Courses/Distributed Systems

Distributed Systems

CS3.401

Prof. Kishore Kothapalli•Monsoon 2025-26•4 credits

Revision Notes/Unit 2 — Models, Events & Logical Time

Unit 2 — Models, Events & Logical Time

The model of a distributed program (processors, channels, events). Lamport's happened-before relation. Logical clocks: scalar (Lamport), vector, and matrix — their properties, comparison, and use-cases. The strong-consistency trap (scalar fails the converse). Physical-time synchronisation: Cristian's, Berkeley, Decentralised averaging, NTP.

Logical Clocks (Scalar / Vector / Matrix) + Physical Time Sync

18 min

Without a global clock, we need a discipline for talking about 'before' and 'after'. Lamport's happened-before relation $(\to)$ captures *causal* ordering; logical clocks (scalar, vector, matrix) implement that relation in code. The trade-off is storage vs information: scalar = 1 int (cheap, but loses converse); vector = n ints (strongly consistent); matrix = n × n ints (also tracks what others know about each other — enables obsolete-message GC).