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feat: implement weighted random peer selection for load balancing

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Use probabilistic distribution based on peer quality scores, giving all peers
opportunity while favoring better-performing ones. Selection probability is
inversely proportional to score.

Part of https://github.com/codex-storage/nim-codex/issues/974
This commit is contained in:
Chrysostomos Nanakos 2025-09-12 22:05:38 +03:00
parent 8e8d9f8e60
commit 0fbca17269
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1 changed files with 33 additions and 2 deletions
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35
codex/blockexchange/engine/engine.nim
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@ -842,8 +842,39 @@ proc blockexcTaskRunner(self: BlockExcEngine) {.async: (raises: []).} =
info "Exiting blockexc task runner"
proc selectRandom*(peers: seq[BlockExcPeerCtx]): BlockExcPeerCtx =
Rng.instance.sample(peers)
proc selectRandom*(
peers: seq[BlockExcPeerCtx]
): BlockExcPeerCtx {.gcsafe, raises: [].} =
if peers.len == 1:
return peers[0]
proc evalPeerScore(peer: BlockExcPeerCtx): float =
let
loadPenalty = peer.blocksRequested.len.float * 2.0
successRate =
if peer.exchanged > 0:
peer.exchanged.float / (peer.exchanged + peer.blocksRequested.len).float
else:
0.5
failurePenalty = (1.0 - successRate) * 5.0
return loadPenalty + failurePenalty
let
scores = peers.mapIt(evalPeerScore(it))
maxScore = scores.max() + 1.0
weights = scores.mapIt(maxScore - it)
var totalWeight = 0.0
for w in weights:
totalWeight += w
var r = rand(totalWeight)
for i, weight in weights:
r -= weight
if r <= 0.0:
return peers[i]
return peers[^1]
proc new*(
T: type BlockExcEngine,