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How to have a background thread call back to JS multiple times? #438
Description
TL;DR How can a long-running thread call a JS function repeatedly, i.e. without the use of AsyncWorker?
AsyncWorker seems to be for one long-running task that calls back to JS land only once, once finished.
In my scenario, a background thread is repeatedly calling back to JS land, something like relaying events that occur in native land to a JS callback. In the past (node.js 0.8), I did this with libuv's uv_async_send(), but I need to upgrade, and since uv and v8 have changed so much I thought I'd migrate straight over to node-addon-api, as it seems to have everything I need.
However I cannot seem to find out how to pull this off with AsyncContext and CallbackScope, which are made for cases like this, if I parse the docs correctly. However, all examples in the documentation and the tests in https://github.com/nodejs/node-addon-api/blob/master/test/ for both classes are (ironically?) working synchronously, and my attempts to migrate their callbacks over to run on a different thread have failed so far.
A nudge in the right direction on how to change either test/asyncworker.cc, test/callbackscope.cc or https://github.com/nodejs/node-addon-api/blob/master/doc/async_context.md#example to work asynchronously would be awesome. If I do get it to work in a reasonable amount of time, I promise I'll add a test and/or an example to node-addon-api.
(In order to be run on the JS thread, I'd expect I'd have to provide something like a function pointer, but I can't find anything to that effect in AsyncContext, CallbackScope, or Function::MakeCallback)
Activity
twoi commentedon Jan 29, 2019
N.B. There is an example that seems to be doing something like I want to do, in plain N-API at https://github.com/nodejs/node-addon-examples/blob/master/async_work_thread_safe_function/node-api/
It is using *_threadsafe_function for that, which is nowhere to be found in the node-addon-api sources, so I guess I have to go N-API for this? (But then, what would AsyncContext and CallbackScope be good for?)
twoi commentedon Jan 29, 2019
So the *_threadsafe_function isn't even part of the N-API included in the latest node-addon-api, which would mean I have to ditch node-addon-api and do everything with lower level N-API?
romandev commentedon Jan 29, 2019
@twoi, I think this issue is duplicated with #312
Yes, for now.
DaAitch commentedon Jan 29, 2019
To your question @twoi thanks for asking.
Short version is (I hope this is still valid):
AsyncWorkeruses nodejs computation threads, good for async short running tasks like algorithms to not block main loop, don't do long running tasks with it, like IO or sleeping/waiting, etc., you only have a few threads (fixed size pool)twoi commentedon Jan 29, 2019
Thanks @romandev @DaAitch
But how can
AsyncContextandCallbackScopebe used asynchronously?twoi commentedon Feb 14, 2019
In the meantime, I've switched to the lower-level N-API to get the job done. It would still be great to be able to do this in node-addon-api, though. I find N-API not as easy to use as node-addon-api.
gabrielschulhof commentedon Feb 14, 2019
@twoi node-addon-api and N-API can be used together. While we don't yet have a node-addon-api wrapper around
napi_threadsafe_functionyou can use the rest of node-addon-api together withnapi_*_threadsafe_function(). You can use things likeinfo.Env()fromNapi::CallbackInfo infoto obtain thenapi_envyou need, because aNapi::Envis automatically cast to anapi_envwhen you pass it into a N-API.gabrielschulhof commentedon Feb 14, 2019
TBH I'm starting to think we should expose
NAPI_THROW_IF_FAILEDandNAPI_THROW_IF_FAILED_VOIDbecause they're super-useful for mixing node-addon-api and N-API.legendecas commentedon Feb 14, 2019
Is there any discussion on this idea? :P having these two in future n-api would be very helpful.
mhdawson commentedon Mar 4, 2019
@legendecas just landed as: #448
mhdawson commentedon Mar 4, 2019
@legendecas any remaining questions that need to be addressed in this issue. If not can you close it?
twoi commentedon Mar 4, 2019
Let me close this. My question has been answered as "currently not possible with (pure) node-addon-api
"