Software Design & Architecture Microservices Distributed Systems Saga Pattern Data Consistency

Saga Pattern for Distributed Transactions

Mind Map Summary

  • Topic: Saga Pattern
  • Definition: A sequence of local transactions where each transaction updates data within a single service and publishes an event or message that triggers the next transaction in the saga. If a transaction fails, the saga executes compensating transactions to undo the preceding transactions.
  • Key Concepts:
    • Local Transaction: A standard ACID transaction performed by a single microservice.
    • Compensating Transaction: An operation that reverses the effect of a previous transaction.
    • Choreography: Each service in the saga subscribes to events from other services and acts accordingly. There is no central coordinator.
    • Orchestration: A central orchestrator (the saga orchestrator) tells the participants what to do. The orchestrator communicates with each service directly.
  • Pros:
    • Maintains Data Consistency: Ensures data consistency across services without tight coupling.
    • No Distributed Locks: Avoids the need for distributed locks, which can be a performance bottleneck.
    • Improved Fault Tolerance: Sagas are designed to handle failures gracefully.
  • Cons:
    • Complex to Implement: The logic for sagas, especially compensating transactions, can be complex.
    • Eventual Consistency: Data is eventually consistent, which may not be suitable for all use cases.
    • Debugging Challenges: Debugging a distributed saga can be difficult.

Practice Exercise

Whiteboard the Saga pattern for a ‘trip booking’ system involving three services: Flights, Hotels, and Payments. Show the sequence of events and compensating transactions for both a successful booking and a failure scenario (e.g., the hotel booking fails).

Answer

Scenario: Successful Trip Booking (Orchestration)

  1. Client -> Trip Booking Orchestrator: CreateTripRequest
  2. Trip Booking Orchestrator -> Flight Service: BookFlightCommand
  3. Flight Service: Books the flight, saves to its DB, and replies to the orchestrator.
  4. Trip Booking Orchestrator -> Hotel Service: BookHotelCommand
  5. Hotel Service: Books the hotel, saves to its DB, and replies to the orchestrator.
  6. Trip Booking Orchestrator -> Payment Service: ProcessPaymentCommand
  7. Payment Service: Processes the payment, saves to its DB, and replies to the orchestrator.
  8. Trip Booking Orchestrator: Marks the trip as booked and replies to the client.

Scenario: Hotel Booking Fails (Orchestration with Compensation)

  1. Client -> Trip Booking Orchestrator: CreateTripRequest
  2. Trip Booking Orchestrator -> Flight Service: BookFlightCommand
  3. Flight Service: Books the flight, saves to its DB, and replies to the orchestrator.
  4. Trip Booking Orchestrator -> Hotel Service: BookHotelCommand
  5. Hotel Service: Fails to book the hotel and replies with an error.
  6. Trip Booking Orchestrator: Initiates compensation.
  7. Trip Booking Orchestrator -> Flight Service: CancelFlightCommand (Compensating Transaction)
  8. Flight Service: Cancels the flight, updates its DB, and replies to the orchestrator.
  9. Trip Booking Orcheator: Marks the trip as failed and replies to the client.