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ringhash: ensure addresses are consistenly hashed across updates #6066

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merged 1 commit into from
Mar 3, 2023
Merged

ringhash: ensure addresses are consistenly hashed across updates #6066

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19 changes: 11 additions & 8 deletions xds/internal/balancer/ringhash/ring.go
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Original file line number Diff line number Diff line change
Expand Up @@ -92,16 +92,19 @@ func newRing(subConns *resolver.AddressMap, minRingSize, maxRingSize uint64, log
//
// A hash is generated for each item, and later the results will be sorted
// based on the hash.
var (
idx int
targetIdx float64
)
var currentHashes, targetHashes float64
for _, scw := range normalizedWeights {
targetIdx += scale * scw.weight
for float64(idx) < targetIdx {
h := xxhash.Sum64String(scw.sc.addr + strconv.Itoa(idx))
items = append(items, &ringEntry{idx: idx, hash: h, sc: scw.sc})
targetHashes += scale * scw.weight
// This index ensures that ring entries corresponding to the same
// address hash to different values. And since this index is
// per-address, these entries hash to the same value across address
// updates.
idx := 0
for currentHashes < targetHashes {
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Why are we accumulating targetHashes and currentHashes instead of resetting them? e.g.

for _, scw := range normalizedWeights {
	targetHashes := scale * scw.weight
	for idx := 0; idx < targetHashes; idx++ {
		h := xxhash.Sum...
		items = append...
	}
}

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Since scw.weight is not a whole number, targetHashes is not a whole number either. In order to ensure that we end up with the correct number of items in the ring, we need to keep a running sum.

I based this change on C-core and Java's implementation. Looks like envoy also does the same: https://github.com/envoyproxy/envoy/blob/main/source/common/upstream/ring_hash_lb.cc#L159.

h := xxhash.Sum64String(scw.sc.addr + "_" + strconv.Itoa(idx))
items = append(items, &ringEntry{hash: h, sc: scw.sc})
idx++
currentHashes++
}
}

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