understanding agenda-group in drools

Question

I tried a sample example to see how agenda-group works. Initially I set the focus of ksession to agenda-group \"ag1\" and fired the rules.

package com.sample

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Answer 1

Agenda groups work like a stack. When you set the focus to a given agenda group, that group is placed on top of the stack. When the engine tries to fire the next activation and there are no more activations in a given group, that group is removed from the top of the stack and the group below it receives the focus again.

So it goes like this (main is the default group that is always present):

* STACK: [MAIN, ag1]

Hello Word fires and activates both "GoodBye" rules
GoodBye fires, activates both "Hello World" rules and sets the focus to "ag2"

* STACK: [MAIN, ag1, ag2]

Hellow World 2 fires, cancels the "Hello World 1" rule and activates both "GoodBye" rules
GoodBye 2 fires because ag2 has the focus

* There are no more activations in ag2 to fire, so ag2 is removed from the stack
* STACK: [MAIN, ag1]
* The "GoodBye" rule is still active in ag1, so it fires

GoodBye fires, activates both "Hello World" rules and sets the focus to "ag2"

* STACK: [MAIN, ag1, ag2]

Hellow World 2 fires, cancels the "Hello World 1" rule and activates both "GoodBye" rules
...

And the loop repeats.

This kind of behavior is very easy to see if you use the audit log in the Eclipse IDE.

Hope this helps.