State Machines — Notes

Interview heuristic

When the prompt mentions a lifecycle ("order", "booking", "cabin"), draw the state diagram first, even before classes. It anchors invariants for the rest of the design.

Common bugs

• Forgetting to handle the "rollback" path from a successful state (refund after shipped).
• Two events racing toward the same state — pin both to optimistic concurrency.
• Implicit transitions hidden in timers / TTLs — make them explicit ("idle → expired after 10 min").

Libraries

• xstate (JS/TS) — visualisable, statecharts.
• Stateless (.NET / Java port).
• Spring StateMachine.
• Akka FSM (Scala / Java).
• Temporal / Cadence — durable workflow state machines.

Refs

• David Harel: "Statecharts: A Visual Formalism" (1987).
• Miro Samek: Practical UML Statecharts in C/C++.
• xstate docs.
• "Designing Reactive Systems with Akka FSM" talks.