Redis源码整体运行流程详解

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/Acceptedxukai/article/details/17842119

本文所引用的源码全部来自Redis2.8.2版本。

Redis源码整体运行流程的相关文件是：redis.h, redis.c, networking.c, ae.h, ae.c。

转载请注明，本文出自：http://blog.csdn.net/acceptedxukai/article/details/17842119

Redis Server端处理Client请求的流程图

main函数

main函数主要的功能为：调用initServerConfig函数，进行默认的redisServer数据结构的参数初始化；调用daemonize函数，为服务器开始守护进程，对于守护进行相关详细信息见http://blog.csdn.net/acceptedxukai/article/details/8743189；调用initServer函数，初始化服务器；调用loadServerConfig函数，读取Redis的配置文件，使用配置文件中的参数替换默认的参数值；调用aeMain函数，开启事件循环，整个服务器开始工作。

initServer函数

该函数主要为初始化服务器，需要初始化的内容比较多，主要有：

1、创建事件循环

server.el = aeCreateEventLoop(server.maxclients+REDIS_EVENTLOOP_FDSET_INCR);

2、创建TCP与UDP Server，启动服务器，完成bind与listen

/* Open the TCP listening socket for the user commands. */
    //server.ipfd是个int数组，启动服务器，完成bind,listen
    if (listenToPort(server.port,server.ipfd,&server.ipfd_count) == REDIS_ERR)
        exit(1);
    /* Open the listening Unix domain socket. */
    if (server.unixsocket != NULL) {
        unlink(server.unixsocket); /* don't care if this fails */
        server.sofd = anetUnixServer(server.neterr,server.unixsocket,server.unixsocketperm);
        if (server.sofd == ANET_ERR) {
            redisLog(REDIS_WARNING, "Opening socket: %s", server.neterr);
            exit(1);
        }
    }

Redis2.8.2 TCP同时支持IPv4与IPv6，同时与之前版本的Redis不同，此版本支持多个TCP服务器，listenToPort函数主要还是调用anetTcpServer函数，完成socket()-->bind()-->listen()，下面详细查看下TCPServer的创建，UDP直接忽略吧，我也不知道UDP具体用在哪。

static int anetListen(char *err, int s, struct sockaddr *sa, socklen_t len) {
    //绑定bind
    if (bind(s,sa,len) == -1) {
        anetSetError(err, "bind: %s", strerror(errno));
        close(s);
        return ANET_ERR;
    }

    /* Use a backlog of 512 entries. We pass 511 to the listen() call because
     * the kernel does: backlogsize = roundup_pow_of_two(backlogsize + 1);
     * which will thus give us a backlog of 512 entries */
    //监听
    if (listen(s, 511) == -1) {
        anetSetError(err, "listen: %s", strerror(errno));
        close(s);
        return ANET_ERR;
    }
    return ANET_OK;
}
static int _anetTcpServer(char *err, int port, char *bindaddr, int af)
{
    int s, rv;
    char _port[6];  /* strlen("65535") */
    struct addrinfo hints, *servinfo, *p;

    snprintf(_port,6,"%d",port);
    memset(&hints,0,sizeof(hints));
    hints.ai_family = af;
    hints.ai_socktype = SOCK_STREAM;
    //套接字地址用于监听绑定
    hints.ai_flags = AI_PASSIVE;    /* No effect if bindaddr != NULL */
    //可以加上hints.ai_protocol = IPPROTO_TCP;

