Feat: add load balancer (#1437)

* add balancer
* add p2c balancer
* add http client selector filter

Co-authored-by: yuemoxi <99347745@qq.com>
Co-authored-by: chenzhihui <zhihui_chen@foxmail.com>

selector/p2c/p2c.go

@@ -0,0 +1,74 @@
+package p2c
+
+import (
+	"context"
+	"math/rand"
+	"sync/atomic"
+	"time"
+
+	"github.com/go-kratos/kratos/v2/selector"
+	"github.com/go-kratos/kratos/v2/selector/node/ewma"
+)
+
+const (
+	forcePick = time.Second * 3
+	// Name is balancer name
+	Name = "p2c"
+)
+
+var _ selector.Balancer = &Balancer{}
+
+// New creates a p2c selector.
+func New() selector.Selector {
+	return &selector.Default{
+		NodeBuilder: &ewma.Builder{},
+		Balancer: &Balancer{
+			r: rand.New(rand.NewSource(time.Now().UnixNano())),
+		},
+	}
+}
+
+// Balancer is p2c selector.
+type Balancer struct {
+	r  *rand.Rand
+	lk int64
+}
+
+// choose two distinct nodes.
+func (s *Balancer) prePick(nodes []selector.WeightedNode) (nodeA selector.WeightedNode, nodeB selector.WeightedNode) {
+	a := s.r.Intn(len(nodes))
+	b := s.r.Intn(len(nodes) - 1)
+	if b >= a {
+		b = b + 1
+	}
+	nodeA, nodeB = nodes[a], nodes[b]
+	return
+}
+
+// Pick pick a node.
+func (s *Balancer) Pick(ctx context.Context, nodes []selector.WeightedNode) (selector.WeightedNode, selector.DoneFunc, error) {
+	if len(nodes) == 0 {
+		return nil, nil, selector.ErrNoAvailable
+	} else if len(nodes) == 1 {
+		done := nodes[0].Pick()
+		return nodes[0], done, nil
+	}
+
+	var pc, upc selector.WeightedNode
+	nodeA, nodeB := s.prePick(nodes)
+	// meta.Weight为服务发布者在discovery中设置的权重
+	if nodeB.Weight() > nodeA.Weight() {
+		pc, upc = nodeB, nodeA
+	} else {
+		pc, upc = nodeA, nodeB
+	}
+
+	// 如果落选节点在forceGap期间内从来没有被选中一次，则强制选一次
+	// 利用强制的机会，来触发成功率、延迟的更新
+	if upc.PickElapsed() > forcePick && atomic.CompareAndSwapInt64(&s.lk, 0, 1) {
+		pc = upc
+		atomic.StoreInt64(&s.lk, 0)
+	}
+	done := pc.Pick()
+	return pc, done, nil
+}