系統設計

1interface Step { name: string; run(): boolean; compensate(): void }
2 
3function runSaga(steps: Step[], log: string[]): boolean {
4  const done: Step[] = [];
5  for (const step of steps) {
6    if (step.run()) {
7      log.push("run " + step.name);
8      done.push(step);
9      continue;
10    }
11    log.push("fail " + step.name);
12    for (const previous of done.reverse()) {
13      previous.compensate();
14      log.push("compensate " + previous.name);
15    }
16    return false;
17  }
18  return true;
19}
20 
21type Event = { id: number };
22interface Db {
23  transaction(work: (tx: Db) => void): void;
24  insert(table: string, row: object): void;
25  unsent(): Event[];
26  markSent(id: number): void;
27}
28interface Broker { publish(event: Event): void }
29 
30// The bug: two systems, two writes, and a crash can land between them.
31function placeOrderDualWrite(db: Db, broker: Broker, order: Event) {
32  db.insert("orders", order);
33  broker.publish({ id: order.id });
34}
35 
36// The fix: the event is a row, committed together with the order.
37function placeOrder(db: Db, order: Event) {
38  db.transaction((tx) => {
39    tx.insert("orders", order);
40    tx.insert("outbox", { id: order.id, sent: false });
41  });
42}
43 
44// Runs in the background; after a crash it simply runs again.
45function relayOnce(db: Db, broker: Broker) {
46  for (const event of db.unsent()) {
47    broker.publish(event);
48    db.markSent(event.id);
49  }
50}
51 
52// Delivery is at least once, so the consumer remembers what it has done.
53class Consumer {
54  private seen = new Set<number>();
55 
56  handle(event: Event, apply: (event: Event) => void): boolean {
57    if (this.seen.has(event.id)) return false;
58    this.seen.add(event.id);
59    apply(event);
60    return true;
61  }
62}