系統設計

1type Strategy = "round-robin" | "random" | "least-connections";
2 
3class LoadBalancer {
4  private active: number[];          // requests in flight, per server
5  private live: number[];            // servers passing health checks
6  private turn = 0;
7 
8  constructor(servers: number) {
9    this.active = new Array(servers).fill(0);
10    this.live = [...this.active.keys()];
11  }
12 
13  markDown(server: number): void {
14    this.live = this.live.filter((s) => s !== server);
15  }
16 
17  pick(strategy: Strategy): number {
18    const live = this.live;
19    let chosen: number;
20    if (strategy === "round-robin") {
21      chosen = live[this.turn % live.length];
22    } else if (strategy === "random") {
23      chosen = live[Math.floor(Math.random() * live.length)];
24    } else {
25      chosen = live[this.turn % live.length];
26      for (let k = 1; k < live.length; k++) {
27        const s = live[(this.turn + k) % live.length];
28        if (this.active[s] < this.active[chosen]) chosen = s;
29      }
30    }
31    this.turn += 1;
32    this.active[chosen] += 1;
33    return chosen;
34  }
35 
36  finish(server: number): void {
37    this.active[server] -= 1;
38  }
39}