[rules-dev] StatefulSession Marshalling

[Mark Proctor]

New code has been added for a higly efficient binary protocol 
marshalling for stateful sessions. While Drools 4 supported standard 
serialisation of Working Memories, it used the default serialisation, 
this is both very slow and memory bloating. It also has the problem that 
it just serialises the user objects with it.

The Marshalling implementation allows for a strategy approach when 
handling the marshalling of user objects. You register different 
strategies for different namespaces. Currently we have two strategies, 
Serialised and Identity. Serialised just writes the user object to the 
stream via standard read/write methods. Identity assigns the user object 
an id, from an int counter, and places it in a map and writes the id to 
the stream. Reading in uses that map to lookup the user object. Identity 
strategy is thus stateful, write method can only be called once read can 
be called repeatedly. Other strategies will be added, such as hibernate 
Placeholder support. Users can of course write there own.

So what does this give?
-Session pause/resume, ideal for moving a session to a new server or 
restarting an existing server.
-General snapshotting, backup. You could do a poor man's high 
availability by snapshotting the session at the end of each transaction. 
Later we will be extending this binary protocol to support journalling 
and also replication via clustering.
-Pre-loaded session templates. If you repeatedly create stateful 
sessions with the same base set of data and the insertion time is 
annoying, now you can just populate the session once and write out to a 
byte[] and use that as a template for further session creations.
-Long term persistence, needed for our process integration work.

You can see the MarshallingTest to look at the integration tests in 
place at the moment.
http://fisheye.jboss.org/browse/JBossRules/trunk/drools-compiler/src/test/java/org/drools/integrationtests/MarshallingTest.java?r=HEAD

In integrationtests SerializationHelper you can see the help methods 
there demonstrating how it all works.
http://fisheye.jboss.org/browse/JBossRules/trunk/drools-compiler/src/test/java/org/drools/integrationtests/SerializationHelper.java?r=HEAD

The DefaultMarshalling uses the Identity strategy if it is not passed a 
strategy factory. The Identity strategy is used throughout the tests.
http://fisheye.jboss.org/browse/JBossRules/trunk/drools-core/src/main/java/org/drools/marshalling/DefaultMarshaller.java?r=HEAD

The read/write marshalling is maintained in two files ReadMarshaller and 
WriteMarshaller, this will allow better handling of backwards 
compatiblity in the future.
http://fisheye.jboss.org/browse/JBossRules/trunk/drools-core/src/main/java/org/drools/marshalling/OutputMarshaller.java?r=HEAD
http://fisheye.jboss.org/browse/JBossRules/trunk/drools-core/src/main/java/org/drools/marshalling/InputMarshaller.java?r=HEAD

It's still very alphaware, but please do start trying it. I need to tidy 
up the debug output and will do that soon, I'll probably make each 
read/write method take a String which will be written to the log when 
debug is on. It currently sorts a number of map/set data structures, 
this is for debugging so that testing can round trip for data loss, I 
will make sure that it can be disabled for production. The following 
types don't yet marshall, not sure how to handle date/time stuff at the 
moment:
Temporal Sessions do not marshall
Scheduled rules, duration attribution do not marhsall

Implementations have not been added for accumulate, collect, from and 
rightinputadapter.

We are just hooking up the process marshalling into this at the moment.

Mark