Event Sourcing: How Storing History Instead of State Changes System Design

Event Sourcing is an architectural pattern that stores every change to a system as an immutable event, rather than saving only the current state as traditional databases do. Each event is recorded in an append-only event store with metadata such as type, payload, timestamp, and version, making the full history permanently available. The current state is derived by replaying these events in sequence, while separate read-optimized views called projections can be built or rebuilt retroactively from the full event history. The pattern pairs naturally with CQRS, separating write and read paths, and is especially valuable in domains like finance, healthcare, and legal systems where a complete audit trail is a business requirement. Key trade-offs include increased conceptual complexity, eventual consistency in projections, the need for event schema versioning, and significant long-term storage growth.