Bounded caches (LRU)

LRU Map pattern

export function createLru(max = 500) {
  const map = new Map();

  return {
    get(key) {
      if (!map.has(key)) return undefined;
      const val = map.get(key);
      map.delete(key);
      map.set(key, val);
      return val;
    },
    set(key, val) {
      if (map.has(key)) map.delete(key);
      map.set(key, val);
      if (map.size > max) {
        map.delete(map.keys().next().value);
      }
    },
    get size() {
      return map.size;
    },
  };
}

TTL layer

function setWithTtl(cache, key, val, ttlMs) {
  cache.set(key, { val, exp: Date.now() + ttlMs });
}

function getWithTtl(cache, key) {
  const entry = cache.get(key);
  if (!entry) return undefined;
  if (Date.now() > entry.exp) {
    cache.delete(key);
    return undefined;
  }
  return entry.val;
}

Libraries

lru-cache (npm) — battle-tested for Node
Redis / Memcached — process-external bounds

flowchart LR
  req[Request] --> cache{In cache?}
  cache -->|yes| hit[Return cached]
  cache -->|no| miss[Compute]
  miss --> evict[Evict oldest if full]
  evict --> store[Store]