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.

INSTRUCTIONS

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.