Event Sourcing — holat o‘rniga eventlar

An’anaviy CRUD’da biz faqat joriy holatni saqlaymiz: UPDATE orders SET status='shipped'. Eski holat ustiga yoziladi va yo‘qoladi.

State emas, faktlar oqimini saqlaymiz. Joriy holat — ularning yig‘indisi.

Tanish analogiya: Git ham shunday ishlaydi — u faqat fayllarning oxirgi holatini emas, har bir commit (o‘zgarish)ni saqlaydi; istalgan vaqtga qaytish, tarixni ko‘rish mumkin. Event Sourcing — ma’lumotlaringiz uchun "git".

Eventlar har aggregat (masalan, har buyurtma) uchun alohida oqimda (stream) saqlanadi. Joriy holatni olish uchun ularni yuklab, tartib bilan qo‘llaymiz — bu rehydration (qayta tiklash).

Misol: bo‘sh buyurtma → OrderCreated (status=draft) → ItemAdded (2 ta mahsulot) → ItemAdded (yana 1 ta) → OrderShipped (status=shipped). Eventlarni boshidan qo‘llasak — aniq joriy holatga kelamiz. Bu deterministik: bir xil eventlar → har doim bir xil holat.

NestJS’ning @nestjs/cqrs moduli Event Sourcing uchun AggregateRoot beradi: u apply() bilan event chiqaradi va loadFromHistory() bilan eventlardan tiklanadi.

1 · Eventlar

// Eventlar — O'TGAN ZAMONDA bo'lib o'tgan faktlar (immutable)
export class OrderCreatedEvent { constructor(public id: string, public userId: string) {} }
export class ItemAddedEvent    { constructor(public id: string, public sku: string, public qty: number) {} }
export class OrderShippedEvent { constructor(public id: string) {} }

2 · Aggregate — eventlardan holat quradi

// Aggregate — eventlardan o'z holatini quradi (@nestjs/cqrs)
export class Order extends AggregateRoot {
  private id: string;
  private items: { sku: string; qty: number }[] = [];
  private status: "draft" | "shipped" = "draft";

  // COMMAND metodlari: tekshiradi va YANGI event chiqaradi (apply)
  createOrder(id: string, userId: string) {
    this.apply(new OrderCreatedEvent(id, userId));        // fakt qo'shildi
  }
  addItem(sku: string, qty: number) {
    if (this.status === "shipped") throw new Error("Yuborilgan buyurtma yopilgan");
    this.apply(new ItemAddedEvent(this.id, sku, qty));
  }

  // EVENT HANDLER'lar: event holatga QANDAY ta'sir qilishini belgilaydi
  onOrderCreatedEvent(e: OrderCreatedEvent) { this.id = e.id; this.status = "draft"; }
  onItemAddedEvent(e: ItemAddedEvent)       { this.items.push({ sku: e.sku, qty: e.qty }); }
  onOrderShippedEvent(e: OrderShippedEvent) { this.status = "shipped"; }
}

3 · Event store — append va rehydration

// Holatni QAYTA QURISH (rehydration) va saqlash (append-only)
async load(orderId: string): Promise<Order> {
  const events = await this.eventStore.getEvents(orderId);  // shu aggregat eventlari
  const order = new Order();
  order.loadFromHistory(events);    // har eventni navbat bilan apply -> joriy holat
  return order;
}

async save(order: Order) {
  const newEvents = order.getUncommittedEvents();           // faqat YANGI eventlar
  await this.eventStore.append(order.id, newEvents);        // APPEND (insert), update emas
  order.commit();                                           // eventlarni publish (projection'larga)
}

Snapshot — minglab eventni qayta o‘ynatmaslik

Yechim — snapshot: davriy ravishda joriy holatni saqlab qo‘yish; yuklashda snapshot’dan boshlab, faqat undan keyingi eventlarni qo‘llash:

// SNAPSHOT: minglab eventni har safar qayta o'ynatish sekin -> davriy holat saqlaymiz
async load(orderId: string): Promise<Order> {
  const snap = await this.snapshots.getLatest(orderId);     // oxirgi snapshot (masalan, v100)
  const order = snap ? Order.fromSnapshot(snap) : new Order();
  const from = snap ? snap.version : 0;
  const events = await this.eventStore.getEvents(orderId, { after: from });  // FAQAT keyingilari
  order.loadFromHistory(events);                            // v100 dan keyingi eventlar
  return order;
}

Nega CQRS bilan juft (9-mavzu)

Event store "buyurtma #5 holati"ni tez beradi, lekin "barcha yuborilgan buyurtmalar" kabi so‘rovni bera olmaydi — buning uchun har birini qayta o‘ynatish kerak bo‘lardi. Yechim: eventlardan projection (CQRS read model) qurish. Shuning uchun ES deyarli doim CQRS bilan keladi:

• Event Sourcing = joriy holat o‘rniga barcha eventlarni (o‘zgarmas, append-only faktlar) saqlash. Holat — eventlarni qayta o‘ynatish (rehydration) natijasi.
• Event = o‘tgan zamondagi fakt (OrderShipped); command = niyat. Event jurnali — haqiqat manbai. Analogiya: bank daftari, git.
• NestJS: AggregateRoot + apply() (event chiqarish) + loadFromHistory() (tiklash); event store append-only.
• Ikki hamroh: snapshot (minglab eventni qayta o‘ynamaslik) va CQRS (projection — o‘qish uchun, chunki ES so‘rovga noqulay).
• Afzallik: audit, tarix, temporal, replay. Qiyinchilik: event versiyalash, o‘chirish (GDPR), murakkablik. Faqat tarix muhim aggregatlarga qo‘lla — CRUD uchun ishlatma.