前面几篇我们已经完全理解了redis的基本功能的实现了。
但单靠基本功能实现，往往还是称不上优秀的项目的。毕竟，我们现在面对的都是复杂的环境，高并发的场景，大数据量的可能。
简而言之，现在的系统一般都需要支持分布式部署，不存在单点问题，才算是一个合格的系统。
而redis作为一个存储系统，单点问题肯定是不行的。
最简单的，就是起码得支持读写分离功能，因为我们面临的许多问题，一般是面对大量的查询问题。而要做到读写分离功能，就是要把主节点的数据同步到从节点上。从而可以让从节点接受读请求，以减轻主节点的读压力。
就让我们来分析下 Redis 是如何进行主从同步数据的吧！主从同步，换个名称也就是数据复制。
0. 主从复制的作用
数据冗余：主从复制实现了数据的热备份，是持久化之外的一种数据冗余方式。
故障恢复：当主节点出现问题时，可以由从节点提供服务，实现快速的故障恢复；实际上是一种服务的冗余。
负载均衡：在主从复制的基础上，配合读写分离，可以由主节点提供写服务，由从节点提供读服务（即写Redis数据时应用连接主节点，读Redis数据时应用连接从节点），分担服务器负载；尤其是在写少读多的场景下，通过多个从节点分担读负载，可以大大提高Redis服务器的并发量。
读写分离：可以用于实现读写分离，主库写、从库读，读写分离不仅可以提高服务器的负载能力，同时可根据需求的变化，改变从库的数量；
高可用基石：除了上述作用以外，主从复制还是哨兵和集群能够实施的基础，因此说主从复制是Redis高可用的基础。
1. Redis 主从复制简介
在主从复制中，数据库分为两类，一类是主库(master)，另一类是同步主库数据的从库(slave)。主库可以进行读写操作，当写操作导致数据变化时会自动同步到从库。而从库一般是只读的(特定情况也可以写,通过参数slave-read-only指定)，并接受来自主库的数据，一个主库可拥有多个从库，而一个从库只能有一个主库。这样就使得redis的主从架构有了两种模式：一类是一主多从如下图1，二类是“链式主从复制”--主->从->主-从如下图2。
2. Redis 主从复制的操作步骤简略说明
1. 首先，你得有至少2个redis server 实例，单机多实例或者多机多实例皆可。
2. 配置主从关系，使用 slaveof master_host master_port; (config rewrite 可直接写入配置文件，避免每次都重新写)
3. 验证主从配置，使用 info Replication;
上面的操作步骤是进行实时操作的，也可以直接将 master/slave 配置放到 redis.conf 中，启动时直接加载。
当master需要使用密码进行访问时，可以使用命令 masterauth 进行授权。
masterauth 123456 # 写到redis.conf配置文件中
config set masterauth 123456 # 通过命令行进行授权
3. 主要同步的实现原理
主从复制大致流程为：
1. slaveof 是我们的开启方法，它会将master信息写入到从节点；
2. 然后与master进行建立连接；
3. 然后master决定复制方式是全量同步还是部分同步；
4. master进行数据准备；
5. 将需要同步的发送给slave节点；
6. 从节点执行发送过来的数据；
但是，我们需要进行深入理解。
3.1. slaveof 命令源码解析
slaveof 为我们操作开启主从复制开启了入口，其接口定义如下：
{"slaveof",slaveofCommand,3,"ast",0,NULL,0,0,0,0,0},
// 用法 slaveof <master_host> <master_port> 建立主从关系
// slaveof no one 取消主从同步
// replication.c
void slaveofCommand(client *c) {
/* SLAVEOF is not allowed in cluster mode as replication is automatically
* configured using the current address of the master node. */
if (server.cluster_enabled) {
addReplyError(c,"SLAVEOF not allowed in cluster mode.");
return;
}
/* The special host/port combination "NO" "ONE" turns the instance
* into a master. Otherwise the new master address is set. */
// slaveof no one, 取消主从同步
if (!strcasecmp(c->argv[1]->ptr,"no") &&
!strcasecmp(c->argv[2]->ptr,"one")) {
if (server.masterhost) {
// 取消当前的master关联，返回客户端目前状态信息，结束
replicationUnsetMaster();
sds client = catClientInfoString(sdsempty(),c);
serverLog(LL_NOTICE,"MASTER MODE enabled (user request from '%s')",
client);
sdsfree(client);
}
} else {
long port;
if ((getLongFromObjectOrReply(c, c->argv[2], &port, NULL) != C_OK))
return;
/* Check if we are already attached to the specified slave */
// 只能和一个 master 建立主从关系
if (server.masterhost && !strcasecmp(server.masterhost,c->argv[1]->ptr)
&& server.masterport == port) {
serverLog(LL_NOTICE,"SLAVE OF would result into synchronization with the master we are already connected with. No operation performed.");
addReplySds(c,sdsnew("+OK Already connected to specified master\r\n"));
return;
}
/* There was no previous master or the user specified a different one,
* we can continue. */
// 设置master信息
replicationSetMaster(c->argv[1]->ptr, port);
// 输出client状态信息
sds client = catClientInfoString(sdsempty(),c);
serverLog(LL_NOTICE,"SLAVE OF %s:%d enabled (user request from '%s')",
server.masterhost, server.masterport, client);
sdsfree(client);
}
addReply(c,shared.ok);
}
// 绑定新的master关联
/* Set replication to the specified master address and port. */
void replicationSetMaster(char *ip, int port) {
sdsfree(server.masterhost);
server.masterhost = sdsnew(ip);
server.masterport = port;
if (server.master) freeClient(server.master);
// slave 不进行阻塞客户端
disconnectAllBlockedClients(); /* Clients blocked in master, now slave. */
// 断开所有 slave 连接
disconnectSlaves(); /* Force our slaves to resync with us as well. */
// cacheMaster 丢弃
replicationDiscardCachedMaster(); /* Don't try a PSYNC. */
// 链式主从复制删除
freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */
// 断开正在连接slave请求
cancelReplicationHandshake();
server.repl_state = REPL_STATE_CONNECT;
server.master_repl_offset = 0;
server.repl_down_since = 0;
}
// 取消master关联
/* Cancel replication, setting the instance as a master itself. */
void replicationUnsetMaster(void) {
if (server.masterhost == NULL) return; /* Nothing to do. */
sdsfree(server.masterhost);
server.masterhost = NULL;
if (server.master) {
if (listLength(server.slaves) == 0) {
/* If this instance is turned into a master and there are no
* slaves, it inherits the replication offset from the master.
* Under certain conditions this makes replicas comparable by
* replication offset to understand what is the most updated. */
server.master_repl_offset = server.master->reploff;
freeReplicationBacklog();
}
freeClient(server.master);
}
replicationDiscardCachedMaster();
cancelReplicationHandshake();
server.repl_state = REPL_STATE_NONE;
}
// blocked.c, 解除所有的阻塞客户端
/* Mass-unblock clients because something changed in the instance that makes
* blocking no longer safe. For example clients blocked in list operations
* in an instance which turns from master to slave is unsafe, so this function
* is called when a master turns into a slave.
*
* The semantics is to send an -UNBLOCKED error to the client, disconnecting
* it at the same time. */
void disconnectAllBlockedClients(void) {
listNode *ln;
listIter li;
listRewind(server.clients,&li);
while((ln = listNext(&li))) {
client *c = listNodeValue(ln);
if (c->flags & CLIENT_BLOCKED) {
addReplySds(c,sdsnew(
"-UNBLOCKED force unblock from blocking operation, "
"instance state changed (master -> slave?)\r\n"));
unblockClient(c);
c->flags |= CLIENT_CLOSE_AFTER_REPLY;
}
}
}
// networking.c, 断开所有的 slave 连接
/* Close all the slaves connections. This is useful in chained replication
* when we resync with our own master and want to force all our slaves to
* resync with us as well. */
void disconnectSlaves(void) {
while (listLength(server.slaves)) {
listNode *ln = listFirst(server.slaves);
freeClient((client*)ln->value);
}
}
// replication.c
/* Free a cached master, called when there are no longer the conditions for
* a partial resync on reconnection. */
void replicationDiscardCachedMaster(void) {
if (server.cached_master == NULL) return;
serverLog(LL_NOTICE,"Discarding previously cached master state.");
server.cached_master->flags &= ~CLIENT_MASTER;
freeClient(server.cached_master);
server.cached_master = NULL;
}
// replication.c
void freeReplicationBacklog(void) {
serverAssert(listLength(server.slaves) == 0);
zfree(server.repl_backlog);
server.repl_backlog = NULL;
}
// replication.c
/* This function aborts a non blocking replication attempt if there is one
* in progress, by canceling the non-blocking connect attempt or
* the initial bulk transfer.
*
* If there was a replication handshake in progress 1 is returned and
* the replication state (server.repl_state) set to REPL_STATE_CONNECT.
*
* Otherwise zero is returned and no operation is perforemd at all. */
int cancelReplicationHandshake(void) {
if (server.repl_state == REPL_STATE_TRANSFER) {
replicationAbortSyncTransfer();
server.repl_state = REPL_STATE_CONNECT;
} else if (server.repl_state == REPL_STATE_CONNECTING ||
slaveIsInHandshakeState())
{
undoConnectWithMaster();
server.repl_state = REPL_STATE_CONNECT;
} else {
return 0;
}
return 1;
}
// networking.c
/* Concatenate a string representing the state of a client in an human
* readable format, into the sds string 's'. */
sds catClientInfoString(sds s, client *client) {
char flags[16], events[3], *p;
int emask;
p = flags;
if (client->flags & CLIENT_SLAVE) {
if (client->flags & CLIENT_MONITOR)
*p++ = 'O';
else
*p++ = 'S';
}
if (client->flags & CLIENT_MASTER) *p++ = 'M';
if (client->flags & CLIENT_MULTI) *p++ = 'x';
if (client->flags & CLIENT_BLOCKED) *p++ = 'b';
if (client->flags & CLIENT_DIRTY_CAS) *p++ = 'd';
if (client->flags & CLIENT_CLOSE_AFTER_REPLY) *p++ = 'c';
if (client->flags & CLIENT_UNBLOCKED) *p++ = 'u';
if (client->flags & CLIENT_CLOSE_ASAP) *p++ = 'A';
if (client->flags & CLIENT_UNIX_SOCKET) *p++ = 'U';
if (client->flags & CLIENT_READONLY) *p++ = 'r';
if (p == flags) *p++ = 'N';
*p++ = '\0';
emask = client->fd == -1 ? 0 : aeGetFileEvents(server.el,client->fd);
p = events;
if (emask & AE_READABLE) *p++ = 'r';
if (emask & AE_WRITABLE) *p++ = 'w';
*p = '\0';
// 可变参数定义: sds sdscatfmt(sds s, char const *fmt, ...)
return sdscatfmt(s,
"id=%U addr=%s fd=%i name=%s age=%I idle=%I flags=%s db=%i sub=%i psub=%i multi=%i qbuf=%U qbuf-free=%U obl=%U oll=%U omem=%U events=%s cmd=%s",
(unsigned long long) client->id,
getClientPeerId(client),
client->fd,
client->name ? (char*)client->name->ptr : "",
(long long)(server.unixtime - client->ctime),
(long long)(server.unixtime - client->lastinteraction),
flags,
client->db->id,
(int) dictSize(client->pubsub_channels),
(int) listLength(client->pubsub_patterns),
(client->flags & CLIENT_MULTI) ? client->mstate.count : -1,
(unsigned long long) sdslen(client->querybuf),
(unsigned long long) sdsavail(client->querybuf),
(unsigned long long) client->bufpos,
(unsigned long long) listLength(client->reply),
(unsigned long long) getClientOutputBufferMemoryUsage(client),
events,
client->lastcmd ? client->lastcmd->name : "NULL");
}
所以，slaveof 只是做简单的验证，然后设置了下 master 信息，然后就返回了。那么是谁在做同步的工作呢？
其实同步任务是由 cron 任务运行的。
3.2. 如何执行同步任务？
因为复制是比较耗性能的东西，如果和用户线程共享处理过程的话，将可能引起并发性能的。所以，redis使用异步 cron 任务的形式实现主从复制功能。
// server.c, 初始化server，注册 cron
void initServer(void) {
...
