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Saga Pattern Definition
What is a Saga?
A Saga is a sequence of local transactions where each transaction updates data within a single service. Each local transaction publishes an event or message that triggers the next local transaction in the saga. If a local transaction fails, the saga executes compensating transactions to undo the changes made by preceding transactions.
Key Characteristics
Distributed Transactions: Spans multiple microservices, each with its own database.
Local Transactions: Each service performs its own ACID transaction.
Event-Driven: Services communicate through events or commands.
Compensations: Rollback mechanism using compensating transactions.
Eventual Consistency: System reaches a consistent state over time.
Saga vs Two-Phase Commit (2PC)
| Feature | Saga Pattern | Two-Phase Commit |
|---|---|---|
| Locking | No distributed locks | Requires locks during commit |
| Performance | Better performance | Performance bottleneck |
| Scalability | Highly scalable | Limited scalability |
| Complexity | Business logic complexity | Protocol complexity |
| Failure Handling | Compensating transactions | Automatic rollback |
| Isolation | Lower isolation | Full isolation |
| NoSQL Support | Yes | No |
| Microservices Fit | Excellent | Poor |
ACID vs BASE
ACID (Traditional Databases):
- Atomicity: All or nothing
- Consistency: Valid state transitions
- Isolation: Concurrent transactions don't interfere
- Durability: Committed data persists
BASE (Saga Pattern):
- Basically Available: System is available most of the time
- Soft state: State may change over time
- Eventual consistency: System becomes consistent eventually
When to Use Saga Pattern
Use the saga pattern when:
- Building distributed transactions across multiple microservices
- Needing to replace 2PC with a more scalable solution
- Services need to maintain eventual consistency
- Handling long-running processes spanning multiple services
- Implementing compensating transactions for failed operations
When NOT to Use Saga Pattern
Avoid the saga pattern when:
- Single service transactions (use local ACID transactions)
- Strong consistency is required immediately
- Simple CRUD operations without cross-service dependencies
- Low transaction volume with simple flows
- Team lacks experience with distributed systems
Migration Path
Many organizations migrate from traditional monolithic systems or 2PC-based systems to sagas:
- From Monolith to Saga: Identify transaction boundaries, extract services gradually, implement sagas incrementally
- From 2PC to Saga: Analyze existing 2PC transactions, design compensating transactions, implement sagas in parallel, monitor and compare results before full migration