launch new multiprocessing task from pv / camonitor callback about pyepics HOT 5 CLOSED

@kingspride One of the slightly tricky parts here is that the monitor callback is registered from the main Python loop but is called from "inside" the C-level CA event loop. That puts some subtle limitations on the work that can be done within each callback. One thing that basically cannot be done is to do other CA work, such as creating PVs or checking the connection status of other PVs. You can get other PV values, but those will essentially be the cached values.

So, what I do and recommend often (though I would hesitate to call this "the right way", is to just set a simple global/class-level field in the callback and then do the work such as "start long running task" from the main loop.

Does that make sense?

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okay. usage of global variables are not the "fine english way" as you'd say in german, but I can definitly use it if there is no other option.

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Well, your runner, run, worker, starter, stopper, and main are global variables, and you are using global throughout your script. Another completely normal approach would be to encapsulate that into a class, and have each of those be attributes/methods of the instance of the object (and then nothing would be global in your script and instead be owned by self).

But also, information and data about CA connections do need to be in "per-process" global data. That is done behind the scenes, but it can be useful to keep in mind that there is a single CA event loop running within the C-level CA library.

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sure, but initially I didnt want to use at least the global statement. maybe its just personal taste.

the script above was already a later version where I tried a lot of things (bluesky and ophyd devices dont work inside pyepics multiprocesses... :( )
so yeah...

I'll try to make my way though it with globals / classes and see where I get.
thanks so far :)

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@kingspride well, yeah there is also the question of "multiprocess" is actually what you want, especially in contrast to running multiple processes (ie, launched by the OS).

I can't help with anything related to ophyd/bluesky - I don't use it.

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