Twisted: 1 thread in the reactor pattern

[jacopo]

On Sep 23, 5:57 pm, exar... at twistedmatrix.com wrote:
> On 06:08 am, jacopo.pe... at gmail.com wrote:
>
> >I am diving into Twisted and Perspective Broker (PB) in particular and
> >I would like to understand more about what happens behind the
> >curtains.
> >Say I have a client and a server on two different machines, the server
> >gets callRemote() 19s in an asynchronous way, these requests are parked
> >in a queue and then served sequentially (not in parallel  13 correct me
> >if I am wrong).
>
> Since, as you point out below, there is only one thread, the remote
> methods can only be invoked one at a time.
>
> However, rather central to the asynchronous operation of Twisted
> libraries and applications, the remote method itself may return before
> the remote call has been completely serviced.  So while only one remote
> method will run at a time, each of the two remote calls may run
> concurrently at some point before they are responded to.
>
> >If everything is implemented in a single thread, how
> >is it possible that while the processor is engaged in the processing
> >triggered by  callRemote() 19s at the same time the reactor is ready to
> >listen/accept new events and put them in a queue? To me it looks like
> >there should be at least 2 processes, one for the reactor and on for
> >the rest.
>
> It isn't possible.  While the remote methods are running, other events
> are not being serviced.  This is what is meant when people describe
> Twisted as a "*cooperative* multitasking" system.  Event handlers (such
> as remote methods) run for a short enough period of time that it doesn't
> matter that the reactor is prevented from accepting new connections (or
> what have you) for the duration of their execution.
>
> Jean-Paul

Jean -Paul, not sure I have understood.
Say I have one server S and two clients C1 and C2 (all on separate
machines).

(a)	C1 requests a remote call of f1() to S, f1() requires 5 minutes of
processing.
(b)	S  puts f1() in a queue and returns immediately a Deferred to
C1.
(c)	Now f1() starts and keeps S’s processor busy for 5 mins
(d)	 after few seconds C2 requests a remote call f2() to S.
(e)	On S the processor is already engaged with f1() but still
“someone” on S is able to accept the request from C2, put it in a
queue (after f1()) and return a Deferred to C2.
(f)	At some point after f1() is finished f2() will start

I believe (b) is what you say “run for a short enough period of time
that it doesn't
matter that the reactor is prevented from accepting new connections
(or
what have you) for the duration of their execution.” ?!

Don’t we have (c) and (e) running at the same time here? How is it
possible to have only one Thread?

Thanks,
 Jacopo