redis复制

redis复制过程
backlog命令协议文本 为psync增量同步，同时写给salves

redis.c 1761 

if (flags & REDIS_PROPAGATE_REPL)
        replicationFeedSlaves(server.slaves,dbid,argv,argc);


replication.c 141 
replicationFeedSlaves

 while((ln = listNext(&li))) {
        redisClient *slave = ln->value;

        /* Don't feed slaves that are still waiting for BGSAVE to start */
        if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START) continue;

        /* Feed slaves that are waiting for the initial SYNC (so these commands
         * are queued in the output buffer until the initial SYNC completes),
         * or are already in sync with the master. */

        /* Add the multi bulk length. */
        addReplyMultiBulkLen(slave,argc);

        /* Finally any additional argument that was not stored inside the
         * static buffer if any (from j to argc). */
        for (j = 0; j < argc; j++)
            addReplyBulk(slave,argv[j]);
    }


sync的过程如下：


客户端执行slave of  host ip，此时slave向master发送sync命令，服务端收到以后，首先检查当前是否在执行bgsave
如在执行的话，继续检查是否存在处于REDIS_REPL_WAIT_BGSAVE_END的slave，如有那太棒了，直接把这个slave的outputbuffer copy到
client中，但此时并不会将这些内容写回slave，因为还没有注册sendReply回调，只是累积在client的outputbuffer中。

listRewind(server.slaves,&li);
        while((ln = listNext(&li))) {
            slave = ln->value;
            if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) break;
        }
        if (ln) {
            /* Perfect, the server is already registering differences for
             * another slave. Set the right state, and copy the buffer. */
            copyClientOutputBuffer(c,slave);
            c->replstate = REDIS_REPL_WAIT_BGSAVE_END;
            redisLog(REDIS_NOTICE,"Waiting for end of BGSAVE for SYNC");
        } else {
            /* No way, we need to wait for the next BGSAVE in order to
             * register differences */
            c->replstate = REDIS_REPL_WAIT_BGSAVE_START;
            redisLog(REDIS_NOTICE,"Waiting for next BGSAVE for SYNC");
        }


updateSlavesWaitingBgsave
在bgsave完成以后，将rdbsend到slave。

if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE, sendBulkToSlave, slave) == AE_ERR) {
                freeClient(slave);
                continue;
}

主要是给slave的write注册sendBulkToSlave回调，在sendBulkToSlave中完成rdb的传输。
接下来注册sendReplyToClient，将累积的reply写回。

回顾一下：

    1.  slave执行sync 服务端检查是否在bgsave，如存在再检查是否存在处于REDIS_REPL_WAIT_BGSAVE_END的other slaver，如存在那就太棒了
        只需将那个slave的outputbuffer copy到自己。

    2. 如何保证数据的一致的呢？ 首先用bgsave生成一个全量的rdb，当然这只是这个时间的内存快照，在此以后的Object将会写在client的outputbuffer中，
       直到client接收到完整rdb，将增量的reply发给client(slave)