系統設計

1interface Database {
2  read(key: string): string;                     // slow: ~10 ms
3}
4 
5class CacheAside {
6  // A Map remembers insertion order: first key = least recently used.
7  private cache = new Map<string, string>();
8  constructor(private capacity: number, private db: Database) {}
9 
10  get(key: string): string {
11    const cached = this.cache.get(key);
12    if (cached !== undefined) {
13      this.cache.delete(key);
14      this.cache.set(key, cached);
15      return cached;
16    }
17    const value = this.db.read(key);
18    if (this.cache.size >= this.capacity) {
19      const oldest = this.cache.keys().next().value!;
20      this.cache.delete(oldest);
21    }
22    this.cache.set(key, value);
23    return value;
24  }
25}