[rules-dev] Lazily Enabled Truth Maintenace (Project Propsoal)

[Mark Proctor]

  Here is another project proposal, this time simpler. I think this one 
has Wolfgang's name on it ;)

http://blog.athico.com/2010/09/lazily-enabled-truth-maintenace.html

Three weeks ago I posted the project idea for "Left and Right Unlinking" 
<http://blog.athico.com/2010/08/left-and-right-unlinking-community.html>. So 
far there are no takers, so if you are interested let me know :)

In the meantime I tried to think of a simpler enhancement that we would 
like to see done.

At the moment Drools has a user setting "MaintainTMSOption" which can be 
true or false. It's a small optimisation that when turned off avoids 
using the equality hashmap that is maintained for all inserted objects.

It would be a much better idea to remove this configuration setting, 
thus simplifying things for end users and have TMS lazily enabled on demand.

For each object type there is an "ObjectTypeConf" configuration object 
that is retrieved every time a working memory action, such as insert, is 
executed. The enabledTMS boolean should be moved there, so there is one 
per object type, by default it is false.

When a working memory action occurs, like insert, it retrieved the 
ObjectTypeConf and checks the maintainTms boolean there, instead of the 
current engine scoped configuration. When a logical insertion occurs and 
the ObjectTypeConf is retrieved if maintainTms is false it sets the 
value to true and then iterates the associated ObjectTypeNode memory 
lazily adding all the objects to the TMS equality map. From then on for 
that ObjectType all inserted objects are added to that equality map.

With this you now have the advantage of TMS being laziy enabled, so the 
minor hashmap operation is no longer used and likewise a small memory 
saving from not populating the map. There is a further advantage that 
this is now fine grained and when enabled only impacts for that specific 
object type.

A further enhancement could use a int counter, instead of a boolean. 
Each logical insertion for that object type increases the counter, each 
retraction decreases the counter; even if automatically retracted if the 
truth is broken for that logical assertion. When the counter reaches 
zero, TMS for that OTN can be disabled. We do not however remove the 
objects from the equality map, as this would cause "churn" if TMS is 
continuously enabled and disabled. Instead when TMS is disabled record 
the current fact counter id. Then if TMS is disabled on a retraction but 
there is a counter id, we can check that counter id to see if the fact 
is prior to TMS being disabled and thus would need to be retracted from 
the equality map.
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