Editing Distributed computing (section)

== Events vs. Messages ==
In distributed systems, [[Event (computing)|events]] represent a fact or state change (e.g., ''OrderPlaced'') and are typically broadcast asynchronously to multiple consumers, promoting loose coupling and scalability. While events generally don’t expect an immediate response, acknowledgment mechanisms are often implemented at the infrastructure level (e.g., Kafka commit offsets, SNS delivery statuses) rather than being an inherent part of the event pattern itself. <ref name=":1">{{Cite book |last=Kleppmann |first=Martin |title=Designing Data-Intensive Applications: The Big Ideas Behind Reliable, Scalable, and Maintainable Systems |date=2017 |publisher=O'Reilly Media |isbn=978-1449373320}}</ref><ref name=":2">{{Cite book |title=Building Event-Driven Microservices: Leveraging Organizational Data at Scale |isbn=978-1492057895}}</ref>

In contrast, [[Message|messages]] serve a broader role, encompassing commands (e.g., ''ProcessPayment''), events (e.g., ''PaymentProcessed''), and documents (e.g., ''DataPayload''). Both events and messages can support various delivery guarantees, including at-least-once, at-most-once, and exactly-once, depending on the technology stack and implementation. However, exactly-once delivery is often achieved through idempotency mechanisms rather than true, infrastructure-level exactly-once semantics. <ref name=":1" /><ref name=":2" />

Delivery patterns for both events and messages include publish/subscribe (one-to-many) and point-to-point (one-to-one). While request/reply is technically possible, it is more commonly associated with messaging patterns rather than pure event-driven systems. Events excel at state propagation and decoupled notifications, while messages are better suited for command execution, workflow orchestration, and explicit coordination. <ref name=":1" /><ref name=":2" />

Modern architectures commonly combine both approaches, leveraging events for distributed state change notifications and messages for targeted command execution and structured workflows based on specific timing, ordering, and delivery requirements. <ref name=":1" /><ref name=":2" />