Eventual Consistency

Problem

Your profile picture is replicated around the world. Requiring every replica to update before acknowledging your change would increase latency and reduce availability. Maybe temporary disagreement is acceptable.

Solution — Eventual Consistency

Allow replicas to temporarily differ while changes propagate.

Vienna → new photo
London → old photo
Tokyo  → old photo

       ↓ propagation

All → new photo

Core idea: eventual consistency accepts temporary disagreement in exchange for other distributed-system benefits.