    /**getaddrinfo(const char *hostname, const char *servicename,
                   const struct addrinfo *hint,struct addrinfo **res);
       hostname:主机名
       servicename: 服务名
       hint: 用于过滤的模板，仅能使用ai_family, ai_flags, ai_protocol, ai_socktype，其余字段为0
       res:得到所有可用的地址
    */
    if ((rv = getaddrinfo(bindaddr,_port,&hints,&servinfo)) != 0) {
        anetSetError(err, "%s", gai_strerror(rv));
        return ANET_ERR;
    }
    //轮流尝试多个地址，找到一个允许连接到服务器的地址时便停止
    for (p = servinfo; p != NULL; p = p->ai_next) {
        if ((s = socket(p->ai_family,p->ai_socktype,p->ai_protocol)) == -1)
            continue;

        if (af == AF_INET6 && anetV6Only(err,s) == ANET_ERR) goto error;
        //设置套接字选项setsockopt,采用地址复用
        if (anetSetReuseAddr(err,s) == ANET_ERR) goto error;
        //bind, listen
        if (anetListen(err,s,p->ai_addr,p->ai_addrlen) == ANET_ERR) goto error;
        goto end;
    }
    if (p == NULL) {
        anetSetError(err, "unable to bind socket");
        goto error;
    }

error:
    s = ANET_ERR;
end:
    freeaddrinfo(servinfo);
    return s;
}
//if server.ipfd_count = 0, bindaddr = NULL
int anetTcpServer(char *err, int port, char *bindaddr)
{
    return _anetTcpServer(err, port, bindaddr, AF_INET);
}

3、将listen的端口加入到事件监听中，进行监听，由aeCreateFileEvent函数完成，其注册的listen端口可读事件处理函数为acceptTcpHandler，这样在listen端口有新连接的时候会调用acceptTcpHandler，后者在accept这个新连接，然后就可以处理后续跟这个客户端连接相关的事件了。

/* Create an event handler for accepting new connections in TCP and Unix
     * domain sockets. */
     //文件事件，用于处理响应外界的操作请求,事件处理函数为acceptTcpHandler/acceptUnixHandler
     //在networking.c
    for (j = 0; j < server.ipfd_count; j++) {
        if (aeCreateFileEvent(server.el, server.ipfd[j], AE_READABLE,
            acceptTcpHandler,NULL) == AE_ERR)
            {
                redisPanic(
                    "Unrecoverable error creating server.ipfd file event.");
            }
    }
    if (server.sofd > 0 && aeCreateFileEvent(server.el,server.sofd,AE_READABLE,
        acceptUnixHandler,NULL) == AE_ERR) redisPanic("Unrecoverable error creating server.sofd file event.");

acceptTcpHandler函数

上面介绍了，initServer完成listen端口后，会加入到事件循环中，该事件为可读事件，并记录处理函数为fe->rfileProc = acceptTcpHandler；该函数分两步操作:用acceptTcpHandler接受这个客户端连接；然第二部初始化这个客户端连接的相关数据，将clientfd加入事件里面，设置的可读事件处理函数为readQueryFromClient，也就是读取客户端请求的函数。

void acceptTcpHandler(aeEventLoop *el, int fd, void *privdata, int mask) {
    int cport, cfd;
    char cip[REDIS_IP_STR_LEN];
    REDIS_NOTUSED(el);//无意义
    REDIS_NOTUSED(mask);
    REDIS_NOTUSED(privdata);

    //cfd为accept函数返回的客户端文件描述符，accept使服务器完成一个客户端的链接
    cfd = anetTcpAccept(server.neterr, fd, cip, sizeof(cip), &cport);
    if (cfd == AE_ERR) {
        redisLog(REDIS_WARNING,"Accepting client connection: %s", server.neterr);
        return;
    }
    redisLog(REDIS_VERBOSE,"Accepted %s:%d", cip, cport);
	//将cfd加入事件循环并设置回调函数为readQueryFromClient，并初始化redisClient
    acceptCommonHandler(cfd,0);
}

第一步很简单即完成accept，主要关注第二步acceptCommonHandler函数

static void acceptCommonHandler(int fd, int flags) {
    redisClient *c;
    if ((c = createClient(fd)) == NULL) {//创建新的客户端
        redisLog(REDIS_WARNING,
            "Error registering fd event for the new client: %s (fd=%d)",
            strerror(errno),fd);
        close(fd); /* May be already closed, just ignore errors */
        return;
    }
    /* If maxclient directive is set and this is one client more... close the
     * connection. Note that we create the client instead to check before
     * for this condition, since now the socket is already set in non-blocking
     * mode and we can send an error for free using the Kernel I/O */
    //当前连接的客户端数目大于服务器最大运行的连接数，则拒绝连接
    if (listLength(server.clients) > server.maxclients) {
        char *err = "-ERR max number of clients reached\r\n";

