A progress bar your boss can spectate without joining

You're running an eight-hour data migration. Five people on the team want to know how far along it is. None of them want to consume a receiver slot or interrupt the stream.

Append ?progress to the pipe URL. Anyone who opens it in a browser gets an HTML dashboard showing bytes transferred, current speed, ETA, and state transitions: idle → waiting → streaming → complete. Anyone who hits it with curl -H "Accept: text/event-stream" gets a clean SSE feed they can pipe into their own scripts.

Up to fifty spectators per path. None of them touch the data: the pipe just forwards transfer state to them. The migration runs at full speed regardless of how many eyes are watching. Your boss bookmarks the progress URL on her phone and stops asking.