系統設計

1interface Origin {
2  fetch(key: string): string;                    // far away: 30-185 ms
3}
4 
5class EdgeCache {
6  // A Map keeps insertion order: the first key is the least recently used.
7  private cache = new Map<string, { body: string; fetchedAt: number }>();
8 
9  constructor(private capacity: number, private ttlSec: number, private origin: Origin) {}
10 
11  get(key: string, now: number): { body: string; hit: boolean } {
12    const entry = this.cache.get(key);
13    if (entry && now - entry.fetchedAt < this.ttlSec) {
14      this.cache.delete(key);
15      this.cache.set(key, entry);
16      return { body: entry.body, hit: true };
17    }
18    if (entry) this.cache.delete(key);
19    const body = this.origin.fetch(key);
20    if (this.cache.size >= this.capacity) {
21      this.cache.delete(this.cache.keys().next().value!);
22    }
23    this.cache.set(key, { body, fetchedAt: now });
24    return { body, hit: false };
25  }
26 
27  // Called on every edge when the origin changes an object.
28  purge(key: string): void {
29    this.cache.delete(key);
30  }
31}