系統設計

1const LOCAL_SEC = 0.002;                               // the primary's own write
2 
3interface Replica {
4  // Ships a write; returns the time the replica will have applied it.
5  send(key: string, version: number, value: string, now: number): number;
6}
7 
8class Primary {
9  private version = 0;
10  private data = new Map<string, { version: number; value: string }>();
11 
12  // How many replicas must confirm before the client hears back:
13  // 0 = asynchronous, 1 = semi-synchronous, all = synchronous.
14  constructor(private replicas: Replica[], private waitFor: number) {}
15 
16  // Returns when the client is told the write succeeded.
17  write(key: string, value: string, now: number): number {
18    const version = ++this.version;
19    this.data.set(key, { version, value });
20    const applied = this.replicas.map((r) => r.send(key, version, value, now));
21    if (this.waitFor === 0) return now + LOCAL_SEC;
22    applied.sort((a, b) => a - b);
23    return Math.max(now + LOCAL_SEC, applied[this.waitFor - 1]);
24  }
25}
26 
27class ReadRouter {
28  private lastWrite = new Map<number, number>();
29  private next = 0;
30 
31  constructor(private replicaCount: number, private readYourWrites: boolean, private windowSec: number) {}
32 
33  wrote(user: number, now: number): void {
34    this.lastWrite.set(user, now);
35  }
36 
37  // 0 = the primary, 1..n = a replica.
38  route(user: number, now: number): number {
39    const last = this.lastWrite.get(user);
40    if (this.readYourWrites && last !== undefined && now - last < this.windowSec) {
41      return 0;
42    }
43    return 1 + (this.next++ % this.replicaCount);
44  }
45}