An ObservableResource is not notified about observation canceling

[aellwein]

I've noticed that a RST message which is supposed to cancel an existing observation, does nothing on ObservableResource. Is there no handler for that to inform the application about the observation cancelation? I think this is also related to issue #13.

[aellwein]

@chrysn
WDYT maybe it is a good idea to introduce an additional cancel_observation method on ObservableResource to notify the resource about observe cancellation? Just to keep it symmetric to the existing add_observation handler?

[chrysn]

Removal of observations generally happens via the cancellation callback passed to the ServerObservation's accept method; this allows an observation to be properly cancelled (pending bugs like this one, when cancellation_callback is probably not properly called, haven't looked properly yet), even when for example it has changed its path in the meantime. (Not a situation I'd expect to be common, but possible for kind of dynamic resources.) When add_obsevation is called, eg. on a Site, the ServerObservation is passed around until it gets accepted by a resource object, and I'm in favor of establishing the relationship to that object at accept time instead of hoping that a parallel cancel_observation would find its way to the very object the add_observation found, however likely it is in most cases.

Do you have a suggestion on how to fix this bug that would be made easier if there were a cancel_observation?

[aellwein]

What i am actually trying to do is to establish a producer for an observation, which is to be notified via cancel_observation (server-side cancellation, not self-triggered) to not produce the notifications anymore (by using asyncio loop, my producer just reschedules itself periodically).

With no expectation of corner cases here i am currently not aware of, i would try a naive approach of looking up the cancel_observation attribute on a ObservableResource (by convention). If this one is present, it is to be called, i think after calling remove() on the server observation. But as i said, just a naive approach, maybe it makes much more sense to somehow let the resource keep the server observation until a cancellation is needed.

[aellwein]

You may close the issue, if you think that such a callback is not necessary.

[chrysn]

I see now what you are aiming for (the resource implementation needs to know when it is being observed, eg. to establish a fnotify); in light of that, my previous answer was not exactly to point.

To give me some leeway in how to implement observations, I try to keep the ServerObservation objects out of the ObservableResource as far as possible; a .cancel_observation would be possible, but as long as the actual question is "am i being observed", would a .update_observation_count (called with 0 or non-zero) method work for you?

[aellwein]

Yes, this is exactly what i am looking for.

[aellwein]

Thank you 👍