Twemproxy support

StackExchange.Redis.Tests/Issues/Issue6.cs

+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using NUnit.Framework;
+
+namespace StackExchange.Redis.Tests.Issues
+{
+    [TestFixture]
+    public class Issue6 :  TestBase
+    {
+        [Test]
+        public void ShouldWorkWithoutEchoOrPing()
+        {
+            using(var conn = Create(proxy: Proxy.Twemproxy))
+            {
+                Console.WriteLine("config: " + conn.Configuration);
+                var db = conn.GetDatabase();
+                var time = db.Ping();
+                Console.WriteLine("ping time: " + time);
+            }
+        }
+    }
+}

StackExchange.Redis.Tests/StackExchange.Redis.Tests.csproj

@@ -70,6 +70,7 @@
     <Compile Include="Expiry.cs" />
     <Compile Include="Config.cs" />
     <Compile Include="FloatingPoint.cs" />
+    <Compile Include="Issues\Issue6.cs" />
     <Compile Include="Keys.cs" />
     <Compile Include="KeysAndValues.cs" />
     <Compile Include="Lists.cs" />

StackExchange.Redis.Tests/TestBase.cs

@@ -144,7 +144,7 @@ protected IServer GetServer(ConnectionMultiplexer muxer)
             string clientName = null, int? syncTimeout = null, bool? allowAdmin = null, int? keepAlive = null,
             int? connectTimeout = null, string password = null, string tieBreaker = null, TextWriter log = null,
             bool fail = true, string[] disabledCommands = null, bool checkConnect = true, bool pause = true, string failMessage = null,
-            string channelPrefix = null, bool useSharedSocketManager = true)
+            string channelPrefix = null, bool useSharedSocketManager = true, Proxy? proxy = null)
         {
             if(pause) Thread.Sleep(500); // get a lot of glitches when hammering new socket creations etc; pace it out a bit
             string configuration = GetConfiguration();
@@ -171,6 +171,7 @@ protected IServer GetServer(ConnectionMultiplexer muxer)
             if (allowAdmin != null) config.AllowAdmin = allowAdmin.Value;
             if (keepAlive != null) config.KeepAlive = keepAlive.Value;
             if (connectTimeout != null) config.ConnectTimeout = connectTimeout.Value;
+            if (proxy != null) config.Proxy = proxy.Value;
             var watch = Stopwatch.StartNew();
             var task = ConnectionMultiplexer.ConnectAsync(config, log ?? Console.Out);
             if (!task.Wait(config.ConnectTimeout >= (int.MaxValue / 2) ? int.MaxValue : config.ConnectTimeout * 2))

StackExchange.Redis/StackExchange/Redis/CommandMap.cs

@@ -9,7 +9,36 @@ namespace StackExchange.Redis
     /// </summary>
     public sealed class CommandMap
     {
-        private static readonly CommandMap @default = CreateImpl(null);
+        private static readonly CommandMap
+            @default = CreateImpl(null, null),
+            twemproxy = CreateImpl(null, exclusions: new HashSet<RedisCommand>
+        {
+            // see https://github.com/twitter/twemproxy/blob/master/notes/redis.md
+            RedisCommand.KEYS, RedisCommand.MIGRATE, RedisCommand.MOVE, RedisCommand.OBJECT, RedisCommand.RANDOMKEY,
+            RedisCommand.RENAME, RedisCommand.RENAMENX, RedisCommand.SORT, RedisCommand.SCAN,
+
+            RedisCommand.BITOP, RedisCommand.MSET, RedisCommand.MSETNX,
+
+            RedisCommand.HSCAN,
+
+            RedisCommand.BLPOP, RedisCommand.BRPOP, RedisCommand.BRPOPLPUSH, // yeah, me neither!
+
+            RedisCommand.SSCAN,
+
+            RedisCommand.ZSCAN,
+
+            RedisCommand.PSUBSCRIBE, RedisCommand.PUBLISH, RedisCommand.PUNSUBSCRIBE, RedisCommand.SUBSCRIBE, RedisCommand.UNSUBSCRIBE,
+
+            RedisCommand.DISCARD, RedisCommand.EXEC, RedisCommand.MULTI, RedisCommand.UNWATCH, RedisCommand.WATCH,
+
+            RedisCommand.SCRIPT,
+
+            RedisCommand.AUTH, RedisCommand.ECHO, RedisCommand.PING, RedisCommand.QUIT, RedisCommand.SELECT,
+
+            RedisCommand.BGREWRITEAOF, RedisCommand.BGSAVE, RedisCommand.CLIENT, RedisCommand.CLUSTER, RedisCommand.CONFIG, RedisCommand.DBSIZE,
+            RedisCommand.DEBUG, RedisCommand.FLUSHALL, RedisCommand.FLUSHDB, RedisCommand.INFO, RedisCommand.LASTSAVE, RedisCommand.MONITOR, RedisCommand.SAVE,
+            RedisCommand.SHUTDOWN, RedisCommand.SLAVEOF, RedisCommand.SLOWLOG, RedisCommand.SYNC, RedisCommand.TIME
+        });
         private readonly byte[][] map;
 
         internal CommandMap(byte[][] map)
@@ -21,6 +50,12 @@ internal CommandMap(byte[][] map)
         /// </summary>
         public static CommandMap Default { get { return @default; } }
 
+        /// <summary>
+        /// The commands available to <a href="twemproxy">https://github.com/twitter/twemproxy</a>
+        /// </summary>
+        /// <remarks>https://github.com/twitter/twemproxy/blob/master/notes/redis.md</remarks>
+        public static CommandMap Twemproxy {  get { return twemproxy; } }
+
         /// <summary>
         /// Create a new CommandMap, customizing some commands
         /// </summary>
@@ -28,8 +63,65 @@ public static CommandMap Create(Dictionary<string, string> overrides)
         {
             if (overrides == null || overrides.Count == 0) return Default;
 
