（十九）— replication主从数据复制的实现
最后更新于：2022-04-01 20:20:54
replication的英文单词的原意是“复制”的意思,replication文件作为我在Data目录下的分析的最后一个文件，足以说明他的重要性，代码量1800+，的确非常难啃。只能说个我看代码下来的大致印象吧，要我画个结构图好好理理这里面各个API的关系图，这个我目前还真做不到。说到主从复制，这个是实现读写分离的最好手段了，也很常见，当用户数达到一定量，当一个服务器承受不了达到上千万的pv时，采取主从数据库的形式也是一般架构师能够想到的一种手段。Redis的主从数据库在我这里就称为主客户端，从客户端，因为客户端中有所属于的db，因为数据库基于客户单本身进行复制操作的。也就是说，一个Redis，存在一个master主客户端，多个slave从客户端，到时实现的就是slave向主客户端进行复制操作。因为API比较多，进行了稍稍的归类:
~~~
/* ---------------------------------- MASTER -------------------------------- */
void createReplicationBacklog(void) /* 创建backlog的buffer */
void resizeReplicationBacklog(long long newsize) /* 调整复制备份日志的大小，当replication backlog被修改的时候 */
void freeReplicationBacklog(void) /* 释放备份日志 */
void feedReplicationBacklog(void *ptr, size_t len) /* 往备份日志中添加添加数据操作，会引起master_repl_offset偏移量的增加 */
void feedReplicationBacklogWithObject(robj *o) /* 往backlog添加数据，以Redis 字符串对象作为参数 */
void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) /* 将主数据库复制到从数据库 */
void replicationFeedMonitors(redisClient *c, list *monitors, int dictid, robj **argv, int argc) /* 发送数据给monitor监听者客户端 */
long long addReplyReplicationBacklog(redisClient *c, long long offset) /* slave从客户单添加备份日志 */
int masterTryPartialResynchronization(redisClient *c) /* 主数据库尝试分区同步 */
void syncCommand(redisClient *c) /* 同步命令函数 */
void replconfCommand(redisClient *c) /* 此函数用于从客户端进行配置复制进程中的执行参数设置 */
void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) /* 给slave客户端发送BULK数据 */
void updateSlavesWaitingBgsave(int bgsaveerr) /* 此方法将用于后台保存进程快结束时调用，更新slave从客户端 */
/* ----------------------------------- SLAVE -------------------------------- */
void replicationAbortSyncTransfer(void) /* 中止与master主数据的同步操作 */
void replicationSendNewlineToMaster(void) /* 从客户端发送空行给主客户端，破坏了原本的协议格式，避免让主客户端检测出从客户端超时的情况 */
void replicationEmptyDbCallback(void *privdata) /* 清空数据库后的回调方法，当老数据被刷新出去之后等待加载新数据的时候调用 */
void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) /* 从客户端读取同步的Sync的BULK数据 */
char *sendSynchronousCommand(int fd, ...) /* 从客户端发送给主客户端同步数据的命令，附上验证信息，和一些参数配置信息 */
int slaveTryPartialResynchronization(int fd) /* 从客户端尝试分区同步操作 */
void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) /* 与主客户端保持同步，期间包括端口号等的确认，socket连接 */
int connectWithMaster(void) /* 连接主客户端 */
void undoConnectWithMaster(void) /* 撤销连接主客户端 */
int cancelReplicationHandshake(void) /* 当已经存在一个复制进程时，中止一个非阻塞的replication复制的尝试 */
void replicationSetMaster(char *ip, int port) /* 设定主客户端的ip地址和端口号 */
void replicationUnsetMaster(void)
void slaveofCommand(redisClient *c)
void roleCommand(redisClient *c)
void replicationSendAck(void) /* 发送ACK包给主客户端 ，告知当前的进程偏移量 */
/* ---------------------- MASTER CACHING FOR PSYNC -------------------------- */
void replicationCacheMaster(redisClient *c) /* 缓存客户端信息 */