/* Create out timers, that's our main way to process background
* operations. */
// 添加 serverCron 到 eventLoop 中，以便后续可以执行定时脚本
if (aeCreateTimeEvent(server.el, 1, serverCron, NULL, NULL) == AE_ERR) {
serverPanic("Can't create event loop timers.");
exit(1);
}
...
}
// ae.c, 添加时间事件
long long aeCreateTimeEvent(aeEventLoop *eventLoop, long long milliseconds,
aeTimeProc *proc, void *clientData,
aeEventFinalizerProc *finalizerProc)
{
long long id = eventLoop->timeEventNextId++;
aeTimeEvent *te;
te = zmalloc(sizeof(*te));
if (te == NULL) return AE_ERR;
te->id = id;
aeAddMillisecondsToNow(milliseconds,&te->when_sec,&te->when_ms);
te->timeProc = proc;
te->finalizerProc = finalizerProc;
te->clientData = clientData;
te->next = eventLoop->timeEventHead;
eventLoop->timeEventHead = te;
return id;
}
// server.c, 主脚本运行入口, 每1秒运行1次
int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
...
/* Replication cron function -- used to reconnect to master and
* to detect transfer failures. */
// 主从复制，连接 master，我们的入口
run_with_period(1000) replicationCron();
...
server.cronloops++;
return 1000/server.hz;
}
// 重点入口: replicationCron()
// replication.c, 主从复制定时脚本
/* Replication cron function, called 1 time per second. */
void replicationCron(void) {
static long long replication_cron_loops = 0;
/* Non blocking connection timeout? */
// 连接超时处理，取消重连
if (server.masterhost &&
(server.repl_state == REPL_STATE_CONNECTING ||
slaveIsInHandshakeState()) &&
(time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
{
serverLog(LL_WARNING,"Timeout connecting to the MASTER...");
cancelReplicationHandshake();
}
/* Bulk transfer I/O timeout? */
// 传输数据超时，取消重连
if (server.masterhost && server.repl_state == REPL_STATE_TRANSFER &&
(time(NULL)-server.repl_transfer_lastio) > server.repl_timeout)
{
serverLog(LL_WARNING,"Timeout receiving bulk data from MASTER... If the problem persists try to set the 'repl-timeout' parameter in redis.conf to a larger value.");
cancelReplicationHandshake();
}
/* Timed out master when we are an already connected slave? */
// slave 会话超时
if (server.masterhost && server.repl_state == REPL_STATE_CONNECTED &&
(time(NULL)-server.master->lastinteraction) > server.repl_timeout)
{
serverLog(LL_WARNING,"MASTER timeout: no data nor PING received...");
freeClient(server.master);
}
/* Check if we should connect to a MASTER */
// 3.2.1. 初次设置master时，一定会进行连接处理
if (server.repl_state == REPL_STATE_CONNECT) {
serverLog(LL_NOTICE,"Connecting to MASTER %s:%d",
server.masterhost, server.masterport);
if (connectWithMaster() == C_OK) {
serverLog(LL_NOTICE,"MASTER <-> SLAVE sync started");
}
}
/* Send ACK to master from time to time.
* Note that we do not send periodic acks to masters that don't
* support PSYNC and replication offsets. */
// 3.2.2. 每次定时任务执行，都会发生 ACK 给master
if (server.masterhost && server.master &&
!(server.master->flags & CLIENT_PRE_PSYNC))
replicationSendAck();
/* 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. */
listIter li;
listNode *ln;
robj *ping_argv[1];
/* First, send PING according to ping_slave_period. */
// 3.2.3. 发送 PING 请求
// 默认 repl_ping_slave_period: 10
if ((replication_cron_loops % server.repl_ping_slave_period) == 0) {
ping_argv[0] = createStringObject("PING",4);
replicationFeedSlaves(server.slaves, server.slaveseldb,
ping_argv, 1);
decrRefCount(ping_argv[0]);
}
/* Second, send a newline to all the slaves in pre-synchronization
* stage, that is, slaves waiting for the master to create the RDB file.
* The newline will be ignored by the slave but will refresh the
* last-io timer preventing a timeout. In this case we ignore the
* ping period and refresh the connection once per second since certain
* timeouts are set at a few seconds (example: PSYNC response). */
// 3.2.4. 向以当前节点为master的slaves 发送空行数据
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START ||
(slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END &&
server.rdb_child_type != RDB_CHILD_TYPE_SOCKET))
{
if (write(slave->fd, "\n", 1) == -1) {
/* Don't worry, it's just a ping. */
}
}
}
/* Disconnect timedout slaves. */
// 断开连接超时的 slaves
if (listLength(server.slaves)) {
listIter li;
listNode *ln;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate != SLAVE_STATE_ONLINE) continue;
if (slave->flags & CLIENT_PRE_PSYNC) continue;
if ((server.unixtime - slave->repl_ack_time) > server.repl_timeout)
{
serverLog(LL_WARNING, "Disconnecting timedout slave: %s",
replicationGetSlaveName(slave));
freeClient(slave);
}
}
}
/* If we have no attached slaves and there is a replication backlog
* using memory, free it after some (configured) time. */
// 如果没有slave 跟随当前节点，一段时间后将backlog 释放掉
if (listLength(server.slaves) == 0 && server.repl_backlog_time_limit &&
server.repl_backlog)
{
time_t idle = server.unixtime - server.repl_no_slaves_since;
if (idle > server.repl_backlog_time_limit) {
freeReplicationBacklog();
serverLog(LL_NOTICE,
"Replication backlog freed after %d seconds "
"without connected slaves.",
(int) server.repl_backlog_time_limit);
}
}
/* If AOF is disabled and we no longer have attached slaves, we can
* free our Replication Script Cache as there is no need to propagate
* EVALSHA at all. */
if (listLength(server.slaves) == 0 &&
server.aof_state == AOF_OFF &&
listLength(server.repl_scriptcache_fifo) != 0)
{
replicationScriptCacheFlush();
}
/* If we are using diskless replication and there are slaves waiting
* in WAIT_BGSAVE_START state, check if enough seconds elapsed and
* start a BGSAVE.
*
* This code is also useful to trigger a BGSAVE if the diskless
* replication was turned off with CONFIG SET, while there were already
* slaves in WAIT_BGSAVE_START state. */
if (server.rdb_child_pid == -1 && server.aof_child_pid == -1) {
time_t idle, max_idle = 0;
int slaves_waiting = 0;
int mincapa = -1;
listNode *ln;
listIter li;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
idle = server.unixtime - slave->lastinteraction;
if (idle > max_idle) max_idle = idle;
slaves_waiting++;
mincapa = (mincapa == -1) ? slave->slave_capa :
(mincapa & slave->slave_capa);
}
}
// 3.2.5. 如果有等待同步的slave, 且等待时间超过 server.repl_diskless_sync_delay, 默认是: 5s
if (slaves_waiting && max_idle > server.repl_diskless_sync_delay) {
/* Start a BGSAVE. Usually with socket target, or with disk target
* if there was a recent socket -> disk config change. */
startBgsaveForReplication(mincapa);
}
}
/* Refresh the number of slaves with lag <= min-slaves-max-lag. */
// 刷新本节点的 从健康节点 数量，以便在需要确保多少节点时才进行写入的场景判定
refreshGoodSlavesCount();
replication_cron_loops++; /* Incremented with frequency 1 HZ. */
}
以上，就是整个主从复制的主体框架了。且以上代码包含了两种角色的运行机制。1: master 的运行; 2. slave 的运行;
slave 的运行过程如下:
1. 从节点每秒运行一次定时任务;
2. 当定时任务发现存在新的主节点后，会调用 connectWithMaster() 尝试与master节点建立网络连接;
3. 建立连接后，由 syncWithMaster() 进行处理后续同步事务;
4. 各种连接超时释放处理;
master 的运行过程如下:
1. 各种连接超时释放处理;
2. 定期进行 PING slave 操作;
3. 向slave写入一个空行,相当于ping操作与slave续租期;
4. 清理连接超时的slaves, 如果一个slave也没有, 则直接把backlog释放掉;
5. 如果未开启磁盘持久化操作，且有等待同步的slaves, 则主动开启一个 bgsave;
从上面的框架中，可以说大部分时候都是在处理各种异常问题和续期问题，但是实际最重要的一个连接master操作却只有一行代码。那么slave连接master之后，是如何进行后续的同步的呢？好像这个定时任务的运行并没有太大的作用呢！
3.3. 从节点如何处理同步操作？
从节点是整个同步操作的操控者，整个同步可以说都是其主导的。从上一节的过程，我们可以看到，只有一个连接master的只剩，所以必定许多工作要这里完成。
实际上，slave连接到master的请求实现，基于 epoll 模型的异步操作，所以，在主框架中，我们只看到一个连接操作。因为连接完成后的操作，是异步执行的。
// replication.c, 连接请求到 master 节点
int connectWithMaster(void) {
int fd;
// 创建socket fd
fd = anetTcpNonBlockBestEffortBindConnect(NULL,
server.masterhost,server.masterport,NET_FIRST_BIND_ADDR);
if (fd == -1) {
serverLog(LL_WARNING,"Unable to connect to MASTER: %s",
strerror(errno));
return C_ERR;
}
// 使用epoll模型进行异步连接
// 连接成功后，由 syncWithMaster 进行事件处理
// 关注 读写事件
if (aeCreateFileEvent(server.el,fd,AE_READABLE|AE_WRITABLE,syncWithMaster,NULL) ==
AE_ERR)
{
close(fd);
serverLog(LL_WARNING,"Can't create readable event for SYNC");
return C_ERR;
}
server.repl_transfer_lastio = server.unixtime;
server.repl_transfer_s = fd;
// 状态变更，以便下次不会再进行连接
server.repl_state = REPL_STATE_CONNECTING;
return C_OK;
}
// anet.c, 建立一个非阻塞的socket连接
int anetTcpNonBlockBestEffortBindConnect(char *err, char *addr, int port,
char *source_addr)
{
// ANET_CONNECT_BE_BINDING 代表将进行重试尽可能建立连接
return anetTcpGenericConnect(err,addr,port,source_addr,
ANET_CONNECT_NONBLOCK|ANET_CONNECT_BE_BINDING);
}
// 与master连接成功后，由 syncWithMaster 进行处理后续事务
// replication.c