-            return CreateImpl(overrides);
+            if (ReferenceEquals(overrides.Comparer, StringComparer.OrdinalIgnoreCase) ||
+                ReferenceEquals(overrides.Comparer, StringComparer.InvariantCultureIgnoreCase))
+            {
+                // that's ok; we're happy with ordinal/invariant case-insensitive
+                // (but not culture-specific insensitive; completely untested)
+            }
+            else
+            {
+                // need case insensitive
+                overrides = new Dictionary<string, string>(overrides, StringComparer.OrdinalIgnoreCase);
+            }
+            return CreateImpl(overrides, null);
+        }
+
+        /// <summary>
+        /// Creates a CommandMap by specifying which commands are available or unavailable
+        /// </summary>
+        public static CommandMap Create(HashSet<string> commands, bool available = true)
+        {
+            
+            if (available)
+            {
+                var dictionary = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                // nix everything
+                foreach (RedisCommand command in Enum.GetValues(typeof(RedisCommand)))
+                {
+                    dictionary[command.ToString()] = null;
+                }
+                if (commands != null)
+                {
+                    // then include (by removal) the things that are available
+                    foreach (string command in commands)
+                    {
+                        dictionary.Remove(command);
+                    }
+                }
+                return CreateImpl(dictionary, null);
+            }
+            else
+            {
+                HashSet<RedisCommand> exclusions = null;
+                if (commands != null)
+                {
+                    // nix the things that are specified
+                    foreach (var command in commands)
+                    {
+                        RedisCommand parsed;
+                        if (Enum.TryParse(command, true, out parsed))
+                        {
+                            (exclusions ?? (exclusions = new HashSet<RedisCommand>())).Add(parsed);
+                        }
+                    }
+                }
+                if (exclusions == null || exclusions.Count == 0) return Default;
+                return CreateImpl(null, exclusions);
+            }
+            
         }
+
         /// <summary>
         /// See Object.ToString()
         /// </summary>
@@ -69,33 +161,37 @@ internal bool IsAvailable(RedisCommand command)
             return map[(int)command] != null;
         }
 
-        private static CommandMap CreateImpl(Dictionary<string, string> overrides)
+        private static CommandMap CreateImpl(Dictionary<string, string> caseInsensitiveOverrides, HashSet<RedisCommand> exclusions)
         {
-            RedisCommand[] values = (RedisCommand[])Enum.GetValues(typeof(RedisCommand));
+            var commands = (RedisCommand[])Enum.GetValues(typeof(RedisCommand));
 
-            byte[][] map = new byte[values.Length][];
+            byte[][] map = new byte[commands.Length][];
             bool haveDelta = false;
-            for (int i = 0; i < values.Length; i++)
+            for (int i = 0; i < commands.Length; i++)
             {
-                int idx = (int)values[i];
-                string name = values[i].ToString(), value = name;
+                int idx = (int)commands[i];
+                string name = commands[i].ToString(), value = name;
 
-                if (overrides != null)
+                if (exclusions != null && exclusions.Contains(commands[i]))
+                {
+                    map[idx] = null;
+                }
+                else
                 {
-                    foreach (var pair in overrides)
+                    if (caseInsensitiveOverrides != null)
                     {
-                        if (string.Equals(name, pair.Key, StringComparison.OrdinalIgnoreCase))
+                        string tmp;
+                        if (caseInsensitiveOverrides.TryGetValue(name, out tmp))
                         {
-                            value = pair.Value;
-                            break;
+                            value = tmp;
                         }
                     }
-                }
-                if (value != name) haveDelta = true;
+                    if (value != name) haveDelta = true;
 
-                haveDelta = true;
-                byte[] val = string.IsNullOrWhiteSpace(value) ? null : Encoding.UTF8.GetBytes(value);
-                map[idx] = val;
+                    haveDelta = true;
+                    byte[] val = string.IsNullOrWhiteSpace(value) ? null : Encoding.UTF8.GetBytes(value);
+                    map[idx] = val;
+                }
             }
             if (!haveDelta && @default != null) return @default;
 

StackExchange.Redis/StackExchange/Redis/CommandTrace.cs

@@ -76,7 +76,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             {
                 switch(result.Type)
                 {
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var parts = result.GetItems();
                         CommandTrace[] arr = new CommandTrace[parts.Length];
                         for (int i = 0; i < parts.Length; i++)

StackExchange.Redis/StackExchange/Redis/ConfigurationOptions.cs

@@ -8,6 +8,22 @@
 
 namespace StackExchange.Redis
 {
+
+    /// <summary>
+    /// Specifies the proxy that is being used to communicate to redis
+    /// </summary>
+    public enum Proxy
+    {
+        /// <summary>
+        /// Direct communication to the redis server(s)
+        /// </summary>
+        None,
+        /// <summary>
+        /// Communication via <a href="https://github.com/twitter/twemproxy">twemproxy</a>
+        /// </summary>
+        Twemproxy
+    }
+
     /// <summary>
     /// The options relevant to a set of redis connections
     /// </summary>
@@ -20,7 +36,7 @@ public sealed class ConfigurationOptions : ICloneable
                         VersionPrefix = "version=", ConnectTimeoutPrefix = "connectTimeout=", PasswordPrefix = "password=",
                         TieBreakerPrefix = "tiebreaker=", WriteBufferPrefix = "writeBuffer=", SslHostPrefix = "sslHost=",
                         ConfigChannelPrefix = "configChannel=", AbortOnConnectFailPrefix = "abortConnect=", ResolveDnsPrefix = "resolveDns=",
-                        ChannelPrefixPrefix = "channelPrefix=";
+                        ChannelPrefixPrefix = "channelPrefix=", ProxyPrefix = "proxy=";
 
         private readonly EndPointCollection endpoints = new EndPointCollection();
 
@@ -31,20 +47,13 @@ public sealed class ConfigurationOptions : ICloneable
 
         private bool? allowAdmin, abortOnConnectFail, resolveDns;
 
+        private Proxy? proxy;
+        private CommandMap commandMap;
         private string clientName, serviceName, password, tieBreaker, sslHost, configChannel;
         private Version defaultVersion;
 
         private int? keepAlive, syncTimeout, connectTimeout, writeBuffer;
-        /// <summary>
-        /// Create a new ConfigurationOptions instance
-        /// </summary>
-        public ConfigurationOptions()
-        {
-            CommandMap = CommandMap.Default;
-        }
-
-
-
+        
         /// <summary>
         /// A LocalCertificateSelectionCallback delegate responsible for selecting the certificate used for authentication; note
         /// that this cannot be specified in the configuration-string.
@@ -63,7 +72,12 @@ public ConfigurationOptions()
         /// Gets or sets the SocketManager instance to be used with these options; if this is null a per-multiplexer
         /// SocketManager is created automatically.
         /// </summary>
-        public SocketManager SocketManager {  get;set; }
+        public SocketManager SocketManager {  get;set; }
+
+        /// <summary>
+        /// Indicates whether admin operations should be allowed
+        /// </summary>
+        public Proxy Proxy { get { return proxy.GetValueOrDefault(); } set { proxy = value; } }
 
