Named volumes with docker-compose

04 Sep 2021 on Tech

While working on a side project that uses docker-compose, I stumbled over a problem. One that I had encountered before but had never properly investigated or solved.

Here is what I want to do:

• Have a bunch of services running within a docker-compose setup
• Have those services use mounted shares - one share, used by more than one container.
• The problematic thing: I want multiple containers to use the same volume!

Make a long story short, here is how it works smoothly:

version: '3'
services:
  service1:
    image: nginx
    container_name: service1
    ports:
      - '81:80'
    volumes:
      - content:/usr/share/nginx/html

  service2:
    image: nginx
    container_name: service2
    ports:
      - '82:80'
    volumes:
      - content:/usr/share/nginx/html

volumes:
  content:
     driver_opts:
           type: none
           device: ./data/content 
           o: bind

That’s what’s going on:

• We have 2 services of the same type: plain nginx containers for demo purposes.
• They both expose their (internal) port 80 to port 81 resp. 82 to the outside world.
• They both use a volume called content that is defined in the volumes section.

The detail that I missed for so long was the volumes section with the driver_opts. And while I ran a couple of tests and everything behaved exactly the way I hoped, I couldn’t find any proper documentation. Here’s what the docker documentation says about driver_opts:

Specify a list of options as key-value pairs to pass to the driver for this volume. Those options are driver-dependent - consult the driver’s documentation for more information.

When investigating how things are working, docker’s inspect tools give some insights: This is the Mounts part of docker inspect service1

 "Mounts": [
            {
                "Type": "volume",
                "Name": "docker-playground_content",
                "Source": "/var/lib/docker/volumes/docker-playground_content/_data",
                "Destination": "/usr/share/nginx/html",
                "Driver": "local",
                "Mode": "rw",
                "RW": true,
                "Propagation": ""
            }
        ]

At first I was sceptic because of this line:

"Source": "/var/lib/docker/volumes/docker-playground_content/_data"

But it turns out my data is not in this docker-managed directory, but where I wanted it. In my case that’s in** ./data/content**. Also the relative path works fine.

Sources

Here are the original sources that helped me most

• Docker documentation - strange enough, it dodn’t help at all…
• This was the most helpful Stackoverflow article.

Versions

Since these kind of setups might be version sensitive, here is my setup:

docker-compose version 1.29.2, build 5becea4c
docker-py version: 5.0.0
CPython version: 3.9.0
OpenSSL version: OpenSSL 1.1.1h  22 Sep 2020

And it runs on my Mac with Big Sur Version 11.5.2 (wit h Intel CPU 😜).

The code can be found on Github.