系統設計

1const BASE_MS = 500, CAP_MS = 16_000;
2 
3type Delivery = { via: "socket"; gateway: string } | { via: "push-notification" };
4 
5class ConnectionRegistry {
6  private byUser = new Map<string, string>();          // user -> gateway
7  private byGateway = new Map<string, Set<string>>();  // gateway -> users
8 
9  connect(user: string, gateway: string): void {
10    this.disconnect(user);
11    this.byUser.set(user, gateway);
12    const users = this.byGateway.get(gateway) ?? new Set<string>();
13    users.add(user);
14    this.byGateway.set(gateway, users);
15  }
16 
17  disconnect(user: string): void {
18    const gateway = this.byUser.get(user);
19    if (gateway === undefined) return;
20    this.byUser.delete(user);
21    this.byGateway.get(gateway)?.delete(user);
22  }
23 
24  send(user: string): Delivery {
25    const gateway = this.byUser.get(user);
26    if (gateway === undefined) return { via: "push-notification" };
27    return { via: "socket", gateway };
28  }
29 
30  // A gateway died: its users must reconnect somewhere else.
31  gatewayDown(gateway: string): string[] {
32    const users = [...(this.byGateway.get(gateway) ?? [])].sort();
33    for (const user of users) this.byUser.delete(user);
34    this.byGateway.delete(gateway);
35    return users;
36  }
37}
38 
39// How long a client waits before reconnect attempt number `attempt`.
40function backoffMs(attempt: number, random: number | null): number {
41  const ceiling = Math.min(CAP_MS, BASE_MS * 2 ** attempt);
42  if (random === null) return ceiling;
43  return random * ceiling;  // full jitter: anywhere below the ceiling
44}