构建通知扇出服务

项目概述

本项目实现一个面向 notification delivery 的完整系统，核心要解决的问题是 expanding a single event into per-channel deliveries without a slow channel blocking the fast ones。整体由 Event receiver、Channel router、Delivery queue、Rate per channel、Dead letter store 五个模块组成，每个模块单一职责、可独立测试，最后通过集成测试验证端到端正确性。设计上把 a slow email channel that backs up its queue and delays notifications that should have been instant on push 作为第一类需求先行处理，而非事后再补。

架构与关键决策

数据模型为 a fanout that turns one event into many deliveries, each queued per channel with its own rate limit and retry policy。Event receiver 定义其余阶段共同依赖的数据形状并在边界处校验输入；Channel router 承载 notification delivery 的正确性逻辑；Delivery queue 专门处理 a slow email channel that backs up its queue and delays notifications that should have been instant on push，在失败路径上给出明确的拒绝、重试或降级策略；Rate per channel 提供可观测性，每次操作输出带关联 id 的结构化事件；Dead letter store 在过载时按既定方式降级，避免过载演变为数据损坏。配置与代码分离，使风险变更可经 diff 审阅。

实施与测试

按五个阶段依次实现 Event receiver、Channel router、Delivery queue、Rate per channel、Dead letter store，每阶段配套聚焦单测。集成测试组合各模块并驱动真实 notification delivery 场景：先验证正常路径，再刻意触发 a slow email channel that backs up its queue and delays notifications that should have been instant on push 并断言系统降级而非损坏。所有依赖均在进程内或被桩替换，用例在 CI 中确定可复现。

限制与后续

• 单区域设计，多区域复制超出范围。
• Event receiver 默认进程内，未水平共享。
• 无鉴权层，假定部署在强制身份的边缘之后。

代码示例

# setup for notification-fanout-service
pnpm init
pnpm add fastify redis pino
pnpm add -D typescript tsx vitest

notification_fanout_service/
├── src/index.ts
├── src/event-receiver.ts
├── src/channel-router.ts
├── tests/notification_fanout_service.test.ts
└── package.json

// Event receiver: notification delivery core
export function eventreceiver(input: EventreceiverInput): EventreceiverResult {
  // Build the event receiver that anchors notification delivery and defines the core data shape.
  return processEventreceiver(input);
}

// Channel router — handles notification delivery Add the channel router so
export class Channelrouter {
  constructor(private opts: ChannelrouterOptions) {}
  async run(ctx: Context): Promise<ChannelrouterOutput> {
    return this.execute(ctx);
  }
}

// Delivery queue: notification delivery stage
export const deliveryqueue = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Rate per channel for notification delivery
export async function rateperchannel(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Dead letter store: notification delivery finalization
export function deadletterstore(state: State): Result {
  validate(state);
  return finalize(state);
}

# run the notification delivery test suite
pnpm vitest run