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peer_tracker.go
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peer_tracker.go
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// Copyright (C) 2019-2023, Ava Labs, Inc. All rights reserved.
// See the file LICENSE for licensing terms.
package sync
import (
"math"
"math/rand"
"sync"
"time"
"github.com/prometheus/client_golang/prometheus"
"go.uber.org/zap"
"github.com/ava-labs/avalanchego/ids"
"github.com/ava-labs/avalanchego/utils/logging"
"github.com/ava-labs/avalanchego/utils/set"
"github.com/ava-labs/avalanchego/utils/wrappers"
"github.com/ava-labs/avalanchego/version"
safemath "github.com/ava-labs/avalanchego/utils/math"
)
const (
bandwidthHalflife = 5 * time.Minute
// controls how eagerly we connect to new peers vs. using
// peers with known good response bandwidth.
desiredMinResponsivePeers = 20
newPeerConnectFactor = 0.1
// The probability that, when we select a peer, we select randomly rather
// than based on their performance.
randomPeerProbability = 0.2
)
// information we track on a given peer
type peerInfo struct {
version *version.Application
bandwidth safemath.Averager
}
// Tracks the bandwidth of responses coming from peers,
// preferring to contact peers with known good bandwidth, connecting
// to new peers with an exponentially decaying probability.
type peerTracker struct {
// Lock to protect concurrent access to the peer tracker
lock sync.Mutex
// All peers we are connected to
peers map[ids.NodeID]*peerInfo
// Peers that we're connected to that we've sent a request to
// since we most recently connected to them.
trackedPeers set.Set[ids.NodeID]
// Peers that we're connected to that responded to the last request they were sent.
responsivePeers set.Set[ids.NodeID]
// Max heap that contains the average bandwidth of peers.
bandwidthHeap safemath.AveragerHeap
averageBandwidth safemath.Averager
log logging.Logger
numTrackedPeers prometheus.Gauge
numResponsivePeers prometheus.Gauge
averageBandwidthMetric prometheus.Gauge
}
func newPeerTracker(
log logging.Logger,
metricsNamespace string,
registerer prometheus.Registerer,
) (*peerTracker, error) {
t := &peerTracker{
peers: make(map[ids.NodeID]*peerInfo),
trackedPeers: make(set.Set[ids.NodeID]),
responsivePeers: make(set.Set[ids.NodeID]),
bandwidthHeap: safemath.NewMaxAveragerHeap(),
averageBandwidth: safemath.NewAverager(0, bandwidthHalflife, time.Now()),
log: log,
numTrackedPeers: prometheus.NewGauge(
prometheus.GaugeOpts{
Namespace: metricsNamespace,
Name: "num_tracked_peers",
Help: "number of tracked peers",
},
),
numResponsivePeers: prometheus.NewGauge(
prometheus.GaugeOpts{
Namespace: metricsNamespace,
Name: "num_responsive_peers",
Help: "number of responsive peers",
},
),
averageBandwidthMetric: prometheus.NewGauge(
prometheus.GaugeOpts{
Namespace: metricsNamespace,
Name: "average_bandwidth",
Help: "average sync bandwidth used by peers",
},
),
}
errs := wrappers.Errs{}
errs.Add(
registerer.Register(t.numTrackedPeers),
registerer.Register(t.numResponsivePeers),
registerer.Register(t.averageBandwidthMetric),
)
return t, errs.Err
}
// Returns true if we're not connected to enough peers.
// Otherwise returns true probabilistically based on the number of tracked peers.
// Assumes p.lock is held.
func (p *peerTracker) shouldTrackNewPeer() bool {
numResponsivePeers := p.responsivePeers.Len()
if numResponsivePeers < desiredMinResponsivePeers {
return true
}
if len(p.trackedPeers) >= len(p.peers) {
// already tracking all the peers
return false
}
// TODO danlaine: we should consider tuning this probability function.
// With [newPeerConnectFactor] as 0.1 the probabilities are:
//
// numResponsivePeers | probability
// 100 | 4.5399929762484854e-05
// 200 | 2.061153622438558e-09
// 500 | 1.9287498479639178e-22
// 1000 | 3.720075976020836e-44
// 2000 | 1.3838965267367376e-87
// 5000 | 7.124576406741286e-218
//
// In other words, the probability drops off extremely quickly.
newPeerProbability := math.Exp(-float64(numResponsivePeers) * newPeerConnectFactor)
return rand.Float64() < newPeerProbability // #nosec G404
}
// Returns a peer that we're connected to.
// If we should track more peers, returns a random peer with version >= [minVersion], if any exist.
// Otherwise, with probability [randomPeerProbability] returns a random peer from [p.responsivePeers].