        /* That's a best effort error message, don't check write errors */
        if (write(c->fd,err,strlen(err)) == -1) {
            /* Nothing to do, Just to avoid the warning... */
        }
        server.stat_rejected_conn++;
        freeClient(c);
        return;
    }
    server.stat_numconnections++;
    c->flags |= flags;
}

createClient函数

此函数用来为新连接的客户端初始化一个redisClient数据结构，该数据结构有比较多的参数，详见redis.h。该函数完成两个操作，第一、为客户端创建事件处理函数readQueryFromClient专门接收客户端发来的指令，第二、初始化redisClient数据结构相关参数。

redisClient *createClient(int fd) {
    redisClient *c = zmalloc(sizeof(redisClient));

    /* passing -1 as fd it is possible to create a non connected client.
     * This is useful since all the Redis commands needs to be executed
     * in the context of a client. When commands are executed in other
     * contexts (for instance a Lua script) we need a non connected client. */
     /**
        因为 Redis 命令总在客户端的上下文中执行，
        有时候为了在服务器内部执行命令，需要使用伪客户端来执行命令
        在 fd == -1 时，创建的客户端为伪终端
     */
    if (fd != -1) {
        //下面三个都是设置socket属性
        anetNonBlock(NULL,fd);//非阻塞
        anetEnableTcpNoDelay(NULL,fd);//no delay
        if (server.tcpkeepalive)
            anetKeepAlive(NULL,fd,server.tcpkeepalive);//keep alive

        //创建一个accept fd的FileEvent事件，事件的处理函数是readQueryFromClient
        if (aeCreateFileEvent(server.el,fd,AE_READABLE,
            readQueryFromClient, c) == AE_ERR)
        {
            close(fd);
            zfree(c);
            return NULL;
        }
    }

    selectDb(c,0);//默认选择第0个db, db.c
    c->fd = fd;//文件描述符
    c->name = NULL;
    c->bufpos = 0;//将指令结果发送给客户端的字符串长度
    c->querybuf = sdsempty();//请求字符串初始化
    c->querybuf_peak = 0;//请求字符串顶峰时的长度值
    c->reqtype = 0;//请求类型
    c->argc = 0;//参数个数
    c->argv = NULL;//参数内容
    c->cmd = c->lastcmd = NULL;//操作指令
    c->multibulklen = 0;//块个数
    c->bulklen = -1;//每个块的长度
    c->sentlen = 0;
    c->flags = 0;//客户类型的标记，比较重要
    c->ctime = c->lastinteraction = server.unixtime;
    c->authenticated = 0;
    c->replstate = REDIS_REPL_NONE;
    c->reploff = 0;
    c->repl_ack_off = 0;
    c->repl_ack_time = 0;
    c->slave_listening_port = 0;
    c->reply = listCreate();//存放服务器应答的数据
    c->reply_bytes = 0;
    c->obuf_soft_limit_reached_time = 0;
    listSetFreeMethod(c->reply,decrRefCountVoid);
    listSetDupMethod(c->reply,dupClientReplyValue);
    c->bpop.keys = dictCreate(&setDictType,NULL);//下面三个参数在list数据阻塞操作时使用
    c->bpop.timeout = 0;
    c->bpop.target = NULL;
    c->io_keys = listCreate();
    c->watched_keys = listCreate();//事务命令CAS中使用
    listSetFreeMethod(c->io_keys,decrRefCountVoid);
    c->pubsub_channels = dictCreate(&setDictType,NULL);
    c->pubsub_patterns = listCreate();
    listSetFreeMethod(c->pubsub_patterns,decrRefCountVoid);
    listSetMatchMethod(c->pubsub_patterns,listMatchObjects);
    // 如果不是伪客户端，那么将客户端加入到服务器客户端列表中
    if (fd != -1) listAddNodeTail(server.clients,c);//添加到server的clients链表
    initClientMultiState(c);//初始化事务指令状态
    return c;
}

