关于Redis中交互的过程

一、Redis启动

• 加载配置（命令行或者配置文件）

• 启动TCP监听，客户端的列表保存在redisserver的clients中

• 启动AE Event Loop事件，异步处理客户请求

事件处理器的主循环

aeMain

void aeMain(aeEventLoop *eventLoop) {
    eventLoop->stop = 0;
    while (!eventLoop->stop) {
        // 如果有需要在事件处理前执行的函数，那么运行它
        if (eventLoop->beforesleep != NULL)
            eventLoop->beforesleep(eventLoop);
        aeProcessEvents(eventLoop, AE_ALL_EVENTS);   // 开始处理事件
    }
}

事件处理框架非常简单，从初始化、服务到结束，分别对应的函数:aeCreateEventLoop、aeMain、aeDeleteEventLoop 其中，aeMain是事件循环的主体函数，它又会调用 aeProcessEvents函数，三个主体函数会调用aeApiCreate、aeApiPool、aeApiFree三个接口函数进行处理。 这三个接口函数又会映射到具体的某一种网络模型中

aeDeleteEventLoop

void aeDeleteEventLoop(aeEventLoop *eventLoop) {
    aeApiFree(eventLoop);
    zfree(eventLoop->events);
    zfree(eventLoop->fired);
    zfree(eventLoop);
}

aeCreateEventLoop

aeEventLoop *aeCreateEventLoop(int setsize) {
    aeEventLoop *eventLoop;
    int i;

    if ((eventLoop = zmalloc(sizeof(*eventLoop))) == NULL) goto err;
    eventLoop->events = zmalloc(sizeof(aeFileEvent)*setsize);
    eventLoop->fired = zmalloc(sizeof(aeFiredEvent)*setsize);
    if (eventLoop->events == NULL || eventLoop->fired == NULL) goto err;
    eventLoop->setsize = setsize;
    eventLoop->lastTime = time(NULL);
    eventLoop->timeEventHead = NULL;
    eventLoop->timeEventNextId = 0;
    eventLoop->stop = 0;
    eventLoop->maxfd = -1;
    eventLoop->beforesleep = NULL;
    if (aeApiCreate(eventLoop) == -1) goto err;
    /* Events with mask == AE_NONE are not set. So let's initialize the
     * vector with it. */
    for (i = 0; i < setsize; i++)
        eventLoop->events[i].mask = AE_NONE;
    return eventLoop;

err:
    if (eventLoop) {
        zfree(eventLoop->events);
        zfree(eventLoop->fired);
        zfree(eventLoop);
    }
    return NULL;
}

处理事件的时候，aeMain函数调用aeProcessEvents函数，在一个循环中处理文件事件和到期的时间事件。 aeProcessEvents函数调用aeSearchNearestTimer函数来查询事件循环中最先要过期的事件，时间复杂度为O(N)。先处理文件事件，然后再处理时间事件。

aeProcessEvents

int aeProcessEvents(aeEventLoop *eventLoop, int flags)
{
    int processed = 0, numevents;

    /* Nothing to do? return ASAP */
    if (!(flags & AE_TIME_EVENTS) && !(flags & AE_FILE_EVENTS)) return 0;

    /* Note that we want call select() even if there are no
     * file events to process as long as we want to process time
     * events, in order to sleep until the next time event is ready
     * to fire. */
    if (eventLoop->maxfd != -1 ||
        ((flags & AE_TIME_EVENTS) && !(flags & AE_DONT_WAIT))) {
        int j;
        aeTimeEvent *shortest = NULL;
        struct timeval tv, *tvp;

        if (flags & AE_TIME_EVENTS && !(flags & AE_DONT_WAIT))
            shortest = aeSearchNearestTimer(eventLoop);
        if (shortest) {
            long now_sec, now_ms;

            /* Calculate the time missing for the nearest
             * timer to fire. */
            aeGetTime(&now_sec, &now_ms);
            tvp = &tv;
            tvp->tv_sec = shortest->when_sec - now_sec;
            if (shortest->when_ms < now_ms) {
                tvp->tv_usec = ((shortest->when_ms+1000) - now_ms)*1000;
                tvp->tv_sec --;
            } else {
                tvp->tv_usec = (shortest->when_ms - now_ms)*1000;
            }
            if (tvp->tv_sec < 0) tvp->tv_sec = 0;
            if (tvp->tv_usec < 0) tvp->tv_usec = 0;
        } else {
            /* If we have to check for events but need to return
             * ASAP because of AE_DONT_WAIT we need to set the timeout
             * to zero */
            if (flags & AE_DONT_WAIT) {
                tv.tv_sec = tv.tv_usec = 0;
                tvp = &tv;
            } else {
                /* Otherwise we can block */
                tvp = NULL; /* wait forever */
            }
        }