// With probability [1-randomPeerProbability] returns the peer in [p.bandwidthHeap] with the highest bandwidth.
func (p *peerTracker) GetAnyPeer(minVersion *version.Application) (ids.NodeID, bool) {
p.lock.Lock()
defer p.lock.Unlock()
if p.shouldTrackNewPeer() {
for nodeID := range p.peers {
// if minVersion is specified and peer's version is less, skip
if minVersion != nil && p.peers[nodeID].version.Compare(minVersion) < 0 {
continue
}
// skip peers already tracked
if p.trackedPeers.Contains(nodeID) {
continue
}
p.log.Debug(
"tracking peer",
zap.Int("trackedPeers", len(p.trackedPeers)),
zap.Stringer("nodeID", nodeID),
)
return nodeID, true
}
}
var (
nodeID ids.NodeID
ok bool
)
useRand := rand.Float64() < randomPeerProbability // #nosec G404
if useRand {
nodeID, ok = p.responsivePeers.Peek()
} else {
nodeID, _, ok = p.bandwidthHeap.Pop()
}
if !ok {
// if no nodes found in the bandwidth heap, return a tracked node at random
return p.trackedPeers.Peek()
}
p.log.Debug(
"peer tracking: popping peer",
zap.Stringer("nodeID", nodeID),
zap.Bool("random", useRand),
)
return nodeID, true
}
// Record that we sent a request to [nodeID].
func (p *peerTracker) TrackPeer(nodeID ids.NodeID) {
p.lock.Lock()
defer p.lock.Unlock()
p.trackedPeers.Add(nodeID)
p.numTrackedPeers.Set(float64(p.trackedPeers.Len()))
}
// Record that we observed that [nodeID]'s bandwidth is [bandwidth].
// Adds the peer's bandwidth averager to the bandwidth heap.
func (p *peerTracker) TrackBandwidth(nodeID ids.NodeID, bandwidth float64) {
p.lock.Lock()
defer p.lock.Unlock()
peer := p.peers[nodeID]
if peer == nil {
// we're not connected to this peer, nothing to do here
p.log.Debug("tracking bandwidth for untracked peer", zap.Stringer("nodeID", nodeID))
return
}
now := time.Now()
if peer.bandwidth == nil {
peer.bandwidth = safemath.NewAverager(bandwidth, bandwidthHalflife, now)
} else {
peer.bandwidth.Observe(bandwidth, now)
}
p.bandwidthHeap.Add(nodeID, peer.bandwidth)
if bandwidth == 0 {
p.responsivePeers.Remove(nodeID)
} else {
p.responsivePeers.Add(nodeID)
// TODO danlaine: shouldn't we add the observation of 0
// to the average bandwidth in the if statement?
p.averageBandwidth.Observe(bandwidth, now)
p.averageBandwidthMetric.Set(p.averageBandwidth.Read())
}
p.numResponsivePeers.Set(float64(p.responsivePeers.Len()))
}
// Connected should be called when [nodeID] connects to this node
func (p *peerTracker) Connected(nodeID ids.NodeID, nodeVersion *version.Application) {
p.lock.Lock()
defer p.lock.Unlock()
peer := p.peers[nodeID]
if peer == nil {
p.peers[nodeID] = &peerInfo{
version: nodeVersion,
}
return
}
// Peer is already connected, update the version if it has changed.
// Log a warning message since the consensus engine should never call Connected on a peer
// that we have already marked as Connected.
if nodeVersion.Compare(peer.version) != 0 {
p.peers[nodeID] = &peerInfo{
version: nodeVersion,
bandwidth: peer.bandwidth,
}
p.log.Warn(
"updating node version of already connected peer",
zap.Stringer("nodeID", nodeID),
zap.Stringer("storedVersion", peer.version),
zap.Stringer("nodeVersion", nodeVersion),
)
} else {
p.log.Warn(
"ignoring peer connected event for already connected peer with identical version",
zap.Stringer("nodeID", nodeID),
)
}
}
// Disconnected should be called when [nodeID] disconnects from this node
func (p *peerTracker) Disconnected(nodeID ids.NodeID) {
p.lock.Lock()
defer p.lock.Unlock()
p.bandwidthHeap.Remove(nodeID)
p.trackedPeers.Remove(nodeID)
p.numTrackedPeers.Set(float64(p.trackedPeers.Len()))
p.responsivePeers.Remove(nodeID)
p.numResponsivePeers.Set(float64(p.responsivePeers.Len()))
delete(p.peers, nodeID)
}
// Returns the number of peers the node is connected to.
func (p *peerTracker) Size() int {
p.lock.Lock()
defer p.lock.Unlock()
return len(p.peers)
}