In Book 1, Ch 22, you built a hash table: a hash function maps keys to bucket indices; collisions are resolved by chaining or probing. In Ch 39, you saw modular arithmetic applied to a ring — the basis of consistent hashing.
This chapter traces Thread 1 (Hashing) into three production systems. Each system uses the same mathematical properties — uniform distribution, determinism, and (in the case of SHA-1) collision resistance — but applies them to different engineering problems. The thread continues in Book 3, Chapters 4–5, where consistent hashing becomes the foundation of distributed database sharding.