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.