Question

I wathced a screen cast on DDD by Greg Young the other day which spoke about persisting all state transitions of an object, instead of it s state when saved, then to load it "replay" all these messages to get the current state back.. This seemed like a really interesting idea, but I m stuck as to what this particular thing is called! I d like to read more about it, but I m having trouble getting any decent results without a real name for it.

Can anyone enlighten me?

screen cast is @ http://www.infoq.com/presentations/greg-young-unshackle-qcon08

Answer 1

The comment by Michael Perry calls it Historic Modeling.

He also says that he has "defined a set of rules and a framework for Historic Modeling:" and provides a couple links: (I don t have enough rep to post them both)

http://correspondence.codeplex.com/

I haven t personally studied or used state transition models, but it does sound interesting.

Answer 2

Martin Fowler calls this Event Sourcing. Greg Young even references Fowler in his series on Distributed Domain Driven Design, in which he clarifies some details regarding the presentation; you might want to read it.

Answer 3

Slightly off on a tangent... Rocky Lhotka has implemented something he calls n-level undo (elsewhere referred to as Multilevel Undo) in his CSLA Business Object framework but this only applies to history of changes in memory and (that I m aware of) doesn t persist down into the DB.

DDD folks tend not to think very highly of CSLA but some of the ideas and implementations contained therein are definitely worth looking into.

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