[Python-Dev] Active Objects in Python

["Martin v. Löwis"]

Bruce Eckel wrote:
> Since the enqueuing process serializes all requests to active objects,
> and since each message-task runs to completion before the next one
> starts, the problem of threads interfering with each other is
> eliminated. Also, the enqueuing process will always happen, so even if
> the queue blocks it will be for a very short, deterministic time. So
> the theory is that Active Object systems cannot deadlock (although I
> believe they can livelock).

My experience with active objects is with SDL (the Specification and
Description Language). There, the primitive communication between
objects is through one-way messages (called signals). Such a system
can deadlock if an object doesn't produce "enough" new outgoing
messages in response to incoming messages.

If all communication is through RPCs, i.e. you owe your caller
a response, then the only way to deadlock such a system is that
all active objects terminate. The interesting question then is
whether an object can terminate while processing a message, and
whether this will cause a response to be sent back.

> But what's not clear is whether this would
> require knowledge of the innards of the Python interpreter (which I
> don't have) or if it could be built using libraries.

As others have pointed out, you have to find a way to deal with the
standard library. In principle, each active object should have its
own copy of the standard library. For some modules/objects, this is
not reasonable (e.g. stdout). In OCCAM, host IO itself is (conceptually)
an occam process, so sys.stdout would have to become an active
object. In that approach, you would essentially have to rewrite
parts of the standard library, wrapping the underlying modules
appropriately.

> BTW: I think that Hoare's Communicating Sequential Processes (CSP)
> basically describes the idea of active objects but without objects.
> That is, I think active objects is CSP applied to OO.

I don't believe this is true. In CSP, communication between processes
is through rendezvous, which is quite different from active objects,
as there is no queue for incoming calls - instead, the receiving process
must expect a specific communication partner just like the sending
process; there is also no inherent request/response mechanism in
CSP.

In CSP, there is plenty of chance for deadlocks. If a process becomes
STOP (i.e. the process that will never again interact), then any
communication attempt with that process also cause the partner to
become STOP.

Regards,
Martin