Ordering Guarantees

Ordering and Causality.

Ordering Guarantees

Ordering is closely related to causality ("happens-before").

Lamport Timestamps

A pair (counter, nodeID). Every node tracks the max counter it has seen. This provides a Total Order consistent with causality. Limitation: You can order events, but you can't decide "who won" immediately (e.g., uniqueness constraint) without checking all nodes.

Total Order Broadcast

To implement a lock service or unique username registry, we need Total Order Broadcast:

  1. Reliable delivery (no messages lost).
  2. Totally ordered delivery (messages delivered to all nodes in the same order). This is equivalent to Consensus.