for more details

https://learnxinyminutes.com/docs/yaml/

Follow the section EXTRA YAML FEATURES

YAML also has a handy feature called 'anchors', which let you easily duplicate content across your document. Both of these keys will have the same value:

anchored_content: &anchor_name This string will appear as the value of two keys. other_anchor: *anchor_name

Anchors can be used to duplicate/inherit properties

base: &base
    name: Everyone has same name

foo: &foo
    <<: *base
    age: 10

bar: &bar
    <<: *base
    age: 20

Answer 2

To complement Tarun's answer, & identifies an anchor and * is an alias referring back to the anchor. It is described as the following in the YAML specification:

In the representation graph, a node may appear in more than one collection. When serializing such data, the first occurrence of the node is identified by an anchor. Each subsequent occurrence is serialized as an alias node which refers back to this anchor.

Sidenote:

For those who want to start using anchors in your docker-compose files, there is more powerful way to make re-usable anchors by using docker-compose YAML extension fields.

version: "3.4"

# x-docker-data is an extension and when docker-compose
# parses the YAML, it will not do anything with it

x-docker-data: &docker-file-info
  build:
    context: .
    dockerfile: Dockerfile

services:
  some_service_a:
    <<: *docker-file-info
    restart: on-failure
    ports:
      - 8080:9090
  some_service_b:
    <<: *docker-file-info
    restart: on-failure
    ports:
      - 8080:9595