构建带 TTL 的 LRU 缓存

项目概述

本项目实现一个面向 bounded caching 的完整系统，核心要解决的问题是 evicting both the least recently used and the expired entries so the cache stays bounded and fresh。整体由 Hash map、Recency list、TTL wheel、Eviction policy、Stats counter 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 TTL wheel with too few buckets that expires entries early or late depending on clock alignment 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 hash map of entries plus a doubly linked list for recency order and a timing wheel for per-entry expiry。Hash map 定义其余阶段共同依赖的数据形状并在边界处校验输入；Recency list 承载 bounded caching 的正确性逻辑；TTL wheel 专门处理 a TTL wheel with too few buckets that expires entries early or late depending on clock alignment，在失败路径上给出明确的拒绝、重试或降级策略；Eviction policy 提供可观测性，每次操作输出带关联 id 的结构化事件；Stats counter 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 Hash map、Recency list、TTL wheel、Eviction policy、Stats counter，每阶段配套聚焦单测。集成测试组合各模块并驱动真实 bounded caching 场景：先验证正常路径，再刻意触发 a TTL wheel with too few buckets that expires entries early or late depending on clock alignment 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。
• Hash map 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 lru-cache-ttl
pnpm init
pnpm add lru-cache fastify
pnpm add -D typescript tsx vitest

lru_cache_ttl/
├── src/index.ts
├── src/hash-map.ts
├── src/recency-list.ts
├── tests/lru_cache_ttl.test.ts
└── package.json

// Hash map: bounded caching core
export function hashmap(input: HashmapInput): HashmapResult {
  // Build the hash map that anchors bounded caching and defines the core data shape.
  return processHashmap(input);
}

// Recency list — handles bounded caching Add the recency list so
export class Recencylist {
  constructor(private opts: RecencylistOptions) {}
  async run(ctx: Context): Promise<RecencylistOutput> {
    return this.execute(ctx);
  }
}

// TTL wheel: bounded caching stage
export const ttlwheel = (next: Next) => async (req: Request) => {
  const out = await transform(req);
  return next(out);
};

// Eviction policy for bounded caching
export async function evictionpolicy(stream: AsyncIterable<Chunk>): Promise<Aggregate> {
  let acc = init();
  for await (const c of stream) acc = merge(acc, c);
  return acc;
}

// Stats counter: bounded caching finalization
export function statscounter(state: State): Result {
  validate(state);
  return finalize(state);
}

# run the bounded caching test suite
pnpm vitest run