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I wonder why they didn't use rendezvous hashing (aka HRW)[0]?

It feels like it would solve all the requirement that they laid out, is fully client side, and doesn't require real time updates for the host list via discovery.

[0] https://en.wikipedia.org/wiki/Rendezvous_hashing

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HRW would cover the simple case, but they needed way more-- e.g. per-request balancing, zone affinity, live health checks, spillover, ramp-ups, etc. Once you need all that dynamic behavior, plain hashing just doesn’t cut it IMO. A custom client-side + discovery setup makes more sense.

the problem is that they want to apply a number of stateful/lookaside load balancing strategies, which become more difficult to do in a fully decentralized system. it’s generally easier to asynchronously aggregate information and either decide routing updates centrally or redistribute that aggregate to inform local decisions.

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