void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) {
char tmpfile[256], *err = NULL;
int dfd, maxtries = 5;
int sockerr = 0, psync_result;
socklen_t errlen = sizeof(sockerr);
UNUSED(el);
UNUSED(privdata);
UNUSED(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 == REPL_STATE_NONE) {
close(fd);
return;
}
/* Check for errors in the socket. */
if (getsockopt(fd, SOL_SOCKET, SO_ERROR, &sockerr, &errlen) == -1)
sockerr = errno;
if (sockerr) {
serverLog(LL_WARNING,"Error condition on socket for SYNC: %s",
strerror(sockerr));
goto error;
}
/* Send a PING to check the master is able to reply without errors. */
if (server.repl_state == REPL_STATE_CONNECTING) {
serverLog(LL_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 = REPL_STATE_RECEIVE_PONG;
/* Send the PING, don't check for errors at all, we have the timeout
* that will take care about this. */
// 发送一个 PING 出去，检查 master 是否可以响应
err = sendSynchronousCommand(SYNC_CMD_WRITE,fd,"PING",NULL);
if (err) goto write_error;
return;
}
/* Receive the PONG command. */
if (server.repl_state == REPL_STATE_RECEIVE_PONG) {
// 同步读取PING结果
err = sendSynchronousCommand(SYNC_CMD_READ,fd,NULL);
/* 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. */
// 没有权限且提示不是请授权类的提示，则发生错误
// 没有调用 auth 前
// -NOAUTH, 代表未授权, 可以进入下一步授权操作
if (err[0] != '+' &&
strncmp(err,"-NOAUTH",7) != 0 &&
strncmp(err,"-ERR operation not permitted",28) != 0)
{
serverLog(LL_WARNING,"Error reply to PING from master: '%s'",err);
sdsfree(err);
goto error;
} else {
serverLog(LL_NOTICE,
"Master replied to PING, replication can continue...");
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_AUTH;
}
/* AUTH with the master if required. */
// 需要输入master密码状态
if (server.repl_state == REPL_STATE_SEND_AUTH) {
if (server.masterauth)
// 发送授权命令
// AUTH master_password
err = sendSynchronousCommand(SYNC_CMD_WRITE,fd,"AUTH",server.masterauth,NULL);
if (err) goto write_error;
server.repl_state = REPL_STATE_RECEIVE_AUTH;
return;
} else {
server.repl_state = REPL_STATE_SEND_PORT;
}
}
/* Receive AUTH reply. */
if (server.repl_state == REPL_STATE_RECEIVE_AUTH) {
// 授权响应，读取结果
// 授权成功响应 +OK, 其他授权失败
err = sendSynchronousCommand(SYNC_CMD_READ,fd,NULL);
if (err[0] == '-') {
serverLog(LL_WARNING,"Unable to AUTH to MASTER: %s",err);
sdsfree(err);
goto error;
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_PORT;
}
/* Set the slave port, so that Master's INFO command can list the
* slave listening port correctly. */
// 发送端口号给master, 以便master可以列举出所有slave的端口号
if (server.repl_state == REPL_STATE_SEND_PORT) {
sds port = sdsfromlonglong(server.port);
// 发送本节点的端口给 master
// 命令: REPLCONF listening-port port
err = sendSynchronousCommand(SYNC_CMD_WRITE,fd,"REPLCONF",
"listening-port",port, NULL);
sdsfree(port);
if (err) goto write_error;
sdsfree(err);
server.repl_state = REPL_STATE_RECEIVE_PORT;
return;
}
/* Receive REPLCONF listening-port reply. */
if (server.repl_state == REPL_STATE_RECEIVE_PORT) {
err = sendSynchronousCommand(SYNC_CMD_READ,fd,NULL);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF listening-port. */
// 忽略失败情况，影响不大，只是个展示问题，且并非所有版本都支持该命令
if (err[0] == '-') {
serverLog(LL_NOTICE,"(Non critical) Master does not understand "
"REPLCONF listening-port: %s", err);
}
sdsfree(err);
server.repl_state = REPL_STATE_SEND_CAPA;
}
/* Inform the master of our capabilities. While we currently send
* just one capability, it is possible to chain new capabilities here
* in the form of REPLCONF capa X capa Y capa Z ...
* The master will ignore capabilities it does not understand. */
if (server.repl_state == REPL_STATE_SEND_CAPA) {
// 发送命令: REPLCONF capa eof
err = sendSynchronousCommand(SYNC_CMD_WRITE,fd,"REPLCONF",
"capa","eof",NULL);
if (err) goto write_error;
sdsfree(err);
server.repl_state = REPL_STATE_RECEIVE_CAPA;
return;
}
/* Receive CAPA reply. */
if (server.repl_state == REPL_STATE_RECEIVE_CAPA) {
err = sendSynchronousCommand(SYNC_CMD_READ,fd,NULL);
/* Ignore the error if any, not all the Redis versions support
* REPLCONF capa. */
if (err[0] == '-') {
serverLog(LL_NOTICE,"(Non critical) Master does not understand "
"REPLCONF capa: %s", err);
}
sdsfree(err);
// 可以进行数据同步了 PSYNC
server.repl_state = REPL_STATE_SEND_PSYNC;
}
/* 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. */
if (server.repl_state == REPL_STATE_SEND_PSYNC) {
// 尝试进行部分同步, 可能为 全量同步、部分同步、或者命令不支持
// PSYNC_WAIT_REPLY, PSYNC_CONTINUE, PSYNC_FULLRESYNC, PSYNC_NOT_SUPPORTED
if (slaveTryPartialResynchronization(fd,0) == PSYNC_WRITE_ERROR) {
err = sdsnew("Write error sending the PSYNC command.");
goto write_error;
}
server.repl_state = REPL_STATE_RECEIVE_PSYNC;
return;
}
/* If reached this point, we should be in REPL_STATE_RECEIVE_PSYNC. */
if (server.repl_state != REPL_STATE_RECEIVE_PSYNC) {
serverLog(LL_WARNING,"syncWithMaster(): state machine error, "
"state should be RECEIVE_PSYNC but is %d",
server.repl_state);
goto error;
}
// 读取 PSYNC 结果
// PSYNC_WAIT_REPLY, PSYNC_CONTINUE, PSYNC_FULLRESYNC, PSYNC_NOT_SUPPORTED
psync_result = slaveTryPartialResynchronization(fd,1);
if (psync_result == PSYNC_WAIT_REPLY) return; /* Try again later... */
/* Note: if PSYNC does not return WAIT_REPLY, it will take care of
* uninstalling the read handler from the file descriptor. */
if (psync_result == PSYNC_CONTINUE) {
serverLog(LL_NOTICE, "MASTER <-> SLAVE sync: Master accepted a Partial Resynchronization.");
return;
}
/* PSYNC failed or is not supported: we want our slaves to resync with us
* as well, if we have any (chained replication case). The mater may
* transfer us an entirely different data set and we have no way to
* incrementally feed our slaves after that. */
// 不能使用 PSYNC 进行同步，断开当前节点的 slaves
// 不允许链式主从
disconnectSlaves(); /* Force our slaves to resync with us as well. */
freeReplicationBacklog(); /* Don't allow our chained slaves to PSYNC. */
/* 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) {
serverLog(LL_NOTICE,"Retrying with SYNC...");
// 不支持 PSYNC, 降级为 SYNC
if (syncWrite(fd,"SYNC\r\n",6,server.repl_syncio_timeout*1000) == -1) {
serverLog(LL_WARNING,"I/O error writing to MASTER: %s",
strerror(errno));
goto error;
}
}
/* Prepare a suitable temp file for bulk transfer */
// 准备从rdb文件中读取数据，最多重试5次(共5s)
// 临时文件名: temp-<1560888xxx>.<pid>.rdb
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) {
serverLog(LL_WARNING,"Opening the temp file needed for MASTER <-> SLAVE synchronization: %s",strerror(errno));
goto error;
}
/* Setup the non blocking download of the bulk file. */
// 使用 epoll 模型进行异步接收master传送过来的rdb文件
// 由 readSyncBulkPayload 函数进行结果处理
if (aeCreateFileEvent(server.el,fd, AE_READABLE,readSyncBulkPayload,NULL)
== AE_ERR)
{
serverLog(LL_WARNING,
"Can't create readable event for SYNC: %s (fd=%d)",
strerror(errno),fd);
goto error;
}
// 保存同步状态
server.repl_state = REPL_STATE_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:
aeDeleteFileEvent(server.el,fd,AE_READABLE|AE_WRITABLE);
close(fd);
server.repl_transfer_s = -1;
server.repl_state = REPL_STATE_CONNECT;
return;
write_error: /* Handle sendSynchronousCommand(SYNC_CMD_WRITE) errors. */
serverLog(LL_WARNING,"Sending command to master in replication handshake: %s", err);
sdsfree(err);
goto error;
}
整个连接成功之后的处理过程还是比较繁杂的，主要逻辑就在 syncWithMaster，主要是在各个状态之间的转换，尤其头疼，不过幸好都是流水式的一步步下来。
1. REPL_STATE_CONNECTING: 待连接状态. slave 发送 PING命令进行主动连接, 然后将状态置为 REPL_STATE_RECEIVE_PONG;
2. REPL_STATE_RECEIVE_PONG: 待master响应状态. slave同步等待结果(其实一般会立即获取到，因为epoll已经准备好，才会调用此状态)，判断是否PING正常后, 将状态置为 REPL_STATE_SEND_AUTH;
3. REPL_STATE_SEND_AUTH: 等待授权状态. slave 发送 auth passwd 给master后, 将状态置为 REPL_STATE_RECEIVE_AUTH;
4. REPL_STATE_RECEIVE_AUTH: 等待授权响应状态. slave同步等待结果, 判断授权通过后, 将状态置为 REPL_STATE_SEND_PORT;
5. REPL_STATE_SEND_PORT: 待发送端口状态. slave发送自身的服务端口给master以便master展示使用, 然后将状态置为 REPL_STATE_RECEIVE_PORT;
6. REPL_STATE_RECEIVE_PORT: 等待端口发送结果. 不论结果如何, 直接将状态置为 REPL_STATE_SEND_CAPA;
7. REPL_STATE_SEND_CAPA: 等待发送capa命令状态. 发送 REPLCONF capa eof 后, 将状态置为 REPL_STATE_RECEIVE_CAPA;
8. REPL_STATE_RECEIVE_CAPA: 等待capa命令发送结果. 不论结果如何, 将状态置为 REPL_STATE_SEND_PSYNC;
9. REPL_STATE_SEND_PSYNC: 等待PSYNC同步命令状态. 尝试使用PSYNC进行部分复制,结果可能是全量复制或部分复制,也可能使用其他版本命令执行, 将状态置为 REPL_STATE_RECEIVE_PSYNC;
10. REPL_STATE_RECEIVE_PSYNC: 等待PSYNC结果. 这是真正接收数据的时候, 是终态, 根据上一次命令的请求方式，接收相应结果进一步处理;
11. 重新注册一个 epoll 事件，用于接收master传输过来的数据，处理方法为 readSyncBulkPayload();
