I would consider to spawn the whole loop in a background task (including open & close of the connection).

Does that really help? Because, even if we spawn it as a different task, we'll still have to wait for that task to finish here, as we can't return from this deregister call before everything is cleared.

as we can't return from this deregister call before everything is cleared.

Why not? This was my proposal. I see clearing all these retained messages as a follow-up task of entity de-registration. But, I agree that this can be discussed.

I see clearing all these retained messages as a follow-up task of entity de-registration. But, I agree that this can be discussed.

Since one of the main highlights of the HTTP API was the feedback after something is complete, returning while the operation is still in-progress in the background feels a bit weird. As a user, I personally prefer the deterministic behaviour, esp since this is a rare operation and hence I'm okay with waiting a bit more until everything is fully complete. But I'll let @reubenmiller also pitch in, if the proposed eventual cleanup is sufficient.