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balancer.go
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balancer.go
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package pool
import (
"context"
"hash/maphash"
"math/rand"
"strconv"
"time"
"github.com/jackc/pgx/v5"
"github.com/jackc/pgx/v5/pgxpool"
"github.com/prometheus/client_golang/prometheus"
"golang.org/x/exp/maps"
"golang.org/x/exp/slices"
"golang.org/x/sync/semaphore"
log "github.com/authzed/spicedb/internal/logging"
)
var (
connectionsPerCRDBNodeCountGauge = prometheus.NewGaugeVec(prometheus.GaugeOpts{
Name: "crdb_connections_per_node",
Help: "the number of connections spicedb has to each crdb node",
}, []string{"pool", "node_id"})
pruningTimeHistogram = prometheus.NewHistogramVec(prometheus.HistogramOpts{
Name: "crdb_pruning_duration",
Help: "milliseconds spent on one iteration of pruning excess connections",
Buckets: []float64{.1, .2, .5, 1, 2, 5, 10, 20, 50, 100},
}, []string{"pool"})
)
func init() {
prometheus.MustRegister(connectionsPerCRDBNodeCountGauge)
prometheus.MustRegister(pruningTimeHistogram)
}
type balancePoolConn[C balanceConn] interface {
Conn() C
Release()
}
type balanceConn interface {
comparable
IsClosed() bool
}
// balanceablePool is an interface that a pool must implement to allow its
// connections to be balanced by the balancer.
type balanceablePool[P balancePoolConn[C], C balanceConn] interface {
ID() string
AcquireAllIdle(ctx context.Context) []P
Node(conn C) uint32
GC(conn C)
MaxConns() uint32
Range(func(conn C, nodeID uint32))
}
// NodeConnectionBalancer attempts to keep the connections managed by a RetryPool balanced between healthy nodes in
// a Cockroach cluster.
// It asynchronously processes idle connections, and kills any to nodes that have too many. When the pool reconnects,
// it will have a different balance of connections, and over time the balancer will bring the counts close to equal.
type NodeConnectionBalancer struct {
nodeConnectionBalancer[*pgxpool.Conn, *pgx.Conn]
}
// NewNodeConnectionBalancer builds a new nodeConnectionBalancer for a given connection pool and health tracker.
func NewNodeConnectionBalancer(pool *RetryPool, healthTracker *NodeHealthTracker, interval time.Duration) *NodeConnectionBalancer {
return &NodeConnectionBalancer{*newNodeConnectionBalancer[*pgxpool.Conn, *pgx.Conn](pool, healthTracker, interval)}
}
// nodeConnectionBalancer is generic over underlying connection types for
// testing purposes. Callers should use the exported NodeConnectionBalancer
type nodeConnectionBalancer[P balancePoolConn[C], C balanceConn] struct {
ticker *time.Ticker
sem *semaphore.Weighted
pool balanceablePool[P, C]
healthTracker *NodeHealthTracker
rnd *rand.Rand
seed int64
}
// newNodeConnectionBalancer is generic over underlying connection types for
// testing purposes. Callers should use the exported NewNodeConnectionBalancer.
func newNodeConnectionBalancer[P balancePoolConn[C], C balanceConn](pool balanceablePool[P, C], healthTracker *NodeHealthTracker, interval time.Duration) *nodeConnectionBalancer[P, C] {
seed := int64(new(maphash.Hash).Sum64())
return &nodeConnectionBalancer[P, C]{
ticker: time.NewTicker(interval),
sem: semaphore.NewWeighted(1),
healthTracker: healthTracker,
pool: pool,
seed: seed,
rnd: rand.New(rand.NewSource(seed)),
}
}
// Prune starts periodically checking idle connections and killing ones that are determined to be unbalanced.
func (p *nodeConnectionBalancer[P, C]) Prune(ctx context.Context) {
for {
select {
case <-ctx.Done():
p.ticker.Stop()
return
case <-p.ticker.C:
if p.sem.TryAcquire(1) {
ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
p.pruneConnections(ctx)
cancel()
p.sem.Release(1)
}
}
}
}
// pruneConnections prunes connections to nodes that have more than MaxConns/(# of nodes)
// This causes the pool to reconnect, which over time will lead to a balanced number of connections
// across each node.