void replicationDiscardCachedMaster(void) /* 当某个客户端将不会再回复的时候，可以释放掉缓存的主客户端 */
void replicationResurrectCachedMaster(int newfd) /* 将缓存客户端复活 */
/* ------------------------- MIN-SLAVES-TO-WRITE --------------------------- */
void refreshGoodSlavesCount(void) /* 更新slave从客户端数量 */
void replicationScriptCacheInit(void)
void replicationScriptCacheFlush(void)
void replicationScriptCacheAdd(sds sha1)
int replicationScriptCacheExists(sds sha1)
void replicationCron(void)
~~~
找一个标准的slave从客户端向主客户端实现同步的操作:
~~~
/* 与主客户端保持同步，期间包括端口号等的确认，socket连接 */
void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) {
char tmpfile[256], *err;
int dfd, maxtries = 5;
int sockerr = 0, psync_result;
socklen_t errlen = sizeof(sockerr);
REDIS_NOTUSED(el);
REDIS_NOTUSED(privdata);
REDIS_NOTUSED(mask);
/* If this event fired after the user turned the instance into a master
* with SLAVEOF NO ONE we must just return ASAP. */
if (server.repl_state == REDIS_REPL_NONE) {
close(fd);
return;
}
/* Check for errors in the socket. */
/* socket连接是否正常 */
if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &sockerr, &errlen) == -1)
sockerr = errno;
if (sockerr) {
aeDeleteFileEvent(server.el,fd,AE_READABLE|AE_WRITABLE);
redisLog(REDIS_WARNING,"Error condition on socket for SYNC: %s",
strerror(sockerr));
goto error;
}
/* If we were connecting, it's time to send a non blocking PING, we want to
* make sure the master is able to reply before going into the actual
* replication process where we have long timeouts in the order of
* seconds (in the meantime the slave would block). */
/* 连接测试，将由主客户端发送PING命令给从客户端，在给定的延迟时间内观察是否有回复 */
if (server.repl_state == REDIS_REPL_CONNECTING) {
redisLog(REDIS_NOTICE,"Non blocking connect for SYNC fired the event.");
/* Delete the writable event so that the readable event remains
* registered and we can wait for the PONG reply. */
aeDeleteFileEvent(server.el,fd,AE_WRITABLE);
server.repl_state = REDIS_REPL_RECEIVE_PONG;
/* Send the PING, don't check for errors at all, we have the timeout
* that will take care about this. */
//发送PING命令
syncWrite(fd,"PING\r\n",6,100);
return;
}
/* Receive the PONG command. */
//收到回复了
if (server.repl_state == REDIS_REPL_RECEIVE_PONG) {
char buf[1024];
/* Delete the readable event, we no longer need it now that there is
* the PING reply to read. */
aeDeleteFileEvent(server.el,fd,AE_READABLE);
/* Read the reply with explicit timeout. */
buf[0] = '\0';
if (syncReadLine(fd,buf,sizeof(buf),
server.repl_syncio_timeout*1000) == -1)
{
redisLog(REDIS_WARNING,
"I/O error reading PING reply from master: %s",
strerror(errno));
goto error;
}
/* We accept only two replies as valid, a positive +PONG reply
* (we just check for "+") or an authentication error.
