distributed-systems

To prevent duplicate processing and lost messages in an event-driven distributed system, implement a transactional outbox pattern for reliable event relay and use idempotent consumers with deduplication logic to safely handle at-least-once delivery semantics.

The saga pattern coordinates multi-service transactions by defining compensating steps for each workflow action. You must choose between orchestration, using a central controller, or choreography, using event-driven reactions, to manage distributed state and rollback logic reliably.

Implement CQRS with event sourcing by persisting state as an immutable event store and generating separate read models. Ensure your projections are rebuildable by designing idempotent consumers that can replay events to reconstruct read model state without data corruption.

Use orchestration for complex distributed workflows requiring explicit state control and compensating transaction visibility, and choose choreography for decentralized event-driven systems where independent services react to events without a central coordinator adding coupling.

Reliable message queues require verification gates including correlation IDs for end-to-end tracing, retry limits with exponential backoff, circuit breakers to prevent cascading failures, dead-letter queue handling, and monitored projection rebuildability to ensure consistent state.