void syncCommand(redisClient *c) {
/* ignore SYNC if aleady slave or in monitor mode */
if (c->flags & REDIS_SLAVE) return;
/* Refuse SYNC requests if we are a slave but the link with our master
* is not ok... */
if (server.masterhost && server.repl_state != REDIS_REPL_CONNECTED) {
addReplyError(c,"Can't SYNC while not connected with my master");
return;
}
/* SYNC can't be issued when the server has pending data to send to
* the client about already issued commands. We need a fresh reply
* buffer registering the differences between the BGSAVE and the current
* dataset, so that we can copy to other slaves if needed. */
if (listLength(c->reply) != 0) {
addReplyError(c,"SYNC is invalid with pending input");
return;
}
redisLog(REDIS_NOTICE,"Slave ask for synchronization");
/* Here we need to check if there is a background saving operation
* in progress, or if it is required to start one */
if (server.rdb_child_pid != -1) {
/* Ok a background save is in progress. Let's check if it is a good
* one for replication, i.e. if there is another slave that is
* registering differences since the server forked to save */
redisClient *slave;
listNode *ln;
listIter li;
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");
}
} else {
/* Ok we don't have a BGSAVE in progress, let's start one */
redisLog(REDIS_NOTICE,"Starting BGSAVE for SYNC");
if (rdbSaveBackground(server.rdb_filename) != REDIS_OK) {
redisLog(REDIS_NOTICE,"Replication failed, can't BGSAVE");
addReplyError(c,"Unable to perform background save");
return;
}
c->replstate = REDIS_REPL_WAIT_BGSAVE_END;
}
c->repldbfd = -1;
c->flags |= REDIS_SLAVE;
c->slaveseldb = 0;
listAddNodeTail(server.slaves,c);
return;
}
void syncCommand(redisClient *c) {
//响应redis的sync指令
/* ignore SYNC if already slave or in monitor mode */
if (c->flags & REDIS_SLAVE) return;
/* Refuse SYNC requests if we are a slave but the link with our master
* is not ok... */
if (server.masterhost && server.repl_state != REDIS_REPL_CONNECTED) {
addReplyError(c,"Can't SYNC while not connected with my master");
return;
}
/* SYNC can't be issued when the server has pending data to send to
* the client about already issued commands. We need a fresh reply
* buffer registering the differences between the BGSAVE and the current
* dataset, so that we can copy to other slaves if needed. */
if (listLength(c->reply) != 0) {//如果服务端还有数据没有发送给客户端，那先不处理sync请求
addReplyError(c,"SYNC is invalid with pending input");
return;
}
redisLog(REDIS_NOTICE,"Slave ask for synchronization");
/* Here we need to check if there is a background saving operation
* in progress, or if it is required to start one */
if (server.rdb_child_pid != -1) {
/* Ok a background save is in progress. Let's check if it is a good
* one for replication, i.e. if there is another slave that is
* registering differences since the server forked to save */
redisClient *slave;
listNode *ln;
listIter li;
listRewind(server.slaves,&li);
//正好现在有RDB程序在保存日志，那么我需要查看一下是否正好有从库在等待这个RDB结束的操作
//如果有，我就可以偷偷的将其尚未发送，累积起来的写操作日志拷贝一份，插队到后面作为从库。
//等待全部RDB文件发送完后将这部分累积的日志发送给这个新从库，其实就是类似插队的意思，插上对后，顺便打一碗饭
while((ln = listNext(&li))) {
slave = ln->value;
if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) break;
}
if (ln) {//正好有其他slave出发了BGSAVE操作，那么就正好可以通过copy同伴的缓冲区的方式追上。
//这些buffer是一个客户端注册成为从库之后，主库会主动的在replicationFeedSlaves
//里面将新写入日志追加到这里的。只是那个时候还不能发送，没注册写事件
/* Perfect, the server is already registering differences for
* another slave. Set the right state, and copy the buffer. */
copyClientOutputBuffer(c,slave);//将要发送给之前的slave的数据拷贝一份，也就是也要发送给新的这个从库。
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 */
//由于之前正好没有客户端在后台保存过程中发送sync，所以只能老老实话等待下一个bgsave了。
c->replstate = REDIS_REPL_WAIT_BGSAVE_START;
redisLog(REDIS_NOTICE,"Waiting for next BGSAVE for SYNC");
}
} else {//没办法了，老老实实进行BGSAVE，
/* Ok we don't have a BGSAVE in progress, let's start one */
redisLog(REDIS_NOTICE,"Starting BGSAVE for SYNC");
if (rdbSaveBackground(server.rdb_filename) != REDIS_OK) {
redisLog(REDIS_NOTICE,"Replication failed, can't BGSAVE");
addReplyError(c,"Unable to perform background save");
return;
}
c->replstate = REDIS_REPL_WAIT_BGSAVE_END;
}
//SYNC命令只是尝试启动RDB快照，真正的数据发送再快照完成后的updateSlavesWaitingBgsave
if (server.repl_disable_tcp_nodelay)
anetDisableTcpNoDelay(NULL, c->fd); /* Non critical if it fails. */
c->repldbfd = -1;
c->flags |= REDIS_SLAVE;//成功，标记为slave
c->slaveseldb = 0;
listAddNodeTail(server.slaves,c);
return;
}
