Persist data only for selected volumes with docker-compose - docker

Question

I have two Docker images for my documentation files: docs:v1 and docs:v2.
They just contain some files in /docs exposed as a VOLUME:
# docker run --rm docs:v1 cat /docs/doc.txt
Version1
# docker run --rm docs:v2 cat /docs/doc.txt
Version2
And I have my app described in this docker-compose.yml (using v1.4):
app:
image: "busybox"
command: /bin/sh -c "cat /docs/doc.txt && echo `date` >> /logs/log.txt"
volumes:
- "/logs"
volumes_from:
- "docs"
docs:
image: "docs:v1"
So basically my app prints the content of the docs and echo the current date in a log file. The log file is also in a VOLUME.
=> What I want it just to be able to update the docs to docs:v2, see that it prints "Version2" as expected and keep the logs intact.
First run:
# docker-compose up
Creating tmp_docs_1...
Creating tmp_app_1...
Attaching to tmp_docs_1, tmp_app_1
app_1 | Version1
...
# docker run --rm --volumes-from tmp_app_1 busybox cat /logs/log.txt
Tue Aug 25 22:09:11 UTC 2015
It works as expected: Prints the Version1 documentation and echo in the logs.
Next I update the yml file with : image: "docs:v2". Then restart my app:
# docker-compose up
Recreating tmp_docs_1...
Recreating tmp_app_1...
Attaching to tmp_docs_1, tmp_app_1
app_1 | Version1
...
# docker run --rm --volumes-from tmp_app_1 busybox cat /logs/log.txt
Tue Aug 25 22:09:11 UTC 2015
Tue Aug 25 22:10:26 UTC 2015
The logs have been updated, that's fine, but my doc is still in Version1 !
It might be surprising, but that's actually the expected behavior. According to the docker docs: "Changes to a data volume will not be included when you update an image."
Right, but I want to be able to see my updated docs, so let's try to delete the docs container and volume:
# docker-compose rm -v docs
Removing tmp_docs_1... done
# docker-compose up
Creating tmp_docs_1...
Starting tmp_app_1...
Attaching to tmp_docs_1, tmp_app_1
app_1 | Version1
...
No luck .. still in Version 1. That's because the app container still points to the old Version1 volume. So let's try to delete the app as well (just the app, not the volumes this time):
# docker-compose rm app
Removing tmp_app_1... done
# docker-compose up
Starting tmp_docs_1...
Creating tmp_app_1...
Attaching to tmp_docs_1, tmp_app_1
app_1 | Version2
Version2: it worked ! Let's check the logs:
# docker run --rm --volumes-from tmp_app_1 busybox cat /logs/log.txt
Tue Aug 25 22:19:21 UTC 2015
Ach ! My old logs are gone.
So here's the question again: how can I update the docs image, see the change in my app and still be able to keep the logs upon restarts ?

Answer 1

You will want to map your volumes to your actual FS. Right now you are creating volumes in the container by using the volume command, these volumes bypass the UFS and persist even if the container is deleted which is why you have to delete the volume to make it act properly. Map the volumes to an external folder on the host O/S and then when you upgrade your apps the logs will be there but the app upgraded.
To be clear about what is occurring:
You create the intial app > volume inits > logs are saved.
Upgrade app > logs are not updated (Changes to a data volume will not be included when you update an image.) (expected behavior).
Delete data volume > upgrade app > new logs are there old logs are gone because they were deleted.
To resolve mount the volumes to the host O/S so they persist but can be written to on upgrade.