redis源码浅析--十一.事件（下）时间事件

环境说明：redis源码版本 5.0.3；我在阅读源码过程做了注释，git地址：https://gitee.com/xiaoangg/redis_annotation
参考书籍：《redis的设计与实现》

目录

事件（上）文件事件

一 时间事件

二 时间事件的实现

三 事件的调度与执行

---

一 时间事件

redis的时间事件分为两大类

1. 定时事件：让程序在指定时间只执行一次；
2. 周期事件：让程序每间隔一段时间执行一次；

一个「时间事件」属性组成：redis的「时间事件」结构体定义位于ae.h/aeTimeEvent

#define AE_NOMORE -1 
/**
 * 时间事件结构 
*/
/* Time event structure */
typedef struct aeTimeEvent {
    long long id; /* time event identifier. */ //事件id，全局唯一id，自增；

    //when_sec和when_ms 记录什么时候执行该事件；阅读 ae.c中processTimeEvents函数可以理解该两个属性的用途
    long when_sec; /* seconds */  // 记录事件到达的时间（秒）
    long when_ms; /* milliseconds */ //记录事件到达的时间（毫秒）（

    aeTimeProc *timeProc;
    aeEventFinalizerProc *finalizerProc; //最终执行的事件处理函数（ae.c中processTimeEvents函数中，可以看到该函数的调用逻辑）
    void *clientData;
    struct aeTimeEvent *prev;
    struct aeTimeEvent *next; //事件双向连表
} aeTimeEvent;

1. id是服务器为事件创建的全局唯一标志。从小到大自增；
2. when_sec和when_ms 记录什么时候执行该事件；阅读 ae.c/processTimeEvents函数可以理解该值作用；
3. timeProc就是「时间事件」的处理函数；
   如果处理函数返回非AE_NOMORE的整数值，表示该事件是周期性事件，服务器会根据返回值更新 when属性（也就是下次执行的时间）；
   
   如果处理函数返回AE_NOMORE，表示是定时事件，执行完毕后会删除该事件；

---

二 时间事件的实现

服务器将所有的「时间事件」放到一个无序链表中（ae.c/aeCreateTimeEvent）。

当事件执行时，会遍历所有链表，判断已经到达的事件，并执行相应的事件处理器；

执行代码位于ae.c/processTimeEvents

/**
 * 「时间事件」处理函数
 * 
 *  返回处理的「事件」的数量
*/
/* Process time events */
static int processTimeEvents(aeEventLoop *eventLoop) {
    int processed = 0;
    aeTimeEvent *te;
    long long maxId;
    time_t now = time(NULL); //获取当前时间戳

    /*
     * 如果系统时间被设置回了过去时间，那么设置lastTime属性为正确的时间，「时间事件」将会随机方式延时执行；
     * 这就意味着之前 预定的操作 不会及时的执行；
     *
     * 我们尝试发现系统时钟偏差，并且当发现时钟偏差时，强制所有「时间事件」尽快的执行；
     * 实践证明尽快处理 比 延时处理造成的危害更小；
    */
    /* If the system clock is moved to the future, and then set back to the
     * right value, time events may be delayed in a random way. Often this
     * means that scheduled operations will not be performed soon enough.
     *
     * Here we try to detect system clock skews, and force all the time
     * events to be processed ASAP when this happens: the idea is that
     * processing events earlier is less dangerous than delaying them
     * indefinitely, and practice suggests it is. */
    if (now < eventLoop->lastTime) {
        te = eventLoop->timeEventHead; 
        while(te) {
            te->when_sec = 0; //设置「时间事件」立即执行
            te = te->next;
        }
    }
    eventLoop->lastTime = now;//设置回当前时间

    //开始遍历「时间事件」链表，执行事件
    te = eventLoop->timeEventHead;
    maxId = eventLoop->timeEventNextId-1;
    while(te) {
        long now_sec, now_ms;
        long long id;

        //如果事件已经删除状态来，将事件从链表中移除
        /* Remove events scheduled for deletion. */
        if (te->id == AE_DELETED_EVENT_ID) {
            aeTimeEvent *next = te->next;
            if (te->prev)
                te->prev->next = te->next;
            else
                eventLoop->timeEventHead = te->next;
            if (te->next)
                te->next->prev = te->prev;
            if (te->finalizerProc) //执行终结处理函数
                te->finalizerProc(eventLoop, te->clientData);
            zfree(te);
            te = next;
            continue;
        }