接下来，我们先看看尝试部分时都做了哪些事，因为这决定了是使用全量复制还是部分复制：
// 尝试进行部分同步
// replication.c
int slaveTryPartialResynchronization(int fd, int read_reply) {
char *psync_runid;
char psync_offset[32];
sds reply;
/* Writing half */
// 第一次调用时， read_reply=0, 即是写动作
// 向 master 写入 PSYNC psync_runid psync_offset
// 即是每次都拉取一部分数据吧
if (!read_reply) {
/* Initially set repl_master_initial_offset to -1 to mark the current
* master run_id and offset as not valid. Later if we'll be able to do
* a FULL resync using the PSYNC command we'll set the offset at the
* right value, so that this information will be propagated to the
* client structure representing the master into server.master. */
server.repl_master_initial_offset = -1;
// 如果已经建立了连接，则 psync_runid, psync_offset 都是可预知的
// 否则 psync_runid = "?", psync_offset="-1";
if (server.cached_master) {
psync_runid = server.cached_master->replrunid;
snprintf(psync_offset,sizeof(psync_offset),"%lld", server.cached_master->reploff+1);
serverLog(LL_NOTICE,"Trying a partial resynchronization (request %s:%s).", psync_runid, psync_offset);
} else {
serverLog(LL_NOTICE,"Partial resynchronization not possible (no cached master)");
psync_runid = "?";
memcpy(psync_offset,"-1",3);
}
/* Issue the PSYNC command */
// 首次发送命令 PSYNC ? -1
// 后续使用实际的信息 PSYNC psync_runid psync_offset
reply = sendSynchronousCommand(SYNC_CMD_WRITE,fd,"PSYNC",psync_runid,psync_offset,NULL);
if (reply != NULL) {
serverLog(LL_WARNING,"Unable to send PSYNC to master: %s",reply);
sdsfree(reply);
aeDeleteFileEvent(server.el,fd,AE_READABLE);
return PSYNC_WRITE_ERROR;
}
return PSYNC_WAIT_REPLY;
}
/* Reading half */
// 读取 PSYNC 的结果
reply = sendSynchronousCommand(SYNC_CMD_READ,fd,NULL);
if (sdslen(reply) == 0) {
/* The master may send empty newlines after it receives PSYNC
* and before to reply, just to keep the connection alive. */
sdsfree(reply);
return PSYNC_WAIT_REPLY;
}
aeDeleteFileEvent(server.el,fd,AE_READABLE);
// +FULLRESYNC 代表需要进行全量复制，否则进行部分复制
// +FULLRESYNC runid offset
if (!strncmp(reply,"+FULLRESYNC",11)) {
char *runid = NULL, *offset = NULL;
/* FULL RESYNC, parse the reply in order to extract the run id
* and the replication offset. */
runid = strchr(reply,' ');
if (runid) {
runid++;
offset = strchr(runid,' ');
if (offset) offset++;
}
// runid 长度为 40
if (!runid || !offset || (offset-runid-1) != CONFIG_RUN_ID_SIZE) {
serverLog(LL_WARNING,
"Master replied with wrong +FULLRESYNC syntax.");
/* This is an unexpected condition, actually the +FULLRESYNC
* reply means that the master supports PSYNC, but the reply
* format seems wrong. To stay safe we blank the master
* runid to make sure next PSYNCs will fail. */
memset(server.repl_master_runid,0,CONFIG_RUN_ID_SIZE+1);
} else {
memcpy(server.repl_master_runid, runid, offset-runid-1);
server.repl_master_runid[CONFIG_RUN_ID_SIZE] = '\0';
server.repl_master_initial_offset = strtoll(offset,NULL,10);
serverLog(LL_NOTICE,"Full resync from master: %s:%lld",
server.repl_master_runid,
server.repl_master_initial_offset);
}
/* We are going to full resync, discard the cached master structure. */
// 全量同步，重置master缓存
replicationDiscardCachedMaster();
sdsfree(reply);
return PSYNC_FULLRESYNC;
}
// 部分复制的情况下，只会返回 +CONTINUE
if (!strncmp(reply,"+CONTINUE",9)) {
/* Partial resync was accepted, set the replication state accordingly */
serverLog(LL_NOTICE,
"Successful partial resynchronization with master.");
// 立即将结果释放，那什么时候处理结果呢？
sdsfree(reply);
// 实际上通过该方法同步数据的
replicationResurrectCachedMaster(fd);
// 继续使用 部分同步
return PSYNC_CONTINUE;
}
/* If we reach this point we received either an error since the master does
* not understand PSYNC, or an unexpected reply from the master.
* Return PSYNC_NOT_SUPPORTED to the caller in both cases. */
// PSYNC 不支持，因处理为降级版本
if (strncmp(reply,"-ERR",4)) {
/* If it's not an error, log the unexpected event. */
serverLog(LL_WARNING,
"Unexpected reply to PSYNC from master: %s", reply);
} else {
serverLog(LL_NOTICE,
"Master does not support PSYNC or is in "
"error state (reply: %s)", reply);
}
sdsfree(reply);
replicationDiscardCachedMaster();
return PSYNC_NOT_SUPPORTED;
}
通过上面的过程，我们可以看清了整个与master是如何协调进行同步的，主要依赖于 PSYNC 的返回值决定。也可以看到，全量同步功能时，注册了一个可读事件的监听，具体处理使用 readSyncBulkPayload 进行承载。
3.4. 全量同步数据的实现方式
通过前面的分析，我们看到全量同时时，注册了一个FileEvent事件，依赖于epoll实现异步操作。具体处理由 readSyncBulkPayload() 进行处理。它负责异步读取master 同步过来的数据，写入aof文件，加载到slave的数据库中。具体如下：
// replication.c
/* Asynchronously read the SYNC payload we receive from a master */
#define REPL_MAX_WRITTEN_BEFORE_FSYNC (1024*1024*8) /* 8 MB */
void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
char buf[4096];
ssize_t nread, readlen;
off_t left;
UNUSED(el);
UNUSED(privdata);
UNUSED(mask);
/* Static vars used to hold the EOF mark, and the last bytes received
* form the server: when they match, we reached the end of the transfer. */
static char eofmark[CONFIG_RUN_ID_SIZE];
static char lastbytes[CONFIG_RUN_ID_SIZE];
static int usemark = 0;
/* If repl_transfer_size == -1 we still have to read the bulk length
* from the master reply. */
// 先读取数据长度
if (server.repl_transfer_size == -1) {
if (syncReadLine(fd,buf,1024,server.repl_syncio_timeout*1000) == -1) {
serverLog(LL_WARNING,
"I/O error reading bulk count from MASTER: %s",
strerror(errno));
goto error;
}
if (buf[0] == '-') {
serverLog(LL_WARNING,
"MASTER aborted replication with an error: %s",
buf+1);
goto error;
} else if (buf[0] == '\0') {
/* At this stage just a newline works as a PING in order to take
* the connection live. So we refresh our last interaction
* timestamp. */
server.repl_transfer_lastio = server.unixtime;
return;
} else if (buf[0] != '$') {
serverLog(LL_WARNING,"Bad protocol from MASTER, the first byte is not '$' (we received '%s'), are you sure the host and port are right?", buf);
goto error;
}
/* There are two possible forms for the bulk payload. One is the
* usual $<count> bulk format. The other is used for diskless transfers
* when the master does not know beforehand the size of the file to
* transfer. In the latter case, the following format is used:
*
* $EOF:<40 bytes delimiter>
*
* At the end of the file the announced delimiter is transmitted. The
* delimiter is long and random enough that the probability of a
* collision with the actual file content can be ignored. */
if (strncmp(buf+1,"EOF:",4) == 0 && strlen(buf+5) >= CONFIG_RUN_ID_SIZE) {
usemark = 1;
memcpy(eofmark,buf+5,CONFIG_RUN_ID_SIZE);
memset(lastbytes,0,CONFIG_RUN_ID_SIZE);
/* Set any repl_transfer_size to avoid entering this code path
* at the next call. */
server.repl_transfer_size = 0;
serverLog(LL_NOTICE,
"MASTER <-> SLAVE sync: receiving streamed RDB from master");
} else {
usemark = 0;
// 读取数据长度, 写入 server.repl_transfer_size, 后续判断是否取完整数据
server.repl_transfer_size = strtol(buf+1,NULL,10);
serverLog(LL_NOTICE,
"MASTER <-> SLAVE sync: receiving %lld bytes from master",
(long long) server.repl_transfer_size);
}
return;
}
/* Read bulk data */
if (usemark) {
readlen = sizeof(buf);
} else {
left = server.repl_transfer_size - server.repl_transfer_read;
readlen = (left < (signed)sizeof(buf)) ? left : (signed)sizeof(buf);
}
nread = read(fd,buf,readlen);
if (nread <= 0) {
serverLog(LL_WARNING,"I/O error trying to sync with MASTER: %s",
(nread == -1) ? strerror(errno) : "connection lost");
cancelReplicationHandshake();
return;
}
server.stat_net_input_bytes += nread;
/* When a mark is used, we want to detect EOF asap in order to avoid
* writing the EOF mark into the file... */
int eof_reached = 0;
if (usemark) {
/* Update the last bytes array, and check if it matches our delimiter.*/
// 更新 最后几个字符
if (nread >= CONFIG_RUN_ID_SIZE) {
memcpy(lastbytes,buf+nread-CONFIG_RUN_ID_SIZE,CONFIG_RUN_ID_SIZE);
} else {
int rem = CONFIG_RUN_ID_SIZE-nread;
memmove(lastbytes,lastbytes+nread,rem);
memcpy(lastbytes+rem,buf,nread);
}
if (memcmp(lastbytes,eofmark,CONFIG_RUN_ID_SIZE) == 0) eof_reached = 1;
}
server.repl_transfer_lastio = server.unixtime;
// 将数据写入到 temp rdb 文件中
if (write(server.repl_transfer_fd,buf,nread) != nread) {
serverLog(LL_WARNING,"Write error or short write writing to the DB dump file needed for MASTER <-> SLAVE synchronization: %s", strerror(errno));
goto error;
}
server.repl_transfer_read += nread;
/* Delete the last 40 bytes from the file if we reached EOF. */
if (usemark && eof_reached) {
