Docker keeps data but creates a new volume from host folder every "run"

Question

It seems to be a misunderstood point from me about volumes. I have a docker-compose file with two services : jobs which is a Flask api built from a Dockerfile (see below), and mongo which is from official MongoDb image.

I have two volumes : - .:/code is linked from my host working directory to /code folder in the container, and a named volume mongodata.

version: "3"
services: 
  jobs:
    build: .
    ports: 
      - "5000:5000"
    volumes: 
      - .:/code
    environment: 
      FLASK_ENV: ${FLASK_ENV}
      FLASK_APP: ${FLASK_APP}
    depends_on: 
      - mongo
  mongo:
    image: "mongo:3.6.21-xenial"
    restart: "always"
    ports: 
      - "27017:27017"
    volumes: 
      - mongodata:/data/db
    environment: 
      MONGO_INITDB_ROOT_USERNAME: ${MONGO_INITDB_ROOT_USERNAME}
      MONGO_INITDB_ROOT_PASSWORD: ${MONGO_INITDB_ROOT_PASSWORD}
volumes: 
  mongodata:

Dockerfile for jobs service :

FROM python:3.7-alpine
WORKDIR /code
ENV FLASK_APP=job-checker
ENV FLASK_ENV=development
COPY requirements.txt requirements.txt
RUN pip install -r requirements.txt
EXPOSE 5000
COPY . .
CMD ["flask", "run", "--host=0.0.0.0"]

Every time I remove these container and re-run, everything is fine, I still have my data in mongodata volume. But when I check the volume list I can see that a new volume is created from - .:/code with a long volume name, for example :

$ docker volume ls
DRIVER              VOLUME NAME
local               55c08cd008a1ed1af8345cef01247cbbb29a0fca9385f78859607c2a751a0053
local               abe9fd0c415ccf7bf8c77346f31c146e0c1feeac58b3e0e242488a155f6a3927
local               job-checker_mongodata

Here I ran docker-compose up, then I removed containers, then ran up again, so I have two volumes from my working folder.

Is this normal that every up create a new volume instead of using the previous one ?

Thanks

Answer 1

Hidden at the end of the Docker Hub mongo image documentation is a note:

This image also defines a volume for /data/configdb...

The image's Dockerfile in turn contains the line

VOLUME /data/db /data/configdb

When you start the container, you mount your own volume over /data/db, but you don't mount anything on the second path. This causes Docker to create an anonymous volume there, which is the volume you're seeing with only a long hex ID.

It should be safe to remove the extra volumes, especially if you're sure they're not attached to a container and they don't have interesting content.

This behavior has nothing to do with the bind mount in the other container; bind mounts never show up in the docker volume ls listing at all.