系統設計

1class Replica {
2  version = 0;
3  value: string | null = null;
4  up = true;
5 
6  store(version: number, value: string): boolean {
7    if (!this.up) return false;                  // unreachable: no ack
8    if (version > this.version) {
9      this.version = version;
10      this.value = value;
11    }
12    return true;
13  }
14}
15 
16class QuorumClient {
17  constructor(private replicas: Replica[], private w: number, private r: number) {}
18 
19  // Sends to every replica; succeeds once w of them have stored it.
20  write(version: number, value: string): boolean {
21    let acks = 0;
22    for (const replica of this.replicas) {
23      if (replica.store(version, value)) acks++;
24    }
25    return acks >= this.w;
26  }
27 
28  // replies: the replicas in the order their answers arrive.
29  read(replies: Replica[]): { version: number; value: string | null } | null {
30    const answered = replies.filter((x) => x.up).slice(0, this.r);
31    if (answered.length < this.r) return null;
32    let newest = answered[0];
33    for (const x of answered) {
34      if (x.version > newest.version) newest = x;
35    }
36    for (const x of answered) {
37      if (x.version < newest.version) x.store(newest.version, newest.value!);
38    }
39    return { version: newest.version, value: newest.value };
40  }
41}