Implement Saga Pattern with Compensating Transactions - The Coordinator

                      TASK
                    

Implementation

The Saga pattern manages long-running transactions by breaking them into a sequence of local transactions with compensating actions for rollback. Unlike 2PC, each step commits immediately, but can be undone by its compensating transaction.

Saga structure:

Transactions: T1, T2, ..., Tn
Compensators: C1, C2, ..., Cn

Execution:
  T1 → T2 → T3 → ... → Tn (forward path)

If Ti fails:
  C(i-1) → C(i-2) → ... → C1 (backward path)

Example e-commerce saga:

T1: ReserveInventory (sku="abc123", quantity=1)
    C1: ReleaseReservation (sku="abc123", quantity=1)

T2: ChargePayment (user_id="u42", amount=99.99)
    C2: RefundPayment (user_id="u42", amount=99.99)

T3: CreateShipment (order_id="o123", address="...")
    C3: CancelShipment (order_id="o123")

Forward execution (happy path):

Request:  {"type": "saga_begin", "msg_id": 1, "saga_id": "saga42", "steps": [
    {"transaction": "ReserveInventory", "params": {"sku": "abc123", "quantity": 1}},
    {"transaction": "ChargePayment", "params": {"user_id": "u42", "amount": 99.99}},
    {"transaction": "CreateShipment", "params": {"order_id": "o123"}}
]}

Response: {"type": "saga_begin_ok", "in_reply_to": 1, "saga_id": "saga42", "status": "pending"}

// Step 1 completes:
{"type": "step_complete", "saga_id": "saga42", "step": 1, "transaction": "ReserveInventory", "result": "reservation_id=r1"}

// Step 2 completes:
{"type": "step_complete", "saga_id": "saga42", "step": 2, "transaction": "ChargePayment", "result": "payment_id=p1"}

// Step 3 completes:
{"type": "step_complete", "saga_id": "saga42", "step": 3, "transaction": "CreateShipment", "result": "shipment_id=s1"}

// Saga complete:
{"type": "saga_complete", "saga_id": "saga42", "status": "completed"}

Rollback execution (failure path):

// Step 1 completes:
{"type": "step_complete", "saga_id": "saga42", "step": 1, "transaction": "ReserveInventory"}

// Step 2 fails:
{"type": "step_failed", "saga_id": "saga42", "step": 2, "transaction": "ChargePayment", "error": "insufficient_funds"}

// Compensating transactions run:
{"type": "compensate", "saga_id": "saga42", "step": 1, "compensator": "ReleaseReservation", "params": {"reservation_id": "r1"}}

// Saga aborted:
{"type": "saga_aborted", "saga_id": "saga42", "status": "aborted", "reason": "Step 2 failed: insufficient_funds"}

Sample Test Cases

Successful saga executionTimeout: 5000ms

Input

{"src":"c0","dest":"saga_orchestrator","body":{"type":"init","msg_id":1}}
{"src":"c1","dest":"saga_orchestrator","body":{"type":"saga_begin","msg_id":2,"saga_id":"saga42","steps":[{"transaction":"ReserveInventory","params":{"sku":"abc123","quantity":1}},{"transaction":"ChargePayment","params":{"user_id":"u42","amount":50}},{"transaction":"CreateShipment","params":{"order_id":"o123"}}]}}

Expected Output

{"src": "saga_orchestrator", "dest": "c0", "body": {"type": "init_ok", "in_reply_to": 1, "msg_id": 0}}

Saga rollback on payment failureTimeout: 5000ms

Input

{"src":"c0","dest":"saga_orchestrator","body":{"type":"init","msg_id":1}}
{"src":"c1","dest":"saga_orchestrator","body":{"type":"saga_begin","msg_id":2,"saga_id":"saga43","steps":[{"transaction":"ReserveInventory","params":{"sku":"abc123","quantity":1}},{"transaction":"ChargePayment","params":{"user_id":"u999","amount":99999}},{"transaction":"CreateShipment","params":{"order_id":"o124"}}]}}

Expected Output

{"src": "saga_orchestrator", "dest": "c0", "body": {"type": "init_ok", "in_reply_to": 1, "msg_id": 0}}

Hints

Hint 1▾

A saga is a sequence of local transactions T1, T2, ..., Tn

Hint 2▾

Each transaction Ti has a compensating transaction Ci

Hint 3▾

If Ti fails, run C(i-1), ..., C1 to rollback

Hint 4▾

Each Ti commits locally (no 2PC lock-in)

Hint 5▾

Example: ReserveInventory (T1) → ChargePayment (T2) → CreateShipment (T3)

Resources

Saga Pattern

Saga pattern documentation from microservices.io

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

saga patterncompensating transactionslocal transactionsrollbacklong-running transactions

main.py

python