What Is Eventual Consistency?
Eventual consistency is a model where distributed database replicas may temporarily serve stale data but will converge to the same state given enough time. After a write, not all replicas are instantly updated — but they will be eventually. It trades immediate consistency for availability and performance.
How Eventual Consistency Works
When you post on social media, your followers in different regions might see the post at slightly different times. The CDN caches update eventually — this is acceptable because seeing a post 2 seconds late is fine. But for bank transfers, eventual consistency is not acceptable.
Key Concepts
- Convergence — All replicas will reach the same state — the system self-heals after temporary inconsistencies
- Read-After-Write Consistency — A stronger guarantee: you always see your own writes immediately, even if others see them later
Frequently Asked Questions
When is eventual consistency acceptable?
Social feeds, analytics dashboards, recommendation engines, search indexes — any case where slightly stale data doesn't cause harm. Unacceptable for financial transactions, inventory counts, or authentication.