Eventual consistency
A guarantee that copies of data will agree once updates stop arriving, but may briefly show different values in the meantime.
Read replicas, caches, search indexes and CQRS read models all lag a little behind the source. A user saves a form and the list still shows the old value; a report misses the last minute. That is fine for many screens and wrong for others, such as an account balance checked before a withdrawal. The design work is deciding which reads must come from the source and how the interface explains the delay.
Related terms
- CQRS (Command Query Responsibility Segregation): Splitting a system into a write side that changes data and a separate read side shaped for queries, kept in sync by events.
- Split-brain: Two parts of a system both believe they hold the truth and accept changes independently, so their data drifts apart.
- Two-phase commit (2PC): A protocol where a coordinator asks every database to prepare a change, then tells all of them to commit or all of them to roll back.
