Practical Event Sourcing in TypeScript

Most applications store the current state and overwrite it on every change. The history is gone. When something breaks, nobody can explain how the system got there.

Event sourcing takes a different approach. Instead of storing what is, you store what happened. Every action becomes an immutable fact in a log. State is derived by replaying those facts. You get a complete audit trail, time-travel queries, and the ability to rebuild any view of your data at any point.

This book teaches event sourcing, CQRS, hexagonal architecture, and domain-driven design through working TypeScript code—no abstract theory. Every concept is introduced by building something, starting with a simple counter and progressing to a full multiplayer Risk game with real-time state broadcast, AI opponents, and production deployment.

The framework used throughout is Act, an open-source event sourcing toolkit for TypeScript that handles state machines, invariants, projections, reactions, and the event store.

You will learn: Event sourcing from first principles; CQRS and read/write separation; business rules as composable invariants; hexagonal architecture with swappable adapters; domain-driven design applied to a real domain; real-time multiplayer with SSE; and production operations, including snapshots, time travel, and projection rebuilds.

Source code: github.com/rotorsoft/act-root

Included:

1) Incremental releases in PDF and EPUB formats, including all future chapters and updates.

2) The complete Risk game source code.