系統設計

1// CP sketch: one shared committed value, assuming an established leader/log.
2// This shows the availability gate only; it is not a consensus protocol.
3// Independent instances do not replicate or become linearizable by this check.
4class MajorityReplica {
5  private value = 0;
6  constructor(private replicas: number) {}
7 
8  increment(reachable: number): number {
9    if (reachable * 2 <= this.replicas) throw new Error("unavailable");
10    this.value += 1;
11    return this.value;
12  }
13 
14  read(reachable: number): number {
15    if (reachable * 2 <= this.replicas) throw new Error("unavailable");
16    return this.value;
17  }
18}
19 
20// AP, last writer wins: one value and its timestamp per replica.
21type Versioned = { value: number; ts: number; site: string };
22 
23class LwwReplica {
24  state: Versioned = { value: 0, ts: -1, site: "" };
25  constructor(private site: string) {}
26 
27  increment(now: number): Versioned {
28    this.state = { value: this.state.value + 1, ts: now, site: this.site };
29    return this.state;
30  }
31 
32  merge(remote: Versioned): void {
33    const s = this.state;
34    if (remote.ts > s.ts || (remote.ts === s.ts && remote.site > s.site)) {
35      this.state = remote;
36    }
37  }
38}
39 
40// AP, CRDT: each replica counts only its own increments; merging keeps
41// the largest count seen per replica, and the value is the sum.
42class GCounter {
43  counts: Record<string, number> = {};
44  constructor(private site: string) {}
45 
46  increment(): void {
47    this.counts[this.site] = (this.counts[this.site] ?? 0) + 1;
48  }
49 
50  merge(remote: Record<string, number>): void {
51    for (const [site, n] of Object.entries(remote)) {
52      this.counts[site] = Math.max(this.counts[site] ?? 0, n);
53    }
54  }
55 
56  value(): number {
57    return Object.values(this.counts).reduce((a, b) => a + b, 0);
58  }
59}