Add weighted round robin load balancer on TCP

Co-authored-by: Mathieu Lonjaret <mathieu.lonjaret@gmail.com>

pkg/tcp/wrr_load_balancer.go

@@ -0,0 +1,122 @@
+package tcp
+
+import (
+	"fmt"
+	"sync"
+
+	"github.com/containous/traefik/v2/pkg/log"
+)
+
+type server struct {
+	Handler
+	weight int
+}
+
+// WRRLoadBalancer is a naive RoundRobin load balancer for TCP services
+type WRRLoadBalancer struct {
+	servers       []server
+	lock          sync.RWMutex
+	currentWeight int
+	index         int
+}
+
+// NewWRRLoadBalancer creates a new WRRLoadBalancer
+func NewWRRLoadBalancer() *WRRLoadBalancer {
+	return &WRRLoadBalancer{
+		index: -1,
+	}
+}
+
+// ServeTCP forwards the connection to the right service
+func (b *WRRLoadBalancer) ServeTCP(conn WriteCloser) {
+	if len(b.servers) == 0 {
+		log.WithoutContext().Error("no available server")
+		return
+	}
+
+	next, err := b.next()
+	if err != nil {
+		log.WithoutContext().Errorf("Error during load balancing: %v", err)
+		conn.Close()
+	}
+	next.ServeTCP(conn)
+}
+
+// AddServer appends a server to the existing list
+func (b *WRRLoadBalancer) AddServer(serverHandler Handler) {
+	w := 1
+	b.AddWeightServer(serverHandler, &w)
+}
+
+// AddWeightServer appends a server to the existing list with a weight
+func (b *WRRLoadBalancer) AddWeightServer(serverHandler Handler, weight *int) {
+	w := 1
+	if weight != nil {
+		w = *weight
+	}
+	b.servers = append(b.servers, server{Handler: serverHandler, weight: w})
+}
+
+func (b *WRRLoadBalancer) maxWeight() int {
+	max := -1
+	for _, s := range b.servers {
+		if s.weight > max {
+			max = s.weight
+		}
+	}
+	return max
+}
+
+func (b *WRRLoadBalancer) weightGcd() int {
+	divisor := -1
+	for _, s := range b.servers {
+		if divisor == -1 {
+			divisor = s.weight
+		} else {
+			divisor = gcd(divisor, s.weight)
+		}
+	}
+	return divisor
+}
+
+func gcd(a, b int) int {
+	for b != 0 {
+		a, b = b, a%b
+	}
+	return a
+}
+
+func (b *WRRLoadBalancer) next() (Handler, error) {
+	b.lock.Lock()
+	defer b.lock.Unlock()
+
+	if len(b.servers) == 0 {
+		return nil, fmt.Errorf("no servers in the pool")
+	}
+
+	// The algo below may look messy, but is actually very simple
+	// it calculates the GCD  and subtracts it on every iteration, what interleaves servers
+	// and allows us not to build an iterator every time we readjust weights
+
+	// GCD across all enabled servers
+	gcd := b.weightGcd()
+	// Maximum weight across all enabled servers
+	max := b.maxWeight()
+
+	for {
+		b.index = (b.index + 1) % len(b.servers)
+		if b.index == 0 {
+			b.currentWeight -= gcd
+			if b.currentWeight <= 0 {
+				b.currentWeight = max
+				if b.currentWeight == 0 {
+					return nil, fmt.Errorf("all servers have 0 weight")
+				}
+			}
+		}
+		srv := b.servers[b.index]
+		if srv.weight >= b.currentWeight {
+			return srv, nil
+		}
+	}
+}