Forward logs from file to journald

It's necessary to setup SystemMaxUse=100G e.g.

The problem is actually with buffering from the Flask application and not with how systemd or journald are ingesting those logs.

This can be counter-intuitive, since as you mentioned, running python3 run.py directly on the command-line works and shows logs properly and also timestamps look correct on the logs.

The former happens because Unix/Linux will typically set up stdout to be unbuffered when connected to a terminal (since it's expecting interaction with an user), but buffered when connected to a file (in case of StandardOutput=file:...) or to a pipe (in case you're logging to the journal, which is the default.)

The latter is because the Python/Flask logger is adding timestamps, so even though it's buffering that output, when it finally issues it into the logs, all the timestamps are there.

Some applications will know this is typically an issue and will set up buffering on stdout appropriately when using it for logs, but this doesn't seem to be the case with this particular Python/Flask setup you are using.

On Python, it's fairly easy to globally change stdout to unbuffered mode, which you can do by:

1. Passing a -u flag to python3 in your command.
2. Setting PYTHONUNBUFFERED=1 in your environment (which you can do in the systemd service unit with an additional Environment=PYTHONUNBUFFERED=1 line.)

You confirmed this worked for your specific case, so that's great!

For non-Python applications suffering from similar issues, there are command-line tools such as unbuffer and stdbuf which can often solve this same problem.

Solutions are usually specific to the kind of application, which is somewhat unfortunate, but often googling or looking for other answers in Stack Exchange (once you know buffering is the issue) will usually lead you to an useful suggestion.