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From "Øyvind Harboe" <oyvind.har...@zylin.com>
Subject Thread safety and Cayenne ( & Tapestry)
Date Fri, 29 Sep 2006 13:46:20 GMT
Where can I read more about Cayenne and behaviour of objects when
accessed from multiple threads simultaneously?


We've screwed up a bit(how much I need to find out :-) when using
Tapestry + Cayenne, but things work surprisingly well even so. That
things appear to work fine is perhaps the scariest thing. I'd rather
they'd break immediately.

Basically we have multiple threads accessing/modifying a list of
CayenneDataObjects.

The problems started in our Visit object implementation in Tapestry:

class Visit
{
  public List getFoo()
  {
    if (list==null)
    {
      List l=someCayenneQuery();
Thread.sleep(5000);  // (A)
      list=l;
    }
    return list;
  }
}


Several problems:

- There is a race condition. Context switch at (A) and a subsequent
getFoo() invocation will will cause any number of someCayenneQuery()
method calls thus each thread may be operating on a different list of
objects.
- Items might be added/removed from multiple threads. A simple
LinkedList() is not thread safe, what about something returned from a
Cayenne query?

- How will a CayenneDataObject respond to multiple threads modifying properties?

- How will a CayenneDataObject respond to multiple threads reading properties?


Possible solutions for us that I'm pondering:

- Read up more on Cayenne to understand the threading model better.
Multiple threads reading/writing the same CayenneDataObjects may be
fine.
- I could single thread all modifications of the session/visit state.
- use serialization somehow to create and write back a copy of each
object per HTTP request.
- Does some framework which can be used with Cayenne & Tapestry exist
that can help me address the issues?
- Can I make my Tapestry application single threaded? (I believe the
answer is Bad Idea :-)


-- 
Øyvind Harboe
http://www.zylin.com

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