Apply per-primary round-robin when selecting replicas - fix #610

StackExchange.Redis/StackExchange/Redis/ServerEndPoint.cs

@@ -341,6 +341,10 @@ internal void AutoConfigure(PhysicalConnection connection)
             }
         }
 
+        int _nextReplicaOffset;
+        internal uint NextReplicaOffset() // used to round-robin between multiple replicas
+            => (uint) System.Threading.Interlocked.Increment(ref _nextReplicaOffset);
+
         internal Task Close()
         {
             var tmp = interactive;

StackExchange.Redis/StackExchange/Redis/ServerSelectionStrategy.cs

@@ -240,9 +240,11 @@ private ServerEndPoint FindSlave(ServerEndPoint endpoint, RedisCommand command)
             if (endpoint.IsSlave && endpoint.IsSelectable(command)) return endpoint;
 
             var slaves = endpoint.Slaves;
-            for (int i = 0; i < slaves.Length; i++)
+            var len = slaves.Length;
+            uint startOffset = len <= 1 ? 0 : endpoint.NextReplicaOffset();
+            for (int i = 0; i < len; i++)
             {
-                endpoint = slaves[i];
+                endpoint = slaves[(int)(((uint)i + startOffset) % len)];
                 if (endpoint.IsSlave && endpoint.IsSelectable(command)) return endpoint;
             }
             return null;

docs/ReleaseNotes.md

 # Release Notes
 
+## 1.2.3
+
+- fix: when using `redis-cluster` with multiple replicas, use round-robin when selecting replica (#610)
+
 ## 1.2.2 (preview):
 
 - **UNAVAILABLE**: .NET 4.0 support is not in this build, due to [a build issue](https://github.com/dotnet/cli/issues/5993) - looking into solutions