Airflow: what do `airflow webserver`, `airflow scheduler` and `airflow worker` exactly do?

Question

I've been working with Airflow for a while now, which was set up by a colleague. Lately I run into several errors, which require me to more in dept know how to fix certain things within Airflow.

I do understand what the 3 processes are, I just don't understand the underlying things that happen when I run them. What exactly happens when I run one of the commands? Can I somewhere see afterwards that they are running? And if I run one of these commands, does this overwrite older webservers/schedulers/workers or add a new one?

Moreover, if I for example run airflow webserver, the screen shows some of the things that are happening. Can I simply get out of this by pressing CTRL + C? Because when I do this, it says things like Worker exiting and Shutting down: Master. Does this mean I'm shutting everything down? How else should I get out of the webserver screen then?

Answer 1

Each process does what they are built to do while they are running (webserver provides a UI, scheduler determines when things need to be run, and workers actually run the tasks).

I think your confusion is that you may be seeing them as commands that tell some sort of "Airflow service" to do something, but they are each standalone commands that start the processes to do stuff. ie. Starting from nothing, you run airflow scheduler: now you have a scheduler running. Run airflow webserver: now you have a webserver running. When you run airflow webserver, it is starting a python flask app. While that process is running, the webserver is running, if you kill command, is goes down.

All three have to be running for airflow as a whole to work (assuming you are using an executor that needs workers). You should only ever had one scheduler running, but if you were to run two processes of airflow webserver (ignoring port conflicts, you would then have two separate http servers running using the same metadata database. Workers are a little different in that you may want multiple worker processes running so you can execute more tasks concurrently. So if you create multiple airflow worker processes, you'll end up with multiple processes taking jobs from the queue, executing them, and updating the task instance with the status of the task.

When you run any of these commands you'll see the stdout and stderr output in console. If you are running them as a daemon or background process, you can check what processes are running on the server.

If you ctrl+c you are sending a signal to kill the process. Ideally for a production airflow cluster, you should have some supervisor monitoring the processes and ensuring that they are always running. Locally you can either run the commands in the foreground of separate shells, minimize them and just keep them running when you need them. Or run them in as a background daemon with the -D argument. ie airflow webserver -D.