Redis源码分析之PSYNC同步

Redis master-slave 同步源码分析

（1）slave 流程分析
（2）master 流程分析

Slave 分析

当Redis 启动后，会每隔 1s 调用 replicationCron （通过 redis自带的serverCron后台线程），即无论是单机、还是Master、还是Slave都会调用这个函数。
我们先来讨论下作为Slave的情况下，replicationCron 函数运行逻辑。
作为slave，这个函数的功能应该能猜到，就是和Master保持连接、握手、接受Master存量数据+接受Master增量数据

/* Replication cron function, called 1 time per second. */
void replicationCron(void) {
    
    static long long replication_cron_loops = 0;

    /*一堆超时判断，先不管*/

    /* Non blocking connection timeout? */
    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();
    }
    
    ......


    /*作为slave，connectWithMaster创建和Master的socket，并且将回调函数设置为 syncWithMaster，并将同步状态设置为 REPL_STATE_CONNECTING */
    /* Check if we should connect to a MASTER */
    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 <-> REPLICA 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. */
    if (server.masterhost && server.master &&
        !(server.master->flags & CLIENT_PRE_PSYNC))
        replicationSendAck();

    ....
}

syncWithMaster 就是作为Slave和Master连接的的握手函数。
syncWithMaster函数就不再详细贴出代码，直接这里逻辑
1、发送ping
2、读pong表示成功继续第3步
3、如果master配置了requirepass ，那么slave必须配置 masterauth，所以就需要发送auth信息。如果不需要发送auth就到第5步
4、接受auth的结果，正确就继续
5、发送slave的port信息，这个port用于告知master连slave时使用哪个端口
6、判断 5 的结果
7、同5，只是发送的是slave 的 ip地址。5和7的端口，实际上目前翻看Redis源码，没有用到。
8、判断 7 的返回结果
9、slave发送自己的同步能力到master，用以和Master对齐同步的方法，最新版本支持 eof 和 psync2。psync是 partial resynchronization 的意思
10、发送 "PSYNC psync_replid psync_offset -1"到Master（全量的化就是 “PSYNC ? -1）
11、如果Master 回复 “+FULLRESYNC psync_replid psync_offset”，表示全量复制；如果回复”+CONTINUE psync_replid"表示部分复制

如果握手完成后，则 和 Master 建立的fd的 read event 变成 readSyncBulkPayload 函数

void syncWithMaster(aeEventLoop *el, int fd, void *privdata, int mask) {
    

	......
    /* Setup the non blocking download of the bulk file. */
    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;
    }
}

readSyncBulkPayload负责读完数据，然后调用 replicationCreateMasterClient，将当前和master连接的fd的callback变成readQueryFromClient
这样，这个连接后续就会接受Master发过来的指令。

void readSyncBulkPayload(aeEventLoop *el, int fd, void *privdata, int mask) {
    
    char buf[4096];
    ssize_t nread, readlen, nwritten;
    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. */

     /*第一次读 repl_transfer_size 是 -1 * Master第一次传输的数据是rdb文件的长度， *格式是有2种，一种是 $len。另一种 是 $EOF:<40 bytes delimiter>，前者表示指定长度，然后slave读指定长度即可，后者是 40字节的结束符 *Master 会在rdb文件发送完成后发送40字节结束符表示自己结束了。 * */
    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 <-> REPLICA sync: receiving streamed RDB from master");
        } else {
    
            usemark = 0;
            server.repl_transfer_size = strtol(buf+1,NULL,10);
            serverLog(LL_NOTICE,
                "MASTER <-> REPLICA sync: receiving %lld bytes from master",
                (long long) server.repl_transfer_size);
        }
        return;
    }

    /*到这就是读文件了 * usemark：表示通过EOF读 *repl_transfer_size：文件总大小 *repl_transfer_read：文件已读大小 */
    /* 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.*/
        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;
    if ((nwritten = 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 <-> REPLICA synchronization: %s",
            (nwritten == -1) ? strerror(errno) : "short write");
        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. */
    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 */
    if (!usemark) {
    
        if (server.repl_transfer_read == server.repl_transfer_size)
            eof_reached = 1;
    }

    /*读完一份，就加载rdb*/
    if (eof_reached) {
    
        int aof_is_enabled = server.aof_state != AOF_OFF;

        /* Ensure background save doesn't overwrite synced data */
        if (server.rdb_child_pid != -1) {
    
            serverLog(LL_NOTICE,
                "Replica is about to load the RDB file received from the "
                "master, but there is a pending RDB child running. "
                "Killing process %ld and removing its temp file to avoid "
                "any race",
                    (long) server.rdb_child_pid);
            kill(server.rdb_child_pid,SIGUSR1);
            rdbRemoveTempFile(server.rdb_child_pid);
        }