客户端的请求指令字符串始终存放在querybuf中，在对querybuf解析后，将指令参数的个数存放在argc中，具体的指令参数存放在argv中；但是服务器应答的结果有两种存储方式：buf字符串、reply列表。

readQueryFromClient函数

readQueryFromClient函数用来读取客户端的请求命令行数据，并调用processInputBuffer函数依照redis通讯协议对数据进行解析。服务器使用最原始的read函数来读取客户端发送来的请求命令，并将字符串存储在querybuf中，根据需要对querybuf进行扩展。

void readQueryFromClient(aeEventLoop *el, int fd, void *privdata, int mask) {
    redisClient *c = (redisClient*) privdata;
    int nread, readlen;
    size_t qblen;
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(mask);

    server.current_client = c;
    readlen = REDIS_IOBUF_LEN; //1024 * 16
    /* 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 == REDIS_REQ_MULTIBULK && c->multibulklen && c->bulklen != -1
        && c->bulklen >= REDIS_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;
    //对querybuf的空间进行扩展
    c->querybuf = sdsMakeRoomFor(c->querybuf, readlen);
    //读取客户端发来的操作指令
    nread = read(fd, c->querybuf+qblen, readlen);
    if (nread == -1) {
        if (errno == EAGAIN) {
            nread = 0;
        } else {
            redisLog(REDIS_VERBOSE, "Reading from client: %s",strerror(errno));
            freeClient(c);
            return;
        }
    } else if (nread == 0) {
        redisLog(REDIS_VERBOSE, "Client closed connection");
        freeClient(c);
        return;
    }
    if (nread) {
        //改变querybuf的实际长度和空闲长度，len += nread, free -= nread;
        sdsIncrLen(c->querybuf,nread);
        c->lastinteraction = server.unixtime;
        if (c->flags & REDIS_MASTER) c->reploff += nread;
    } else {
        server.current_client = NULL;
        return;
    }
    //客户端请求的字符串长度大于服务器最大的请求长度值
    if (sdslen(c->querybuf) > server.client_max_querybuf_len) {
        sds ci = getClientInfoString(c), bytes = sdsempty();

        bytes = sdscatrepr(bytes,c->querybuf,64);
        redisLog(REDIS_WARNING,"Closing client that reached max query buffer length: %s (qbuf initial bytes: %s)", ci, bytes);
        sdsfree(ci);
        sdsfree(bytes);
        freeClient(c);
        return;
    }
    //解析请求
    processInputBuffer(c);
    server.current_client = NULL;
}

processInputBuffer函数主要用来处理请求的解析工作，redis有两种解析方式；行指令解析与多重指令解析，行指令解析直接忽略，下面详解多重指令解析。

void processInputBuffer(redisClient *c) {
    /* Keep processing while there is something in the input buffer */
    while(sdslen(c->querybuf)) {
        /* Immediately abort if the client is in the middle of something. */
        if (c->flags & REDIS_BLOCKED) return;

        /* REDIS_CLOSE_AFTER_REPLY closes the connection once the reply is
         * written to the client. Make sure to not let the reply grow after
         * this flag has been set (i.e. don't process more commands). */
        if (c->flags & REDIS_CLOSE_AFTER_REPLY) return;

        /* Determine request type when unknown. */
        //当请求类型未知时，先确定属于哪种请求
        if (!c->reqtype) {
            if (c->querybuf[0] == '*') {
                c->reqtype = REDIS_REQ_MULTIBULK;//多重指令解析
            } else {
                c->reqtype = REDIS_REQ_INLINE;//按行解析
            }
        }

        if (c->reqtype == REDIS_REQ_INLINE) {
            if (processInlineBuffer(c) != REDIS_OK) break;
        } else if (c->reqtype == REDIS_REQ_MULTIBULK) {
            if (processMultibulkBuffer(c) != REDIS_OK) break;
        } else {
            redisPanic("Unknown request type");
        }

