Group-Coordination

The River Crossing Problem The River Crossing Problem, inspired by Allen Downey’s “The Little Book of Semaphores,” demonstrates constrained group formation and safety-critical synchronization. It models scenarios where groups must form under specific rules before proceeding - common in distributed systems and resource allocation. The Scenario A river crossing with: Two types of people: Hackers (H) and Employees (E) A boat that holds exactly 4 people Random arrivals on the riverbank Safety rules for boarding: ...

The Santa Claus Problem The Santa Claus problem is a delightful synchronization challenge by J.A. Trono (1994). It demonstrates group coordination, prioritization, and conditional wake-up patterns. The Scenario Characters: Santa Claus (1) Reindeer (9) Elves (10) The rules: Santa sleeps in his shop at the North Pole Santa can be awakened by either: All 9 reindeer return from vacation → prepare sleigh, deliver toys Any 3 elves have problems → get help from Santa Reindeer have priority over elves Santa helps 3 elves at a time, then goes back to sleep After delivering toys, Santa unhitches reindeer and goes back to sleep The Challenge Coordination: Wake Santa only when conditions met Grouping: Exactly 9 reindeer OR 3 elves Priority: Reindeer interrupt elf help Fairness: All elves should eventually get help Real-World Applications This pattern models many production scenarios: ...