Time, Clocks, and the Ordering of Events conceptpaper
Lamport, 1978.
Lamport, 1978. The foundational paper on logical time in distributed systems. Defines the happens-before relation, introduces logical clocks (now called Lamport clocks), and proves that a partial order based on causality is the most we can hope for in an asynchronous system. The intellectual basis for vector clocks, version vectors, distributed snapshots, and consensus-based total ordering. One of the most-cited papers in computer science; routinely re-read by distributed-systems engineers.