gist of the idea of a sticky_random balancer

go/vt/vtgateproxy/mysql_server.go

@@ -200,6 +200,7 @@ func (ph *proxyHandler) ComQuery(c *mysql.Conn, query string, callback func(*sql
 		return mysql.NewSQLErrorFromError(err)
 	}
 
+	ctx = context.WithValue(ctx, CONN_ID_KEY, int(c.ConnectionID))
 	result, err := ph.proxy.Execute(ctx, session, query, make(map[string]*querypb.BindVariable))
 
 	if err := mysql.NewSQLErrorFromError(err); err != nil {

go/vt/vtgateproxy/sticky_random_balancer.go

@@ -0,0 +1,82 @@
+/*
+ *
+ * Copyright 2017 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+// Sticky random is a derivative based on the round_robin balancer which uses a Context
+// variable to maintain client-side affinity to a given connection.
+
+package vtgateproxy
+
+import (
+	"math/rand/v2"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/balancer/base"
+	"google.golang.org/grpc/grpclog"
+)
+
+type ConnIdKey string
+
+const CONN_ID_KEY = ConnIdKey("ConnId")
+
+const Name = "sticky_random"
+
+var logger = grpclog.Component("sticky_random")
+
+// newBuilder creates a new roundrobin balancer builder.
+func newStickyRandomBuilder() balancer.Builder {
+	return base.NewBalancerBuilder(Name, &stickyPickerBuilder{}, base.Config{HealthCheck: true})
+}
+
+func init() {
+	balancer.Register(newStickyRandomBuilder())
+}
+
+type stickyPickerBuilder struct{}
+
+func (*stickyPickerBuilder) Build(info base.PickerBuildInfo) balancer.Picker {
+	logger.Infof("stickyRandomPicker: Build called with info: %v", info)
+	if len(info.ReadySCs) == 0 {
+		return base.NewErrPicker(balancer.ErrNoSubConnAvailable)
+	}
+	scs := make([]balancer.SubConn, 0, len(info.ReadySCs))
+	for sc := range info.ReadySCs {
+		scs = append(scs, sc)
+	}
+	return &stickyPicker{
+		subConns: scs,
+	}
+}
+
+type stickyPicker struct {
+	// subConns is the snapshot of the  balancer when this picker was
+	// created. The slice is immutable.
+	subConns []balancer.SubConn
+}
+
+func (p *stickyPicker) Pick(info balancer.PickInfo) (balancer.PickResult, error) {
+
+	subConnsLen := len(p.subConns)
+
+	connId := info.Ctx.Value(CONN_ID_KEY).(int)
+	if connId == 0 {
+		connId = rand.IntN(subConnsLen) // shouldn't happen
+	}
+
+	sc := p.subConns[connId%subConnsLen]
+	return balancer.PickResult{SubConn: sc}, nil
+}