Consensus in a Synchronous System

• Possible because known upper bound on message latencies and clock drifts/skews
• In a system with n machines and f failures, if we run algorithm for f=1 rounds, we will reach consensus

Consensus in Async System

• FLP proof proves that this is impossible
• Generally use to unbounded message latencies, etc.

Terminology:

• Schedule: a series of events
• Univalent: could be only either 0-valent or 1-valent, basically decision is reached
• Bivalent: still has not reached a decision, outcome could be either 0- or 1-valent

FLP: Lemma 1

• Disjoint schedules are commutative
• Apply S1 and S2 in any order and the result would be the same