sysdesign-dual-write-event-sourcing

The dual-write anti-pattern occurs when a single user action updates two data stores simultaneously, risking inconsistency. It requires an explicit source of truth and a structured trade-off evaluation to decide if patterns like transactional outbox or event sourcing are needed.

Compare transactional outbox, change data capture, and event sourcing using a four-axis trade-off scoring framework. Evaluate explicit tradeoffs for each pattern against your source of truth requirements to determine which approach best fits your data consistency needs.

Use event sourcing when your system needs a complete audit log and complex state reconstruction, accepting added complexity. Choose transactional outbox when you only need reliable message delivery alongside database writes without full event replay capabilities.

Document your dual-write pattern decision by producing an Architecture Decision Record containing a trade-off table, system diagrams, and a risk or reconciliation plan. This ADR locks in the chosen pattern and defines a clear rollback plan before implementation.

Event sourcing tradeoffs involve increased system complexity, eventual consistency challenges, and infrastructure overhead. A structured scoring framework helps evaluate whether the benefits of event replay and auditability outweigh these operational costs for your specific scenario.

Yes, dual-write tradeoff evaluation applies to search index updates, cache updates, and DB+Kafka scenarios. The framework helps identify your explicit source of truth and guides pattern selection across any two-store write path your system requires.