Repo to demontrate the statefulness of containers. I was recently confused by this as a container as I was playing with seemed to be wiping state between restarts ¯\_(ツ)_/¯ .
From prior docker experience, I thought that containers were stateful but I wanted to be sure as a sanity check.
There is a basic python script python-test/create_files.py that creates a file 1.txt and every second thereafter creates a new file <inc_num>.txt (ie, 2.txt, then 3.txt etc).
If containers are stateless, then I would expect the directory would be empty and the script would start at 1.txt again when restarting the container. If stateful, then the script should pick up at the highest number and create new files from this number.
In docker-test: docker build . -t ubuntu_python
Then, in python-test: docker build . -t py-script
In any directory (with docker in the $PATH): docker run --name py-fun py-script.
This creates a container with a name of py-fun, you can check this with docker ps, you should see something like this:
$ docker ps
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
eeaa42fce560 py-script "python create_files…" 9 seconds ago Up 9 seconds py-fun
You can now go into the container and see all the new files being created:
$ docker exec -it py-fun ls
1.txt 2.txt 3.txt 4.txt create_files.py
Now restart py-fun:
py-fun
$ docker exec -it py-fun ls
1.txt 12.txt 15.txt 18.txt 3.txt 6.txt 9.txt
10.txt 13.txt 16.txt 19.txt 4.txt 7.txt create_files.py
11.txt 14.txt 17.txt 2.txt 5.txt 8.txt
$ docker restart py-fun
py-fun
$ docker exec -it py-fun ls
1.txt 14.txt 19.txt 23.txt 28.txt 32.txt 4.txt 9.txt
10.txt 15.txt 2.txt 24.txt 29.txt 33.txt 5.txt create_files.py
11.txt 16.txt 20.txt 25.txt 3.txt 34.txt 6.txt
12.txt 17.txt 21.txt 26.txt 30.txt 35.txt 7.txt
13.txt 18.txt 22.txt 27.txt 31.txt 36.txt 8.txt
If the containers are stateless, it should pick off from the snapshotted image every time the container is started, but the txt files that were created previously are still present.
On the other hand , if we re-run the image:
$ docker run --name py-fun2 py-script
And check the new container:
$ docker exec -it py-fun2 ls
1.txt 2.txt 3.txt 4.txt 5.txt create_files.py
It clearly starts off at the snapshotted state from the image.
You can also use docker commit, a very handly docker command I've picked up in the past few weeks as described here: https://stackoverflow.com/a/49204476 , to demonstrate the same thing.