Definition

Eventual consistency is a weak consistency model where if no new updates are made, all replicas of data will eventually converge to the same state.

Core concept

Updates may arrive at replicas at different times, so reads might get stale data for a while. However, given enough time without updates, all copies will match. This model guarantees the system will become consistent, but not immediately.

Use cases

Important in distributed, partition-tolerant systems (AP in CAP). E.g., DNS, Amazon’s Dynamo, Cassandra. It allows the system to remain available and partition-tolerant by allowing temporary divergence.

Trade-offs

Inconsistent reads are possible. The application must tolerate reading stale data for some window. Suitable for non-critical data, or where stale reads won’t cause harm.

Example

When updating a user’s profile, one server might show the new name instantly, while another may lag by a few seconds. Eventually, all servers reflect the update.

References

“Eventual consistency… after some time with no updates, all data replicas will eventually converge to a consistent state”[[19]](https://www.geeksforgeeks.org/system-design/eventual-consistency-in-distributive-systems-learn-system-design/ "Eventual Consistency in Distributed Systems | Learn …").