DataContext is not thread-safe, as it is intended for access by a
single user.
DataDomain and the rest of the stack underneath the DataContext is
thread-safe.
If a there is a chance of multiple threads belonging to the same user
to access the context (and its DataObjects) simultaneously, you'll
have to synchronize manually.
Andrus
On Sep 29, 2006, at 9:46 AM, Øyvind Harboe wrote:
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
