Saga Pattern - Cloud Patterns

Definition

The Saga Pattern manages distributed transactions across multiple microservices by breaking them into a sequence of local transactions. Each transaction updates the database and publishes an event or message to trigger the next step. If a step fails, compensating transactions are executed to undo previous changes.

When to Use It

Managing distributed transactions across microservices
You need to maintain data consistency without 2PC (two-phase commit)
Long-running business processes spanning multiple services
Systems that require eventual consistency
When ACID transactions across services are not feasible
Complex workflows with multiple steps

Pros & Cons

✓ Pros

Maintains data consistency across services
No distributed locks required
Supports long-running transactions
Better availability than 2PC
Scalable approach
Clear rollback mechanism

✗ Cons

Complex to design and implement
Compensating transactions add complexity
Debugging is challenging
Eventual consistency (not immediate)
Requires careful error handling
Testing can be difficult

Architecture Diagram

Order Saga: Choreography-Based Example
=======================================

Success Flow:
─────────────

┌───────────┐    ┌─────────────┐    ┌─────────────┐    ┌──────────┐
│  Order    │    │  Payment    │    │  Inventory  │    │ Shipping │
│  Service  │    │   Service   │    │   Service   │    │ Service  │
└─────┬─────┘    └──────┬──────┘    └──────┬──────┘    └────┬─────┘
      │                 │                  │                 │
  1.  │ CreateOrder     │                  │                 │
      ├────────────────►│                  │                 │
  2.  │                 │ ProcessPayment   │                 │
      │                 ├─────────────────►│                 │
  3.  │                 │                  │ ReserveStock    │
      │                 │                  ├────────────────►│
  4.  │                 │                  │                 │ Ship
      │                 │                  │                 │ ✓
      │                 │                  │                 │
      │◄────────────────┴──────────────────┴─────────────────┘
      │                  Success Events
      │

Failure Flow (with Compensation):
─────────────────────────────────

┌───────────┐    ┌─────────────┐    ┌─────────────┐
│  Order    │    │  Payment    │    │  Inventory  │
│  Service  │    │   Service   │    │   Service   │
└─────┬─────┘    └──────┬──────┘    └──────┬──────┘
      │                 │                  │
  1.  │ CreateOrder     │                  │
      ├────────────────►│                  │
  2.  │                 │ ProcessPayment   │
      │                 ├─────────────────►│
  3.  │                 │                  │ FAIL! ✗
      │                 │                  │
  4.  │                 │ RefundPayment    │ (Compensate)
      │                 │◄─────────────────┤
  5.  │ CancelOrder     │ (Compensate)     │
      │◄────────────────┤                  │
      │                 │                  │
      │              Order Cancelled       │

Key Concepts:
• Each service performs local transaction
• Compensating transactions undo changes
• Events drive the saga forward
• Idempotency is crucial