From 370736e85449522716683605c66f649c31847850 Mon Sep 17 00:00:00 2001
From: dv <deepakv@microsoft.com>
Date: Fri, 21 May 2021 15:40:52 -0700
Subject: [PATCH] exponential Replication check (#19)

* exponential Replication check

* updating comment

* updating comment

* updating comment
---
 .../ConnectionMultiplexer.cs                  |  7 -----
 src/StackExchange.Redis/PhysicalBridge.cs     | 15 +++++++---
 src/StackExchange.Redis/ServerEndPoint.cs     | 29 +++++++++++++++++--
 3 files changed, 37 insertions(+), 14 deletions(-)

diff --git a/src/StackExchange.Redis/ConnectionMultiplexer.cs b/src/StackExchange.Redis/ConnectionMultiplexer.cs
index 2e6146dcf..43ef1649d 100644
--- a/src/StackExchange.Redis/ConnectionMultiplexer.cs
+++ b/src/StackExchange.Redis/ConnectionMultiplexer.cs
@@ -3175,13 +3175,6 @@ public Task<long> PublishReconfigureAsync(CommandFlags flags = CommandFlags.None
         /// <param name="key">The <see cref="RedisKey"/> to determine the hash slot for.</param>
         public int GetHashSlot(RedisKey key) => ServerSelectionStrategy.HashSlot(key);
 
-        internal void MarkServerEndpointsForReplicationRoleRefresh()
-        {
-            foreach (var s in servers.Values)
-            {
-                ((ServerEndPoint)s).ForceReplicationCheck = true;
-            }
-        }
     }
 
     internal enum WriteResult
diff --git a/src/StackExchange.Redis/PhysicalBridge.cs b/src/StackExchange.Redis/PhysicalBridge.cs
index 50f0d68f0..2877b8cba 100644
--- a/src/StackExchange.Redis/PhysicalBridge.cs
+++ b/src/StackExchange.Redis/PhysicalBridge.cs
@@ -427,10 +427,17 @@ internal void OnDisconnected(ConnectionFailureType failureType, PhysicalConnecti
             oldState = default(State); // only defined when isCurrent = true
             if (isCurrent = (physical == connection))
             {
-                Multiplexer.MarkServerEndpointsForReplicationRoleRefresh();
-
                 Trace("Bridge noting disconnect from active connection" + (isDisposed ? " (disposed)" : ""));
                 oldState = ChangeState(State.Disconnected);
+                if(oldState == State.ConnectedEstablished && !ServerEndPoint.IsReplica)
+                {
+                    // if the disconnected endpoint was a master endpoint run info replication
+                    // more frequently on it's replica with exponential increments
+                    foreach (var r in ServerEndPoint.Replicas)
+                    {
+                        r.ForceExponentiallyReplicationCheck();
+                    }
+                }
                 physical = null;
 
                 if (!isDisposed && Interlocked.Increment(ref failConnectCount) == 1)
@@ -538,10 +545,10 @@ internal void OnHeartbeat(bool ifConnectedOnly)
                             }
                             tmp.OnBridgeHeartbeat();
                             int writeEverySeconds = ServerEndPoint.WriteEverySeconds,
-                                checkConfigSeconds = Multiplexer.RawConfig.ConfigCheckSeconds;
+                                checkConfigSeconds = ServerEndPoint.ConfigCheckSeconds;
 
                             if (state == (int)State.ConnectedEstablished && ConnectionType == ConnectionType.Interactive
-                                && ((checkConfigSeconds > 0 && ServerEndPoint.LastInfoReplicationCheckSecondsAgo >= checkConfigSeconds) || ServerEndPoint.ForceReplicationCheck)
+                                && ((checkConfigSeconds > 0 && ServerEndPoint.LastInfoReplicationCheckSecondsAgo >= checkConfigSeconds))
                                 && ServerEndPoint.CheckInfoReplication())
                             {
                                 // that serves as a keep-alive, if it is accepted
diff --git a/src/StackExchange.Redis/ServerEndPoint.cs b/src/StackExchange.Redis/ServerEndPoint.cs
index b516588af..1b516af5e 100644
--- a/src/StackExchange.Redis/ServerEndPoint.cs
+++ b/src/StackExchange.Redis/ServerEndPoint.cs
@@ -56,7 +56,7 @@ public ServerEndPoint(ConnectionMultiplexer multiplexer, EndPoint endpoint)
             databases = 0;
             writeEverySeconds = config.KeepAlive > 0 ? config.KeepAlive : 60;
             serverType = ServerType.Standalone;
-
+            ConfigCheckSeconds = Multiplexer.RawConfig.ConfigCheckSeconds;
             // overrides for twemproxy
             if (multiplexer.RawConfig.Proxy == Proxy.Twemproxy)
             {
@@ -537,10 +537,14 @@ public EndPoint MasterEndPoint
             set { SetConfig(ref masterEndPoint, value); }
         }
 
+        internal volatile int ConfigCheckSeconds;
+
         internal bool CheckInfoReplication()
         {
             lastInfoReplicationCheckTicks = Environment.TickCount;
-            ForceReplicationCheck = false;
+
+            ResetExponentiallyReplicationCheck();
+
             PhysicalBridge bridge;
             if (version >= RedisFeatures.v2_8_0 && Multiplexer.CommandMap.IsAvailable(RedisCommand.INFO)
                 && (bridge = GetBridge(ConnectionType.Interactive, false)) != null)
@@ -556,7 +560,26 @@ internal bool CheckInfoReplication()
         }
 
         private int lastInfoReplicationCheckTicks;
-        internal volatile bool ForceReplicationCheck;
+        [ThreadStatic]
+        private static Random r;
+
+
+        // Forces frequent replication check starting from 1 second upto max ConfigCheckSeconds with an exponential increment
+        internal void ForceExponentiallyReplicationCheck()
+        {
+                ConfigCheckSeconds = 1;  // start checking info replication more frequently
+        }
+
+        private void ResetExponentiallyReplicationCheck()
+        {
+            if (ConfigCheckSeconds < Multiplexer.RawConfig.ConfigCheckSeconds)
+            {
+                r = r ?? new Random();
+                var newExponentialConfigCheck = ConfigCheckSeconds * 2;
+                var jitter = r.Next(ConfigCheckSeconds + 1, newExponentialConfigCheck);
+                ConfigCheckSeconds = Math.Min(jitter, Multiplexer.RawConfig.ConfigCheckSeconds);
+            }
+        }
 
         private int _heartBeatActive;
         internal void OnHeartbeat()