         /// <summary>
         /// Indicates whether admin operations should be allowed
@@ -83,7 +97,24 @@ public ConfigurationOptions()
         /// <summary>
         /// The command-map associated with this configuration
         /// </summary>
-        public CommandMap CommandMap { get; set; }
+        public CommandMap CommandMap
+        {
+            get
+            {
+                if (commandMap != null) return commandMap;
+                switch(Proxy)
+                {
+                    case Redis.Proxy.Twemproxy:
+                        return CommandMap.Twemproxy;
+                    default:
+                        return CommandMap.Default;
+                }
+            }
+            set {
+                if (value == null) throw new ArgumentNullException("value");
+                commandMap = value;
+            }
+        }
 
         /// <summary>
         /// Channel to use for broadcasting and listening for configuration change notification
@@ -180,7 +211,8 @@ public ConfigurationOptions Clone()
                 configChannel = configChannel,
                 abortOnConnectFail = abortOnConnectFail,
                 resolveDns = resolveDns,
-                CommandMap = CommandMap,
+                proxy = proxy,
+                commandMap = commandMap,
                 CertificateValidationCallback = CertificateValidationCallback,
                 CertificateSelectionCallback = CertificateSelectionCallback,
                 ChannelPrefix = ChannelPrefix.Clone(),
@@ -217,7 +249,8 @@ public override string ToString()
             Append(sb, AbortOnConnectFailPrefix, abortOnConnectFail);
             Append(sb, ResolveDnsPrefix, resolveDns);
             Append(sb, ChannelPrefixPrefix, (string)ChannelPrefix);
-            CommandMap.AppendDeltas(sb);
+            Append(sb, ProxyPrefix, proxy);
+            if(commandMap != null) commandMap.AppendDeltas(sb);
             return sb.ToString();
         }
 
@@ -301,9 +334,10 @@ void Clear()
             allowAdmin = abortOnConnectFail = resolveDns = null;
             defaultVersion = null;
             endpoints.Clear();
+            commandMap = null;
+
             CertificateSelection = null;
-            CertificateValidation = null;
-            CommandMap = CommandMap.Default;
+            CertificateValidation = null;            
             ChannelPrefix = default(RedisChannel);
             SocketManager = null;
         }
@@ -396,12 +430,16 @@ private void DoParse(string configuration)
                         {
                             int tmp;
                             if (Format.TryParseInt32(value.Trim(), out tmp)) WriteBuffer = tmp;
+                        } else if(IsOption(option, ProxyPrefix))
+                        {
+                            Proxy tmp;
+                            if (Enum.TryParse(option, true, out tmp)) Proxy = tmp;
                         }
                         else if(option[0]=='$')
                         {
                             RedisCommand cmd;
                             option = option.Substring(1, idx-1);
-                            if (Enum.TryParse<RedisCommand>(option, true, out cmd))
+                            if (Enum.TryParse(option, true, out cmd))
                             {
                                 if (map == null) map = new Dictionary<string, string>(StringComparer.InvariantCultureIgnoreCase);
                                 map[option] = value;
@@ -418,7 +456,10 @@ private void DoParse(string configuration)
                         if (ep != null && !endpoints.Contains(ep)) endpoints.Add(ep);
                     }
                 }
-                this.CommandMap = CommandMap.Create(map);
+                if (map != null && map.Count != 0)
+                {
+                    this.CommandMap = CommandMap.Create(map);
+                }
             }
         }
     }

StackExchange.Redis/StackExchange/Redis/ConnectionMultiplexer.cs

@@ -793,10 +793,15 @@ private ConnectionMultiplexer(ConfigurationOptions configuration)
             IncludeDetailInExceptions = true;
             
             this.configuration = configuration;
-            this.CommandMap = configuration.CommandMap;
-            this.CommandMap.AssertAvailable(RedisCommand.PING);
-            this.CommandMap.AssertAvailable(RedisCommand.ECHO);
-            if (!string.IsNullOrWhiteSpace(configuration.Password)) this.CommandMap.AssertAvailable(RedisCommand.AUTH);
+            
+            var map = this.CommandMap = configuration.CommandMap;
+            if (!string.IsNullOrWhiteSpace(configuration.Password)) map.AssertAvailable(RedisCommand.AUTH);
+
+            if(!map.IsAvailable(RedisCommand.ECHO) && !map.IsAvailable(RedisCommand.PING) && !map.IsAvailable(RedisCommand.TIME))
+            { // I mean really, give me a CHANCE! I need *something* to check the server is available to me...
+                // see also: SendTracer (matching logic)
+                map.AssertAvailable(RedisCommand.EXISTS);
+            }
 
             PreserveAsyncOrder = true; // safest default
             this.timeoutMilliseconds = configuration.SyncTimeout;
@@ -857,6 +862,7 @@ internal static long LastGlobalHeartbeatSecondsAgo
         /// </summary>
         public ISubscriber GetSubscriber(object asyncState = null)
         {
+            if (RawConfig.Proxy == Proxy.Twemproxy) throw new NotSupportedException("The pub/sub API is not available via twemproxy");
             return new RedisSubscriber(this, asyncState);
         }
         /// <summary>
@@ -865,6 +871,7 @@ public ISubscriber GetSubscriber(object asyncState = null)
         public IDatabase GetDatabase(int db = 0, object asyncState = null)
         {
             if (db < 0) throw new ArgumentOutOfRangeException("db");
+            if (db != 0 && RawConfig.Proxy == Proxy.Twemproxy) throw new NotSupportedException("Twemproxy only supports database 0");
             return new RedisDatabase(this, db, asyncState);
         }
 