        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 <-> REPLICA synchronization: %s", strerror(errno));
            cancelReplicationHandshake();
            return;
        }
        serverLog(LL_NOTICE, "MASTER <-> REPLICA sync: Flushing old data");
        /* We need to stop any AOFRW fork before flusing and parsing * RDB, otherwise we'll create a copy-on-write disaster. */
        if(aof_is_enabled) stopAppendOnly();
        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 <-> REPLICA sync: Loading DB in memory");
        rdbSaveInfo rsi = RDB_SAVE_INFO_INIT;
        if (rdbLoad(server.rdb_filename,&rsi) != C_OK) {
    
            serverLog(LL_WARNING,"Failed trying to load the MASTER synchronization DB from disk");
            cancelReplicationHandshake();
            /* Re-enable the AOF if we disabled it earlier, in order to restore * the original configuration. */
            if (aof_is_enabled) restartAOFAfterSYNC();
            return;
        }
        /* Final setup of the connected slave <- master link */
        zfree(server.repl_transfer_tmpfile);
        close(server.repl_transfer_fd);
        replicationCreateMasterClient(server.repl_transfer_s,rsi.repl_stream_db);
        server.repl_state = REPL_STATE_CONNECTED;
        server.repl_down_since = 0;
        /* After a full resynchroniziation we use the replication ID and * offset of the master. The secondary ID / offset are cleared since * we are starting a new history. */
        memcpy(server.replid,server.master->replid,sizeof(server.replid));
        server.master_repl_offset = server.master->reploff;
        clearReplicationId2();
        /* Let's create the replication backlog if needed. Slaves need to * accumulate the backlog regardless of the fact they have sub-slaves * or not, in order to behave correctly if they are promoted to * masters after a failover. */
        if (server.repl_backlog == NULL) createReplicationBacklog();

        serverLog(LL_NOTICE, "MASTER <-> REPLICA 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 (aof_is_enabled) restartAOFAfterSYNC();
    }
    return;

error:
    cancelReplicationHandshake();
    return;
}

总结一下，Redis 的 psync 模式
（1）slave 往 master connect，然后握手。
（2）slave 发送 sync/psync 往 Master。
（3）Master 判断 是否全量还是增量推送，然后回复给slave，全量模式下，master立刻发送 rdb文件，增量模式下，master就发送指令。
（4）如果是全量模式，在推送rdb文件的后，Master会把推送rdb文件这个窗口期内的数据+后续的数据，以指令方式推送，其实就是增量部分了。
（5）增量部分的发送，调用了call->propagate->replicationFeedSlaves->addReply 来发送至 slave， addReply 通常用于 处理完Client的请求然后发送响应，这里被用来同数据给Salve。但addReply只是写入Redis自己的缓冲区，然后等主循环的下一次循环，beforeSleep->handleClientsWithPendingWrites将数据发送出去。

Master 流程分析

看到Salve流程，基本就能猜出Master的流程。

1、Master处理 psync指令，判断是否是全量同步还是增量同步
2、masterTryPartialResynchronization->addReplyReplicationBacklog

实际上，Master开不开AOF和RDB并不影响 同步。

Master 的 backlog逻辑，对Master任意写操作，都会触发feedReplicationBacklog，目的是将数据写入自己的缓冲区，然后记录一些偏移量。
缓冲区作用
（1）全量同步rdb时比较耗时，在RDB文件处理完成前，的这部分数据需要保留下来，作为增量数据传输
（2）Slave和Master全量同步之后增量同步，此时如果发生断网，那么Slave重连后，无条件全量同步肯定不能接受，Master需要保存部分数据，这部分数据如果恰巧包含了断网
期间Salve未收到的数据，则执行增量同步。

Redis 的backlog是一个环形的数据，所以不存在溢出，但是如果一个断网期间，Master写入的数据，实打实的超过一圈，那么也没救了，因为写入一圈，必然覆盖了部分旧数据，
相当于丢数据了所以看到不能增量同步了，需要全量。

几个offset说明一下意思
（1）repl_backlog_idx，下次待写入的数据的起始地址，所以下面代码可以看到，repl_backlog_idx 满一圈后，置为0，然后从0开始再写。
（2）repl_backlog_size，buffer的总长度，配置文件配置的，repl_backlog 就是根据这个大小申请出来的buffer
（3）master_repl_offset，从0开始累加的一个值
（4）repl_backlog_histlen 值 也是从0开始的一个值，最大为 repl_backlog_size

反正，大家不要尝试从字面角度或者源码角度理解这几个值，比较复杂，但是，核心目的很简单，这几个值就是需要让Master自己知道，Slave psync时发送过来的offset开始的指令，Master自己是否好保存。

void feedReplicationBacklog(void *ptr, size_t len) {
    
    unsigned char *p = ptr;
    printf("p %s\n",p);
    server.master_repl_offset += len;

    /* This is a circular buffer, so write as much data we can at every * iteration and rewind the "idx" index if we reach the limit. */
    while(len) {
    
        size_t thislen = server.repl_backlog_size - server.repl_backlog_idx;
        if (thislen > len) thislen = len;
        memcpy(server.repl_backlog+server.repl_backlog_idx,p,thislen);
        server.repl_backlog_idx += thislen;
        if (server.repl_backlog_idx == server.repl_backlog_size)
            server.repl_backlog_idx = 0;
        len -= thislen;
        p += thislen;
        server.repl_backlog_histlen += thislen;
    }
    if (server.repl_backlog_histlen > server.repl_backlog_size)
        server.repl_backlog_histlen = server.repl_backlog_size;
    /* Set the offset of the first byte we have in the backlog. */
    server.repl_backlog_off = server.master_repl_offset -
                              server.repl_backlog_histlen + 1;
}