When you are dealing with events, the processing works a bit differently. First, we have to clarify: Are you inserting events or plain facts? If it has a timestamp, it has to be handled as an event. The distinction matters because in an event-driven scenario, only events would be inserted "from the outside" (i.e., from outside the session), and then plain facts (representing the system state) get inserted/updated/deleted in response to these incoming events (via rule consequences). The only time you *might* insert plain facts "from the outside" is at the beginning, before the start of event processing, to define an initial state of the system. (If anyone can think of a good reason to insert plain facts *during* event processing other than in response to events, please let us know - always willing to learn something new :-).)

In your example, rather than inserting an object (that represents system state) again with modified state, you would insert a "StateChangeEvent" into the session, and a rule would update the object (fact) in response to this event - that's what is meant by event-driven processing or event-driven architecture. Think of your system state as getting updated in response to external events, and the rule engine is in charge of performing the updates.

So, if your objects (object[ABC]) are events, then the situation of inserting the same event (as in reference identity: ref1 == ref2) twice doesn't make sense. Honestly, I don't know what the rule engine would do in this case, but I'd say you have to consider the behavior undefined. Each state change should be represented by a separate, new event.

I strongly recommend calling fireAllRules() after every single event insertion (event != fact, event is a special kind of fact). It may be possible to do batched inserts of events, but the overhead of calling fireAllRules() after every single event insertion should be minimal. Keep in mind, either way the rule engine has to process each event in turn in order to maintain the event-driven semantics. In an event-driven setting, you would not find yourself doing batch inserts like this, anyway. Events are processed as they arrive. If you have multiple event streams and inevitable delays due to messaging, then it is on you as the rule author to ensure that your rules don't break due to unpredictable event ordering. I also recommend synchronizing on the session in case you are inserting events from multiple threads (I have empirical evidence that the StatefulKnowledgeSession is *not* thread-safe). In a single-event-stream scenario, where events arrive in chronological order (and even if not), you want to insert and fire immediately upon event arrival (and, yes, events could come from an asynchronous messaging queue).

At least this approach has worked well for me. Happy to hear feedback from others.


On Thu, Dec 22, 2011 at 10:45 AM, Skiddlebop <lmalson@cisco.com> wrote:
Thanks for the link :) 'Twas an interesting view! A few complexities still
remain. I may be ill-informed about this matter so please correct me if I'm
totally off-base in any of these statements.

Alexandre states that the KnowledgeSession is thread safe, which is contrary
to the documentation (if he is is referring to a StatefulKnowledgeSession).
However, it seems like he is actually using a StatelessKnowledgeSession
pulling his facts from Redis. Our requirements are different, we are dealing
with real-time data, not historical data, and this real-time data will all
be stored in a long-running StatefulKnowledgeSession. So every fact in that
StatefulKnowledgeSession will be an active real-time representation of an
object's state. However, we do share a similar approach in that we are also
"going async" using queues. The reasoning for choosing the queue for us is
based on a variety of considerations, the most relevant for this
conversation is the fact that, based on the un-thread-safe nature of the
StatefulKnowledgeSession, we anticipate processing one record at a time (one
call to fireAllRules() per event). Does this seem like a reasonable

I have considered inserting multiple events (as facts) per call to
FireAllRules(), but am uncertain of how the conflict resolution should work
if say for instance two of those facts correspond to the same field of the
same object. Here's a pseudo-code example outlining my concern:

Key assumption: It is NOT possible to concurrently access a
StatefulKnowledgeSession from multiple threads or separate parallel REST
service calls.

//Assume KnowledgeBase has already been built and sks is our long-running
global singleton //StatefulKnowledgeSession which has continuity between
individual REST web-service sessions.

//Begin Pseudo-Code Snippet.

//objectA with Data Member "Integer A" Value 1
//objectA with Data Member "Integer A" Value 2 (this is an update to the
same object which has a higher //index in the chronologically sorted queue).

//End Pseudo-Code Snippet.

As you can see, we enter multiple facts into the session for a single call
to FireAllRules(), and objectA contains a conflict because there are two
separate events relating to an update to the same object (objectA) and the
same field (integerA).

Does this seem like a reasonable or feasible approach? What would happen in
this scenario? Should we just try to work around it by pre-determining such

Which of the two is advised? Single Event per SKS fireAllRules() call or
Multiple Events per fireAllRules() call? Again, performance is important,
but we must temper such desires with our considerations of data-integrity :)

I REALLY REALLY Appreciate the help you've provided thus far.

Thanks for all the shoes,
Lucas (Skiddlebop)

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