Thanks Yichun and Jeff for your inputs and sorry for late ack.

I had a brief look at ngx_drizzle, I see that it needs the library to
expose underlying fd, which is not possible in our case, but looks an
interesting solution. The other cosocket API mechanism seem to be using
unix domain socket as per the docs, so this would be similar to using
pipe/socket to handle async apis where the callback could trigger event on
the pipe/socket. Please correct me if am missing something? (never worked
on lua :) )

Use case is very similar to image optimization case in pagespeed. I am
guessing cosocket approach can also fit here?

@Jeff regarding using pipes to trigger event, does it create one pipe per
request or just one pipe where say module can
- trigger the async API,
- queue the nginx_request and proceed with other requests, (not sure this
is possible with nginx?, as i think nginx handler cannot return something
like NGX_AGAIN to resume handler where it called async api)
- when there is event on pipe, nginx pipe_event_handler can figure out
which request's data is ready and respond

Easiest approach I can think of is to use domain socket in upstream form,
but wanted to avoid as it it would mean additional socket descriptor per
request.

If you can help understand at design level w.r.t nginx, will be faster to
adapt :)

Thanks again!!
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