add subscriber coordinator

weed/mq/client/sub_client/subscribe.go

@@ -11,9 +11,14 @@ import (
 
 func (sub *TopicSubscriber) Subscribe() error {
 	util.RetryUntil("subscribe", func() error {
+		// ask balancer for brokers of the topic
 		if err := sub.doLookup(sub.bootstrapBroker); err != nil {
 			return fmt.Errorf("lookup topic %s/%s: %v", sub.ContentConfig.Namespace, sub.ContentConfig.Topic, err)
 		}
+		// treat the first broker as the topic leader
+		// connect to the leader broker
+
+		// subscribe to the topic
 		if err := sub.doProcess(); err != nil {
 			return fmt.Errorf("subscribe topic %s/%s: %v", sub.ContentConfig.Namespace, sub.ContentConfig.Topic, err)
 		}

weed/mq/client/sub_client/subscriber.go

@@ -6,11 +6,13 @@ import (
 )
 
 type SubscriberConfiguration struct {
-	ClientId         string
-	GroupId          string
-	GroupInstanceId  string
-	BootstrapServers []string
-	GrpcDialOption   grpc.DialOption
+	ClientId          string
+	GroupId           string
+	GroupInstanceId   string
+	GroupMinimumPeers int32
+	GroupMaximumPeers int32
+	BootstrapServers  []string
+	GrpcDialOption    grpc.DialOption
 }
 
 type ContentConfiguration struct {