redis StackExchange 主备 实现 demo

网上关于redis高可用基本都是用redis-sentinel 哨兵 或者 redis cluster 集群来实现， 但是有没有更简单的方式，比如我现在就只有2个redis实例。我试验的结果是我们可用采用主备的方式来实现（我们的实际需求很简单，有2个redis实例分布在不同的计算机，在一个实例down掉后我们的应用程序有继续读写redis，主从配置可用手动修改）。需求很简单， 实现也就很简单。首先下载 https://github.com/StackExchange/StackExchange.Redis 源码。启动StackExchange.Redis-master\Redis Configs里面的主从2个实例，我最终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",  },
                                { "127.0.0.1",  }
                            },
                AllowAdmin =true,
            };

             conn = ConnectionMultiplexer.Connect(option);
            database = conn.GetDatabase();

            Random rand = new Random();

            while (true)
            {
                string val = "gavin_" + rand.Next(, ).ToString();
                TestWriteRead(val);
                Thread.Sleep();
            }

        }

        static void TestWriteRead(string value) {
            string key = "gavinteststring";
            try
            {
                database.StringSet(key, value);
                Console.WriteLine($"写入{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[],CommandFlags.FireAndForget);
                    }

                }
                database.StringSet(key, value);
                Console.WriteLine($"写入{key}={value}成功");
                //Console.WriteLine($"写入{key}={value}失败:"+ex.ToString());
               // Console.ReadKey();
            }
            string temp = string.Empty;
            try
            {
                temp=database.StringGet(key);
                Console.WriteLine($"读取{key}={temp}成功");
            }
            catch (Exception ex)
            {
                Console.WriteLine($"读取{key}失败:" + ex.ToString());
            }
        }
    }

大家请先忽略我catch里面的code，当我把redis的master关闭后，程序报错：

No connection is available to service this operation: SET gavinteststring; 远程主机强迫关闭了一个现有的连接。; IOCP: (Busy=0,Free=1000,Min=4,Max=1000), WORKER: (Busy=0,Free=1023,Min=4,Max=1023), Local-CPU: 100%

找到源码后发现在ConnectionMultiplexer的ExecuteSyncImpl方法里面有这么一段：

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

也就是说StackExchange没有找到redis的服务器实例，继续跟踪code发现具体查找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 = ; 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;
        }

因为主的server已经down掉了，所以可用访问的server就是Slave，但是这里的flags默认是CommandFlags.DemandMaster。所以是找不到server。那么我们把现在的从的server改为主的server如：  server.MakeMaster(ReplicationChangeOptions.All); 我以为就可以了，但是还是不行。 后来我想 如果我把主的也改为从是否可以了  server.SlaveOf(points[1],CommandFlags.FireAndForget);（我测试的时候还用过quit方法，调试有，但是release的时候说没有该方法）。运行效果如下

后来把上面的code简单封装为一个方法：

  void ChangeMaster(IDatabase database)
        {
            var mex = database.Multiplexer;
            var endpoints = mex.GetEndPoints();
            if (endpoints.Count() < )
            {
                return;
            }
            //多个endpoint 才切换主备服务器
            List<EndPoint> connectedPoints = new List<EndPoint>();
            List<EndPoint> disconnetedPoints = new List<EndPoint>();
            foreach (var item in endpoints)
            {
                //判断哪些服务器可以连接
                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("没有可用的redis服务器");
            }
            mex.GetServer(connectedPoint).MakeMaster(ReplicationChangeOptions.All);
            for (int i = ; i < connectedPoints.Count; i++)
            {
                mex.GetServer(connectedPoints[i]).SlaveOf(connectedPoint, CommandFlags.FireAndForget);
            }
            foreach (var item in disconnetedPoints)
            {
                mex.GetServer(item).SlaveOf(connectedPoint, CommandFlags.FireAndForget);
            }
        }

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

我们知道读写redis的时候都是Message包

protected Message(int db, CommandFlags flags, RedisCommand command)
        {
            bool dbNeeded = RequiresDatabase(command);
            if (db < )
            {
                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-is
                    break;
                case CommandFlags.PreferMaster:
                case CommandFlags.PreferSlave:
                default: // we will run this on the master, then
                    flags = 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 about
            return (everything & ~(CommandFlags.DemandMaster | CommandFlags.DemandSlave | CommandFlags.PreferMaster | CommandFlags.PreferSlave))
                            | masterSlave;
        }

这里根据我们的Command来判断是否必须是Master主库，如果是 就代用SetMasterOnly来设置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;
            }
        }

如果我们执行脚本则是用的ScriptEvalMessage类，其构造函数：

   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 = ; i < keys.Length; i++)
                    keys[i].AssertNotNull();
                this.keys = keys;
                for (int i = ; i < values.Length; i++)
                    values[i].AssertNotNull();
                this.values = values;
            }     

也就是说 执行脚本可以在从库上执行，很多查询语句也可以在从库上执行。