Distributed Transactions

**The problem:** In microservices, a business operation may span multiple services, each with its own database. How do you ensure all-or-nothing consistency? **2-Phase Commit (2PC):** 1. Prepare phase: coordinator asks all participants if they can commit 2. Commit phase: if all say yes, coordinator tells all to commit - Problem: blocking, coordinator is SPOF, poor performance **Saga Pattern (preferred):** Break transaction into a sequence of local transactions with compensating actions. **Types:** - **Choreography**: Services emit events, other services react - Pros: loose coupling, simple - Cons: hard to track, can become complex - **Orchestration**: Central coordinator directs the flow - Pros: easier to understand, centralized logic - Cons: coordinator is a dependency **Example (Order Saga):** 1. Create Order → 2. Reserve Inventory → 3. Process Payment → 4. Ship If step 3 fails: Compensate step 2 (release inventory), Compensate step 1 (cancel order) **Key requirement:** Each step must be idempotent (safe to retry).