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40 customers, 4 cells, one of them hot and one customer locked to the EU. How do you decide where a new tenant goes?

Modulo hashing reshuffles everyone the day you add a cell. Consistent hashing fixes that and then loses to the two constraints that actually decide placement in an enterprise deployment: jurisdiction and who must not share a blast radius with whom.

Updated Nov 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

Modulo hashing reshuffles everyone the day you add a cell. Consistent hashing fixes that and then loses to the two constraints that actually decide placement in an enterprise deployment: jurisdiction and who must not share a blast radius with whom.

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FEDITOR'S NOTE

The senior signal is knowing consistent hashing well enough to say where it stops being the right tool. A candidate who reaches for it and stops has answered a distributed-systems question; a candidate who reaches for it and then notices that a regulated tenant cannot be placed by a hash function has answered a deployment question. The reserve probe is the rebalance: moving a tenant between cells means moving their data, and a design that treats placement as stateless has not thought about what a cell contains. Watch for candidates who propose per-tenant weights without saying where the weight comes from, and for anyone who never asks whether two tenants are competitors.

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