Question about asyncio and blocking operations

[Maxime Steisel]

Le 28 janv. 2016 22:52, "Ian Kelly" <ian.g.kelly at gmail.com> a écrit :
>
> On Thu, Jan 28, 2016 at 2:23 PM, Maxime S <maxischmeii at gmail.com> wrote:
> >
> > 2016-01-28 17:53 GMT+01:00 Ian Kelly <ian.g.kelly at gmail.com>:
> >>
> >> On Thu, Jan 28, 2016 at 9:40 AM, Frank Millman <frank at chagford.com>
wrote:
> >>
> >> > The caller requests some data from the database like this.
> >> >
> >> >    return_queue = asyncio.Queue()
> >> >    sql = 'SELECT ...'
> >> >    request_queue.put((return_queue, sql))
> >>
> >> Note that since this is a queue.Queue, the put call has the potential
> >> to block your entire event loop.
> >>
> >
> > Actually, I don't think you actually need an asyncio.Queue.
> >
> > You could use a simple deque as a buffer, and call fetchmany() when it
is
> > empty, like that (untested):
>
> True. The asyncio Queue is really just a wrapper around a deque with
> an interface designed for use with the producer-consumer pattern. If
> the producer isn't a coroutine then it may not be appropriate.
>
> This seems like a nice suggestion. Caution is advised if multiple
> cursor methods are executed concurrently since they would be in
> different threads and the underlying cursor may not be thread-safe.
> --
> https://mail.python.org/mailman/listinfo/python-list

Indeed, the run_in_executor call should probably protected by an
asyncio.Lock.

But it is a pretty strange idea to call two fetch*() method concurrently
anyways.