WHY "WHAT HAPPENED FIRST?" IS ONE OF THE HARDEST QUESTIONS IN LARGE-SCALE SYSTEMS

Summary from the TLDR Web Dev newsletter

Determining the precise order of events in large-scale distributed systems is challenging due to imperfect clock synchronization across computers. Network latency and clock drift make it impossible to rely on wall-clock time for accurate ordering. Instead, computer scientists use logical clocks to track the "happened-before" relationship between events, which assign increasing counters to events and messages, making sure that if event A caused event B, A's logical timestamp will be lower than B's.