Well, when I read about "commands" modifying the model and stored in a log for future replay it sounds a lot like Prevayler with the luxury of automatic "transaction command object" creation based on parameters extracted via a concern.
With Prevayler, you are supposed to create snapshots of your model in order to avoid replaying all the commands. Those snapshots could be considered a cache(command(t)) of your model to optimise replaying at t+1. One question is :if your model data is the result of applying a queue of commands, how do you want to store it? When you think about model migration, storing all events since the beginning looks efficient since you only have to modify the command's code (and BTW you get a historic model for the same price) and replay everything. When you apply a command, you probably need to know the last value of other entities, supposedly stored in some kind of snapshot. So when you think about efficiency, you need direct access to the last value, the snapshot. Now, what about an "entityStore" containing a queue of events mapped to an entity identity ? What about calculating the actual model's value "on the fly" applying only a specific queue of commands (BTW commands must obviously be deterministic). You actually extract a reverse tree of commands to apply to get the final entity value. The snapshot is only an "optimisation" or a cache of the last entity calculation. When you want to migrate your commands, you simply clear the cache, you don't replay anything "in advance". One problem I had with Prevayler is "snapshot generation". It takes a lot of resources (O(model's value size)). I partly solved it by partitioning my model in smaller snapshots. But I still had a problem with "dead" old data... like last year "order lines", they were still present in the snapshot ! So I had to remove "old values", and it was a mess. This seems to be solved here, I can let complete branches of the "reverse tree of commands" go to sleep gracefully because I will never traverse those branches anymore. The added value here is that I could still migrate those "dead data" to a new version of commands since I will only reapply those commands when I really need to 'calculate' the latest value of my model's entities... Anyone still following me ? I will try to resume. If you think of an entity as a result of the application of a queue of commands using other entities you should be able to create a historical representation of your complete model on the fly, the snapshot. But if you don't consider the snapshot as a whole, but as a map of entity values, you should be able to find out the entity last value by replaying a smaller queue of commands (and cache it for future use). Is it clearer or helpful ? Cheers, Philippe David Leangen-19 wrote: > > >> What about using Prevayler ? >> http://www.prevayler.org/wiki/ > > For what? I'm not sure I understand why you are proposing this. > > Recently, I have been using Prevayler a lot. It's great for small > projects that can be kept in memory. I don't see the connection with > this thread, though... > > Can you clarify? > > > Cheers, > =David > > > > _______________________________________________ > qi4j-dev mailing list > [email protected] > http://lists.ops4j.org/mailman/listinfo/qi4j-dev > > -- View this message in context: http://www.nabble.com/Migration-support%2C-Event-Feeds%2C-and-others...-tp24602284p24622918.html Sent from the Qi4j-dev mailing list archive at Nabble.com. _______________________________________________ qi4j-dev mailing list [email protected] http://lists.ops4j.org/mailman/listinfo/qi4j-dev
