Implement Orchestration-Based Saga - The Coordinator

                      TASK
                    

Implementation

In an orchestration-based saga, a central orchestrator manages the saga lifecycle. It sends commands to services, tracks completion, and initiates compensating transactions on failure.

Orchestrator state machine:

States:
  PENDING → Step 1 executing
  Step 1 complete → Step 2 executing
  Step 2 complete → Step 3 executing
  ...
  All steps complete → COMPLETED
  Any step fails → COMPENSATING → ABORTED

Command/reply pattern:

// Orchestrator sends command:
{"type": "ChargePayment", "saga_id": "saga42", "step": 2, "params": {"user_id": "u42", "amount": 99.99}}

// Service replies:
{"type": "ChargePayment_ok", "saga_id": "saga42", "step": 2, "result": {"payment_id": "p1"}}

// Or fails:
{"type": "ChargePayment_failed", "saga_id": "saga42", "step": 2, "error": "insufficient_funds"}

Orchestrator state structure:

interface SagaState {
  saga_id: string;
  status: "PENDING" | "COMPLETED" | "ABORTED" | "COMPENSATING";
  current_step: number;
  completed_steps: number[];
  compensating_steps: number[];
  steps: Array<{
    transaction: string;
    params: any;
    status: "PENDING" | "COMPLETED" | "FAILED" | "COMPENSATED";
    result?: any;
  }>;
}

Example orchestration execution:

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

// Orchestrator sends command to inventory:
{"type": "ReserveInventory", "saga_id": "saga42", "step": 1, "params": {"sku": "abc123", "quantity": 1}}

// Inventory replies:
{"type": "ReserveInventory_ok", "saga_id": "saga42", "step": 1, "result": {"reservation_id": "r1"}}

// Orchestrator sends command to payment:
{"type": "ChargePayment", "saga_id": "saga42", "step": 2, "params": {"user_id": "u42", "amount": 99.99}}

// Payment fails:
{"type": "ChargePayment_failed", "saga_id": "saga42", "step": 2, "error": "insufficient_funds"}

// Orchestrator initiates compensation:
{"type": "ReleaseReservation", "saga_id": "saga42", "step": 1, "compensating": true}

// Inventory compensates:
{"type": "ReleaseReservation_ok", "saga_id": "saga42", "step": 1}

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

Sample Test Cases

Orchestrator completes saga successfullyTimeout: 5000ms

Input

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

Expected Output

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

Orchestrator compensates on failureTimeout: 5000ms

Input

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

Expected Output

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

Hints

Hint 1▾

A saga orchestrator sends commands to services and listens for replies

Hint 2▾

The orchestrator maintains the saga state machine (pending, completed, aborted)

Hint 3▾

On failure, orchestrator sends compensating commands in reverse order

Hint 4▾

Orchestrator persists state for recovery after crashes

Hint 5▾

Example: orchestrator → ChargePayment → payment service → orchestrator

Resources

Saga Orchestration Pattern

IBM documentation on saga orchestration

                      OVERVIEW
                    

Theoretical Hub

Concept overview coming soon

Key Concepts

orchestrationsaga orchestratorcentral coordinatorstate machinecommand patterns

main.py

python