@@ -897,7 +904,7 @@ public IServer GetServer(IPAddress host, int port)
         public IServer GetServer(EndPoint endpoint, object asyncState = null)
         {
             if (endpoint == null) throw new ArgumentNullException("endpoint");
-
+            if (RawConfig.Proxy == Proxy.Twemproxy) throw new NotSupportedException("The server API is not available via twemproxy");
             var server = (ServerEndPoint)servers[endpoint];
             if (server == null) throw new ArgumentException("The specified endpoint is not defined", "endpoint");
             return new RedisServer(this, server, asyncState);
@@ -1079,10 +1086,10 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
                     }
                     foreach (var server in serverSnapshot)
                     {
-                        server.Activate(RedisCommand.ECHO);
+                        server.Activate(ConnectionType.Interactive);
                         if (this.CommandMap.IsAvailable(RedisCommand.SUBSCRIBE))
                         {
-                            server.Activate(RedisCommand.SUBSCRIBE);
+                            server.Activate(ConnectionType.Subscription);
                         }
                     }
                 }
@@ -1110,8 +1117,8 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
                     if (reconfigureAll && server.IsConnected)
                     {
                         LogLocked(log, "Refreshing {0}...", Format.ToString(server.EndPoint));
-                        // note that these will be processed synchronously *BEFORE* the PONG is processed,
-                        // so we know that the configuration will be up to date if we see the PONG
+                        // note that these will be processed synchronously *BEFORE* the tracer is processed,
+                        // so we know that the configuration will be up to date if we see the tracer
                         server.AutoConfigure(null);
                     }
                     available[i] = server.SendTracer();
@@ -1159,6 +1166,7 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
 
                             switch (server.ServerType)
                             {
+                                case ServerType.Twemproxy:
                                 case ServerType.Standalone:
                                     servers[i].ClearUnselectable(UnselectableFlags.ServerType);
                                     standaloneCount++;
@@ -1202,7 +1210,7 @@ internal async Task<bool> ReconfigureAsync(bool first, bool reconfigureAll, Text
 
                 if (clusterCount == 0)
                 {
-                    this.serverSelectionStrategy.ServerType = ServerType.Standalone;
+                    this.serverSelectionStrategy.ServerType = RawConfig.Proxy == Proxy.Twemproxy ? ServerType.Twemproxy : ServerType.Standalone;
                     var preferred = await NominatePreferredMaster(log, servers, useTieBreakers, tieBreakers, masters).ObserveErrors().ForAwait();
                     foreach (var master in masters)
                     {
@@ -1437,12 +1445,16 @@ private bool TryPushMessageToBridge<T>(Message message, ResultProcessor<T> proce
                     throw ExceptionFactory.MasterOnly(IncludeDetailInExceptions, message.Command, message, server);
                 }
 
-                if (server.ServerType == ServerType.Cluster)
+                switch(server.ServerType)
                 {
-                    if (message.GetHashSlot(ServerSelectionStrategy) == ServerSelectionStrategy.MultipleSlots)
-                    {
-                        throw ExceptionFactory.MultiSlot(IncludeDetailInExceptions, message);
-                    }
+                    case ServerType.Cluster:
+                    case ServerType.Twemproxy: // strictly speaking twemproxy uses a different hashing algo, but the hash-tag behavior is
+                                               // the same, so this does a pretty good job of spotting illegal commands before sending them
+                        if (message.GetHashSlot(ServerSelectionStrategy) == ServerSelectionStrategy.MultipleSlots)
+                        {
+                            throw ExceptionFactory.MultiSlot(IncludeDetailInExceptions, message);
+                        }
+                        break;
                 }
                 if (!server.IsConnected)
                 {

StackExchange.Redis/StackExchange/Redis/Message.cs

@@ -278,6 +278,9 @@ public static bool IsMasterOnly(RedisCommand command)
             {
                 case RedisCommand.APPEND:
                 case RedisCommand.BITOP:
+                case RedisCommand.BLPOP:
+                case RedisCommand.BRPOP:
+                case RedisCommand.BRPOPLPUSH:
                 case RedisCommand.DECR:
                 case RedisCommand.DECRBY:
                 case RedisCommand.DEL:
@@ -305,6 +308,7 @@ public static bool IsMasterOnly(RedisCommand command)
                 case RedisCommand.MIGRATE:
                 case RedisCommand.MOVE:
                 case RedisCommand.MSET:
+                case RedisCommand.MSETNX:
                 case RedisCommand.PERSIST:
                 case RedisCommand.PEXPIRE:
                 case RedisCommand.PEXPIREAT:

StackExchange.Redis/StackExchange/Redis/PhysicalBridge.cs

@@ -236,11 +236,8 @@ internal void KeepAlive()
             switch (connectionType)
             {
                 case ConnectionType.Interactive:
-                    if (commandMap.IsAvailable(RedisCommand.PING))
-                    {
-                        msg = Message.Create(-1, CommandFlags.FireAndForget, RedisCommand.PING);
-                        msg.SetSource(ResultProcessor.DemandPONG, null);
-                    }
+                    msg = serverEndPoint.GetTracerMessage(false);
+                    msg.SetSource(ResultProcessor.Tracer, null);
                     break;
                 case ConnectionType.Subscription:
                     if (commandMap.IsAvailable(RedisCommand.UNSUBSCRIBE))

StackExchange.Redis/StackExchange/Redis/PhysicalConnection.cs

@@ -294,7 +294,7 @@ internal Message GetSelectDatabaseCommand(int targetDatabase, Message message)
                 var serverEndpoint = bridge.ServerEndPoint;
                 int available = serverEndpoint.Databases;
 
-                if (!serverEndpoint.HasDatabases) // only db0 is available on cluster
+                if (!serverEndpoint.HasDatabases) // only db0 is available on cluster/twemproxy
                 {
                     if (targetDatabase != 0)
                     { // should never see this, since the API doesn't allow it; thus not too worried about ExceptionFactory
@@ -538,7 +538,7 @@ int EnsureSpaceAndComputeBytesToRead()
         void MatchResult(RawResult result)
         {
             // check to see if it could be an out-of-band pubsub message
-            if (connectionType == ConnectionType.Subscription && result.Type == ResultType.Array)
+            if (connectionType == ConnectionType.Subscription && result.Type == ResultType.MultiBulk)
             {   // out of band message does not match to a queued message
                 var items = result.GetItems();
                 if (items.Length >= 3 && items[0].Assert(message))

