[rules-dev] RFC: stream concept
Mark Proctor
mproctor at codehaus.org
Wed Jul 27 10:26:47 EDT 2011
On 27/07/2011 14:13, Edson Tirelli wrote:
>
> Hi Wolfgang,
>
> Thanks for the feedback. My comments.
>
> 2011/7/27 Wolfgang Laun <wolfgang.laun at gmail.com
> <mailto:wolfgang.laun at gmail.com>>
>
> On 26 July 2011 21:42, Edson Tirelli <ed.tirelli at gmail.com
> <mailto:ed.tirelli at gmail.com>> wrote:
>
>
> Hi all,
>
> As you all know, we use "entry-points" to represent streams
> in Drools, but in fact, entry points are a much more general
> abstraction than just streams. For instance, they create
> partitions in the RETE's alpha network, they support all Rete
> concepts like truth maintenance, they share a centralized fact
> handle factory (that is possibly a contention point), etc.
>
> Event Streams in general can have a much more specialized
> implementation. For instance, as events are immutable,
>
>
> "Immutable" certainly not by a Drools definition? Something with
> @role(event) is just a fact, and I can modify an event as required
> by my application.
>
>
> Drools (and most CEP products) do not enforce immutability because
> we are dealing with "representations of events" and not the events
> themselves. Representation of events are usually subject to data
> enrichment and so it would not be good to enforce immutability at the
> engine level. OTOH, the documentation states (not only for Drools)
> that event representations should never be modified (other than data
> enrichment) since they represent something that happened and the past
> can't be changed. In other words, a voice call event in a telecom
> system could be enriched by rules by adding customer name for a given
> calling number, but the calling number from the event should never
> change, nor the timestamp for when the call was placed, etc. (from now
> on I will continue to refer to "representations of events" just as
> "events").
>
> In such scenarios, TMS for events become just an overhead.
TMS itself is now disabled by default and only enabled per object type,
per entry point. However there is still some cost involved, but it's
minimal.
ObjectTypeConf typeConf = this.typeConfReg.getObjectTypeConf(
this.entryPoint, object );
if ( logical && !typeConf.isTMSEnabled()) {
enableTMS(object, typeConf);
}
// check if the object already exists in the WM
handle = (InternalFactHandle) this.objectStore.getHandleForObject( object );
So this really is quite minimal. We need to get the typeConf anyway I
think as it contains things like the cache of OTNs. If it's not a
logical insertion and TMS is not enabled we do nothing. I suspecct in
that situation even the FactHandle itself can be delayed until it
reaches the beta network.
In short TMS overhead should not be a consideration in design choices
for streams.
>
> there is no need for truth maintenance;
>
>
> A fact derived from events could be managed by the TMS - why not?
> Or do you mean s.th <http://s.th>. else?
>
>
> A fact derived from events exclusively would be an event and again
> immutable. If the justification is immutable, there would be no need
> for TMS. On the other hand, if a justification contain non-events as
> part of it, it could be mutable, and in this case the justified fact
> should not be modeled as an event. If it is not an event, TMS should
> apply. The good thing about we discussing this is that it made me
> realize that some users might not be so strict when modeling their
> systems, and maybe instead of disabling TMS for "streams of events",
> this should be a per object type configuration?
This is already done and automatic, done by leo. You don't pay for TMS
unless you logically insert something.
>
> Because of things like the ones above, I was considering
> creating an explicit concept for streams in Drools. They would
> be a first class concept in the engine and would be handled
> appropriately. They would be orthogonal to entry-points, and
> be used exclusively for events.
>
>
> It's difficult (for me) to see what a "stream" should imply. Can
> you provide a concise definition? A "stream" should be for role
> event only, and its facts should be immutable (?) - what else?
>
> Are there any good use cases for these "streams" that cannot be
> readily dealt with using events?
>
>
> Streams, from a user's perspective, are just a sequence of events that
> are channeled into the system, exactly like entry-points, but
> specialized for events. I am not convinced we should create this
> concept nor am I convinced that we should continue using entry points.
> The use of streams would only make explicit to users that they are
> exclusive for events (you can use any type of facts in entry-points)
> and the algorithms behind the scene are exclusive for events (entry
> points, if used like that, would select algorithms based on
> heuristics, transparently to users).
My personally opinion is much of what you are suggesting would be good
design for entrypoints. Lazy TMS (already done), delayed FactHandle
creation util beta node (great idea). I suspect we can do a little
refactoring of EntryPoints to push more towards delayed execution at the
Beta level if we look into it. As the FactHandle has a reference to the
EntryPoint that it belongs to, having a FactHandleFactory per EntryPoint
shouldn't be a problem. This is actually realted to Wolfgang's previous
email about facthandle behaviour when used on the correct/incorrect
entry point.
On a separate issue it might be we prefer "streams" as a conceptual
level to allow users to understand different use cases. i.e. a stream
might be specific to a given re-usable named window. Using the term
"entry point" there might field a bit wierd. But I'd rather sort out the
technical implementation first, figure out the use cases and see whether
then users need the additional representation.
Mark
>
>
> The other option that we have would be to continue to hide
> the implementation behind the concept of entry-points and to
> select algorithm details by compile time analysis. Although it
> seems like a simpler solution, it has the potential of
> confusing users
>
>
> Most of the time, the confusion of Drools users stems from the
> lack of clear documentation - not from the complexitiy of features.
>
>
> Touché! :)
>
>
> --
> Edson Tirelli
> JBoss Drools Core Development
> JBoss by Red Hat @ www.jboss.com <http://www.jboss.com>
>
>
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> rules-dev at lists.jboss.org
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