網(wǎng)上關(guān)于redis高可用基本都是用redis-sentinel 哨兵 或者 redis cluster 集群來(lái)實(shí)現(xiàn)， 但是有沒(méi)有更簡(jiǎn)單的方式，比如我現(xiàn)在就只有2個(gè)redis實(shí)例。我試驗(yàn)的結(jié)果是我們可用采用主備的方式來(lái)實(shí)現(xiàn)（我們的實(shí)際需求很簡(jiǎn)單，有2個(gè)redis實(shí)例分布在不同的計(jì)算機(jī)，在一個(gè)實(shí)例down掉后我們的應(yīng)用程序有繼續(xù)讀寫(xiě)redis，主從配置可用手動(dòng)修改）。需求很簡(jiǎn)單， 實(shí)現(xiàn)也就很簡(jiǎn)單。首先下載?https://github.com/StackExchange/StackExchange.Redis 源碼。啟動(dòng)StackExchange.Redis-master\Redis Configs里面的主從2個(gè)實(shí)例，我最終demo的code如下：

class Program{static IDatabase database;static ConnectionMultiplexer conn;static void Main(string[] args){ConfigurationOptions option = new ConfigurationOptions() {EndPoints ={{ "127.0.0.1", 6379 },{ "127.0.0.1", 6380 }},AllowAdmin =true, };conn = ConnectionMultiplexer.Connect(option);database = conn.GetDatabase();Random rand = new Random();while (true){string val = "gavin_" + rand.Next(1, 999999).ToString();TestWriteRead(val);Thread.Sleep(100);}}static void TestWriteRead(string value) {string key = "gavinteststring";try{database.StringSet(key, value);Console.WriteLine($"寫(xiě)入{key}={value}成功");}catch (Exception ex){var points = conn.GetEndPoints();foreach (var item in points){var server = conn.GetServer(item);if (server.IsConnected){server.MakeMaster(ReplicationChangeOptions.All);}else{server.SlaveOf(points[1],CommandFlags.FireAndForget);}}database.StringSet(key, value);Console.WriteLine($"寫(xiě)入{key}={value}成功");//Console.WriteLine($"寫(xiě)入{key}={value}失敗:"+ex.ToString());// Console.ReadKey(); }string temp = string.Empty;try{temp=database.StringGet(key);Console.WriteLine($"讀取{key}={temp}成功");}catch (Exception ex){Console.WriteLine($"讀取{key}失敗:" + ex.ToString());}}} View Code

大家請(qǐng)先忽略我catch里面的code，當(dāng)我把redis的master關(guān)閉后，程序報(bào)錯(cuò)：

No connection is available to service this operation: SET gavinteststring; 遠(yuǎn)程主機(jī)強(qiáng)迫關(guān)閉了一個(gè)現(xiàn)有的連接。; IOCP: (Busy=0,Free=1000,Min=4,Max=1000), WORKER: (Busy=0,Free=1023,Min=4,Max=1023), Local-CPU: 100%

找到源碼后發(fā)現(xiàn)在ConnectionMultiplexer的ExecuteSyncImpl方法里面有這么一段：

if (!TryPushMessageToBridge(message, processor, source, ref server))
{
throw ExceptionFactory.NoConnectionAvailable(IncludeDetailInExceptions, message.Command, message, server, GetServerSnapshot());
}

也就是說(shuō)StackExchange沒(méi)有找到redis的服務(wù)器實(shí)例，繼續(xù)跟蹤code發(fā)現(xiàn)具體查找server的code在ConnectionMultiplexer的AnyConnected方法里面：

internal ServerEndPoint AnyConnected(ServerType serverType, uint startOffset, RedisCommand command, CommandFlags flags){var tmp = serverSnapshot;int len = tmp.Length;ServerEndPoint fallback = null;for (int i = 0; i < len; i++){var server = tmp[(int)(((uint)i + startOffset) % len)];if (server != null && server.ServerType == serverType && server.IsSelectable(command)){if (server.IsSlave){switch (flags){case CommandFlags.DemandSlave:case CommandFlags.PreferSlave:return server;case CommandFlags.PreferMaster:fallback = server;break;}} else{switch (flags){case CommandFlags.DemandMaster:case CommandFlags.PreferMaster:return server;case CommandFlags.PreferSlave:fallback = server;break;}}}}return fallback;}