StackExchange.Redis/StackExchange/Redis/RawResult.cs

@@ -34,7 +34,7 @@ public RawResult(ResultType resultType, byte[] buffer, int offset, int count)
         public RawResult(RawResult[] arr)
         {
             if (arr == null) throw new ArgumentNullException("arr");
-            this.resultType = ResultType.Array;
+            this.resultType = ResultType.MultiBulk;
             this.offset = 0;
             this.count = arr.Length;
             this.arr = arr;
@@ -59,7 +59,7 @@ public override string ToString()
                     return string.Format("{0}: {1}", resultType, GetString());
                 case ResultType.BulkString:
                     return string.Format("{0}: {1} bytes", resultType, count);
-                case ResultType.Array:
+                case ResultType.MultiBulk:
                     return string.Format("{0}: {1} items", resultType, count);
                 default:
                     return "(unknown)";

StackExchange.Redis/StackExchange/Redis/RedisBase.cs

@@ -16,16 +16,29 @@ internal RedisBase(ConnectionMultiplexer multiplexer, object asyncState)
             this.asyncState = asyncState;
         }
 
-
+        private ResultProcessor.TimingProcessor.TimerMessage GetTimerMessage(CommandFlags flags)
+        {
+            // do the best we can with available commands
+            var map = multiplexer.CommandMap;
+            if(map.IsAvailable(RedisCommand.PING))
+                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.PING);
+            if(map.IsAvailable(RedisCommand.TIME))
+                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.TIME);
+            if (map.IsAvailable(RedisCommand.ECHO))
+                return ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.ECHO, RedisLiterals.PING);
+            // as our fallback, we'll do something odd... we'll treat a key like a value, out of sheer desperation
+            // note: this usually means: twemproxy - in which case we're fine anyway, since the proxy does the routing
+            return ResultProcessor.TimingProcessor.CreateMessage(0, flags, RedisCommand.EXISTS, (RedisValue)multiplexer.UniqueId);
+        }
         public virtual TimeSpan Ping(CommandFlags flags = CommandFlags.None)
         {
-            var msg = ResultProcessor.TimingProcessor.CreateMessage(flags, RedisCommand.PING);
+            var msg = GetTimerMessage(flags);
             return ExecuteSync(msg, ResultProcessor.ResponseTimer);
         }
 
         public virtual Task<TimeSpan> PingAsync(CommandFlags flags = CommandFlags.None)
         {
-            var msg = ResultProcessor.TimingProcessor.CreateMessage(flags, RedisCommand.PING);
+            var msg = GetTimerMessage(flags);
             return ExecuteAsync(msg, ResultProcessor.ResponseTimer);
         }
 

StackExchange.Redis/StackExchange/Redis/RedisCommand.cs

@@ -11,6 +11,9 @@ enum RedisCommand
         BITCOUNT,
         BITOP,
         BITPOS,
+        BLPOP,
+        BRPOP,
+        BRPOPLPUSH,
 
         CLIENT,
         CLUSTER,
@@ -50,6 +53,7 @@ enum RedisCommand
         HLEN,
         HMGET,
         HMSET,
+        HSCAN,
         HSET,
         HSETNX,
         HVALS,
@@ -78,8 +82,11 @@ enum RedisCommand
         MONITOR,
         MOVE,
         MSET,
+        MSETNX,
         MULTI,
 
+        OBJECT,
+
         PERSIST,
         PEXPIRE,
         PEXPIREAT,

StackExchange.Redis/StackExchange/Redis/RedisDatabase.cs

@@ -1688,6 +1688,7 @@ private Message GetStringSetMessage(KeyValuePair<RedisKey, RedisValue>[] values,
                 case 0: return null;
                 case 1: return GetStringSetMessage(values[0].Key, values[0].Value, null, when, flags);
                 default:
+                    WhenAlwaysOrNotExists(when);
                     int slot = ServerSelectionStrategy.NoSlot, offset = 0;
                     var args = new RedisValue[values.Length * 2];
                     var serverSelectionStrategy = multiplexer.ServerSelectionStrategy;
@@ -1697,7 +1698,7 @@ private Message GetStringSetMessage(KeyValuePair<RedisKey, RedisValue>[] values,
                         args[offset++] = values[i].Value;
                         slot = serverSelectionStrategy.CombineSlot(slot, values[i].Key);
                     }
-                    return Message.CreateInSlot(Db, slot, flags, RedisCommand.MSET, args);
+                    return Message.CreateInSlot(Db, slot, flags, when == When.NotExists ? RedisCommand.MSETNX : RedisCommand.MSET, args);
             }
         }
         Message GetStringSetMessage(RedisKey key, RedisValue value, TimeSpan? expiry = null, When when = When.Always, CommandFlags flags = CommandFlags.None)
@@ -1780,10 +1781,10 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                 {
                     switch (result.Type)
                     {
-                        case ResultType.Array:
+                        case ResultType.MultiBulk:
                             var arr = result.GetItems();
                             long i64;
-                            if (arr.Length == 2 && arr[1].Type == ResultType.Array && arr[0].TryGetInt64(out i64))
+                            if (arr.Length == 2 && arr[1].Type == ResultType.MultiBulk && arr[0].TryGetInt64(out i64))
                             {
                                 var sscanResult = new SetScanResult(i64, arr[1].GetItemsAsValues());
                                 SetResult(message, sscanResult);

StackExchange.Redis/StackExchange/Redis/RedisLiterals.cs

@@ -51,6 +51,7 @@ public static readonly RedisValue
             LOAD = "LOAD",
             EXISTS = "EXISTS",
             FLUSH = "FLUSH",
+            PING = "PING",
 
             // DO NOT CHANGE CASE: these are configuration settings and MUST be as-is
             databases = "databases",
@@ -62,8 +63,9 @@ public static readonly RedisValue
             server = "server",
             Wildcard = "*";
 
-        public static readonly byte[] OK = Encoding.UTF8.GetBytes("OK");
+        public static readonly byte[] BytesOK = Encoding.UTF8.GetBytes("OK");
         public static readonly byte[] ByteWildcard = { (byte)'*' };
+        public static readonly byte[] BytesPONG = Encoding.UTF8.GetBytes("PONG");
 
 
         internal static RedisValue Get(Bitwise operation)

StackExchange.Redis/StackExchange/Redis/RedisResult.cs

@@ -20,7 +20,7 @@ internal static RedisResult TryCreate(PhysicalConnection connection, RawResult r
                     case ResultType.SimpleString:
                     case ResultType.BulkString:
                         return new SingleRedisResult(result.AsRedisValue());
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var items = result.GetItems();
                         var arr = new RedisResult[items.Length];
                         for (int i = 0; i < arr.Length; i++)