        numevents = aeApiPoll(eventLoop, tvp);
        for (j = 0; j < numevents; j++) {
            aeFileEvent *fe = &eventLoop->events[eventLoop->fired[j].fd];
            int mask = eventLoop->fired[j].mask;
            int fd = eventLoop->fired[j].fd;
            int rfired = 0;

|       /* note the fe->mask & mask & ... code: maybe an already processed
             * event removed an element that fired and we still didn't
             * processed, so we check if the event is still valid. */
            if (fe->mask & mask & AE_READABLE) {
                rfired = 1;
                fe->rfileProc(eventLoop,fd,fe->clientData,mask);
            }
            if (fe->mask & mask & AE_WRITABLE) {
                if (!rfired || fe->wfileProc != fe->rfileProc)
                    fe->wfileProc(eventLoop,fd,fe->clientData,mask);
            }
            processed++;
        }
    }
    /* Check time events */
    if (flags & AE_TIME_EVENTS)
        processed += processTimeEvents(eventLoop);

    return processed; /* return the number of processed file/time events */
}

二、接受客户端TCP连接流程

acceptTcpHandler()该函数会调用acceptCommonHander()，而acceptCommonHander()又会调用createClient()来为该client创建一个redisClient对象，最终，redis会根据用户输入的命令通过查找命令表调用已经写好的命令执行函数

acceptTcpHandler

void acceptTcpHandler(aeEventLoop *el, int fd, void *privdata, int mask) {
    int cport, cfd, max = MAX_ACCEPTS_PER_CALL;
    char cip[REDIS_IP_STR_LEN];
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(mask);
    REDIS_NOTUSED(privdata);

    while(max--) {
        cfd = anetTcpAccept(server.neterr, fd, cip, sizeof(cip), &cport);
        if (cfd == ANET_ERR) {
            if (errno != EWOULDBLOCK)
                redisLog(REDIS_WARNING,
                    "Accepting client connection: %s", server.neterr);
            return;
        }
        redisLog(REDIS_VERBOSE,"Accepted %s:%d", cip, cport);
        acceptCommonHandler(cfd,0);
    }
}

client输入get命令，redis server最终会调用getCommand函数，client输入set命令，redis最终会调用setCommand函数

redis执行完用户的一个命令后，会将结果写入到redisClient对象中的reply list中，而sendReplyToClient函数会不断的从该list中数据，异步地发送给client。需要注意的是，sendReplyToClient函数也是通过aeCreateFileEvent注册的

三、处理客户请求流程

通过processInputBuffer()来解析querybuf, 若c->querybuf存在多条命令，则依次解析并处理这些命令

processInputBuffer

void processInputBuffer(redisClient *c) {
    /* Keep processing while there is something in the input buffer */
    while(sdslen(c->querybuf)) {
        /* Return if clients are paused. */
        if (!(c->flags & REDIS_SLAVE) && clientsArePaused()) return;

        /* 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. */
        if (!c->reqtype) {
            if (c->querybuf[0] == '*') {
                c->reqtype = REDIS_REQ_MULTIBULK;
            } else {
                c->reqtype = REDIS_REQ_INLINE;
            }
        }

        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. */
            if (processCommand(c) == REDIS_OK)
                resetClient(c);
        }
    }
}

• 如果是telnet发送的裸协议数据是没有*打头的表示参数个数的辅助信息，用processInlineBuffer()函数解析输入

• 其他则通过processMultibulkBuffer()函数解析

• 若解析函数返回REDIS_ERR，则等待下一次read()，是因为客户端缓存数据还没构成一条命令即不满足Redis协议格式；否则返回REDIS_OK, 处理命令

四、响应客户流程

数据读取 readQueryFromClient

调用系统函数read来读取客户端传送过来的数据， 调用read后对读取过程中所遇到的情况：

• 系统中断(nread == -1 && errno == EAGAIN)

• 读取出错(nread == -1 && errno != EAGAIN) freeClient()

• 客户端关闭(nread == 0) freeClient()

• 超过读取数据限制(1GB)则报错。 读取完后进入processInputBuffer进行协议解析

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;
    /* 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;
    c->querybuf = sdsMakeRoomFor(c->querybuf, readlen);
    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) {
        sdsIncrLen(c->querybuf,nread);
        c->lastinteraction = server.unixtime;
        if (c->flags & REDIS_MASTER) c->reploff += nread;
        server.stat_net_input_bytes += nread;
    } else {
        server.current_client = NULL;
        return;
    }
    if (sdslen(c->querybuf) > server.client_max_querybuf_len) {
        sds ci = catClientInfoString(sdsempty(),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;
    }
    processInputBuffer(c);
    server.current_client = NULL;
}  

