Workflow Engine Design

Build the engine layer — not individual idempotent operations (see idempotent-financial-workflows), but the runtime that sequences them durably.

Orchestration vs Choreography

• Orchestration: central coordinator holds explicit state; easier to observe, debug, and audit. Use for multi-step money flows and compliance-sensitive sequences.
• Choreography: services react to events on a shared bus; decoupled, harder to trace. Use for loose fan-out notifications.
• Hybrid: orchestrator per bounded context, events between contexts.

State Machine Design

• Name every state and every valid transition before writing code.
• Store state durably before executing side effects — never derive current state from side-effect logs.
• Terminal states: completed, failed, cancelled, compensated.
• Persist the full input/output of each step; a replay must reproduce the same decisions without re-running external calls.

Durable Execution Primitives