We can think as three broad kinds of code changes here: new features, defect fixes, and temporal logic.

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For internal stuff this is really easy to fix, all you need to do is make the fix and reprocess the events. Your application state is now fixed to what it should have been. For many situations this is really rather nice.

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The third case is where the logic itself changes over time, a rule along the lines of \"charge $10 before November 18 and $15 afterwords\". This kind of stuff needs to actually go into the domain model itself. The domain model should be able to run events at any time with the correct rules for the event processing. You can do this with conditional logic, but this will get messy if you have much temporal logic. The better route is to hook strategy objects into a Temporal Property: something likechargingRules.get(aDate).process(anEvent). Take a look at Agreement Dispatcher for this kind of style.

CQRS / EventSourcing