源码版本：Redis 2.4.4
redis的主从复制实现简单却功能强大，其具有以下特点：
1. 一个master支持多个slave连接，slave可以接受其他slave的连接
2. 主从同步时，master和slave都是非阻塞的
redis主从复制可以用来：
1. data redundancy
2. slave作为master的扩展，提供一些read-only的服务
3. 可以将数据持久化放在slave做，从而提升master性能
通过简单的配置slave（master端无需配置），用户就能使用redis的主从复制
相关配置（redis.conf）：
slaveof <masterip> <masterport>
表示该redis服务作为slave，masterip和masterport分别为master 的ip和port
masterauth <master-password>
如果master设置了安全密码，则此处设置为相应的密码
slave-serve-stale-data yes
当slave丢失master或者同步正在进行时，如果发生对slave的服务请求：
slave-serve-stale-data设置为yes则slave依然正常提供服务
slave-serve-stale-data设置为no则slave返回client错误："SYNC with master in progress"
repl-ping-slave-period 10
slave发送PINGS到master的时间间隔
repl-timeout 60
IO超时时间
代码：
slave端
slave状态：
/* Slave replication state - slave side */
#define REDIS_REPL_NONE 0 /* No active replication */
#define REDIS_REPL_CONNECT 1 /* Must connect to master */
#define REDIS_REPL_CONNECTING 2 /* Connecting to master */
#define REDIS_REPL_TRANSFER 3 /* Receiving .rdb from master */
#define REDIS_REPL_CONNECTED 4 /* Connected to master */
初始化时设置
server.replstate = REDIS_REPL_CONNECT
即slave需要连接master
slave周期性调用replicationCron，查看slave状态：
[cpp]
void replicationCron(void) { /*判断是否IO超时*/ if (server.masterhost && server.replstate == REDIS_REPL_TRANSFER && (time(NULL)-server.repl_transfer_lastio) > server.repl_timeout) { redisLog(REDIS_WARNING,"Timeout receiving bulk data from MASTER..."); replicationAbortSyncTransfer(); //终止连接，并设置server.replstate = REDIS_REPL_CONNECT; } /* Timed out master when we are an already connected slave? */ if (server.masterhost && server.replstate == REDIS_REPL_CONNECTED && (time(NULL)-server.master->lastinteraction) > server.repl_timeout) { redisLog(REDIS_WARNING,"MASTER time out: no data nor PING received..."); freeClient(server.master); } /* Check if we should connect to a MASTER */ if (server.replstate == REDIS_REPL_CONNECT) { redisLog(REDIS_NOTICE,"Connecting to MASTER..."); if (connectWithMaster() == REDIS_OK) { //连接master redisLog(REDIS_NOTICE,"MASTER <-> SLAVE sync started"); } } /* If we have attached slaves, PING them from time to time. * So slaves can implement an explicit timeout to masters, and will * be able to detect a link disconnection even if the TCP connection * will not actually go down. */ if (!(server.cronloops % (server.repl_ping_slave_period*10))) { listIter li; listNode *ln; listRewind(server.slaves,&li); while((ln = listNext(&li))) { redisClient *slave = ln->value; /* Don't ping slaves that are in the middle of a bulk transfer * with the master for first synchronization. */ if (slave->replstate == REDIS_REPL_SEND_BULK) continue; if (slave->replstate == REDIS_REPL_ONLINE) { /* If the slave is online send a normal ping */ addReplySds(slave,sdsnew("PING\r\n")); } else { /* Otherwise we are in the pre-synchronization stage. * Just a newline will do the work of refreshing the * connection last interaction time, and at the same time * we'll be sure that being a single char there are no * short-write problems. */ if (write(slave->fd, "\n", 1) == -1) { /* Don't worry, it's just a ping. */ } } } } }