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The random_choose selection policy is meant to implement power-of-d-choices: sample d available upstreams uniformly, then pick the least-loaded of that sample. The sampling loop, however, was not a correct reservoir sample (Algorithm R): it never filled the first k reservoir slots unconditionally, instead writing every candidate to a random slot j = rand(i+1), which can evict an earlier candidate while leaving another slot nil. It also derived j from the upstream's index in the pool rather than from the number of available upstreams seen, skewing the sample whenever unavailable upstreams precede available ones. As a result the reservoir frequently held fewer than min(k, available) upstreams, so the least-load comparison often never happened. Most notably, with two upstreams and 'random_choose 2' (the canonical power-of-two-choices setup), half of all requests were routed to the more-loaded upstream even when it was saturated and the other idle -- identical behavior to plain 'random'. The nil slots this leaves behind were also the cause of the panic reported in #3810, which was patched by skipping nils in leastRequests rather than by fixing the sampling. Replace the loop with a standard Algorithm R reservoir sample over the available upstreams: fill the first k slots, then replace a random slot with probability k/n. Every available upstream is now sampled uniformly and the reservoir always holds min(k, available) candidates. Co-authored-by: Claude Fable 5 <noreply@anthropic.com> |
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| standard | ||