Programming Tutorials

  You have successfully implemented Event Sourcing using the Axon Framework. Your event store is immutable, capturing every state change your application has ever seen. It works perfectly until your domain logic evolves. You add a new mandatory field to an existing event class. You deploy the new version. Suddenly, your replay fails, or your projection groups stall. The logs are flooded with serialization exceptions: com.fasterxml.jackson.databind.exc.MismatchedInputException: Missing required creator property 'currency' This is the "Schema Evolution" problem. Since you cannot modify the historical events in your append-only store, you must bridge the gap between the old data structure and your new Java class definition. The solution is not to relax your serialization rules, but to implement  Upcasters . The Root Cause: The Immutability Paradox In a CRUD-based system, schema changes are handled via database migration scripts (e.g., Flyway or Liquibase). You simply run...