Twisted: 1 thread in the reactor pattern

[exarkun at twistedmatrix.com]

On 06:08 am, jacopo.pecci 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