构建 HTTP 缓存层

项目概述

本项目实现一个面向 http response caching 的完整系统，核心要解决的问题是 honoring cache-control and etag semantics so cached responses stay fresh without stale reads。整体由 Key normalizer、Freshness checker、Validation revalidate、Vary splitter、Purge endpoint 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 response with a missing vary header that returns one content type to a client that expected another 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 key-value store of responses keyed by method and normalized URL, with vary headers splitting entries per representation。Key normalizer 定义其余阶段共同依赖的数据形状并在边界处校验输入；Freshness checker 承载 http response caching 的正确性逻辑；Validation revalidate 专门处理 a response with a missing vary header that returns one content type to a client that expected another，在失败路径上给出明确的拒绝、重试或降级策略；Vary splitter 提供可观测性，每次操作输出带关联 id 的结构化事件；Purge endpoint 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 Key normalizer、Freshness checker、Validation revalidate、Vary splitter、Purge endpoint，每阶段配套聚焦单测。集成测试组合各模块并驱动真实 http response caching 场景：先验证正常路径，再刻意触发 a response with a missing vary header that returns one content type to a client that expected another 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。
• Key normalizer 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 http-cache-layer
pnpm init
pnpm add express lru-cache undici
pnpm add -D typescript tsx vitest

http_cache_layer/
├── src/index.ts
├── src/key-normalizer.ts
├── src/freshness-checker.ts
├── tests/http_cache_layer.test.ts
└── package.json

// Key normalizer: http response caching core
export function keynormalizer(input: KeynormalizerInput): KeynormalizerResult {
  // Build the key normalizer that anchors http response caching and defines the core data shape.
  return processKeynormalizer(input);
}

// Freshness checker — handles http response caching Add the freshness checker so
export class Freshnesschecker {
  constructor(private opts: FreshnesscheckerOptions) {}
  async run(ctx: Context): Promise<FreshnesscheckerOutput> {
    return this.execute(ctx);
  }
}

// Validation revalidate: http response caching stage
export const validationrevalidate = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Vary splitter for http response caching
export async function varysplitter(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Purge endpoint: http response caching finalization
export function purgeendpoint(state: State): Result {
  validate(state);
  return finalize(state);
}

# run the http response caching test suite
pnpm vitest run