Anna McDonald explains the concept of derivative event sourcing:
If you happen to be the proud owner of a single order service, then you are all set to begin.
But what if you have more than one order service?
Something that tends to happen at companies that have been around for more than a sprint is the accumulation of technical debt. Sometimes that debt takes the form of duplicate applications. Mergers happen and you adopt other applications that, for reasons beyond your control, cannot be retired or rewritten right away. In other words, sometimes you end up with more than one order service—enter derivative event sourcing!
This is a nice article for real-life scenarios where you don’t get to build nice, well-designed services from scratch.