[System Design] Banking Microservices Architecture: Event Sourcing, CQRS & Saga Patterns for Core Banking

                    ┌─────────────────────────────────────┐
  CHANNELS          │  Mobile App  │  Internet Banking  │  ATM/POS  │
                    └──────────────────────┬──────────────────────────┘
                                           │ REST/gRPC
                    ┌──────────────────────▼──────────────────────────┐
  API GATEWAY       │         API Gateway (Auth, Rate Limit, Routing)  │
                    └──────────────────────┬──────────────────────────┘
                                           │
         ┌─────────────────────────────────┼──────────────────────────┐
         │                                 │                          │
  ┌──────▼──────┐               ┌──────────▼─────────┐    ┌──────────▼──────────┐
  │ CIF Service │               │  Account Service   │    │ Payment Service     │
  │ (Customer)  │               │  (CASA, GL)        │    │ (Transfers, Fees)   │
  └─────────────┘               └────────────────────┘    └─────────────────────┘
         │                                 │                          │
         └─────────────────────────────────┼──────────────────────────┘
                                           │ Events (Kafka/Dapr)
                    ┌──────────────────────▼──────────────────────────┐
  EVENT BUS         │              Message Broker (Kafka / Redis)      │
                    └──────────────────────┬──────────────────────────┘
                                           │
         ┌─────────────────────────────────┼──────────────────────────┐
         │                                 │                          │
  ┌──────▼──────┐               ┌──────────▼─────────┐    ┌──────────▼──────────┐
  │ Loan Service│               │ Notification Svc   │    │ Reporting Service   │
  │ (Lending)   │               │ (SMS, Push, Email) │    │ (CQRS Read Side)    │
  └─────────────┘               └────────────────────┘    └─────────────────────┘

// Events in the Account domain
type AccountOpened struct {
    AccountID    string
    CIFNumber    string
    Currency     string
    OpenedAt     time.Time
}

type MoneyDeposited struct {
    AccountID     string
    Amount        int64
    TransactionID string
    OccurredAt    time.Time
}

type MoneyWithdrawn struct {
    AccountID     string
    Amount        int64
    TransactionID string
    OccurredAt    time.Time
}

// Calculate balance by replaying events
func calculateBalance(events []Event) int64 {
    var balance int64
    for _, event := range events {
        switch e := event.(type) {
        case MoneyDeposited:
            balance += e.Amount
        case MoneyWithdrawn:
            balance -= e.Amount
        }
    }
    return balance
}

WRITE SIDE (Command)                READ SIDE (Query)
────────────────────────            ──────────────────────────
POST /transfers            →        Materialized Views
POST /accounts             →        Elasticsearch Index
PUT /loans/repay           →        Redis Cache

↓ Event Published ↓                ↑ Subscribe & Update ↑
         └──────────────────────────┘
              (Event Bus / Kafka)

Account Service                Payment Service           Notification Service
      │                               │                          │
      │── TransferInitiated ──────────▶│                          │
      │                               │── PaymentSubmitted ──────▶│
      │                               │                          │── SMS Sent
      │◀── PaymentCompleted ──────────│                          │
      │                               │                          │
   (release hold)                                            (done)

If Payment fails:
      │◀── PaymentFailed ─────────────│
      │                               │
   (cancel hold, refund)

-- Outbox table: written in the same transaction as business data
CREATE TABLE outbox_events (
    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    topic       VARCHAR(100) NOT NULL,  -- 'account.transfer.completed'
    payload     JSONB        NOT NULL,
    status      VARCHAR(20)  NOT NULL DEFAULT 'PENDING',
    created_at  TIMESTAMPTZ  NOT NULL DEFAULT NOW(),
    published_at TIMESTAMPTZ
);

-- Inside the same Database Transaction:
-- 1. Update account balance
-- 2. Write ledger entries  
-- 3. INSERT into outbox_events

-- Separate worker running periodically:
-- SELECT * FROM outbox_events WHERE status = 'PENDING'
-- → Publish to Kafka
-- → UPDATE status = 'PUBLISHED'

POST /v1/transfers                    → Initiate transfer command
  Header: Idempotency-Key: <uuid>
  Body: { from, to, amount, currency }
  Response: { transfer_id, status: "PROCESSING" }

GET  /v1/transfers/{transfer_id}      → Check result
  Response: { status: "COMPLETED" | "FAILED", ... }