[kepler-dev] Handling shared-resources

[Edward A. Lee]

This is a very interesting question.

There are some major potential pitfalls with shared resources
in actor-oriented models, but there are also patterns that work.
The biggest pitfall is unexpected nondeterminism.  Patterns
that could work:

 - Represent the resource in a token, and pass the token
   around. For your example, in order to write to a file,
   an actor would need to receive a token at its input
   representing that file. Passing the token to multiple
   actors at the same time would result in nondeterministic
   interleaving of the writes.  The token would need to
   somehow implement a reference counting mechanism to
   keep track of how many references to it are in use,
   and close the file only when these drop to zero.

 - Use the Rendezvous domain. Rendezvous is all about
   simultaneous access to shared resources.
   A pool of resources could be offered using the
   ResourcePool actor in the Rendezvous domain...

 - The Metropolis project (a sister project in Chess) has made
   a major effort in this direction. Metropolis has so called
   "quantity managers" that would not be hard to emulate in
   Ptolemy II and that model resource sharing.
   It would be a worthwhile project to provide a systematic way
   to do this...

Edward

At 09:32 AM 3/7/2007, Norbert Podhorszki wrote:
>Hi,
>
>Does anyone handle a "resource" in a workflow "shared" among actors?
>
>I mean like org.kepler.actor.Logger that opens a file once but many 
>instances of that actor in the workflow can write into the same file. So 
>there is no need to drag channels from many actors to one single Logger 
>actor. The file is then (supposed to be) closed at the end of the workflow 
>execution once.
>
>Or, the org.kepler.actor.ssh.* actors that share an established ssh 
>session to perform remote operations. The session should be closed at the 
>very end of the execution.
>
>I have learnt from the recent thread "Wrapup sequence question" that I was 
>wrong about that actors can be aware of the finalization stage of the 
>model/workflow, I have put the finalization of shared resources into the 
>wrapup() method. However, under PN, individual actors can wrap-up 
>surprisingly early, even before another actors start working.
>
>My question is to those who handle shared resources: When and how do you 
>close/finalize them?
>
>I am not sure that actor-oriented modelling allows shared resources at 
>all, or they are just examples of violating it.
>
>Best regards
>Norbert
>
>
>      Norbert Podhorszki
>    ------------------------------------
>      University of California, Davis
>      Department of Computer Science
>      1 Shields Ave, 2236 Kemper Hall
>      Davis, CA 95616
>      (530) 752-5076
>      pnorbert at cs.ucdavis.edu
>      ----------------------------------
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------------ 
Edward A. Lee
Chair of EECS and Robert S. Pepper Distinguished Professor
231 Cory Hall, UC Berkeley, Berkeley, CA 94720-1770
phone: 510-642-0253, fax: 510-642-2845
eal at eecs.Berkeley.EDU, http://ptolemy.eecs.berkeley.edu/~eal