Outbox Pattern

When a customer places an order on the Composable Commerce Platform, seven events need to happen in sequence across four independent services: Order created → Payment authorized → Stock reserved → Fulfillment triggered → Notification sent → Loyalty points awarded → Shipping label generated. Any of these can fail. The network can fail. The database can fail. A third-party payment gateway can time out. Without a reliability mechanism, a 2% failure rate on any step means 2% of all orders are stuck in an inconsistent state, requiring manual intervention. ...

Prerequisite: Part 8 of the System Design Masterclass. Read Part 7: Idempotent API Design first — compensating transactions in Saga must be idempotent. Answer-first: The Saga Pattern coordinates distributed transactions across microservices by decomposing a large transaction into a sequence of local transactions. If any step fails, the system automatically executes compensating transactions in reverse order to undo completed steps. Each local transaction must be idempotent. What Are the Problems with 2PC in Microservices? Answer-first: Two-Phase Commit (2PC) is a blocking protocol with a coordinator single point of failure. If the coordinator crashes between the Prepare and Commit phases, all participants are blocked indefinitely with locks held — a catastrophic failure mode in microservices. These are the same core banking distributed transaction challenges seen in legacy systems. ...

← Previous | Series hub | Next → Chapter 4: Eliminating the Dual-Write Nightmare When your Golang application migrates from a Monolith to Event-Driven Microservices, you will immediately face an architectural nightmare: the Dual-Write Problem. 1. What is the Dual-Write Problem? Answer-first: Dual-Write occurs when an app attempts to write to a Database and publish to a Message Broker (Kafka) simultaneously. Without a distributed transaction, network failures will cause the two systems to fall out of sync. ...