StackExchange.Redis/StackExchange/Redis/RedisServer.cs

@@ -547,10 +547,10 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                 {
                     switch (result.Type)
                     {
-                        case ResultType.Array:
+                        case ResultType.MultiBulk:
                             var arr = result.GetItems();
                             long i64;
-                            if (arr.Length == 2 && arr[1].Type == ResultType.Array && arr[0].TryGetInt64(out i64))
+                            if (arr.Length == 2 && arr[1].Type == ResultType.MultiBulk && arr[0].TryGetInt64(out i64))
                             {
                                 var keysResult = new KeysScanResult(i64, arr[1].GetItemsAsKeys());
                                 SetResult(message, keysResult);

StackExchange.Redis/StackExchange/Redis/RedisSubscriber.cs

@@ -17,7 +17,7 @@ public override TimeSpan Ping(CommandFlags flags = CommandFlags.None)
         {
             // can't use regular PING, but we can unsubscribe from something random that we weren't even subscribed to...
             RedisValue channel = Guid.NewGuid().ToByteArray();
-            var msg = ResultProcessor.TimingProcessor.CreateMessage(flags, RedisCommand.UNSUBSCRIBE, channel);
+            var msg = ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.UNSUBSCRIBE, channel);
             return ExecuteSync(msg, ResultProcessor.ResponseTimer);
         }
 
@@ -25,7 +25,7 @@ public override Task<TimeSpan> PingAsync(CommandFlags flags = CommandFlags.None)
         {
             // can't use regular PING, but we can unsubscribe from something random that we weren't even subscribed to...
             RedisValue channel = Guid.NewGuid().ToByteArray();
-            var msg = ResultProcessor.TimingProcessor.CreateMessage(flags, RedisCommand.UNSUBSCRIBE, channel);
+            var msg = ResultProcessor.TimingProcessor.CreateMessage(-1, flags, RedisCommand.UNSUBSCRIBE, channel);
             return ExecuteAsync(msg, ResultProcessor.ResponseTimer);
         }
 

StackExchange.Redis/StackExchange/Redis/RedisTransaction.cs

@@ -395,14 +395,14 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                     switch (result.Type)
                     {
                         case ResultType.SimpleString:
-                            if (tran.IsAborted && result.Assert(RedisLiterals.OK))
+                            if (tran.IsAborted && result.Assert(RedisLiterals.BytesOK))
                             {
                                 connection.Multiplexer.Trace("Acknowledging UNWATCH (aborted electively)");
                                 SetResult(message, false);
                                 return true;
                             }
                             break;
-                        case ResultType.Array:
+                        case ResultType.MultiBulk:
                             if (!tran.IsAborted)
                             {
                                 var arr = result.GetItems();

StackExchange.Redis/StackExchange/Redis/ResultProcessor.cs

@@ -12,13 +12,13 @@ abstract class ResultProcessor
     {
         public static readonly ResultProcessor<bool>
             Boolean = new BooleanProcessor(),
-            DemandOK = new ExpectBasicStringProcessor(RedisLiterals.OK),
+            DemandOK = new ExpectBasicStringProcessor(RedisLiterals.BytesOK),
             DemandPONG = new ExpectBasicStringProcessor("PONG"),
             DemandZeroOrOne = new DemandZeroOrOneProcessor(),
             AutoConfigure = new AutoConfigureProcessor(),
-            EstablishConnection = new EstablishConnectionProcessor(),
             TrackSubscriptions = new TrackSubscriptionsProcessor(),
-            Tracer = new TracerProcessor();
+            Tracer = new TracerProcessor(false),
+            EstablishConnection = new TracerProcessor(true);
 
         public static readonly ResultProcessor<double>
             Double = new DoubleProcessor();
@@ -176,7 +176,7 @@ public sealed class TrackSubscriptionsProcessor : ResultProcessor<bool>
         {
             protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
             {
-                if(result.Type == ResultType.Array)
+                if(result.Type == ResultType.MultiBulk)
                 {
                     var items = result.GetItems();
                     long count;
@@ -192,9 +192,9 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
         
         public sealed class TimingProcessor : ResultProcessor<TimeSpan>
         {
-            public static Message CreateMessage(CommandFlags flags, RedisCommand command, RedisValue value = default(RedisValue))
+            public static TimerMessage CreateMessage(int db, CommandFlags flags, RedisCommand command, RedisValue value = default(RedisValue))
             {
-                return new TimerMessage(flags, command, value);
+                return new TimerMessage(db, flags, command, value);
             }
             protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
             {
@@ -203,7 +203,8 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                     return false;
                 }
                 else
-                {
+                {   // don't check the actual reply; there are multiple ways of constructing
+                    // a timing message, and we don't actually care about what approach was used
                     var timingMessage = message as TimerMessage;
                     TimeSpan duration;
                     if (timingMessage != null)
@@ -221,12 +222,12 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                 }
             }
 
-            sealed class TimerMessage : Message
+            internal sealed class TimerMessage : Message
             {
                 public readonly Stopwatch Watch;
                 private readonly RedisValue value;
-                public TimerMessage(CommandFlags flags, RedisCommand command, RedisValue value)
-                    : base(-1, flags, command)
+                public TimerMessage(int db, CommandFlags flags, RedisCommand command, RedisValue value)
+                    : base(db, flags, command)
                 {
                     this.Watch = Stopwatch.StartNew();
                     this.value = value;
@@ -356,7 +357,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                         }
                         SetResult(message, true);
                         return true;
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         if (message != null && message.Command == RedisCommand.CONFIG)
                         {
                             var arr = result.GetItems();
@@ -489,7 +490,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                 switch (result.Type)
                 {
                     case ResultType.SimpleString:
-                        if(result.Assert(RedisLiterals.OK))
+                        if(result.Assert(RedisLiterals.BytesOK))
                         {
                             SetResult(message, true);
                         } else
@@ -501,7 +502,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                     case ResultType.BulkString:
                         SetResult(message, result.GetBoolean());
                         return true;
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var items = result.GetItems();
                         if(items.Length == 1)
                         { // treat an array of 1 like a single reply (for example, SCRIPT EXISTS)
@@ -619,7 +620,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
                             return true;
                         }
                         break;
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var arr = result.GetItems();
                         switch(arr.Length)
                         {
@@ -646,45 +647,6 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             }
         }
 