func (p *nodeConnectionBalancer[P, C]) pruneConnections(ctx context.Context) {
start := time.Now()
defer func() {
pruningTimeHistogram.WithLabelValues(p.pool.ID()).Observe(float64(time.Since(start).Milliseconds()))
}()
conns := p.pool.AcquireAllIdle(ctx)
defer func() {
// release all acquired idle conns back
for _, c := range conns {
c.Release()
}
}()
// bucket connections by healthy node
healthyConns := make(map[uint32][]P, 0)
for _, c := range conns {
id := p.pool.Node(c.Conn())
if p.healthTracker.IsHealthy(id) {
if healthyConns[id] == nil {
healthyConns[id] = make([]P, 0, 1)
}
healthyConns[id] = append(healthyConns[id], c)
} else {
p.pool.GC(c.Conn())
}
}
nodeCount := uint32(p.healthTracker.HealthyNodeCount())
if nodeCount == 0 {
nodeCount = 1
}
connectionCounts := make(map[uint32]uint32)
p.pool.Range(func(conn C, nodeID uint32) {
connectionCounts[nodeID]++
})
log.Ctx(ctx).Trace().
Str("pool", p.pool.ID()).
Any("counts", connectionCounts).
Msg("connections per node")
// Delete metrics for nodes we no longer have connections for
p.healthTracker.RLock()
for node := range p.healthTracker.nodesEverSeen {
if _, ok := connectionCounts[node]; !ok {
connectionsPerCRDBNodeCountGauge.DeletePartialMatch(map[string]string{
"pool": p.pool.ID(),
"node_id": strconv.FormatUint(uint64(node), 10),
})
}
}
p.healthTracker.RUnlock()
nodes := maps.Keys(connectionCounts)
slices.Sort(nodes)
// Shuffle nodes in place deterministically based on the initial seed.
// This will always generate the same distribution for the life of the
// program, but prevents the same nodes from getting all the "extra"
// connections when they don't divide evenly over nodes.
p.rnd.Seed(p.seed)
p.rnd.Shuffle(len(nodes), func(i, j int) {
nodes[j], nodes[i] = nodes[i], nodes[j]
})
initialPerNodeMax := p.pool.MaxConns() / nodeCount
for i, node := range nodes {
count := connectionCounts[node]
connectionsPerCRDBNodeCountGauge.WithLabelValues(
p.pool.ID(),
strconv.FormatUint(uint64(node), 10),
).Set(float64(count))
perNodeMax := initialPerNodeMax
// Assign MaxConns%(# of nodes) nodes an extra connection. This ensures that
// the sum of all perNodeMax values exactly equals the pool MaxConns.
// Without this, we will either over or underestimate the perNodeMax.
// If we underestimate, the balancer will fight the pool, and if we overestimate,
// it's possible for the difference in connections between nodes to differ by up to
// the number of nodes.
if p.healthTracker.HealthyNodeCount() == 0 ||
uint32(i) < p.pool.MaxConns()%uint32(p.healthTracker.HealthyNodeCount()) {
perNodeMax++
}
// Need to remove excess connections above the perNodeMax
numToPrune := count - perNodeMax
if count <= perNodeMax {
continue
}
log.Ctx(ctx).Info().
Uint32("node", node).
Uint32("poolmaxconns", p.pool.MaxConns()).
Uint32("conncount", count).
Uint32("nodemaxconns", perNodeMax).
Msg("node connections require pruning")
// Prune half of the distance we're trying to cover. This will prune more connections if the gap between
// desired and target is large.
if numToPrune > 1 {
numToPrune >>= 1
}
if uint32(len(healthyConns[node])) < numToPrune {
numToPrune = uint32(len(healthyConns[node]))
}
if numToPrune == 0 {
continue
}
for _, c := range healthyConns[node][:numToPrune] {
log.Ctx(ctx).Debug().Str("pool", p.pool.ID()).Uint32("node", node).Msg("pruning connection")
p.pool.GC(c.Conn())
}
log.Ctx(ctx).Info().Str("pool", p.pool.ID()).Uint32("node", node).Uint32("prunedCount", numToPrune).Msg("pruned connections")
}
}