Redis源码分析：主从复制
源码版本：2.4.4
更新2014.3.17，base 2.8.7
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：
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. */
}
}
}
}
}
当server.replstate == REDIS_REPL_CONNECT时，slave连接master，连接成功后，slave执行syncWithMaster函数，syncWithMaster将向master发送SYNC命令
int connectWithMaster(void) {
int fd;
fd = anetTcpNonBlockConnect(NULL,server.masterhost,server.masterport);
if (fd == -1) {
redisLog(REDIS_WARNING,"Unable to connect to MASTER: %s",
strerror(errno));
return REDIS_ERR;
}
if (aeCreateFileEvent(server.el,fd,AE_READABLE|AE_WRITABLE,syncWithMaster,NULL) ==
AE_ERR)
{
close(fd);
redisLog(REDIS_WARNING,"Can't create readable event for SYNC");
return REDIS_ERR;
}
server.repl_transfer_s = fd;
server.replstate = REDIS_REPL_CONNECTING;
return REDIS_OK;
}
master端：
master对于slave的连接和client的连接统一处理，在接收到slave发出的SYNC命令后，执行syncCommand,syncCommand 将查看当前状态，如果正在做快照，则等待，否则启动后台进程做快照。
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.replstate != 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.bgsavechildpid != -1) {
.....
} else {
/* Ok we don't have a BGSAVE in progress, let's start one */
redisLog(REDIS_NOTICE,"Starting BGSAVE for SYNC");
if (rdbSaveBackground(server.dbfilename) != 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;
}
在完成快照后，执行updateSlavesWaitingBgsave函数，updateSlavesWaitingBgsave将查看当前master的各个slave的状态，如果发现有在等待bgsave完成的，则注册事件sendBulkToSlave，sendBulkToSlave将快照文件发送给slave
void updateSlavesWaitingBgsave(int bgsaveerr) {
listNode *ln;
int startbgsave = 0;
listIter li;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
redisClient *slave = ln->value;
if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START) {
startbgsave = 1;
slave->replstate = REDIS_REPL_WAIT_BGSAVE_END;
} else if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_END) {
struct redis_stat buf;
if (bgsaveerr != REDIS_OK) {
freeClient(slave);
redisLog(REDIS_WARNING,"SYNC failed. BGSAVE child returned an error");
continue;
}
if ((slave->repldbfd = open(server.dbfilename,O_RDONLY)) == -1 ||
redis_fstat(slave->repldbfd,&buf) == -1) {
freeClient(slave);
redisLog(REDIS_WARNING,"SYNC failed. Can't open/stat DB after BGSAVE: %s", strerror(errno));
continue;
}
slave->repldboff = 0;
slave->repldbsize = buf.st_size;
slave->replstate = REDIS_REPL_SEND_BULK;
aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE); //删除之前的写回调
if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE, sendBulkToSlave, slave) == AE_ERR) { //注册新的写回调
freeClient(slave);
continue;
}
}
}
if (startbgsave) {
if (rdbSaveBackground(server.dbfilename) != REDIS_OK) {
listIter li;
listRewind(server.slaves,&li);
redisLog(REDIS_WARNING,"SYNC failed. BGSAVE failed");
while((ln = listNext(&li))) {
redisClient *slave = ln->value;
if (slave->replstate == REDIS_REPL_WAIT_BGSAVE_START)
freeClient(slave);
}
}
}
}为了避免阻塞应用，每次只传输16K数据
void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) {
......
lseek(slave->repldbfd,slave->repldboff,SEEK_SET); //指针移动到上次发送的位置
buflen = read(slave->repldbfd,buf,REDIS_IOBUF_LEN); //读取16K数据
......
if ((nwritten = write(fd,buf,buflen)) == -1) { //传输数据到slave
if (errno != EAGAIN) {
redisLog(REDIS_WARNING,"Write error sending DB to slave: %s",
strerror(errno));
freeClient(slave);
}
return;
}
slave->repldboff += nwritten; //更新已发送位置
......
}
在slave完成第一次的同步后，后续如果master接收到改变db状态的命令，则调用replicationFeedSlaves将相应变更发送slave
/* Call() is the core of Redis execution of a command */
void call(redisClient *c) {
long long dirty, start = ustime(), duration;
dirty = server.dirty;
c->cmd->proc(c);
dirty = server.dirty-dirty;
duration = ustime()-start;
slowlogPushEntryIfNeeded(c->argv,c->argc,duration);
if (server.appendonly && dirty > 0)
feedAppendOnlyFile(c->cmd,c->db->id,c->argv,c->argc);
if ((dirty > 0 || c->cmd->flags & REDIS_CMD_FORCE_REPLICATION) &&
listLength(server.slaves))
replicationFeedSlaves(server.slaves,c->db->id,c->argv,c->argc);
if (listLength(server.monitors))
replicationFeedMonitors(server.monitors,c->db->id,c->argv,c->argc);
server.stat_numcommands++;
}
总结：
1. redis主从复制，并没有增加太多额外代码，但是功能强大，支持多个slave，并且支持slave作为master。
2. redis虽然宣称主从复制无阻塞，但是，由于redis使用单线程服务，而和slave的交互由处理线程统一处理，因此，对性能有一定影响