Processing
What happens in the background after the upload and why it never runs inside a page request.
Never inside a page request
Processing a video takes minutes. Run inside a page request it would block one of the workers serving your site for that time — with two parallel uploads the site stalls. So it always runs as a background job.
The order has a reason
The file is cleaned first, then the derived versions are made. The other way round, a poster image would be taken from the uncleaned file — technically unremarkable, factually wrong.
The job starts after the save
It is only dispatched once the database row is actually written. Without that order a worker picks it up, finds nothing and silently skips processing — the worst kind of failure, because it looks like success.
What you see of it
The lesson's state and, once finished, poster and duration. If a lesson stays stuck in processing, the worker is not running — see Installation.