Microservices Communication Patterns (Saga, Circuit Breaker) · 课时

构建幂等事件消费者

使用幂等消费者、收件箱模式和发件箱模式，让协同式 Saga 的参与者能够安全应对重复事件和乱序事件。

构建幂等事件消费者 是 CoddyKit 上的免费 Microservices Communication Patterns (Saga, Circuit Breaker) 课时。这是第 4 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Microservices Communication Patterns (Saga, Circuit Breaker) 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Microservices Communication Patterns (Saga, Circuit Breaker) 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

Events Get Redelivered

In a choreography saga, services react to events from a broker. Brokers deliver at least once, so the same event can arrive twice — and sometimes out of order. Naive handlers double-charge or double-ship.

The Goal: Idempotent Consumers

An idempotent consumer can process the same event repeatedly with no extra effect. This is essential for correctness in any event-driven saga.

Track Processed Event Ids

Give every event a unique id. Before handling, check whether you have already processed that id; if so, skip and acknowledge.

if (inbox.exists(event.id())) { ack(); return; }
handle(event);
inbox.save(event.id());
ack();

The Inbox Pattern

The inbox pattern stores processed event ids in a table inside the same database transaction as the business change. Insert + work commit together, so a redelivery is rejected by the unique constraint.

BEGIN;
  INSERT INTO inbox(event_id) VALUES (?); -- unique
  UPDATE orders SET status = 'PAID' WHERE id = ?;
COMMIT;

Why Atomicity Is Key

If you ack the event but crash before committing the business change, the work is lost. Doing both in one transaction guarantees they succeed or fail together.

The Dual-Write Problem

A participant must often update its DB and publish a new event. Doing these as two separate calls risks one succeeding and the other failing — the classic dual-write problem.

The Outbox Pattern

The outbox pattern solves it: write the outgoing event into an outbox table in the same transaction as the business change. A separate relay publishes from the outbox to the broker.

BEGIN;
  UPDATE orders SET status = 'SHIPPED' WHERE id = ?;
  INSERT INTO outbox(payload) VALUES (?);
COMMIT;

Relaying the Outbox

A poller or change-data-capture (CDC) tool like Debezium reads new outbox rows and publishes them, then marks them sent. Publishing is at-least-once, so downstream consumers must be idempotent too.

Handling Out-of-Order Events

Events may arrive out of order. Use version numbers or sequence ids and ignore stale events, or design handlers that converge to the correct state regardless of order.

if (event.version() <= current.version()) return; // stale

Compensations Must Be Idempotent Too

When a saga fails, compensating events also flow through the broker and can be redelivered. A compensation (e.g. refund) must itself be idempotent, or you refund twice.

Putting It Together

A robust choreography participant: consume via the inbox (dedup), do work and write the outbox in one transaction, relay the outbox to the broker, and make every handler — including compensations — idempotent.

Quick Check

Test your event-consumer knowledge.

Recap

You hardened choreography consumers:

Brokers deliver at-least-once, so events repeat and may reorder
Idempotent consumers process duplicates safely
The inbox pattern dedups within the business transaction
The outbox pattern solves the dual-write problem; a relay/CDC publishes it
Handle out-of-order events and make compensations idempotent too

常见问题解答

「构建幂等事件消费者」课时是免费的吗？

是的 — 「构建幂等事件消费者」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Microservices Communication Patterns (Saga, Circuit Breaker) 课程的其余内容，请升级到 CoddyKit PRO。 Microservices Communication Patterns (Saga, Circuit Breaker) 课程共包含 4 节课。

「构建幂等事件消费者」这节课中我会学到什么？

使用幂等消费者、收件箱模式和发件箱模式，让协同式 Saga 的参与者能够安全应对重复事件和乱序事件。你通过在浏览器中直接运行的动手代码来练习 Microservices Communication Patterns (Saga, Circuit Breaker)，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Microservices Communication Patterns (Saga, Circuit Breaker) 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Microservices Communication Patterns (Saga, Circuit Breaker) 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 4 节课，共 4 节。

「构建幂等事件消费者」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Microservices Communication Patterns (Saga, Circuit Breaker) 课中编写并运行代码吗？

能。每节 Microservices Communication Patterns (Saga, Circuit Breaker) 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 Microservices Communication Patterns (Saga, Circuit Breaker)