diff --git a/gee-rpc/day7-registry/registry/registry.go b/gee-rpc/day7-registry/registry/registry.go
index 0d50b4d..2c22244 100644
--- a/gee-rpc/day7-registry/registry/registry.go
+++ b/gee-rpc/day7-registry/registry/registry.go
@@ -3,6 +3,7 @@ package registy
 import (
 	"log"
 	"net/http"
+	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -54,6 +55,7 @@ func (r *Registry) aliveServers() []string {
 			delete(r.servers, addr)
 		}
 	}
+	sort.Strings(alive)
 	return alive
 }
 
diff --git a/gee-rpc/day7-registry/xclient/discovery.go b/gee-rpc/day7-registry/xclient/discovery.go
index f823a7b..783ec4a 100644
--- a/gee-rpc/day7-registry/xclient/discovery.go
+++ b/gee-rpc/day7-registry/xclient/discovery.go
@@ -49,15 +49,16 @@ func (d *MultiServersDiscovery) Update(servers []string) error {
 func (d *MultiServersDiscovery) Get(mode SelectMode) (string, error) {
 	d.mu.Lock()
 	defer d.mu.Unlock()
-	if len(d.servers) == 0 {
+	n := len(d.servers)
+	if n == 0 {
 		return "", errors.New("rpc discovery: no available servers")
 	}
 	switch mode {
 	case RandomSelect:
-		return d.servers[d.r.Intn(len(d.servers))], nil
+		return d.servers[d.r.Intn(n)], nil
 	case RoundRobinSelect:
-		s := d.servers[d.index]
-		d.index = (d.index + 1) % len(d.servers)
+		s := d.servers[d.index%n] // servers could be updated, so mode n to ensure safety
+		d.index = (d.index + 1) % n
 		return s, nil
 	default:
 		return "", errors.New("rpc discovery: not supported select mode")