Scaling

Consistent Hashing

Map keys and nodes onto a ring; a key belongs to the first node clockwise. Adding a node moves only ~1/N of keys.

Why interviewers ask about it

hash(key) % N remaps ~91% of keys when going from 10 to 11 nodes. Virtual nodes are required, not optional, for even distribution.

Placing each physical node at many ring positions (typically 100–200) so arcs are even.

One partition receiving a disproportionate share of traffic, usually from a celebrity or whale key.

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