构建带健康检查的负载均衡器

项目概述

本项目实现一个面向 traffic distribution 的完整系统，核心要解决的问题是 spreading requests across upstreams and ejecting unhealthy ones before clients see failures。整体由 Upstream pool、Active health probe、Passive failure counter、Ejection policy、Pick algorithm 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 health check that passes against a stub endpoint while real requests fail, hiding a degraded upstream 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 list of upstreams each carrying a health state and a rolling failure count, with a picker that skips ejected endpoints。Upstream pool 定义其余阶段共同依赖的数据形状并在边界处校验输入；Active health probe 承载 traffic distribution 的正确性逻辑；Passive failure counter 专门处理 a health check that passes against a stub endpoint while real requests fail, hiding a degraded upstream，在失败路径上给出明确的拒绝、重试或降级策略；Ejection policy 提供可观测性，每次操作输出带关联 id 的结构化事件；Pick algorithm 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 Upstream pool、Active health probe、Passive failure counter、Ejection policy、Pick algorithm，每阶段配套聚焦单测。集成测试组合各模块并驱动真实 traffic distribution 场景：先验证正常路径，再刻意触发 a health check that passes against a stub endpoint while real requests fail, hiding a degraded upstream 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。
• Upstream pool 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 load-balancer-health
pnpm init
pnpm add fastify undici
pnpm add -D typescript tsx vitest

load_balancer_health/
├── src/index.ts
├── src/upstream-pool.ts
├── src/active-health-probe.ts
├── tests/load_balancer_health.test.ts
└── package.json

// Upstream pool: traffic distribution core
export function upstreampool(input: UpstreampoolInput): UpstreampoolResult {
  // Build the upstream pool that anchors traffic distribution and defines the core data shape.
  return processUpstreampool(input);
}

// Active health probe — handles traffic distribution Add the active health probe
export class Activehealthprobe {
  constructor(private opts: ActivehealthprobeOptions) {}
  async run(ctx: Context): Promise<ActivehealthprobeOutput> {
    return this.execute(ctx);
  }
}

// Passive failure counter: traffic distribution stage
export const passivefailurecounter = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Ejection policy for traffic distribution
export async function ejectionpolicy(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Pick algorithm: traffic distribution finalization
export function pickalgorithm(state: State): Result {
  validate(state);
  return finalize(state);
}

# run the traffic distribution test suite
pnpm vitest run