-        sealed class EstablishConnectionProcessor : ResultProcessor<bool>
-        {
-            static readonly byte[] expected = Encoding.UTF8.GetBytes("PONG"), authFail = Encoding.UTF8.GetBytes("ERR operation not permitted"),
-                loading = Encoding.UTF8.GetBytes("LOADING ");
-            public override bool SetResult(PhysicalConnection connection, Message message, RawResult result)
-            {
-                var final = base.SetResult(connection, message, result);
-                if (result.IsError)
-                {
-                    if (result.Assert(authFail))
-                    {
-                        connection.RecordConnectionFailed(ConnectionFailureType.AuthenticationFailure);
-                    } else if(result.AssertStarts(loading))
-                    {
-                        connection.RecordConnectionFailed(ConnectionFailureType.Loading);
-                    }
-                    else
-                    {
-                        connection.RecordConnectionFailed(ConnectionFailureType.ProtocolFailure);
-                    }
-                }
-                return final;
-            }
-
-            protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
-            {
-                if(result.Assert(expected))
-                {
-                    connection.Bridge.OnFullyEstablished(connection);
-                    SetResult(message, true);
-                    return true;
-                }
-                else
-                {
-                    connection.RecordConnectionFailed(ConnectionFailureType.ProtocolFailure);
-                    return false;
-                }
-            }
-        }
         sealed class ExpectBasicStringProcessor : ResultProcessor<bool>
         {
             private readonly byte[] expected;
@@ -797,7 +759,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             {
                 switch (result.Type)
                 {
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var arr = result.GetItemsAsKeys();
                         SetResult(message, arr);
                         return true;
@@ -846,7 +808,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             {
                 switch (result.Type)
                 {
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var arr = result.GetItemsAsValues();
 
                         SetResult(message, arr);
@@ -861,7 +823,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             {
                 switch (result.Type)
                 {
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var arr = result.GetItems();
                         RedisChannel[] final;
                         if (arr.Length == 0)
@@ -982,7 +944,7 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
             {
                 switch (result.Type)
                 {
-                    case ResultType.Array:
+                    case ResultType.MultiBulk:
                         var arr = result.GetItems();
                         int count = arr.Length / 2;
                         KeyValuePair<TKey, TValue>[] pairs;
@@ -1013,7 +975,7 @@ private class SortedSetWithScoresProcessor : ResultProcessor<KeyValuePair<RedisV
         {
             protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
             {
-                if(result.Type == ResultType.Array)
+                if(result.Type == ResultType.MultiBulk)
                 {
                     var items = result.GetItems();
                     var arr = new KeyValuePair<RedisValue, double>[items.Length / 2];
@@ -1034,11 +996,67 @@ protected override bool SetResultCore(PhysicalConnection connection, Message mes
 
         private class TracerProcessor : ResultProcessor<bool>
         {
+            private readonly bool establishConnection;
+
+            public TracerProcessor(bool establishConnection)
+            {
+                this.establishConnection = establishConnection;
+            }
             protected override bool SetResultCore(PhysicalConnection connection, Message message, RawResult result)
             {
-                bool happy = result.Type == ResultType.BulkString && result.Assert(connection.Multiplexer.UniqueId);
-                SetResult(message, happy);
-                return true; // we'll always acknowledge that we saw a non-error response
+                bool happy;
+                switch(message.Command)
+                {
+                    case RedisCommand.ECHO:
+                        happy = result.Type == ResultType.BulkString && (!establishConnection || result.Assert(connection.Multiplexer.UniqueId));
+                        break;
+                    case RedisCommand.PING:
+                        happy = result.Type == ResultType.SimpleString && result.Assert(RedisLiterals.BytesPONG);
+                        break;
+                    case RedisCommand.TIME:
+                        happy = result.Type == ResultType.MultiBulk && result.GetItems().Length == 2;
+                        break;
+                    case RedisCommand.EXISTS:
+                        happy = result.Type == ResultType.Integer;
+                        break;
+                    default:
+                        happy = true;
+                        break;
+                }
+                if(happy)
+                {
+                    if(establishConnection) connection.Bridge.OnFullyEstablished(connection);
+                    SetResult(message, happy);
+                    return true;
+                }
+                else
+                {
+                    connection.RecordConnectionFailed(ConnectionFailureType.ProtocolFailure);
+                    return false;
+                }
+            }
+
+            static readonly byte[] expected = RedisLiterals.BytesPONG, authFail = Encoding.UTF8.GetBytes("ERR operation not permitted"),
+                loading = Encoding.UTF8.GetBytes("LOADING ");
+            public override bool SetResult(PhysicalConnection connection, Message message, RawResult result)
+            {
+                var final = base.SetResult(connection, message, result);
+                if (result.IsError)
+                {
+                    if (result.Assert(authFail))
+                    {
+                        connection.RecordConnectionFailed(ConnectionFailureType.AuthenticationFailure);
+                    }
+                    else if (result.AssertStarts(loading))
+                    {
+                        connection.RecordConnectionFailed(ConnectionFailureType.Loading);
+                    }
+                    else
+                    {
+                        connection.RecordConnectionFailed(ConnectionFailureType.ProtocolFailure);
+                    }
+                }
+                return final;
             }
         }
 

StackExchange.Redis/StackExchange/Redis/ResultType.cs

@@ -7,6 +7,6 @@ internal enum ResultType : byte
         Error = 2,
         Integer = 3,
         BulkString = 4,
-        Array = 5
+        MultiBulk = 5
     }
 }

StackExchange.Redis/StackExchange/Redis/ServerEndPoint.cs

@@ -58,6 +58,13 @@ public ServerEndPoint(ConnectionMultiplexer multiplexer, EndPoint endpoint)
             writeEverySeconds = config.KeepAlive;
             interactive = CreateBridge(ConnectionType.Interactive);
             serverType = ServerType.Standalone;
+
+            // overrides for twemproxy
+            if (multiplexer.RawConfig.Proxy == Proxy.Twemproxy)
+            {
+                databases = 1;
+                serverType = ServerType.Twemproxy;
+            }
         }
 
         public ClusterConfiguration ClusterConfiguration { get; private set; }
@@ -126,6 +133,18 @@ public void Dispose()
             if (tmp != null) tmp.Dispose();
         }
 