if (ftruncate(server.repl_transfer_fd,
server.repl_transfer_read - CONFIG_RUN_ID_SIZE) == -1)
{
serverLog(LL_WARNING,"Error truncating the RDB file received from the master for SYNC: %s", strerror(errno));
goto error;
}
}
/* Sync data on disk from time to time, otherwise at the end of the transfer
* we may suffer a big delay as the memory buffers are copied into the
* actual disk. */
// 缓冲达到一定值后，直接刷盘
// REPL_MAX_WRITTEN_BEFORE_FSYNC: 8M
if (server.repl_transfer_read >=
server.repl_transfer_last_fsync_off + REPL_MAX_WRITTEN_BEFORE_FSYNC)
{
off_t sync_size = server.repl_transfer_read -
server.repl_transfer_last_fsync_off;
rdb_fsync_range(server.repl_transfer_fd,
server.repl_transfer_last_fsync_off, sync_size);
server.repl_transfer_last_fsync_off += sync_size;
}
/* Check if the transfer is now complete */
// 传输完成
if (!usemark) {
if (server.repl_transfer_read == server.repl_transfer_size)
eof_reached = 1;
}
if (eof_reached) {
// 直接将临时 rdb 文件改名为正式的 rdb 文件，从而实现数据替换
if (rename(server.repl_transfer_tmpfile,server.rdb_filename) == -1) {
serverLog(LL_WARNING,"Failed trying to rename the temp DB into dump.rdb in MASTER <-> SLAVE synchronization: %s", strerror(errno));
cancelReplicationHandshake();
return;
}
serverLog(LL_NOTICE, "MASTER <-> SLAVE sync: Flushing old data");
// 清空原来的数据，刷入新数据
signalFlushedDb(-1);
emptyDb(
-1,
server.repl_slave_lazy_flush ? EMPTYDB_ASYNC : EMPTYDB_NO_FLAGS,
replicationEmptyDbCallback);
/* Before loading the DB into memory we need to delete the readable
* handler, otherwise it will get called recursively since
* rdbLoad() will call the event loop to process events from time to
* time for non blocking loading. */
aeDeleteFileEvent(server.el,server.repl_transfer_s,AE_READABLE);
serverLog(LL_NOTICE, "MASTER <-> SLAVE sync: Loading DB in memory");
// 重新载入 rdb 文件，从而完成同步操作
if (rdbLoad(server.rdb_filename) != C_OK) {
serverLog(LL_WARNING,"Failed trying to load the MASTER synchronization DB from disk");
cancelReplicationHandshake();
return;
}
/* Final setup of the connected slave <- master link */
zfree(server.repl_transfer_tmpfile);
close(server.repl_transfer_fd);
// 设置 master 信息，以便下次直接使用
replicationCreateMasterClient(server.repl_transfer_s);
serverLog(LL_NOTICE, "MASTER <-> SLAVE sync: Finished with success");
/* Restart the AOF subsystem now that we finished the sync. This
* will trigger an AOF rewrite, and when done will start appending
* to the new file. */
if (server.aof_state != AOF_OFF) {
int retry = 10;
// 重新关联 aof 文件，以便后续写入aof正常
stopAppendOnly();
while (retry-- && startAppendOnly() == C_ERR) {
serverLog(LL_WARNING,"Failed enabling the AOF after successful master synchronization! Trying it again in one second.");
sleep(1);
}
if (!retry) {
serverLog(LL_WARNING,"FATAL: this slave instance finished the synchronization with its master, but the AOF can't be turned on. Exiting now.");
exit(1);
}
}
}
return;
error:
cancelReplicationHandshake();
return;
}
以上就是全量复制功能实现了，大体步骤为：
1. 先读取整体数据长度;(肯定是master发来的数据了)
2. 依次读取就绪数据，将其定入临时aof文件 temp-<unixtime>.<pid>.aof;
3. 达到一定缓冲数量后，强制刷盘;
4. master 传输完成后，slave将临时aof文件重命名为正式的aof文件;
5. slave 清空原来db数据;
6. 禁用aof文件的监听，载入新的aof数据，重新开启监听;
7. aof 先停止再启动，重新关联新文件;
3.5. 部分复制的实现
前面我们看到有个 slaveTryPartialResynchronization(), 是做部分同步检测的，但是它只会返回几个状态，好像返回后都没有做什么后续处理。只有全量同步时，我们看到了如上逻辑。那么部分同步是如何实现的呢？其中有个 +CONTINUE 的状态值得我们注意：
...
// 部分复制的情况下，只会返回 +CONTINUE
if (!strncmp(reply,"+CONTINUE",9)) {
/* Partial resync was accepted, set the replication state accordingly */
serverLog(LL_NOTICE,
"Successful partial resynchronization with master.");
// 立即将结果释放，那什么时候处理结果呢？
sdsfree(reply);
// 实际上通过该方法同步数据的
replicationResurrectCachedMaster(fd);
// 继续使用 部分同步
return PSYNC_CONTINUE;
}
...
就这上面这个，返回 CONTINUE 后，外部逻辑只是返回，所以肯定是 replicationResurrectCachedMaster() 做了处理。而这个处理，应该是读取后续的数据没错了！
// replication.c, 使用 cacheMaster 做 PSYNC 处理复制数据
/* Turn the cached master into the current master, using the file descriptor
* passed as argument as the socket for the new master.
*
* This function is called when successfully setup a partial resynchronization
* so the stream of data that we'll receive will start from were this
* master left. */
void replicationResurrectCachedMaster(int newfd) {
server.master = server.cached_master;
server.cached_master = NULL;
server.master->fd = newfd;
server.master->flags &= ~(CLIENT_CLOSE_AFTER_REPLY|CLIENT_CLOSE_ASAP);
server.master->authenticated = 1;
server.master->lastinteraction = server.unixtime;
server.repl_state = REPL_STATE_CONNECTED;
/* Re-add to the list of clients. */
listAddNodeTail(server.clients,server.master);
// 添加file事件，epoll事件, 由 readQueryFromClient 进行事件处理
if (aeCreateFileEvent(server.el, newfd, AE_READABLE,
readQueryFromClient, server.master)) {
serverLog(LL_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. */
// 如果有待发送数据，建立一个 写的 fileEvent 事件
if (clientHasPendingReplies(server.master)) {
if (aeCreateFileEvent(server.el, newfd, AE_WRITABLE,
sendReplyToClient, server.master)) {
serverLog(LL_WARNING,"Error resurrecting the cached master, impossible to add the writable handler: %s", strerror(errno));
freeClientAsync(server.master); /* Close ASAP. */
}
}
}
// 接下来，我们查看下 当master发送数据过来时，部分复制是如何实现的
// networking.c, 从 master 中读取数据， privdata = server.master
void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) {
client *c = (client*) privdata;
int nread, readlen;
size_t qblen;
UNUSED(el);
UNUSED(mask);
// PROTO_IOBUF_LEN: 1024*16
// PROTO_MBULK_BIG_ARG: 1024*32
readlen = PROTO_IOBUF_LEN;
/* If this is a multi bulk request, and we are processing a bulk reply
* that is large enough, try to maximize the probability that the query
* buffer contains exactly the SDS string representing the object, even
* at the risk of requiring more read(2) calls. This way the function
* processMultiBulkBuffer() can avoid copying buffers to create the
* Redis Object representing the argument. */
if (c->reqtype == PROTO_REQ_MULTIBULK && c->multibulklen && c->bulklen != -1
&& c->bulklen >= PROTO_MBULK_BIG_ARG)
{
int remaining = (unsigned)(c->bulklen+2)-sdslen(c->querybuf);
if (remaining < readlen) readlen = remaining;
}
qblen = sdslen(c->querybuf);
if (c->querybuf_peak < qblen) c->querybuf_peak = qblen;
c->querybuf = sdsMakeRoomFor(c->querybuf, readlen);
// 读取请求命令
nread = read(fd, c->querybuf+qblen, readlen);
if (nread == -1) {
if (errno == EAGAIN) {
return;
} else {
serverLog(LL_VERBOSE, "Reading from client: %s",strerror(errno));
freeClient(c);
return;
}
} else if (nread == 0) {
serverLog(LL_VERBOSE, "Client closed connection");
freeClient(c);
return;
}
sdsIncrLen(c->querybuf,nread);
c->lastinteraction = server.unixtime;
if (c->flags & CLIENT_MASTER) c->reploff += nread;
server.stat_net_input_bytes += nread;
// 超出最大限制，不处理
if (sdslen(c->querybuf) > server.client_max_querybuf_len) {
sds ci = catClientInfoString(sdsempty(),c), bytes = sdsempty();
bytes = sdscatrepr(bytes,c->querybuf,64);
serverLog(LL_WARNING,"Closing client that reached max query buffer length: %s (qbuf initial bytes: %s)", ci, bytes);
sdsfree(ci);
sdsfree(bytes);
freeClient(c);
return;
}
// 处理 querybuf 数据, 其实就和普通的客户端写请求一样的处理方式
processInputBuffer(c);
}
处理master 部分同步过来的数据，重新在 slave 执行一次即可，基于epoll的事件监听，可以持续处理同步数据。
所以，部分复制，其实就是重新在slave端执行与master相同的请求就好了。这个processInputBuffer()过程在前面的文章已经介绍过。
3.6. PSYNC 命令实现原理
从上面可以看出，PSYNC是整个主从复制过程的重要操作，那么 PSYNC 都是怎么实现的呢？大体上应该是一个范围查找响应的过程，但是细节必然很多。我们可以先自己想想，要处理的点大概有哪些呢？
1. 第一次调用时，即 PSYNC ? -1 如何处理？
2. 后续调用时 即 PSYNC psync_runid psync_offset 如何处理？
3. 响应结构是如何的？比如如何响应+CONTINUE？
我们就通过源码来解答这些问题吧！
首先是 PSYNC 的定义: 可以看到，sync 和 psync 居然是一样的实现？
// 差别是 sync 的参数只有一个，而 psync 的参数是3个
{"sync",syncCommand,1,"ars",0,NULL,0,0,0,0,0},
{"psync",syncCommand,3,"ars",0,NULL,0,0,0,0,0},
具体实现：
// 用法: PSYNC run_id offset
// replication.c
/* SYNC and PSYNC command implemenation. */
void syncCommand(client *c) {
/* ignore SYNC if already slave or in monitor mode */
// SYNC 命令只能调用成功一次，后续就直接忽略了
if (c->flags & CLIENT_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 != REPL_STATE_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. */
// 还有输出未完成时不能再进行处理
if (clientHasPendingReplies(c)) {
addReplyError(c,"SYNC and PSYNC are invalid with pending output");
return;
}
serverLog(LL_NOTICE,"Slave %s asks for synchronization",
replicationGetSlaveName(c));
/* Try a partial resynchronization if this is a PSYNC command.
