Distributed Systems — Episode 5

The fifth episode studies coordination without a shared clock or memory. Physical clocks drift; UTC provides a reference, but network delay limits precision. Internal synchronization aligns processes with one another, while external synchronization aligns them with a reference without moving time backward.

Logical clocks replace exact time with a happens-before relation. A Lamport clock preserves causal order but cannot identify independent events; vector clocks add one counter per process and reveal concurrency. Mutual exclusion is compared through a coordinator, distributed agreement, and a token ring with different message and failure costs.

Leader election begins with bully and ring algorithms and then moves to Raft, where logs, terms, and a majority connect election to safe replication. Proof of Work and Proof of Stake represent another election class: the right to propose state follows computational cost or economic stake, producing different incentives and risks.

Peer sampling maintains a refreshed neighbor set instead of a full network list. Distributed matching in publish–subscribe connects events to subscriptions without one bottleneck. The finale separates physical positioning through GPS from logical proximity measured by latency, hops, or another overlay metric.