volumes_from instruction - docker compose - docker

Question

With the below docker-compose.yml file:
test:
build: ../../
dockerfile: docker/dev/Dockerfile
volumes_from:
- cachev
cachev:
build: ../../
dockerfile: docker/dev/Dockerfile
volumes:
- /build
entrypoint: "true"
cachev service in above file launches volume container that creates anonymous volume in /var/lib/docker/ folder in docker host and creates mount point /cache within volume container(xx_cachev).
Does volumes_from instruction under test service create /build mount point in xx_test container? that points to /build mount point of xx_cachev container?

Answer 1

From volumes_from docs:
Mount all of the volumes from another service or container...
So the short answer is yes:
volumes_from mounts /build volume defined by cachev service inside test service.
Long answer:
To answer your question let's run the test service:
docker compose up test
Before answering your question, let's make sure the description is clear:
cachev service in above file launches volume container...
It's just regular container which exits immediately because of entrypoint: "true".
docker ps -a should show:
ac68a33abe59 cache "true" 16 hours ago Exited (0) 4 minutes ago cache_1
But before it exits it creates volumes specified in volumes:. So we can call it volume container if its volumes are used by other service, for caching for instance.
that creates anonymous volume in /var/lib/docker/ folder in docker host
Agree. - /build is anonymous volume. Can be verified by viewing all container mounts:
docker inspect [cachev_container_id] --format '{{json .Mounts}}' | jq
should show something like:
{
"Type": "volume",
"Name": "1ec7ff7c72bfb5a3259ed54be5b156ea694be6c8d932bcb3fa6e657cbcaea378",
"Source": "/var/lib/docker/volumes/1ec7ff7c72bfb5a3259ed54be5b156ea694be6c8d932bcb3fa6e657cbcaea378/_data",
"Destination": "/build",
"Driver": "local",
"Mode": "",
"RW": true,
"Propagation": ""
}
jq is great utility for working with jsons in bash. Install it for the above command to work.
and creates mount point /cache within volume container(xx_cachev).
Don't see any evidence of mounts in cachev: service spec you provided.
If you add mapping - /tmp/cache:/cache to its volumes section and run docker compose up test again and inspect the exited container you should see:
{
"Type": "bind",
"Source": "/tmp/cache",
"Destination": "/cache",
"Mode": "rw",
"RW": true,
"Propagation": "rprivate"
}
Please, note that docker inspect [cachev_service_id] --format '{{json .Mounts}}' | jq will show all container mounts including those specified in docker/dev/Dockerfile using VOLUME instruction.
To answer to your question we need to inspect test service container:
docker inspect [test_container_id] --format '{{json .Mounts}}' | jq:
would show all the volumes specified in docker/dev/Dockerfile if any and all the volumes of cachev thanks to volumes_from instruction.
You can see that both test and cache containers have:
{
"Type": "volume",
"Name": "1ec7ff7c72bfb5a3259ed54be5b156ea694be6c8d932bcb3fa6e657cbcaea378",
"Source": "/var/lib/docker/volumes/1ec7ff7c72bfb5a3259ed54be5b156ea694be6c8d932bcb3fa6e657cbcaea378/_data",
"Destination": "/build",
"Driver": "local",
"Mode": "",
"RW": true,
"Propagation": ""
}
in their mounts and this volume survives subsequent runs of docker compose up test

Answer 2

Yes, you can verify by executing a command inside both containers. if creat file in test container under touch /build/fromtest.txt path, it will be visible in cacheV container on the same path /build/fromtest.txt.
volumes_from
Mount all of the volumes from another service or container
compose-file-volumes_from
A demo you can try
test:
image: alpine
command: sh -c "touch /build/fromtest.txt && echo hell from test-container && ls /build/"
volumes_from:
- cachev
cachev:
image: node:alpine
command: sh -c "touch /build/fromcache.txt && echo hello from cache-container && ls /build/"
volumes:
- /build
log will be
Recreating compose-volume_cachev_1 ... done
Recreating compose-volume_test_1 ... done
Attaching to compose-volume_cachev_1, compose-volume_test_1
cachev_1 | hello from cache-container
test_1 | hell from test-container
test_1 | fromcache.txt
test_1 | fromtest.txt
cachev_1 | fromcache.txt
cachev_1 | fromtest.txt