2个容器之间的Docker卷

Question

Let's say we have the following docker-compose.yml file

version: '3'

services:
  c1:
    build:
      context: .
      dockerfile: 1.Dockerfile
    volumes:
      - data:/folder
  c2:
    build:
      context: .
      dockerfile: 2.Dockerfile
    volumes:
      - data:/folder
    depends_on: 
      - c1

volumes: 
  data:

with the following 1.Dockerfile

FROM ubuntu:latest
RUN mkdir -p /folder/
RUN touch /folder/1.txt
VOLUME /folder

and the following 2.Dockerfile

FROM ubuntu:latest
RUN mkdir -p /folder/
RUN touch /folder/2.txt
VOLUME /folder

whenever i do docker-compose up

then do

docker-compose run --rm c2 bash
ls folder

or

docker-compose run --rm c1 bash
ls folder

i always get the folder from c1 no matter what, isn't c2 supposed to overwrite c1 's volume

Answer 1

You can read about the behavior you are describing in the docs here: https://docs.docker.com/storage/volumes/#populate-a-volume-using-a-container

If you start a container which creates a new volume, as above, and the container has files or directories in the directory to be mounted (such as /app/ above), the directory's contents are copied into the volume. The container then mounts and uses the volume, and other containers which use the volume also have access to the pre-populated content.

So what is happening is that your volume is initiated with the data from your c1 container when it is created.

Then the pre-populated volume is mounted to both c1 and c2.

Data pre-population to the volume happens create time. After that the volume is mapped with that data that was populated during creation.