Recreate container on stop with docker-compose - docker

Question

I am trying to set up a multi-container service with docker-compose.
Some of the containers need to be restarted from a fresh container (eg. the file system should be like in the image) when they restart.
How can I achieve this?
I've found the restart: always option I can put on my service in the docker-compose.yml file, but that doesn't give me a fresh file system as it uses the same container.
I've also seen the --force-recreate option of docker-compose up, but that doesn't apply as that only recreates the containers when the command is runned.
EDIT:
This is probably not a docker-compose issue, but more of a general docker question: What is the best way to make sure a container is in a fresh state when it is restarted? With fresh state, I mean a state identical to that of a brand new container from the same image. Restarted is the docker command docker restart or docker stop and docker start.

Answer 1

In docker, immutability typically refers to the image layers. They are immutable, and any changes are pushed to a container specific copy-on-write layer of the filesystem. That container specific layer will last for the lifetime of the container. So to have those files not-persist, you have two options:
Recreate the container instead of just restart it
Don't write the changes to the container filesystem, and don't write them to any persistent volumes.
You cannot do #1 with a restart policy by it's very definition. A restart policy gives you the same container filesystem, with the application restarted. But if you use docker's swarm mode, it will recreate containers when they exit, so if you can migrate to swarm mode, you can achieve this result.
Option #2 looks more difficult than it is. If you aren't writing to the container filesystem, or to a volume, then where? The answer is a tmpfs volume that is only stored in memory and is lost as soon as the container exits. In compose, this is a tmpfs: /data/dir/to/not/persist line. Here's an example on the docker command line.
First, let's create a container with a tmpfs mounted at /data, add some content, and exit the container:
$ docker run -it --tmpfs /data --name no-persist busybox /bin/sh
/ # ls -al /data
total 4
drwxrwxrwt 2 root root 40 Apr 7 21:50 .
drwxr-xr-x 1 root root 4096 Apr 7 21:50 ..
/ # echo 'do not save' >>/data/tmp-data.txt
/ # cat /data/tmp-data.txt
do not save
/ # ls -al /data
total 8
drwxrwxrwt 2 root root 60 Apr 7 21:51 .
drwxr-xr-x 1 root root 4096 Apr 7 21:50 ..
-rw-r--r-- 1 root root 12 Apr 7 21:51 tmp-data.txt
/ # exit
Easy enough, it behaves as a normal container, let's restart it and check the directory contents:
$ docker restart no-persist
no-persist
$ docker attach no-persist
/ # ls -al /data
total 4
drwxr-xr-x 2 root root 40 Apr 7 21:51 .
drwxr-xr-x 1 root root 4096 Apr 7 21:50 ..
/ # echo 'still do not save' >>/data/do-not-save.txt
/ # ls -al /data
total 8
drwxr-xr-x 2 root root 60 Apr 7 21:52 .
drwxr-xr-x 1 root root 4096 Apr 7 21:50 ..
-rw-r--r-- 1 root root 18 Apr 7 21:52 do-not-save.txt
/ # exit
As you can see, the directory returned empty, and we can add data as needed back to the directory. The only downside of this is the directory will be empty even if you have content in the image at that location. I've tried combinations of named volumes, or using the mount syntax and passing the volume-nocopy option to 0, without luck. So if you need the directory to be initialized, you'll need to do that as part of your container entrypoint/cmd by copying from another location.

Answer 2

In order to not persist any changes to your containers it is enough that you don't map any directory from host to the container.
In this way, every time the containers runs (with docker run or docker-compose up ), it starts with a fresh file system.
docker-compose down also removes the containers, deleting any data.

Answer 3

The best solution I have found so far, is for the container itself to make sure to clean up when starting or stopping. I solve this by cleaning up when starting.
I copy my app files to /srv/template with the docker COPY directive in my Dockerfile, and have something like this in my ENTRYPOINT script:
rm -rf /srv/server/
cp -r /srv/template /srv/server
cd /srv/server