Designing Saga Pattern for Distributed Transactions
1. Understanding Saga Pattern Concepts
| Concept | Detail |
|---|
| Saga | Sequence of local transactions |
| Compensation | Inverse op for completed step on failure |
| No locks | Eventual consistency across services |
| When | Distributed transactions across microservices |
2. Designing Choreography-Based Sagas
OrderSvc → OrderPlaced ──▶ PaymentSvc → PaymentCaptured ──▶ ShippingSvc
fail ─▶ OrderSvc compensate (cancel)
| Pro | Con |
|---|
| No central coordinator | Hard to visualize/debug |
| Loose coupling | Cyclic dependencies risk |
| Resilient | Implicit workflow |
3. Designing Orchestration-Based Sagas
| Component | Detail |
|---|
| Orchestrator | Central state machine; calls services |
| Tools | Temporal, Camunda, AWS Step Functions, Cadence |
| Pros | Visible flow, easier debugging |
| Cons | Orchestrator becomes critical SPOF; coupled |
4. Designing Compensating Transactions
| Original | Compensation |
|---|
| Reserve inventory | Release inventory |
| Charge card | Refund / void |
| Send email | Send cancellation email (no undo) |
| Create order | Cancel order |
Warning: Some operations cannot be perfectly undone (emails, payments). Use semantic compensation (refund, retraction) and accept side effects.
5. Designing Saga State Management
| Storage | Detail |
|---|
| Event-sourced state | Sequence of saga events |
| Persistent state machine | Current step + context in DB |
| Workflow engine | Temporal/Cadence durable history |
| Idempotency | Steps must be re-executable |
6. Designing Saga Failure Handling
| Failure | Action |
|---|
| Step transient error | Retry with backoff |
| Step business failure | Compensate prior steps |
| Compensation fails | Alert + manual intervention |
| Orchestrator crash | Resume from durable state |
7. Designing Saga Rollback Strategy
| Approach | Detail |
|---|
| Backward recovery | Run compensations in reverse order |
| Forward recovery | Skip failed step; continue if possible |
| Mixed | Partial completion + manual review |
8. Designing Saga Timeout Handling
| Type | Detail |
|---|
| Step timeout | Service didn't respond → retry/compensate |
| Saga timeout | Whole saga exceeds budget → abort |
| Wait timer | "Wait 24h then cancel" |
| Tool support | Temporal, Step Functions native |
9. Designing Saga Observability
| Signal | Detail |
|---|
| Trace ID propagation | Correlate across services |
| Saga state log | Step transitions + duration |
| Dashboards | Active sagas, completion rate |
| Alerts | Stuck/aborted sagas |
10. Designing Saga Testing Strategy
| Test | Detail |
|---|
| Happy path | All steps succeed |
| Each step failure | Verify compensations |
| Idempotency | Replay each step |
| Crash recovery | Kill orchestrator mid-flight |
| Timeout | Inject delay; verify behavior |
11. Designing Long-Running Sagas
| Concern | Mitigation |
|---|
| Days/weeks duration | Durable workflow engine |
| Schema changes | Workflow versioning |
| External signals | Wait for human approval |
| State migration | Tools for in-flight saga upgrade |
12. Designing Saga Recovery
| Mechanism | Detail |
|---|
| Persistent state | Resume after orchestrator restart |
| Replay events | Reconstruct in-flight saga |
| Manual intervention | Admin tooling to retry/abort |
| Reconciliation | Periodic job to find stuck sagas |