        /* Multibulk processing could see a <= 0 length. */
        if (c->argc == 0) {
            resetClient(c);
        } else {
            /* Only reset the client when the command was executed. */
            //执行相应指令
            if (processCommand(c) == REDIS_OK)
                resetClient(c);
        }
    }
}

多重指令解析的处理函数为processMultibulkBuffer，下面先简单介绍下Redis的通讯协议：

以下是这个协议的一般形式：
*< 参数数量 > CR LF
$< 参数 1 的字节数量 > CR LF
< 参数 1 的数据 > CR LF
...
$< 参数 N 的字节数量 > CR LF
< 参数 N 的数据 > CR LF
举个例子，以下是一个命令协议的打印版本：
*3
$3
SET
$3
foo
$3
bar
这个命令的实际协议值如下：
"*3\r\n$3\r\nSET\r\n$3\r\foo\r\n$3\r\bar\r\n"

/**
    例：querybuf = "*3\r\n$3\r\nSET\r\n$3\r\nfoo\r\n$3\r\nbar\r\n"
*/
int processMultibulkBuffer(redisClient *c) {
    char *newline = NULL;
    int pos = 0, ok;
    long long ll;

    if (c->multibulklen == 0) {//参数数目为0，表示这是新的请求指令
        /* The client should have been reset */
        redisAssertWithInfo(c,NULL,c->argc == 0);

        /* Multi bulk length cannot be read without a \r\n */
        newline = strchr(c->querybuf,'\r');
        if (newline == NULL) {
            if (sdslen(c->querybuf) > REDIS_INLINE_MAX_SIZE) {
                addReplyError(c,"Protocol error: too big mbulk count string");
                setProtocolError(c,0);
            }
            return REDIS_ERR;
        }

        /* Buffer should also contain \n */
        if (newline-(c->querybuf) > ((signed)sdslen(c->querybuf)-2))
            return REDIS_ERR;

        /* We know for sure there is a whole line since newline != NULL,
         * so go ahead and find out the multi bulk length. */
        redisAssertWithInfo(c,NULL,c->querybuf[0] == '*');
        //将字符串转为long long整数，转换得到的结果存到ll中,ll就是后面参数的个数
        ok = string2ll(c->querybuf+1,newline-(c->querybuf+1),&ll);
        if (!ok || ll > 1024*1024) {
            addReplyError(c,"Protocol error: invalid multibulk length");
            setProtocolError(c,pos);
            return REDIS_ERR;
        }

        pos = (newline-c->querybuf)+2;//跳过\r\n
        if (ll <= 0) {//参数个数小于0，表示后面的参数数目大于等于绝对值ll
             /** s = sdsnew("Hello World");
             * sdsrange(s,1,-1); => "ello World"
             */
            sdsrange(c->querybuf,pos,-1);//querybuf="$3\r\nSET\r\n$3\r\nfoo\r\n$3\r\nbar\r\n"
            return REDIS_OK;
        }

        c->multibulklen = ll;//得到指令参数个数

        /* Setup argv array on client structure */
        if (c->argv) zfree(c->argv);
        c->argv = zmalloc(sizeof(robj*) * c->multibulklen);//申请参数内存空间
    }

    redisAssertWithInfo(c,NULL,c->multibulklen > 0);
    /**
        开始抽取字符串
        querybuf = "*3\r\n$3\r\nSET\r\n$3\r\nfoo\r\n$3\r\nbar\r\n"
        pos = 4
    */
    while(c->multibulklen) {
        /* Read bulk length if unknown */
        if (c->bulklen == -1) {//参数的长度为-1，这里用来处理每个参数的字符串长度值
            /**newline = "\r\nSET\r\n$3\r\nfoo\r\n$3\r\nbar\r\n"*/
            newline = strchr(c->querybuf+pos,'\r');
            if (newline == NULL) {
                if (sdslen(c->querybuf) > REDIS_INLINE_MAX_SIZE) {
                    addReplyError(c,"Protocol error: too big bulk count string");
                    setProtocolError(c,0);
                }
                break;
            }

