Get notify on server event

[fcollonval]

Problem

The initial question on Gitter was:

A server extension running long actions (mamba-org/gator#132) can delay shutting down the server. Is there a way to be notified the server wants to shutdown?

But there may be other events worth notifying.

Proposed Solution

I would suggest callback registration like the on implemented for widget events: https://ipywidgets.readthedocs.io/en/latest/examples/Widget%20Events.html

Additional context

[blink1073]

Thanks for raising the issue!

[echarles]

just curious, what "network channel" are you thinking to for such events: an existing/new websocket channel, or comm channel...?

[fcollonval]

just curious, what "network channel" are you thinking to for such events: an existing/new websocket channel, or comm channel...?

Actually in the reported case, I only need the main server to notify the server extension. So there is no need for communication with the frontend or the kernels.

[vidartf]

So we discussed this in today's dev meeting, and noted that it would be nice to add a generic event framework for things like this. That would allow events including kernelspec added/modified, file modified, kernel started/stopped - really any REST call other than GET. We could also add file system polling for contents events.

Such a framework should be able to emit both in-process on the server side, and to the client side (ideally with events defined by JSON schemas). It was noted that this sounded very similar to some of the work that is going on in the telemetry effort (#364, https://github.com/jupyter/telemetry), so in order to avoid having two parallel event frameworks it would be beneficial to share as much as possible.

A proposed solution was to merge the in-process emitter logic from the telemetry effort, and use this as a "private" event framework for now. Alternatively, we could create a trait on ServerApp that is the "status", and could be observed using trailets observer, where "shutting down" could be one such status.