* Note that older versions of Redis replied with "operation not
* permitted" instead of using a proper error code, so we test
* both. */
if (buf[0] != '+' &&
strncmp(buf,"-NOAUTH",7) != 0 &&
strncmp(buf,"-ERR operation not permitted",28) != 0)
{
redisLog(REDIS_WARNING,"Error reply to PING from master: '%s'",buf);
goto error;
} else {
redisLog(REDIS_NOTICE,
"Master replied to PING, replication can continue...");
}
}
/* AUTH with the master if required. */
//auth身份验证
if(server.masterauth) {
err = sendSynchronousCommand(fd,"AUTH",server.masterauth,NULL);
if (err[0] == '-') {
redisLog(REDIS_WARNING,"Unable to AUTH to MASTER: %s",err);
sdsfree(err);
goto error;
}
sdsfree(err);
}
/* Set the slave port, so that Master's INFO command can list the
* slave listening port correctly. */
/* 设置从客户端监听端口 */
{
sds port = sdsfromlonglong(server.port);
err = sendSynchronousCommand(fd,"REPLCONF","listening-port",port,
NULL);
sdsfree(port);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF listening-port. */
if (err[0] == '-') {
redisLog(REDIS_NOTICE,"(Non critical) Master does not understand REPLCONF listening-port: %s", err);
}
sdsfree(err);
}
/* Try a partial resynchonization. If we don't have a cached master
* slaveTryPartialResynchronization() will at least try to use PSYNC
* to start a full resynchronization so that we get the master run id
* and the global offset, to try a partial resync at the next
* reconnection attempt. */
psync_result = slaveTryPartialResynchronization(fd);
if (psync_result == PSYNC_CONTINUE) {
redisLog(REDIS_NOTICE, "MASTER <-> SLAVE sync: Master accepted a Partial Resynchronization.");
return;
}
/* Fall back to SYNC if needed. Otherwise psync_result == PSYNC_FULLRESYNC
* and the server.repl_master_runid and repl_master_initial_offset are
* already populated. */
if (psync_result == PSYNC_NOT_SUPPORTED) {
redisLog(REDIS_NOTICE,"Retrying with SYNC...");
if (syncWrite(fd,"SYNC\r\n",6,server.repl_syncio_timeout*1000) == -1) {
redisLog(REDIS_WARNING,"I/O error writing to MASTER: %s",
strerror(errno));
goto error;
}
}
/* Prepare a suitable temp file for bulk transfer */
while(maxtries--) {
snprintf(tmpfile,256,
"temp-%d.%ld.rdb",(int)server.unixtime,(long int)getpid());
dfd = open(tmpfile,O_CREAT|O_WRONLY|O_EXCL,0644);
if (dfd != -1) break;
sleep(1);
}
if (dfd == -1) {
redisLog(REDIS_WARNING,"Opening the temp file needed for MASTER <-> SLAVE synchronization: %s",strerror(errno));
goto error;
}
/* Setup the non blocking download of the bulk file. */
if (aeCreateFileEvent(server.el,fd, AE_READABLE,readSyncBulkPayload,NULL)
== AE_ERR)
{
redisLog(REDIS_WARNING,
"Can't create readable event for SYNC: %s (fd=%d)",
strerror(errno),fd);
goto error;
}
server.repl_state = REDIS_REPL_TRANSFER;
server.repl_transfer_size = -1;
server.repl_transfer_read = 0;
server.repl_transfer_last_fsync_off = 0;
server.repl_transfer_fd = dfd;
server.repl_transfer_lastio = server.unixtime;
server.repl_transfer_tmpfile = zstrdup(tmpfile);
return;
error:
close(fd);
server.repl_transfer_s = -1;
server.repl_state = REDIS_REPL_CONNECT;
return;
}
~~~
在replication中，要一个cacheMaster的概念，就是可以临时缓存主客户端的信息，一般用于突然master和slave断开连接的时候，可以下次进行主从同步的时候快速恢复:
~~~
/* 缓存客户端信息 */
void replicationCacheMaster(redisClient *c) {
listNode *ln;
redisAssert(server.master != NULL && server.cached_master == NULL);
redisLog(REDIS_NOTICE,"Caching the disconnected master state.");
/* Remove from the list of clients, we don't want this client to be
* listed by CLIENT LIST or processed in any way by batch operations. */
//首先移除此客户端
ln = listSearchKey(server.clients,c);
redisAssert(ln != NULL);
listDelNode(server.clients,ln);
/* Save the master. Server.master will be set to null later by
* replicationHandleMasterDisconnection(). */
//保存为缓存客户端
server.cached_master = server.master;
/* Remove the event handlers and close the socket. We'll later reuse
* the socket of the new connection with the master during PSYNC. */
//删除在这个客户端上的读写事件
aeDeleteFileEvent(server.el,c->fd,AE_READABLE);
aeDeleteFileEvent(server.el,c->fd,AE_WRITABLE);
close(c->fd);
/* Set fd to -1 so that we can safely call freeClient(c) later. */
c->fd = -1;
/* Invalidate the Peer ID cache. */
if (c->peerid) {
sdsfree(c->peerid);
c->peerid = NULL;
}
/* Caching the master happens instead of the actual freeClient() call,
* so make sure to adjust the replication state. This function will
* also set server.master to NULL. */
replicationHandleMasterDisconnection();
}
~~~
当想让这个master的复活的时候，调用下面的方法:
~~~
/* Turn the cached master into the current master, using the file descriptor
* passed as argument as the socket for the new master.