            /* Buffer should also contain \n */
            if (newline-(c->querybuf) > ((signed)sdslen(c->querybuf)-2))
                break;

            //每个字符串以$开头，后面的数字表示其长度
            if (c->querybuf[pos] != '$') {
                addReplyErrorFormat(c,
                    "Protocol error: expected '$', got '%c'",
                    c->querybuf[pos]);
                setProtocolError(c,pos);
                return REDIS_ERR;
            }

            //得到字符串的长度值，ll
            ok = string2ll(c->querybuf+pos+1,newline-(c->querybuf+pos+1),&ll);
            if (!ok || ll < 0 || ll > 512*1024*1024) {
                addReplyError(c,"Protocol error: invalid bulk length");
                setProtocolError(c,pos);
                return REDIS_ERR;
            }

            //pos = 8
            pos += newline-(c->querybuf+pos)+2;//跳过\r\n "SET\r\n$3\r\nfoo\r\n$3\r\nbar\r"
            if (ll >= REDIS_MBULK_BIG_ARG) {//字符串长度超过1024*32，需要扩展
                size_t qblen;

                /* If we are going to read a large object from network
                 * try to make it likely that it will start at c->querybuf
                 * boundary so that we can optimize object creation
                 * avoiding a large copy of data. */
                 /**
                    sdsrange(querybuf,pos,-1)是将[pos,len-1]之间的字符串使用memmove前移，
                    然后后面的直接截断
                 */
                sdsrange(c->querybuf,pos,-1);//"SET\r\n$3\r\nfoo\r\n$3\r\nbar\r"
                pos = 0;
                qblen = sdslen(c->querybuf);
                /* Hint the sds library about the amount of bytes this string is
                 * going to contain. */
                if (qblen < ll+2)//这里只会到最后一个字符串才可能为True，并且数据不完整，数据不完整是由于redis使用非阻塞的原因
                    c->querybuf = sdsMakeRoomFor(c->querybuf,ll+2-qblen);
            }
            c->bulklen = ll;
        }

        /* Read bulk argument */
        //读取参数，没有\r\n表示数据不全，也就是说服务器接收到的数据不完整
        if (sdslen(c->querybuf)-pos < (unsigned)(c->bulklen+2)) {
            /* Not enough data (+2 == trailing \r\n) */
            break;
        } else {//数据完整
            /* Optimization: if the buffer contains JUST our bulk element
             * instead of creating a new object by *copying* the sds we
             * just use the current sds string. */
            if (pos == 0 &&
                c->bulklen >= REDIS_MBULK_BIG_ARG &&
                (signed) sdslen(c->querybuf) == c->bulklen+2)
            {//数据刚好完整，那么就直接使用c->querybuf,然后清空querybuf，注意这里只可能在最后一个字符串才可能出现
                c->argv[c->argc++] = createObject(REDIS_STRING,c->querybuf);
                sdsIncrLen(c->querybuf,-2); /* remove CRLF */
                c->querybuf = sdsempty();
                /* Assume that if we saw a fat argument we'll see another one
                 * likely... */
                c->querybuf = sdsMakeRoomFor(c->querybuf,c->bulklen+2);
                pos = 0;
            } else {
                //抽取出具体的字符串,比如SET,建立一个stringObject
                c->argv[c->argc++] =
                    createStringObject(c->querybuf+pos,c->bulklen);
                pos += c->bulklen+2;//跳过\r\n
            }
            c->bulklen = -1;
            c->multibulklen--;
        }
    }

    /**
        由于采用的是非阻塞读取客户端数据的方式，那么如果c->multibulklen != 0，那么就表示
        数据没有接收完全，首先需要将当前的querybuf数据截断
    */
    /* Trim to pos */
    if (pos) sdsrange(c->querybuf,pos,-1);

