Monotonic Writes

You update your username from "alice" to "alice_dev", then update your profile picture. If these writes land on different replicas in different order, some users see the new picture with the old username.

Monotonic writes guarantee that writes from a single client are applied in order. Write A before B means every replica applies A before B. In multi-master or leaderless systems, this is not guaranteed by default.

Write ordering can be enforced by:

Single leader, all writes go through one node that serializes them. Simple but creates a bottleneck.

Session-based write routing, route all writes from one session to the same node. That node applies them sequentially.

Lamport timestamps, attach logical timestamps to writes. Replicas apply writes in timestamp order.