सSaral Shiksha Yojna
Courses/Distributed Systems

Distributed Systems

CS3.401
Prof. Kishore Kothapalli•Monsoon 2025-26•4 credits
Revision Notes/Unit 4 — Causal Order Message Delivery

Unit 4 — Causal Order Message Delivery

When messages must be delivered in causal order (not just FIFO). Birman-Schiper-Stephenson (BSS) algorithm: two delivery conditions on vector clocks. Causal order vs FIFO vs total order. Use cases: replicated databases, group communication, distributed debugging.

BSS Algorithm + Causal vs FIFO vs Total Order

8 min

FIFO says 'messages from the same sender arrive in order'. Causal order says 'if $\text{send}(m_1) \to \text{send}(m_2)$ and both target the same destination, $m_1$ is delivered before $m_2$' — even if the senders are different. This matters when an effect's cause comes through a different path. Total order goes further: all processes deliver every message in the same global order.