    /* We're done when c->multibulk == 0 */
    if (c->multibulklen == 0) return REDIS_OK;

    /* Still not read to process the command */

    return REDIS_ERR;
}

processCommand与call函数

客户端指令解析完之后，需要执行该指令，执行指令的两个函数为processCommand与call函数，这两个函数除了单纯的执行指令外，还做了许多其他的工作，这里不详解，看代码仅仅找到指令如何执行还是很简单的。

指令执行完之后，需要将得到的结果集返回给客户端，这部分是如何工作的，下面开始分析。

在networking.c中可以发现许多以addRelpy为前缀的函数名，这些函数都是用来处理各种不同类型的结果的，我们以典型的addReply函数为例，进行分析。

addReply函数

该函数第一步工作就是调用prepareClientToWrite函数为客户端创建一个写文件事件，事件的处理函数即将结果集发送给客户端的函数为sendReplyToClient.

int prepareClientToWrite(redisClient *c) {
    if (c->flags & REDIS_LUA_CLIENT) return REDIS_OK;
    if ((c->flags & REDIS_MASTER) &&
        !(c->flags & REDIS_MASTER_FORCE_REPLY)) return REDIS_ERR;
    if (c->fd <= 0) return REDIS_ERR; /* Fake client */
    if (c->bufpos == 0 && listLength(c->reply) == 0 &&
        (c->replstate == REDIS_REPL_NONE ||
         c->replstate == REDIS_REPL_ONLINE) &&
        aeCreateFileEvent(server.el, c->fd, AE_WRITABLE,
        sendReplyToClient, c) == AE_ERR) return REDIS_ERR;
    return REDIS_OK;
}

第二步，就是根据相应的条件，将得到的结果rboj数据存储到buf中或者reply链表中。对于存储的策略：redis优先将数据存储在固定大小的buf中，也就是redisClient结构体buf[REDIS_REPLY_CHUNK_BYTES]里，默认大小为16K。如果有数据没有发送完或c->buf空间不足，就会放到c->reply链表里面，链表每个节点都是内存buf，后来的数据放入最后面。具体的处理函数为_addReplyToBuffer和_addReplyStringToList两个函数。

void addReply(redisClient *c, robj *obj) {
    if (prepareClientToWrite(c) != REDIS_OK) return;

    /* This is an important place where we can avoid copy-on-write
     * when there is a saving child running, avoiding touching the
     * refcount field of the object if it's not needed.
     *
     * If the encoding is RAW and there is room in the static buffer
     * we'll be able to send the object to the client without
     * messing with its page. */
    if (obj->encoding == REDIS_ENCODING_RAW) {//字符串类型
        //是否能将数据追加到c->buf中
        if (_addReplyToBuffer(c,obj->ptr,sdslen(obj->ptr)) != REDIS_OK)
            _addReplyObjectToList(c,obj);//添加到c->reply链表中
    } else if (obj->encoding == REDIS_ENCODING_INT) {//整数类型
        /* Optimization: if there is room in the static buffer for 32 bytes
         * (more than the max chars a 64 bit integer can take as string) we
         * avoid decoding the object and go for the lower level approach. */
         //追加到c->buf中
        if (listLength(c->reply) == 0 && (sizeof(c->buf) - c->bufpos) >= 32) {
            char buf[32];
            int len;

            len = ll2string(buf,sizeof(buf),(long)obj->ptr);//整型转string
            if (_addReplyToBuffer(c,buf,len) == REDIS_OK)
                return;
            /* else... continue with the normal code path, but should never
             * happen actually since we verified there is room. */
        }
        obj = getDecodedObject(obj);//64位整数，先转换为字符串
        if (_addReplyToBuffer(c,obj->ptr,sdslen(obj->ptr)) != REDIS_OK)
            _addReplyObjectToList(c,obj);
        decrRefCount(obj);
    } else {
        redisPanic("Wrong obj->encoding in addReply()");
    }
}