因?yàn)橹鞯膕erver已經(jīng)down掉了，所以可用訪問(wèn)的server就是Slave，但是這里的flags默認(rèn)是CommandFlags.DemandMaster。所以是找不到server。那么我們把現(xiàn)在的從的server改為主的server如：? server.MakeMaster(ReplicationChangeOptions.All); 我以為就可以了，但是還是不行。 后來(lái)我想 如果我把主的也改為從是否可以了? server.SlaveOf(points[1],CommandFlags.FireAndForget);（我測(cè)試的時(shí)候還用過(guò)quit方法，調(diào)試有，但是release的時(shí)候說(shuō)沒(méi)有該方法）。運(yùn)行效果如下

?后來(lái)把上面的code簡(jiǎn)單封裝為一個(gè)方法：

void ChangeMaster(IDatabase database){var mex = database.Multiplexer;var endpoints = mex.GetEndPoints();if (endpoints.Count() < 2){return;}//多個(gè)endpoint 才切換主備服務(wù)器List<EndPoint> connectedPoints = new List<EndPoint>();List<EndPoint> disconnetedPoints = new List<EndPoint>();foreach (var item in endpoints){//判斷哪些服務(wù)器可以連接var server = mex.GetServer(item);if (server.IsConnected){connectedPoints.Add(item);}else{disconnetedPoints.Add(item);}}var connectedPoint = connectedPoints.FirstOrDefault();if (connectedPoint == null){throw new Exception("沒(méi)有可用的redis服務(wù)器");}mex.GetServer(connectedPoint).MakeMaster(ReplicationChangeOptions.All);for (int i = 1; i < connectedPoints.Count; i++){mex.GetServer(connectedPoints[i]).SlaveOf(connectedPoint, CommandFlags.FireAndForget);}foreach (var item in disconnetedPoints){mex.GetServer(item).SlaveOf(connectedPoint, CommandFlags.FireAndForget);}} View Code

?-----------------------------------------2017-4-14--------------------------------------------------------

我們知道讀寫(xiě)redis的時(shí)候都是Message包

protected Message(int db, CommandFlags flags, RedisCommand command){bool dbNeeded = RequiresDatabase(command);if (db < 0){if (dbNeeded){throw ExceptionFactory.DatabaseRequired(false, command);}}else{if (!dbNeeded){throw ExceptionFactory.DatabaseNotRequired(false, command);}}bool masterOnly = IsMasterOnly(command);Db = db;this.command = command;this.flags = flags & UserSelectableFlags;if (masterOnly) SetMasterOnly();createdDateTime = DateTime.UtcNow;createdTimestamp = System.Diagnostics.Stopwatch.GetTimestamp();} internal void SetMasterOnly(){switch (GetMasterSlaveFlags(flags)){case CommandFlags.DemandSlave:throw ExceptionFactory.MasterOnly(false, command, null, null);case CommandFlags.DemandMaster:// already fine as-isbreak;case CommandFlags.PreferMaster:case CommandFlags.PreferSlave:default: // we will run this on the master, thenflags = SetMasterSlaveFlags(flags, CommandFlags.DemandMaster);break;}} internal static CommandFlags SetMasterSlaveFlags(CommandFlags everything, CommandFlags masterSlave){// take away the two flags we don't want, and add back the ones we care aboutreturn (everything & ~(CommandFlags.DemandMaster | CommandFlags.DemandSlave | CommandFlags.PreferMaster | CommandFlags.PreferSlave))| masterSlave;}

