Hmm, let see if I can convert my thought into words :). 

To persist is to update or insert automatically, depending on if the ID 
already exists in the datastore for the object. So persist method figures 
out insert or update. 

You can do things after commit, but you won't be able to roll back. You 
don't have to use a transaction, but a transaction will keep track what 
happens, so if a fail happens at the last moment, you can roll back to the 
original record/object that exist before you started the transaction. You 
don't have to persist in a transaction, but if you do, you have the option 
to roll back on a failure.

Hope that helps?
Brandon Donnelson
http://gwt-examples.googlecode.com

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