Versioning Events Without Breaking Everything
Imagine a city library that has been collecting catalog cards for over a hundred years. In 1920, librarians recorded "Author" and "Title." In 1970, they added the ISBN. In 1990, "Author" became "Authors" (plural, to accommodate co-authors). In 2020, they introduced e-book formats and licensing information.
Here's the thing: the old cards are still there. You can't "update" a card from 1920. And yet, the modern library system must understand all of them, from the handwritten notes of a century ago to yesterday's digital acquisition.
Event sourcing faces the same challenge. Events are immutable facts. Once written, they stay forever. But requirements change, domains evolve, and mistakes get discovered. How do you version something that can't be changed?