数据解析

从readQueryFromClient()函数读取客户端传过来的数据，进入processInputBuffer()函数进行协议解析，可以把processInputBuffer函数看作是输入数据的协议解析器

Redis支持两种协议，一种是inline，一种是multibulk。inline协议是老协议，现在一般只在命令行下的redis客户端使用，其他情况一般是使用multibulk协议。

如果客户端传送的数据的第一个字符时‘*’，那么传送数据将被当做multibulk协议处理，否则将被当做inline协议处理。Inline协议的具体解析函数是processInlineBuffer()，multibulk协议的具体解析函数是processMultibulkBuffer()。 当协议解析完毕，即客户端传送的数据已经解析出命令字段和参数字段，接下来进行命令处理，命令处理函数是processCommand。

发送数据 sendReplyToClient

void sendReplyToClient(aeEventLoop *el, int fd, void *privdata, int mask) {
    redisClient *c = privdata;
    int nwritten = 0, totwritten = 0, objlen;
    size_t objmem;
    robj *o;
    REDIS_NOTUSED(el);
    REDIS_NOTUSED(mask);

    while(c->bufpos > 0 || listLength(c->reply)) {
        if (c->bufpos > 0) {
            nwritten = write(fd,c->buf+c->sentlen,c->bufpos-c->sentlen);
            if (nwritten <= 0) break;
            c->sentlen += nwritten;
            totwritten += nwritten;

            /* If the buffer was sent, set bufpos to zero to continue with
             * the remainder of the reply. */
            if (c->sentlen == c->bufpos) {
                c->bufpos = 0;
                c->sentlen = 0;
            }
        } else {
            o = listNodeValue(listFirst(c->reply));
            objlen = sdslen(o->ptr);
            objmem = getStringObjectSdsUsedMemory(o);

            if (objlen == 0) {
                listDelNode(c->reply,listFirst(c->reply));
                c->reply_bytes -= objmem;
                continue;
            }

            nwritten = write(fd, ((char*)o->ptr)+c->sentlen,objlen-c->sentlen);
            if (nwritten <= 0) break;
            c->sentlen += nwritten;
            totwritten += nwritten;

            /* If we fully sent the object on head go to the next one */
            if (c->sentlen == objlen) {
                listDelNode(c->reply,listFirst(c->reply));
                c->sentlen = 0;
                c->reply_bytes -= objmem;
            }
        }
        /* Note that we avoid to send more than REDIS_MAX_WRITE_PER_EVENT
         * bytes, in a single threaded server it's a good idea to serve
         * other clients as well, even if a very large request comes from
         * super fast link that is always able to accept data (in real world
         * scenario think about 'KEYS *' against the loopback interface).
         *
         * However if we are over the maxmemory limit we ignore that and
         * just deliver as much data as it is possible to deliver. */
        server.stat_net_output_bytes += totwritten;
        if (totwritten > REDIS_MAX_WRITE_PER_EVENT &&
            (server.maxmemory == 0 ||
             zmalloc_used_memory() < server.maxmemory)) break;
    }
    if (nwritten == -1) {
        if (errno == EAGAIN) {
            nwritten = 0;
        } else {
            redisLog(REDIS_VERBOSE,
                "Error writing to client: %s", strerror(errno));
            freeClient(c);
            return;
        }
    }
    if (totwritten > 0) {
        /* For clients representing masters we don't count sending data
         * as an interaction, since we always send REPLCONF ACK commands
         * that take some time to just fill the socket output buffer.
         * We just rely on data / pings received for timeout detection. */
        if (!(c->flags & REDIS_MASTER)) c->lastinteraction = server.unixtime;
    }
    if (c->bufpos == 0 && listLength(c->reply) == 0) {
        c->sentlen = 0;
        aeDeleteFileEvent(server.el,c->fd,AE_WRITABLE);

        /* Close connection after entire reply has been sent. */
        if (c->flags & REDIS_CLOSE_AFTER_REPLY) freeClient(c);
    }
}

通过调用系统函数write给客户端发送数据，如果缓冲区有数据就把缓冲区的数据发送给客户端，缓冲区的数据发送完了，如果有排队数据，则继续发送。

• 发送缓冲区(c->buf)的内容

• 发送回复链表(c->reply)的内容

写入异常处理

• 被系统中断(nwritten == -1 && errno == EAGAIN)

• 写数据出错(nwritten == -1 && errno != EAGAIN)，释放客户端freeClient()

参考文章

http://arc8.riaos.com/?p=6061

https://github.com/microheart/annotated_memcached/blob/master/note/Redis/Redis_main_flow.md

http://blog.csdn.net/ordeder/article/details/12791359

http://blog.nosqlfan.com/tags/redis