Event Ordering and Consistency Models

An ordering rule describes which events must precede others. A consistency model describes which results clients may observe from reads and writes. This lesson connects those ideas and shows why agreeing on an event sequence is not enough to guarantee every form of correctness.

The previous lesson introduced Lamport clocks and vector clocks. Lamport clocks preserve happened-before in one direction; vector clocks can also distinguish concurrent events under the stated model. Neither clock alone is a protocol for delivering the same event set to every replica.

While these tools clarify relationships between events, they don’t provide a definitive protocol for ordering those events across the system. This leads to a bigger question: Given what we know about causal and concurrent relationships, how should a system decide the final order of events?

That’s where ordering models come into play.

Ordering models define the rules a distributed system follows to determine the sequence in which events are processed or applied across all nodes. Each model offers a distinct balance between consistency, performance, and user experience, enabling system designers to select the approach that best suits their requirements.