* If it fails, we continue with usual full resynchronization, however
* when this happens masterTryPartialResynchronization() already
* replied with:
*
* +FULLRESYNC <runid> <offset>
*
* So the slave knows the new runid and offset to try a PSYNC later
* if the connection with the master is lost. */
// 事实上，psync 和 sync 的实现还是区别对待的
// psync 将会优先尝试部分复制
if (!strcasecmp(c->argv[0]->ptr,"psync")) {
// 部分复制将不会重置 flags, 即每次 psync 都会成功运行
if (masterTryPartialResynchronization(c) == C_OK) {
server.stat_sync_partial_ok++;
return; /* No full resync needed, return. */
} else {
char *master_runid = c->argv[1]->ptr;
/* Increment stats for failed PSYNCs, but only if the
* runid is not "?", as this is used by slaves to force a full
* resync on purpose when they are not albe to partially
* resync. */
if (master_runid[0] != '?') server.stat_sync_partial_err++;
}
} else {
/* If a slave uses SYNC, we are dealing with an old implementation
* of the replication protocol (like redis-cli --slave). Flag the client
* so that we don't expect to receive REPLCONF ACK feedbacks. */
c->flags |= CLIENT_PRE_PSYNC;
}
// 以下为全量复制
/* Full resynchronization. */
server.stat_sync_full++;
/* Setup the slave as one waiting for BGSAVE to start. The following code
* paths will change the state if we handle the slave differently. */
c->replstate = SLAVE_STATE_WAIT_BGSAVE_START;
if (server.repl_disable_tcp_nodelay)
anetDisableTcpNoDelay(NULL, c->fd); /* Non critical if it fails. */
c->repldbfd = -1;
// 添加slave 到master的从节点集合中, 设置 SLAVE 标识，表示已执行过 SYNC 操作
c->flags |= CLIENT_SLAVE;
listAddNodeTail(server.slaves,c);
/* CASE 1: BGSAVE is in progress, with disk target. */
// 如果 rdb 存储已在进行中，即 BGSAVE 已经在运行
// 此种是对于后来进行主从同步的客户端，只需告知正在运行 BGSAVE 即可
if (server.rdb_child_pid != -1 &&
server.rdb_child_type == RDB_CHILD_TYPE_DISK)
{
/* 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. */
client *slave;
listNode *ln;
listIter li;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END) break;
}
/* To attach this slave, we check that it has at least all the
* capabilities of the slave that triggered the current BGSAVE. */
if (ln && ((c->slave_capa & slave->slave_capa) == slave->slave_capa)) {
/* Perfect, the server is already registering differences for
* another slave. Set the right state, and copy the buffer. */
copyClientOutputBuffer(c,slave);
replicationSetupSlaveForFullResync(c,slave->psync_initial_offset);
serverLog(LL_NOTICE,"Waiting for end of BGSAVE for SYNC");
} else {
/* No way, we need to wait for the next BGSAVE in order to
* register differences. */
serverLog(LL_NOTICE,"Waiting for next BGSAVE for SYNC");
}
/* CASE 2: BGSAVE is in progress, with socket target. */
} else if (server.rdb_child_pid != -1 &&
server.rdb_child_type == RDB_CHILD_TYPE_SOCKET)
{
/* There is an RDB child process but it is writing directly to
* children sockets. We need to wait for the next BGSAVE
* in order to synchronize. */
serverLog(LL_NOTICE,"Waiting for next BGSAVE for SYNC");
/* CASE 3: There is no BGSAVE is progress. */
} else {
// master 不持久化方式下，不启动 bgsave
if (server.repl_diskless_sync && (c->slave_capa & SLAVE_CAPA_EOF)) {
/* Diskless replication RDB child is created inside
* replicationCron() since we want to delay its start a
* few seconds to wait for more slaves to arrive. */
if (server.repl_diskless_sync_delay)
serverLog(LL_NOTICE,"Delay next BGSAVE for SYNC");
} else {
/* Target is disk (or the slave is not capable of supporting
* diskless replication) and we don't have a BGSAVE in progress,
* let's start one. */
// 主动开启一个后台 BGSAVE
if (startBgsaveForReplication(c->slave_capa) != C_OK) return;
}
}
// 如果是第一个 slave, 则创建backlog
if (listLength(server.slaves) == 1 && server.repl_backlog == NULL)
createReplicationBacklog();
// 最后，直接return, 说明响应没有一个统一的格式，各自情况各自判断就好
return;
}
// 3.6.1. 后台 BGSAVE 的触发
// replication.c
/* Start a BGSAVE for replication goals, which is, selecting the disk or
* socket target depending on the configuration, and making sure that
* the script cache is flushed before to start.
*
* The mincapa argument is the bitwise AND among all the slaves capabilities
* of the slaves waiting for this BGSAVE, so represents the slave capabilities
* all the slaves support. Can be tested via SLAVE_CAPA_* macros.
*
* Side effects, other than starting a BGSAVE:
*
* 1) Handle the slaves in WAIT_START state, by preparing them for a full
* sync if the BGSAVE was succesfully started, or sending them an error
* and dropping them from the list of slaves.
*
* 2) Flush the Lua scripting script cache if the BGSAVE was actually
* started.
*
* Returns C_OK on success or C_ERR otherwise. */
int startBgsaveForReplication(int mincapa) {
int retval;
int socket_target = server.repl_diskless_sync && (mincapa & SLAVE_CAPA_EOF);
listIter li;
listNode *ln;
serverLog(LL_NOTICE,"Starting BGSAVE for SYNC with target: %s",
socket_target ? "slaves sockets" : "disk");
if (socket_target)
// 直接向socket中写入数据同步
retval = rdbSaveToSlavesSockets();
else
// 存储到磁盘rdb 文件中
retval = rdbSaveBackground(server.rdb_filename);
/* If we failed to BGSAVE, remove the slaves waiting for a full
* resynchorinization from the list of salves, inform them with
* an error about what happened, close the connection ASAP. */
if (retval == C_ERR) {
serverLog(LL_WARNING,"BGSAVE for replication failed");
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
slave->flags &= ~CLIENT_SLAVE;
listDelNode(server.slaves,ln);
addReplyError(slave,
"BGSAVE failed, replication can't continue");
slave->flags |= CLIENT_CLOSE_AFTER_REPLY;
}
}
return retval;
}
/* If the target is socket, rdbSaveToSlavesSockets() already setup
* the salves for a full resync. Otherwise for disk target do it now.*/
if (!socket_target) {
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
// 依次响应 slave 端 +FULLRESYNC <master_runid> <master_offset>
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
replicationSetupSlaveForFullResync(slave,
getPsyncInitialOffset());
}
}
}
/* Flush the script cache, since we need that slave differences are
* accumulated without requiring slaves to match our cached scripts. */
// lua 脚本相关，略
if (retval == C_OK) replicationScriptCacheFlush();
return retval;
}
// rdb.c, 后台保存数据到 filename 中
int rdbSaveBackground(char *filename) {
pid_t childpid;
long long start;
if (server.rdb_child_pid != -1) return C_ERR;
server.dirty_before_bgsave = server.dirty;
server.lastbgsave_try = time(NULL);
start = ustime();
// 使用fork() 创建子进程进行 bgsave
// 所以，bgsave 应该是个很耗内存的事
if ((childpid = fork()) == 0) {
int retval;
/* Child */
// fork() 出的子进程执行此代码区域
closeListeningSockets(0);
redisSetProcTitle("redis-rdb-bgsave");
// 所以，整个耗时的操作都在 rdbSave() 中了
retval = rdbSave(filename);
if (retval == C_OK) {
size_t private_dirty = zmalloc_get_private_dirty();
if (private_dirty) {
serverLog(LL_NOTICE,
"RDB: %zu MB of memory used by copy-on-write",
private_dirty/(1024*1024));
}
}
// 执行完rdbSave()后，直接退出子进程
// 此处的退出操作，并不会清理进程 I/O 缓冲，以便将来方便使用
exitFromChild((retval == C_OK) ? 0 : 1);
} else {
/* Parent */
// 父进程执行此代码区域
server.stat_fork_time = ustime()-start;
server.stat_fork_rate = (double) zmalloc_used_memory() * 1000000 / server.stat_fork_time / (1024*1024*1024); /* GB per second. */
latencyAddSampleIfNeeded("fork",server.stat_fork_time/1000);
if (childpid == -1) {
server.lastbgsave_status = C_ERR;
serverLog(LL_WARNING,"Can't save in background: fork: %s",
strerror(errno));
return C_ERR;
}
// 记录子进程信息
serverLog(LL_NOTICE,"Background saving started by pid %d",childpid);
server.rdb_save_time_start = time(NULL);
server.rdb_child_pid = childpid;
server.rdb_child_type = RDB_CHILD_TYPE_DISK;
// bgsave 期间禁止dict进行扩容
updateDictResizePolicy();
return C_OK;
}
return C_OK; /* unreached */
}
// replication.c, 响应客户端需要进行全量复制
/* Send a FULLRESYNC reply in the specific case of a full resynchronization,
* as a side effect setup the slave for a full sync in different ways:
*
* 1) Remember, into the slave client structure, the offset we sent
* here, so that if new slaves will later attach to the same
* background RDB saving process (by duplicating this client output
* buffer), we can get the right offset from this slave.
* 2) Set the replication state of the slave to WAIT_BGSAVE_END so that
* we start accumulating differences from this point.
* 3) Force the replication stream to re-emit a SELECT statement so
* the new slave incremental differences will start selecting the
* right database number.