這里根據(jù)我們的Command來(lái)判斷是否必須是Master主庫(kù)，如果是 就代用SetMasterOnly來(lái)設(shè)置flags，那么那些指令需要Master了：

public static bool IsMasterOnly(RedisCommand command){switch (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:case RedisCommand.EXPIRE:case RedisCommand.EXPIREAT:case RedisCommand.FLUSHALL:case RedisCommand.FLUSHDB:case RedisCommand.GETSET:case RedisCommand.HDEL:case RedisCommand.HINCRBY:case RedisCommand.HINCRBYFLOAT:case RedisCommand.HMSET:case RedisCommand.HSET:case RedisCommand.HSETNX:case RedisCommand.INCR:case RedisCommand.INCRBY:case RedisCommand.INCRBYFLOAT:case RedisCommand.LINSERT:case RedisCommand.LPOP:case RedisCommand.LPUSH:case RedisCommand.LPUSHX:case RedisCommand.LREM:case RedisCommand.LSET:case RedisCommand.LTRIM:case RedisCommand.MIGRATE:case RedisCommand.MOVE:case RedisCommand.MSET:case RedisCommand.MSETNX:case RedisCommand.PERSIST:case RedisCommand.PEXPIRE:case RedisCommand.PEXPIREAT:case RedisCommand.PFADD:case RedisCommand.PFMERGE:case RedisCommand.PSETEX:case RedisCommand.RENAME:case RedisCommand.RENAMENX:case RedisCommand.RESTORE:case RedisCommand.RPOP:case RedisCommand.RPOPLPUSH:case RedisCommand.RPUSH:case RedisCommand.RPUSHX:case RedisCommand.SADD:case RedisCommand.SDIFFSTORE:case RedisCommand.SET:case RedisCommand.SETBIT:case RedisCommand.SETEX:case RedisCommand.SETNX:case RedisCommand.SETRANGE:case RedisCommand.SINTERSTORE:case RedisCommand.SMOVE:case RedisCommand.SPOP:case RedisCommand.SREM:case RedisCommand.SUNIONSTORE:case RedisCommand.ZADD:case RedisCommand.ZINTERSTORE:case RedisCommand.ZINCRBY:case RedisCommand.ZREM:case RedisCommand.ZREMRANGEBYLEX:case RedisCommand.ZREMRANGEBYRANK:case RedisCommand.ZREMRANGEBYSCORE:case RedisCommand.ZUNIONSTORE:return true;default:return false;}}

如果我們執(zhí)行腳本則是用的ScriptEvalMessage類(lèi)，其構(gòu)造函數(shù)：

private sealed class ScriptEvalMessage : Message, IMultiMessage{private readonly RedisKey[] keys;private readonly string script;private readonly RedisValue[] values;private byte[] asciiHash, hexHash;public ScriptEvalMessage(int db, CommandFlags flags, string script, RedisKey[] keys, RedisValue[] values): this(db, flags, ResultProcessor.ScriptLoadProcessor.IsSHA1(script) ? RedisCommand.EVALSHA : RedisCommand.EVAL, script, null, keys, values){if (script == null) throw new ArgumentNullException(nameof(script));}public ScriptEvalMessage(int db, CommandFlags flags, byte[] hash, RedisKey[] keys, RedisValue[] values): this(db, flags, RedisCommand.EVAL, null, hash, keys, values){if (hash == null) throw new ArgumentNullException(nameof(hash));}private ScriptEvalMessage(int db, CommandFlags flags, RedisCommand command, string script, byte[] hexHash, RedisKey[] keys, RedisValue[] values): base(db, flags, command){this.script = script;this.hexHash = hexHash;if (keys == null) keys = RedisKey.EmptyArray;if (values == null) values = RedisValue.EmptyArray;for (int i = 0; i < keys.Length; i++)keys[i].AssertNotNull();this.keys = keys;for (int i = 0; i < values.Length; i++)values[i].AssertNotNull();this.values = values;}

也就是說(shuō) 執(zhí)行腳本可以在從庫(kù)上執(zhí)行，很多查詢(xún)語(yǔ)句也可以在從庫(kù)上執(zhí)行。

?

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