系統設計

1type Rendition = { name: string; kbps: number };
2type Video = { status: string; ready: Rendition[] };
3 
4const LADDER: Rendition[] = [ // lowest first, so a video is playable soonest
5  { name: "360p", kbps: 600 }, { name: "480p", kbps: 1000 },
6  { name: "720p", kbps: 2500 }, { name: "1080p", kbps: 5000 },
7];
8 
9class VideoService {
10  private meta = new Map<string, Video>();          // metadata database
11  private jobs: { id: string; r: Rendition }[] = []; // transcoding queue
12  private store = new Map<string, number>();        // object storage: key -> bytes
13  private edge = new Map<string, number>();         // CDN edge, oldest first
14 
15  constructor(private edgeCapacity: number) {}
16 
17  requestUpload(id: string): string {
18    this.meta.set(id, { status: "uploading", ready: [] });
19    return `https://storage.example/raw/${id}?signature=...`;
20  }
21 
22  onUploaded(id: string): void {
23    this.meta.get(id)!.status = "processing";
24    for (const r of LADDER) this.jobs.push({ id, r });
25  }
26 
27  runNextJob(seconds: number): void {
28    const { id, r } = this.jobs.shift()!;
29    this.store.set(`v/${id}/${r.name}`, seconds * r.kbps * 125);
30    const video = this.meta.get(id)!;
31    video.ready.push(r);
32    if (video.ready.length === LADDER.length) video.status = "ready";
33  }
34 
35  playback(id: string, bandwidthKbps: number): string {
36    const ready = this.meta.get(id)!.ready;
37    let pick = ready[0];
38    for (const r of ready) if (r.kbps <= bandwidthKbps * 0.8) pick = r;
39    return `v/${id}/${pick.name}`;
40  }
41 
42  fetch(key: string): "hit" | "miss" {
43    const cached = this.edge.get(key);
44    if (cached !== undefined) {
45      this.edge.delete(key);
46      this.edge.set(key, cached);
47      return "hit";
48    }
49    const bytes = this.store.get(key)!;
50    this.edge.set(key, bytes);
51    if (this.edge.size > this.edgeCapacity) {
52      this.edge.delete(this.edge.keys().next().value!);
53    }
54    return "miss";
55  }
56}