[rules-dev] StatefulKnowledgeSession: thread-safe

[Wolfgang Laun]

(I don't want to confuse things by adding to the recent thread on the
user list. And I do hope that I got this right. Please read carefully
and correct me. Thanks.)

The statement "StatefulKnowledgeSession is thread-safe" must be
annotated with caveats so that users don't overload this basically
simple statement with meaning that isn't logically implied by it.

StatefulKnowledgeSession is thread-safe means that any two threads
cannot interact in such a way that it would corrupt data protected by
the implementing object. It does not mean that all code executed on
behalf of the data maintained by a Stateful Knowledge Session object
is thread-safe. In particular, the execution of a consequence is not a
critical section from the session's point of view.

The existence of an activation implies that the LHS conditions are
true for the objects playing a part. As soon as one thread begins
actions based on these preconditions and is on its way to modifying
one or more facts, any other thread doing likewise with any of the
participating objects in this or another RHS is running into a race
condition.

If a consequence execution is a critical section for some application
object, it is the user's responsibility to achieve proper
synchronisation, either for entire consequences or for individual fact
objects.

-W