本文主要说明Redis监视器的实现代码。

I、监视器理论部分

1、客户端通过执行MONITOR命令可以将自己变为一个监视器，实时地接受并打印出服务器当前处理的命令请求的相关信息，如：

此时，当其他客户端向服务器发送一条命令请求时，服务器除了会处理这条命令请求之外，还会将这条命令请求的信息发送给所有监视器：

II、代码实现

2.1 成为监视器

redisServer维护一个monitors的链表，记录自己的监视器，每次收到MONITOR命令之后，只需将客户端追加到表尾即可：

/*src/redis.c/monitorCommand*/
void monitorCommand(redisClient *c) {
    /* ignore MONITOR if already slave or in monitor mode */
    if (c->flags & REDIS_SLAVE) return;
        c->flags |= (REDIS_SLAVE|REDIS_MONITOR);
        listAddNodeTail(server.monitors,c);
        addReply(c,shared.ok);  //回复OK
}

2.2 向监视器传播命令

call函数中有对于监视器命令传播的实现：

// call() 函数是执行命令的核心函数，这里只看监视器部分
/*src/redis.c/call*/
/* Call() is the core of Redis execution of a command */
void call(redisClient *c, int flags) {
    long long dirty, start = ustime(), duration;
    int client_old_flags = c->flags;
    /* Sent the command to clients in MONITOR mode, only if the commands are
    * not generated from reading an AOF. */
    if (listLength(server.monitors) &&
        !server.loading &&
        !(c->cmd->flags & REDIS_CMD_SKIP_MONITOR))
        {
        replicationFeedMonitors(c,server.monitors,c->db->id,c->argv,c->argc);
    }
    ......
}  

call主要调用了replicationFeedMonitors，这个函数的作用就是将命令打包为协议，发送给监视器：

// 将协议发给 Monitor
/*src/replication.c/replicationFeedMonitors*/
void replicationFeedMonitors(redisClient *c, list *monitors, int dictid, robj **argv, int argc) {
    listNode *ln;
    listIter li;
    int j;
    sds cmdrepr = sdsnew("+");
    robj *cmdobj;
    struct timeval tv;

    // 获取时间戳
    gettimeofday(&tv,NULL);
    cmdrepr = sdscatprintf(cmdrepr,"%ld.%06ld ",(long)tv.tv_sec,(long)tv.tv_usec);
    if (c->flags & REDIS_LUA_CLIENT) {
        cmdrepr = sdscatprintf(cmdrepr,"[%d lua] ",dictid);
    } else if (c->flags & REDIS_UNIX_SOCKET) {
        cmdrepr = sdscatprintf(cmdrepr,"[%d unix:%s] ",dictid,server.unixsocket);
    } else {
        cmdrepr = sdscatprintf(cmdrepr,"[%d %s] ",dictid,getClientPeerId(c));
    }

    // 获取命令和参数
    for (j = 0; j < argc; j++) {
        if (argv[j]->encoding == REDIS_ENCODING_INT) {
            cmdrepr = sdscatprintf(cmdrepr, "\"%ld\"", (long)argv[j]->ptr);
        } else {
            cmdrepr = sdscatrepr(cmdrepr,(char*)argv[j]->ptr,
                        sdslen(argv[j]->ptr));
        }
        if (j != argc-1)
            cmdrepr = sdscatlen(cmdrepr," ",1);
    }
    cmdrepr = sdscatlen(cmdrepr,"\r\n",2);
    cmdobj = createObject(REDIS_STRING,cmdrepr);

    // 将内容发送给所有 MONITOR 
    listRewind(monitors,&li);
    while((ln = listNext(&li))) {
        redisClient *monitor = ln->value;
        addReply(monitor,cmdobj);
    }
    decrRefCount(cmdobj);
}

【参考】
[1] 《Redis设计与实现》
[2] 《Redis源码日志》