+        public PhysicalBridge GetBridge(ConnectionType type, bool create = true)
+        {
+            if (isDisposed) return null;
+            switch (type)
+            {
+                case ConnectionType.Interactive:
+                    return interactive ?? (create ? interactive = CreateBridge(ConnectionType.Interactive) : null);
+                case ConnectionType.Subscription:
+                    return subscription ?? (create ? subscription = CreateBridge(ConnectionType.Subscription) : null);
+            }
+            return null;
+        }
         public PhysicalBridge GetBridge(RedisCommand command, bool create = true)
         {
             if (isDisposed) return null;
@@ -195,18 +214,26 @@ public bool TryEnqueue(Message message)
             return bridge != null && bridge.TryEnqueue(message, isSlave);
         }
 
-        internal void Activate(RedisCommand command)
+        internal void Activate(ConnectionType type)
         {
-            GetBridge(command, true);
+            GetBridge(type, true);
         }
 
         internal void AutoConfigure(PhysicalConnection connection)
         {
+            if (serverType == ServerType.Twemproxy)
+            {
+                // don't try to detect configuration; all the config commands are disabled, and
+                // the fallback master/slave detection won't help
+                return;
+            }
+
             var commandMap = multiplexer.CommandMap;
             const CommandFlags flags = CommandFlags.FireAndForget | CommandFlags.HighPriority | CommandFlags.NoRedirect;
 
             var features = GetFeatures();
             Message msg;
+
             if (commandMap.IsAvailable(RedisCommand.CONFIG))
             {
                 if (multiplexer.RawConfig.KeepAlive <= 0)
@@ -261,7 +288,7 @@ internal Task Close()
         {
             var tmp = interactive;
             Task result;
-            if (tmp == null || !tmp.IsConnected)
+            if (tmp == null || !tmp.IsConnected || !multiplexer.CommandMap.IsAvailable(RedisCommand.QUIT))
             {
                 result = CompletedTask<bool>.Default(null);
             }
@@ -466,10 +493,8 @@ void Handshake(PhysicalConnection connection)
                 multiplexer.Trace("Auto-configure...");
                 AutoConfigure(connection);
             }
-            multiplexer.Trace("Sending critical ping");
-            msg = Message.Create(-1, CommandFlags.FireAndForget, RedisCommand.PING);
-            msg.SetInternalCall();
-            WriteDirectOrQueueFireAndForget(connection, msg, ResultProcessor.EstablishConnection);
+            multiplexer.Trace("Sending critical tracer");
+            WriteDirectOrQueueFireAndForget(connection, GetTracerMessage(true), ResultProcessor.EstablishConnection);
 
 
             // note: this **must** be the last thing on the subscription handshake, because after this
@@ -499,9 +524,42 @@ private void SetConfig<T>(ref T field, T value, [CallerMemberName] string caller
 
         internal Task<bool> SendTracer()
         {
-            var msg = Message.Create(-1, CommandFlags.NoRedirect | CommandFlags.HighPriority, RedisCommand.ECHO,(RedisValue) multiplexer.UniqueId);
+            return QueueDirectAsync(GetTracerMessage(false), ResultProcessor.Tracer);
+        }
+
+        internal Message GetTracerMessage(bool assertIdentity)
+        {
+            // different configurations block certain commands, as can ad-hoc local configurations, so
+            // we'll do the best with what we have available.
+            // note that the muxer-ctor asserts that one of ECHO, PING, TIME of GET is available
+            // see also: TracerProcessor
+            var map = multiplexer.CommandMap;
+            Message msg;
+            const CommandFlags flags = CommandFlags.NoRedirect | CommandFlags.FireAndForget;
+            if (assertIdentity && map.IsAvailable(RedisCommand.ECHO))
+            {
+                msg = Message.Create(-1, flags, RedisCommand.ECHO, (RedisValue)multiplexer.UniqueId);
+            }
+            else if (map.IsAvailable(RedisCommand.PING))
+            {
+                msg = Message.Create(-1, flags, RedisCommand.PING);
+            }
+            else if (map.IsAvailable(RedisCommand.TIME))
+            {
+                msg = Message.Create(-1, flags, RedisCommand.TIME);
+            }
+            else if(!assertIdentity && map.IsAvailable(RedisCommand.ECHO))
+            {
+                // we'll use echo as a PING substitute if it is all we have (in preference to EXISTS)
+                msg = Message.Create(-1, flags, RedisCommand.ECHO, (RedisValue)multiplexer.UniqueId);
+            }
+            else
+            {
+                map.AssertAvailable(RedisCommand.EXISTS);
+                msg = Message.Create(0, flags, RedisCommand.EXISTS, (RedisValue)multiplexer.UniqueId);
+            }
             msg.SetInternalCall();
-            return QueueDirectAsync(msg, ResultProcessor.Tracer);
+            return msg;
         }
 
         internal int GetOutstandingCount(RedisCommand command, out int inst, out int qu, out int qs, out int qc, out int wr, out int wq)

StackExchange.Redis/StackExchange/Redis/ServerSelectionStrategy.cs

@@ -108,10 +108,16 @@ public ServerEndPoint Select(Message message)
         {
             if (message == null) throw new ArgumentNullException("message");
             int slot = NoSlot;
-            if (serverType == ServerType.Cluster)
+            switch (serverType)
             {
-                slot = message.GetHashSlot(this);
-                if (slot == MultipleSlots) throw ExceptionFactory.MultiSlot(multiplexer.IncludeDetailInExceptions, message);
+                case ServerType.Cluster:
+                case ServerType.Twemproxy: // strictly speaking twemproxy uses a different hashing algo, but the hash-tag behavior is
+                                           // the same, so this does a pretty good job of spotting illegal commands before sending them
+
+                    slot = message.GetHashSlot(this);
+                    if (slot == MultipleSlots) throw ExceptionFactory.MultiSlot(multiplexer.IncludeDetailInExceptions, message);
+                    break;
+
             }
             return Select(slot, message.Command, message.Flags);
         }

StackExchange.Redis/StackExchange/Redis/ServerType.cs

@@ -16,6 +16,10 @@ public enum ServerType
         /// <summary>
         /// Distributed redis-cluster server
         /// </summary>
-        Cluster
+        Cluster,
+        /// <summary>
+        /// Distributed redis installation via <a href="https://github.com/twitter/twemproxy">twemproxy</a>
+        /// </summary>
+        Twemproxy
     }
 }