*
* This funciton is called when successfully setup a partial resynchronization
* so the stream of data that we'll receive will start from were this
* master left. */
/* 将缓存客户端复活 */
void replicationResurrectCachedMaster(int newfd) {
//将cached_master赋值为主客户端
server.master = server.cached_master;
server.cached_master = NULL;
server.master->fd = newfd;
server.master->flags &= ~(REDIS_CLOSE_AFTER_REPLY|REDIS_CLOSE_ASAP);
server.master->authenticated = 1;
server.master->lastinteraction = server.unixtime;
server.repl_state = REDIS_REPL_CONNECTED;
/* Re-add to the list of clients. */
//重新添加入客户端列表中
listAddNodeTail(server.clients,server.master);
if (aeCreateFileEvent(server.el, newfd, AE_READABLE,
readQueryFromClient, server.master)) {
redisLog(REDIS_WARNING,"Error resurrecting the cached master, impossible to add the readable handler: %s", strerror(errno));
freeClientAsync(server.master); /* Close ASAP. */
}
/* We may also need to install the write handler as well if there is
* pending data in the write buffers. */
if (server.master->bufpos || listLength(server.master->reply)) {
if (aeCreateFileEvent(server.el, newfd, AE_WRITABLE,
sendReplyToClient, server.master)) {
redisLog(REDIS_WARNING,"Error resurrecting the cached master, impossible to add the writable handler: %s", strerror(errno));
freeClientAsync(server.master); /* Close ASAP. */
}
}
}
~~~
当然如果确定在未来不糊在使用缓存的master的时，可以彻底摧毁:
~~~
/* Free a cached master, called when there are no longer the conditions for
* a partial resync on reconnection. */
/* 当某个客户端将不会再回复的时候，可以释放掉缓存的主客户端 */
void replicationDiscardCachedMaster(void) {
if (server.cached_master == NULL) return;
redisLog(REDIS_NOTICE,"Discarding previously cached master state.");
server.cached_master->flags &= ~REDIS_MASTER;
//直接释放客户端
freeClient(server.cached_master);
//server的缓存客户端赋值为NULL
server.cached_master = NULL;
}
~~~
在这里面靠的就是server.cached_master属性。slave在和master连接的时候，要进行master的ip地址和Port端口的确认：
~~~
/* Set replication to the specified master address and port. */
/* 设定主客户端的ip地址和端口号 */
void replicationSetMaster(char *ip, int port) {
sdsfree(server.masterhost);
server.masterhost = sdsdup(ip);
server.masterport = port;
//设置完毕之后，断开所有的连接，中止replication进程
if (server.master) freeClient(server.master);
disconnectSlaves(); /* Force our slaves to resync with us as well. */
replicationDiscardCachedMaster(); /* Don't try a PSYNC. */
freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */
cancelReplicationHandshake();
server.repl_state = REDIS_REPL_CONNECT;
server.master_repl_offset = 0;
}
~~~
主从复制的实现其实还有很多细节和步骤的。稍稍分析了一下，以后有机会研究的更深入一点
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