Distributed Locks, Outbox and Inbox Patterns: Solving Consistency in Distributed Systems

Distributed systems can behave correctly within each individual component yet still produce incorrect outcomes across components, because separate services do not share a common transaction boundary. Failures most often occur in the gaps between independently operating components rather than within any single component itself. Distributed locking addresses concurrent ownership of shared resources, ensuring only one service instance acts on a shared resource at a given time, though it cannot physically halt a stalled process from continuing after a lock expires. The Outbox pattern bridges the gap between a committed database transaction and reliable delivery to asynchronous messaging systems like Kafka or RabbitMQ, while the Inbox pattern ensures incoming messages are processed exactly once within a transactional business context. Understanding which consistency boundary needs protection — rather than simply categorising patterns by component type — is the key to selecting the right coordination mechanism.