*
* Normally this function should be called immediately after a successful
* BGSAVE for replication was started, or when there is one already in
* progress that we attached our slave to. */
int replicationSetupSlaveForFullResync(client *slave, long long offset) {
char buf[128];
int buflen;
slave->psync_initial_offset = offset;
slave->replstate = SLAVE_STATE_WAIT_BGSAVE_END;
/* We are going to accumulate the incremental changes for this
* slave as well. Set slaveseldb to -1 in order to force to re-emit
* a SLEECT statement in the replication stream. */
server.slaveseldb = -1;
/* Don't send this reply to slaves that approached us with
* the old SYNC command. */
if (!(slave->flags & CLIENT_PRE_PSYNC)) {
buflen = snprintf(buf,sizeof(buf),"+FULLRESYNC %s %lld\r\n",
server.runid,offset);
if (write(slave->fd,buf,buflen) != buflen) {
freeClientAsync(slave);
return C_ERR;
}
}
return C_OK;
}
// rdb.c, 子进程bgsave 数据过程
/* Save the DB on disk. Return C_ERR on error, C_OK on success. */
int rdbSave(char *filename) {
char tmpfile[256];
FILE *fp;
rio rdb;
int error = 0;
// 先使用临时文件写数据，然后再更名为 rdb正式文件
snprintf(tmpfile,256,"temp-%d.rdb", (int) getpid());
fp = fopen(tmpfile,"w");
if (!fp) {
serverLog(LL_WARNING, "Failed opening .rdb for saving: %s",
strerror(errno));
return C_ERR;
}
rioInitWithFile(&rdb,fp);
// rdbSaveRio 主dump数据的关键实现
if (rdbSaveRio(&rdb,&error) == C_ERR) {
errno = error;
goto werr;
}
/* Make sure data will not remain on the OS's output buffers */
if (fflush(fp) == EOF) goto werr;
if (fsync(fileno(fp)) == -1) goto werr;
if (fclose(fp) == EOF) goto werr;
/* Use RENAME to make sure the DB file is changed atomically only
* if the generate DB file is ok. */
if (rename(tmpfile,filename) == -1) {
serverLog(LL_WARNING,"Error moving temp DB file on the final destination: %s", strerror(errno));
unlink(tmpfile);
return C_ERR;
}
serverLog(LL_NOTICE,"DB saved on disk");
server.dirty = 0;
server.lastsave = time(NULL);
server.lastbgsave_status = C_OK;
return C_OK;
werr:
serverLog(LL_WARNING,"Write error saving DB on disk: %s", strerror(errno));
fclose(fp);
unlink(tmpfile);
return C_ERR;
}
// replication.c, 针对第一个进行主从复制的 slave, 需要触发 backlog 的初始化
void createReplicationBacklog(void) {
serverAssert(server.repl_backlog == NULL);
server.repl_backlog = zmalloc(server.repl_backlog_size);
server.repl_backlog_histlen = 0;
server.repl_backlog_idx = 0;
/* When a new backlog buffer is created, we increment the replication
* offset by one to make sure we'll not be able to PSYNC with any
* previous slave. This is needed because we avoid incrementing the
* master_repl_offset if no backlog exists nor slaves are attached. */
server.master_repl_offset++;
/* We don't have any data inside our buffer, but virtually the first
* byte we have is the next byte that will be generated for the
* replication stream. */
server.repl_backlog_off = server.master_repl_offset+1;
}
// 3.6.2. 部分复制时的处理方式
// replication.c, 部分复制尝试
/* This function handles the PSYNC command from the point of view of a
* master receiving a request for partial resynchronization.
*
* On success return C_OK, otherwise C_ERR is returned and we proceed
* with the usual full resync. */
int masterTryPartialResynchronization(client *c) {
long long psync_offset, psync_len;
char *master_runid = c->argv[1]->ptr;
char buf[128];
int buflen;
/* Is the runid of this master the same advertised by the wannabe slave
* via PSYNC? If runid changed this master is a different instance and
* there is no way to continue. */
// run_id 发生了变化，则需要重新同步
if (strcasecmp(master_runid, server.runid)) {
/* Run id "?" is used by slaves that want to force a full resync. */
if (master_runid[0] != '?') {
serverLog(LL_NOTICE,"Partial resynchronization not accepted: "
"Runid mismatch (Client asked for runid '%s', my runid is '%s')",
master_runid, server.runid);
} else {
serverLog(LL_NOTICE,"Full resync requested by slave %s",
replicationGetSlaveName(c));
}
goto need_full_resync;
}
/* We still have the data our slave is asking for? */
if (getLongLongFromObjectOrReply(c,c->argv[2],&psync_offset,NULL) !=
C_OK) goto need_full_resync;
// offset 超出范围，使用全量同步
if (!server.repl_backlog ||
psync_offset < server.repl_backlog_off ||
psync_offset > (server.repl_backlog_off + server.repl_backlog_histlen))
{
serverLog(LL_NOTICE,
"Unable to partial resync with slave %s for lack of backlog (Slave request was: %lld).", replicationGetSlaveName(c), psync_offset);
if (psync_offset > server.master_repl_offset) {
serverLog(LL_WARNING,
"Warning: slave %s tried to PSYNC with an offset that is greater than the master replication offset.", replicationGetSlaveName(c));
}
goto need_full_resync;
}
/* If we reached this point, we are able to perform a partial resync:
* 1) Set client state to make it a slave.
* 2) Inform the client we can continue with +CONTINUE
* 3) Send the backlog data (from the offset to the end) to the slave. */
c->flags |= CLIENT_SLAVE;
c->replstate = SLAVE_STATE_ONLINE;
c->repl_ack_time = server.unixtime;
c->repl_put_online_on_ack = 0;
listAddNodeTail(server.slaves,c);
/* We can't use the connection buffers since they are used to accumulate
* new commands at this stage. But we are sure the socket send buffer is
* empty so this write will never fail actually. */
// 响应客户端 +CONTINUE
buflen = snprintf(buf,sizeof(buf),"+CONTINUE\r\n");
if (write(c->fd,buf,buflen) != buflen) {
freeClientAsync(c);
return C_OK;
}
// 输出部分同步的数据
psync_len = addReplyReplicationBacklog(c,psync_offset);
serverLog(LL_NOTICE,
"Partial resynchronization request from %s accepted. Sending %lld bytes of backlog starting from offset %lld.",
replicationGetSlaveName(c),
psync_len, psync_offset);
/* Note that we don't need to set the selected DB at server.slaveseldb
* to -1 to force the master to emit SELECT, since the slave already
* has this state from the previous connection with the master. */
refreshGoodSlavesCount();
return C_OK; /* The caller can return, no full resync needed. */
need_full_resync:
/* We need a full resync for some reason... Note that we can't
* reply to PSYNC right now if a full SYNC is needed. The reply
* must include the master offset at the time the RDB file we transfer
* is generated, so we need to delay the reply to that moment. */
return C_ERR;
}
// replication.c, 根据偏移量响应从节点数据
/* Feed the slave 'c' with the replication backlog starting from the
* specified 'offset' up to the end of the backlog. */
long long addReplyReplicationBacklog(client *c, long long offset) {
long long j, skip, len;
serverLog(LL_DEBUG, "[PSYNC] Slave request offset: %lld", offset);
if (server.repl_backlog_histlen == 0) {
serverLog(LL_DEBUG, "[PSYNC] Backlog history len is zero");
return 0;
}
serverLog(LL_DEBUG, "[PSYNC] Backlog size: %lld",
server.repl_backlog_size);
serverLog(LL_DEBUG, "[PSYNC] First byte: %lld",
server.repl_backlog_off);
serverLog(LL_DEBUG, "[PSYNC] History len: %lld",
server.repl_backlog_histlen);
serverLog(LL_DEBUG, "[PSYNC] Current index: %lld",
server.repl_backlog_idx);
/* Compute the amount of bytes we need to discard. */
// 重点就是 计算出需要同步的点
skip = offset - server.repl_backlog_off;
serverLog(LL_DEBUG, "[PSYNC] Skipping: %lld", skip);
/* Point j to the oldest byte, that is actaully our
* server.repl_backlog_off byte. */
j = (server.repl_backlog_idx +
(server.repl_backlog_size-server.repl_backlog_histlen)) %
server.repl_backlog_size;
serverLog(LL_DEBUG, "[PSYNC] Index of first byte: %lld", j);
/* Discard the amount of data to seek to the specified 'offset'. */
j = (j + skip) % server.repl_backlog_size;
/* Feed slave with data. Since it is a circular buffer we have to
* split the reply in two parts if we are cross-boundary. */
len = server.repl_backlog_histlen - skip;
serverLog(LL_DEBUG, "[PSYNC] Reply total length: %lld", len);
while(len) {
long long thislen =
((server.repl_backlog_size - j) < len) ?
(server.repl_backlog_size - j) : len;
serverLog(LL_DEBUG, "[PSYNC] addReply() length: %lld", thislen);
addReplySds(c,sdsnewlen(server.repl_backlog + j, thislen));
len -= thislen;
j = 0;
}
return server.repl_backlog_histlen - skip;
}
//3.6.3. 全量复制时如何响应客户端
// 因为前面我们看到只是响应了一个 FULLRESYNC <master_runid> <master_offset> 的标识而已
// 实际上，这也是一个后台脚本在运行时处理的
// replication.c,
/* This function is called at the end of every background saving,
* or when the replication RDB transfer strategy is modified from
* disk to socket or the other way around.
*
* The goal of this function is to handle slaves waiting for a successful
* background saving in order to perform non-blocking synchronization, and
* to schedule a new BGSAVE if there are slaves that attached while a
* BGSAVE was in progress, but it was not a good one for replication (no
* other slave was accumulating differences).
*
* The argument bgsaveerr is C_OK if the background saving succeeded
* otherwise C_ERR is passed to the function.