/**
    Server将数据发送给Client，有两种存储数据的缓冲形式，具体参见redisClient结构体
    1、Response buffer
        int bufpos; //回复
        char buf[REDIS_REPLY_CHUNK_BYTES]; //长度为16 * 1024
    2、list *reply;
        unsigned long reply_bytes; Tot bytes of objects in reply list
        int sentlen;            已发送的字节数
    如果已经使用reply的形式或者buf已经不够存储，那么就将数据添加到list *reply中
    否则将数据添加到buf中
*/
int _addReplyToBuffer(redisClient *c, char *s, size_t len) {
    size_t available = sizeof(c->buf)-c->bufpos;//计算出c->buf的剩余长度

    if (c->flags & REDIS_CLOSE_AFTER_REPLY) return REDIS_OK;

    /* If there already are entries in the reply list, we cannot
     * add anything more to the static buffer. */
    if (listLength(c->reply) > 0) return REDIS_ERR;

    /* Check that the buffer has enough space available for this string. */
    if (len > available) return REDIS_ERR;

    //回复数据追加到buf中
    memcpy(c->buf+c->bufpos,s,len);
    c->bufpos+=len;
    return REDIS_OK;
}

/**
    1、如果链表长度为0: 新建一个节点并直接将robj追加到链表的尾部
    2、链表长度不为0: 首先取出链表的尾部节点
        1)、尾部节点的字符串长度 + robj中ptr字符串的长度 <= REDIS_REPLY_CHUNK_BYTES:
            将robj->ptr追加到尾节点的tail->ptr后面
        2)、反之: 新建一个节点并直接将robj追加到链表的尾部
*/
void _addReplyObjectToList(redisClient *c, robj *o) {
    robj *tail;

    if (c->flags & REDIS_CLOSE_AFTER_REPLY) return;

    //链表长度为0
    if (listLength(c->reply) == 0) {
        incrRefCount(o);//增加引用次数
        listAddNodeTail(c->reply,o);//添加到链表末尾
        c->reply_bytes += zmalloc_size_sds(o->ptr); //计算o->ptr的占用内存大小
    } else {
        //取出链表尾中的数据
        tail = listNodeValue(listLast(c->reply));

        /* Append to this object when possible. */
        // 如果最后一个节点所保存的回复加上新回复内容总长度小于等于 REDIS_REPLY_CHUNK_BYTES
        // 那么将新回复追加到节点回复当中。
        if (tail->ptr != NULL &&
            sdslen(tail->ptr)+sdslen(o->ptr) <= REDIS_REPLY_CHUNK_BYTES)
        {
            c->reply_bytes -= zmalloc_size_sds(tail->ptr);
            tail = dupLastObjectIfNeeded(c->reply);
            tail->ptr = sdscatlen(tail->ptr,o->ptr,sdslen(o->ptr));
            c->reply_bytes += zmalloc_size_sds(tail->ptr);
        } else {//为新回复单独创建一个节点
            incrRefCount(o);
            listAddNodeTail(c->reply,o);
            c->reply_bytes += zmalloc_size_sds(o->ptr);
        }
    }
    // 如果突破了客户端的最大缓存限制，那么关闭客户端
    asyncCloseClientOnOutputBufferLimitReached(c);
}

sendReplyToClient函数

终于到了最后一步，把c->buf与c->reply中的数据发送给客户端即可，发送同样使用的是最原始的write函数。发送完成之后，redis将当前客户端释放，并且删除写事件，代码比较简单，不详细解释。

小结

本文粗略的介绍了Redis整体运行的流程，从服务器的角度，介绍Redis是如何初始化，创建socket，接收客户端请求，解析请求及指令的执行，反馈执行的结果集给客户端等。如果读者想更深入的了解Redis的运行机制，需要亲自阅读源码，本文可以用作参考。同时也是学习linux socket编程的好工具，原本简简单单的socket->bind->listen->accept->read->write也可以用来做许多高效的业务，是Linux socket学习的不二选择。