[jboss-dev-forums] [Design of JBoss jBPM] - batch-size of jbpm runtime objects

[tom.baeyens@jboss.com]

recently someone queried us about the batch size of the jbpm runtime objects and why they were not set to a proper default.  based on Max's input on what batch-size on the class level actually is, i think that jBPM can't really define a good batch-size except the default for 1.

"Max" wrote : 
  | > | Is there a reason why batch-size isn't set to a reasonable default 
  | > | on the various jBPM runtime objects?
  | >
  | > would that make sense ?
  | 
  | depends.
  | 
  | Let me see if i can explain this via pure email (i normally do fancy drawings and gestures to explain the voodoo-magic)
  | 
  | Basically/naivelly speaking batch-size defaults to 1.
  | 
  | Meaning that when Hibernate is asked to get some object by id  from the db, e.g 42 it will try and load this single object by id 42.
  | 
  | By increasing this on the class level to eg. <class name="Blah"  
  | batch-size="5" ..>
  | then instead of just loading the one(1) matching the given id(42) hibernate will look in its cache and see if there is up-to 5 more non-initialized proxies that can be fetched. If there is hibernate will do something like "select ... from Blah where id in (42, 32, 4, 3, 17)"
  | instead of just id = 42. Hence optimally a reduction in requests to the db of factor 5.
  | 
  | So, batch-size > 1 on classes make sense if you often have a bunch of un-initialized objects in the session that it would be relevant to get in batches of selects instead of single selects.
  | 
  | The above is for class instances; but the same is valid for collections.  
  | They too
  | have a batch-size option. This is not about getting a 20 element collection in e.g. 4 selects it is about loading multiple complete collections in fewer selects.
  | 
  | The same logic as above applies. e.g. blah.getChildren() is internally viewed as "get the children collection for blah with id 42" (assuming blah's id is
  | 42 of course).
  | Here the default is to get that one collection, but alternatively Hibernate can look around and see if there are more uninitialized *collections* of the same role (Blah.children) and load those in one go. Again, this can be a very efficient optimization to e.g. load hiearchies of entities.
  | 
  | There are alternatives such as fetch="subselect" which instead of using in
  | (x,y,z) creates
  | a sub query select based on what you just fetched. This requires a bit more drawings for me to explain (and i actually did that when you here - remember ? :)
  | 
  | >  should i have a look at what that is ?
  | 
  | yes.
  | 

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