* The 'type' argument is the type of the child that terminated
* (if it had a disk or socket target). */
void updateSlavesWaitingBgsave(int bgsaveerr, int type) {
listNode *ln;
int startbgsave = 0;
int mincapa = -1;
listIter li;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) {
startbgsave = 1;
mincapa = (mincapa == -1) ? slave->slave_capa :
(mincapa & slave->slave_capa);
}
// 当bgsave 完成后， replstate 将变为 SLAVE_STATE_WAIT_BGSAVE_END
// 代表可以进行发送 rdb 文件了
// 同样，基于epoll io模型，进行高效发送文件
else if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_END) {
struct redis_stat buf;
/* If this was an RDB on disk save, we have to prepare to send
* the RDB from disk to the slave socket. Otherwise if this was
* already an RDB -> Slaves socket transfer, used in the case of
* diskless replication, our work is trivial, we can just put
* the slave online. */
if (type == RDB_CHILD_TYPE_SOCKET) {
serverLog(LL_NOTICE,
"Streamed RDB transfer with slave %s succeeded (socket). Waiting for REPLCONF ACK from slave to enable streaming",
replicationGetSlaveName(slave));
/* Note: we wait for a REPLCONF ACK message from slave in
* order to really put it online (install the write handler
* so that the accumulated data can be transfered). However
* we change the replication state ASAP, since our slave
* is technically online now. */
slave->replstate = SLAVE_STATE_ONLINE;
slave->repl_put_online_on_ack = 1;
slave->repl_ack_time = server.unixtime; /* Timeout otherwise. */
} else {
if (bgsaveerr != C_OK) {
freeClient(slave);
serverLog(LL_WARNING,"SYNC failed. BGSAVE child returned an error");
continue;
}
if ((slave->repldbfd = open(server.rdb_filename,O_RDONLY)) == -1 ||
redis_fstat(slave->repldbfd,&buf) == -1) {
freeClient(slave);
serverLog(LL_WARNING,"SYNC failed. Can't open/stat DB after BGSAVE: %s", strerror(errno));
continue;
}
slave->repldboff = 0;
slave->repldbsize = buf.st_size;
slave->replstate = SLAVE_STATE_SEND_BULK;
slave->replpreamble = sdscatprintf(sdsempty(),"$%lld\r\n",
(unsigned long long) slave->repldbsize);
aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE);
// 注册一个写事件到 epoll 中，由 sendBulkToSlave 进行具体的发送逻辑
if (aeCreateFileEvent(server.el, slave->fd, AE_WRITABLE, sendBulkToSlave, slave) == AE_ERR) {
freeClient(slave);
continue;
}
}
}
}
if (startbgsave) startBgsaveForReplication(mincapa);
}
// replication.c, 发送 rdb 文件到从节点
void sendBulkToSlave(aeEventLoop *el, int fd, void *privdata, int mask) {
client *slave = privdata;
UNUSED(el);
UNUSED(mask);
char buf[PROTO_IOBUF_LEN];
ssize_t nwritten, buflen;
/* Before sending the RDB file, we send the preamble as configured by the
* replication process. Currently the preamble is just the bulk count of
* the file in the form "$<length>\r\n". */
if (slave->replpreamble) {
nwritten = write(fd,slave->replpreamble,sdslen(slave->replpreamble));
if (nwritten == -1) {
serverLog(LL_VERBOSE,"Write error sending RDB preamble to slave: %s",
strerror(errno));
freeClient(slave);
return;
}
server.stat_net_output_bytes += nwritten;
sdsrange(slave->replpreamble,nwritten,-1);
if (sdslen(slave->replpreamble) == 0) {
sdsfree(slave->replpreamble);
slave->replpreamble = NULL;
/* fall through sending data. */
} else {
return;
}
}
/* If the preamble was already transfered, send the RDB bulk data. */
lseek(slave->repldbfd,slave->repldboff,SEEK_SET);
buflen = read(slave->repldbfd,buf,PROTO_IOBUF_LEN);
if (buflen <= 0) {
serverLog(LL_WARNING,"Read error sending DB to slave: %s",
(buflen == 0) ? "premature EOF" : strerror(errno));
freeClient(slave);
return;
}
if ((nwritten = write(fd,buf,buflen)) == -1) {
if (errno != EAGAIN) {
serverLog(LL_WARNING,"Write error sending DB to slave: %s",
strerror(errno));
freeClient(slave);
}
return;
}
slave->repldboff += nwritten;
server.stat_net_output_bytes += nwritten;
// 一次次地写入socket中，直到传输完成
if (slave->repldboff == slave->repldbsize) {
close(slave->repldbfd);
slave->repldbfd = -1;
aeDeleteFileEvent(server.el,slave->fd,AE_WRITABLE);
putSlaveOnline(slave);
}
}
PSYNC 也是主从同步的重要命令，它决定是全量复制还是部分复制。全量复制时，得决定是否开启 BGSAVE 操作；而部分复制时则只需把offset后的数据发送回slave即可完成数据同步。
4. 如何持续同步？
也叫增量同步。前面我们看这么多东西，其实也只做到了初次的全量复制和部分复制功能。那么第一次复制之后呢，后续又是如何持续同步的呢？
想想前面，既然有一个定时任务一直在运行，由它来实现可能是个不错的想法。从节点一直向其发送ping命令，而master节点则一直将自身的数据写入slave中，从而完成持续同步。
事实上，每个写动作，都会有一个事件传播的操作。而这个操作里，就会有一个检测 slave 情况的设定，而非cron去处理。就是 replicationFeedSlaves():
// 将命令传播给slaves
// 触发的场景如: 很多写操作, 特别的:某个key过期,
// replication.c
void replicationFeedSlaves(list *slaves, int dictid, robj **argv, int argc) {
listNode *ln;
listIter li;
int j, len;
char llstr[LONG_STR_SIZE];
/* If there aren't slaves, and there is no backlog buffer to populate,
* we can return ASAP. */
if (server.repl_backlog == NULL && listLength(slaves) == 0) return;
/* We can't have slaves attached and no backlog. */
serverAssert(!(listLength(slaves) != 0 && server.repl_backlog == NULL));
/* Send SELECT command to every slave if needed. */
if (server.slaveseldb != dictid) {
robj *selectcmd;
/* For a few DBs we have pre-computed SELECT command. */
if (dictid >= 0 && dictid < PROTO_SHARED_SELECT_CMDS) {
selectcmd = shared.select[dictid];
} else {
int dictid_len;
dictid_len = ll2string(llstr,sizeof(llstr),dictid);
selectcmd = createObject(OBJ_STRING,
sdscatprintf(sdsempty(),
"*2\r\n$6\r\nSELECT\r\n$%d\r\n%s\r\n",
dictid_len, llstr));
}
/* Add the SELECT command into the backlog. */
if (server.repl_backlog) feedReplicationBacklogWithObject(selectcmd);
/* Send it to slaves. */
listRewind(slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
if (slave->replstate == SLAVE_STATE_WAIT_BGSAVE_START) continue;
addReply(slave,selectcmd);
}
if (dictid < 0 || dictid >= PROTO_SHARED_SELECT_CMDS)
decrRefCount(selectcmd);
}
server.slaveseldb = dictid;
/* Write the command to the replication backlog if any. */
if (server.repl_backlog) {
char aux[LONG_STR_SIZE+3];
/* Add the multi bulk reply length. */
aux[0] = '*';
len = ll2string(aux+1,sizeof(aux)-1,argc);
aux[len+1] = '\r';
aux[len+2] = '\n';
feedReplicationBacklog(aux,len+3);
for (j = 0; j < argc; j++) {
long objlen = stringObjectLen(argv[j]);
/* We need to feed the buffer with the object as a bulk reply
* not just as a plain string, so create the $..CRLF payload len
* and add the final CRLF */
aux[0] = '$';
len = ll2string(aux+1,sizeof(aux)-1,objlen);
aux[len+1] = '\r';
aux[len+2] = '\n';
feedReplicationBacklog(aux,len+3);
feedReplicationBacklogWithObject(argv[j]);
feedReplicationBacklog(aux+len+1,2);
}
}
/* Write the command to every slave. */
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = ln->value;
/* Don't feed slaves that are still waiting for BGSAVE to start */
// 只有初始化完成后的从节点，才会推送同步写操作
if (slave->replstate == SLAVE_STATE_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]);
}
}
写操作的命令传播，是在 call() 调用实际的数据操作里统一封装的，避免了到处写相同的代码。
// server.c, 执行命令核心方法包装
// 调用如: processCommand().call(c,CMD_CALL_FULL); 会以最大能力处理命令
/* Call() is the core of Redis execution of a command.
*
* The following flags can be passed:
* CMD_CALL_NONE No flags.
* CMD_CALL_SLOWLOG Check command speed and log in the slow log if needed.
* CMD_CALL_STATS Populate command stats.
* CMD_CALL_PROPAGATE_AOF Append command to AOF if it modified the dataset
* or if the client flags are forcing propagation.
* CMD_CALL_PROPAGATE_REPL Send command to salves if it modified the dataset
* or if the client flags are forcing propagation.
* CMD_CALL_PROPAGATE Alias for PROPAGATE_AOF|PROPAGATE_REPL.
* CMD_CALL_FULL Alias for SLOWLOG|STATS|PROPAGATE.
*
* The exact propagation behavior depends on the client flags.
* Specifically:
*
* 1. If the client flags CLIENT_FORCE_AOF or CLIENT_FORCE_REPL are set
* and assuming the corresponding CMD_CALL_PROPAGATE_AOF/REPL is set
* in the call flags, then the command is propagated even if the
* dataset was not affected by the command.
* 2. If the client flags CLIENT_PREVENT_REPL_PROP or CLIENT_PREVENT_AOF_PROP
* are set, the propagation into AOF or to slaves is not performed even
* if the command modified the dataset.
*
* Note that regardless of the client flags, if CMD_CALL_PROPAGATE_AOF
* or CMD_CALL_PROPAGATE_REPL are not set, then respectively AOF or
* slaves propagation will never occur.
*
* Client flags are modified by the implementation of a given command
* using the following API:
*
* forceCommandPropagation(client *c, int flags);
* preventCommandPropagation(client *c);
* preventCommandAOF(client *c);
* preventCommandReplication(client *c);
*
*/
void call(client *c, int flags) {
long long dirty, start, duration;
int client_old_flags = c->flags;
...
/* Call the command. */
dirty = server.dirty;
start = ustime();
c->cmd->proc(c);
duration = ustime()-start;
dirty = server.dirty-dirty;
if (dirty < 0) dirty = 0;
...
// 此处将需要传播的命令传播到 slave
/* Propagate the command into the AOF and replication link */
if (flags & CMD_CALL_PROPAGATE &&
(c->flags & CLIENT_PREVENT_PROP) != CLIENT_PREVENT_PROP)
{
int propagate_flags = PROPAGATE_NONE;
/* Check if the command operated changes in the data set. If so
* set for replication / AOF propagation. */
if (dirty) propagate_flags |= (PROPAGATE_AOF|PROPAGATE_REPL);
/* If the client forced AOF / replication of the command, set
* the flags regardless of the command effects on the data set. */
if (c->flags & CLIENT_FORCE_REPL) propagate_flags |= PROPAGATE_REPL;
if (c->flags & CLIENT_FORCE_AOF) propagate_flags |= PROPAGATE_AOF;
/* However prevent AOF / replication propagation if the command
* implementatino called preventCommandPropagation() or similar,
* or if we don't have the call() flags to do so. */
if (c->flags & CLIENT_PREVENT_REPL_PROP ||
!(flags & CMD_CALL_PROPAGATE_REPL))
propagate_flags &= ~PROPAGATE_REPL;
if (c->flags & CLIENT_PREVENT_AOF_PROP ||
!(flags & CMD_CALL_PROPAGATE_AOF))
propagate_flags &= ~PROPAGATE_AOF;
/* Call propagate() only if at least one of AOF / replication
* propagation is needed. */
// 如果需要传播命令，则调用 propagate(), propagate 会决定写 AOF 或者 slaves
if (propagate_flags != PROPAGATE_NONE)
propagate(c->cmd,c->db->id,c->argv,c->argc,propagate_flags);
}
/* Restore the old replication flags, since call() can be executed
* recursively. */
c->flags &= ~(CLIENT_FORCE_AOF|CLIENT_FORCE_REPL|CLIENT_PREVENT_PROP);
c->flags |= client_old_flags &
(CLIENT_FORCE_AOF|CLIENT_FORCE_REPL|CLIENT_PREVENT_PROP);
...
server.stat_numcommands++;
}
/* Propagate the specified command (in the context of the specified database id)
* to AOF and Slaves.
*
* flags are an xor between:
* + PROPAGATE_NONE (no propagation of command at all)
* + PROPAGATE_AOF (propagate into the AOF file if is enabled)
* + PROPAGATE_REPL (propagate into the replication link)
*
* This should not be used inside commands implementation. Use instead
* alsoPropagate(), preventCommandPropagation(), forceCommandPropagation().
*/
void propagate(struct redisCommand *cmd, int dbid, robj **argv, int argc,
int flags)
{
// 写 AOF 文件
if (server.aof_state != AOF_OFF && flags & PROPAGATE_AOF)
feedAppendOnlyFile(cmd,dbid,argv,argc);
// 写slave
if (flags & PROPAGATE_REPL)
replicationFeedSlaves(server.slaves,dbid,argv,argc);
}
其实整个同步过程并不太复杂，大体就是建立连接然后复制数据然后恢复数据的过程，只是要实现的时候，代码还是不会太少。
当然，这里面会有很多要注意的点:
1. 如何不影响性能？
2. 如何保证低延迟？
3. 如何安全地复制？
4. 如何检测异常？
5. 如何保证高可用性？
