Three levels of filtering

[David Hull <dmh@tibco.com>]

However you choose to denote the controlling expression, WSN currently 
provides for three levels of message filtering: Precondition, Topic and 
Selector.

Topic has special status in that we define one possible syntax for it, 
namely WS-Topics, and in that we allow a NotificationProducer to 
advertise which topics it supports and whether this set may vary over time.

The other two, as I understand it, are to be applied before and after 
topic filtering.  That is, a notification that does not meet the 
precondition is (conceptually) never even produced, while a message that 
does not meet the selector is (conceptually) produced, sent on the 
appropriate topic, but never delivered to the consumer.

I say "conceptually" because it's not clear to me how anyone could tell 
the difference.  Precondition is not a global property.  If I make one 
subscription with precondition "only on even-numbered Thursdays" and 
another that says "always", the notification will always need to be 
produced.  Conversely, if there is only one active subscription on a 
topic and the selector is never met, there is never any need to produce 
the notification.

Actually, I believe I've heard a slightly different interpretation of 
Precondition, Topic and Selector.  Like the previous one, it isn't 
explicit in the WS-BN spec.  I believe it came out of the Chicago F2F, 
but it may well be documented explicitly elsewhere:

Precondition can be based on anything known to the producer.  Topic 
information is carried on the message envelope, outside the payload 
proper.  Selector operates on the message content (perhaps including the 
topic?).  So for example, the precondition can be "only if it's raining 
in Topeka" even if the message itself carries no weather information.  
Strictly speaking, topic filtering could be subsumed by selector 
filtering, but calling it out explicitly allows for easier optimization 
and a better fit with existing messaging paradigms.

The larger point here is that there is a plausible argument from Occam's 
razor that "filtering is filtering."  I'm not saying I buy that, but I'd 
like a clear and convincing counterargument to explain why we have three 
different levels.  As it stands, I believe there is a plausible argument 
for combining either Topic and Selector (both depend on the message 
context) or Precondition and Selector (Topic, with its special status, 
vs. everything else).