        /**
         * 确保不处理 由当前「时间事件」创建的「时间事件」
         * 注意当前这个检查是无用的，因为新创建的「时间事件」总是添加到链表的头部；
         * 但是当修改了实现细节 也许这个判断会起作用，所以先留在这里；
        */
        /* Make sure we don't process time events created by time events in
         * this iteration. Note that this check is currently useless: we always
         * add new timers on the head, however if we change the implementation
         * detail, this check may be useful again: we keep it here for future
         * defense. */
        if (te->id > maxId) {
            te = te->next;
            continue;
        }

        aeGetTime(&now_sec, &now_ms); //获取当前的时间戳 秒 和 毫秒

        //当前时间已经超过了 「时间事件」的「执行时间点」，开始执行；
        if (now_sec > te->when_sec ||
            (now_sec == te->when_sec && now_ms >= te->when_ms))
        {
            int retval;

            id = te->id;
            retval = te->timeProc(eventLoop, id, te->clientData);
            processed++;
            if (retval != AE_NOMORE) { //返回非AE_NOMORE 说明是周期性事件，更新下次执行时间
                aeAddMillisecondsToNow(retval,&te->when_sec,&te->when_ms);
            } else { //否则删除事件
                te->id = AE_DELETED_EVENT_ID;
            }
        }
        te = te->next;
    }
    return processed;
}

目前redis版本只有一个时间事件:servce.c/serverCron;在benchmark模式下，也只使用了两个时间事件。所以无序链表并不会影响事件的执行性能；

---

三 事件的调度与执行

redis的事件调度函数位于ae.c/aeProcessEvents

/*
 * 事件调度函数
 * 处理每个挂起的「时间事件」，然后处理每一个挂起的「文件事件」；
 * 
 * 没有特殊的标记时 函数将休眠，直到触发「文件事件」 或是「时间事件」发生；
*/
/* Process every pending time event, then every pending file event
 * (that may be registered by time event callbacks just processed).
 * Without special flags the function sleeps until some file event
 * fires, or when the next time event occurs (if any).
 *
 * If flags is 0, the function does nothing and returns.
 * if flags has AE_ALL_EVENTS set, all the kind of events are processed.
 * if flags has AE_FILE_EVENTS set, file events are processed.
 * if flags has AE_TIME_EVENTS set, time events are processed.
 * if flags has AE_DONT_WAIT set the function returns ASAP until all
 * if flags has AE_CALL_AFTER_SLE EP set, the af tersleep callback is called.
 * the events that's possible to process without to wait are processed.
 *
 * The function returns the number of events processed. */
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 || //如果注册过文件事件
        ((flags & AE_TIME_EVENTS) && !(flags & AE_DONT_WAIT))) {
        int j;
        aeTimeEvent *shortest = NULL;
        struct timeval tv, *tvp;

        //查询最近要发生的「时间事件」
        if (flags & AE_TIME_EVENTS && !(flags & AE_DONT_WAIT))
            shortest = aeSearchNearestTimer(eventLoop);
        
        if (shortest) {
            long now_sec, now_ms;

            aeGetTime(&now_sec, &now_ms);
            tvp = &tv;

            //距离最近的「时间事件」还有多长时间触发；
            /* How many milliseconds we need to wait for the next
             * time event to fire? */
            long long ms =
                (shortest->when_sec - now_sec)*1000 +
                shortest->when_ms - now_ms;

            if (ms > 0) {
                tvp->tv_sec = ms/1000;
                tvp->tv_usec = (ms % 1000)*1000;
            } else {
                tvp->tv_sec = 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) { //立即执行「时间事件」
                tv.tv_sec = tv.tv_usec = 0;
                tvp = &tv;
            } else {
                /* Otherwise we can block */
                tvp = NULL; /* wait forever */
            }
        }

        //apiPoll的堵塞时间是由最近的「时间事件」来决定，必要最近的时间事件还有2s触发，则apiPoll就堵塞2s
        //这样可以避免服务器对「时间事件」频繁轮询；
        /* Call the multiplexing API, will return only on timeout or when
         * some event fires. */
        numevents = aeApiPoll(eventLoop, tvp);

        /* After sleep callback. */
        if (eventLoop->aftersleep != NULL && flags & AE_CALL_AFTER_SLEEP)
            eventLoop->aftersleep(eventLoop);

        //处理所有的文件事件
        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 fired = 0; /* Number of events fired for current fd. */

            /* Normally we execute the readable event first, and the writable
             * event laster. This is useful as sometimes we may be able
             * to serve the reply of a query immediately after processing the
             * query.
             *
             * However if AE_BARRIER is set in the mask, our application is
             * asking us to do the reverse: never fire the writable event
             * after the readable. In such a case, we invert the calls.
             * This is useful when, for instance, we want to do things
             * in the beforeSleep() hook, like fsynching a file to disk,
             * before replying to a client. */
            int invert = fe->mask & AE_BARRIER;

            /* 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.
             *
             * Fire the readable event if the call sequence is not
             * inverted. */
            if (!invert && fe->mask & mask & AE_READABLE) {
                fe->rfileProc(eventLoop,fd,fe->clientData,mask);
                fired++;
            }

            /* Fire the writable event. */
            if (fe->mask & mask & AE_WRITABLE) {
                if (!fired || fe->wfileProc != fe->rfileProc) {
                    fe->wfileProc(eventLoop,fd,fe->clientData,mask);
                    fired++;
                }
            }

            /* If we have to invert the call, fire the readable event now
             * after the writable one. */
            if (invert && fe->mask & mask & AE_READABLE) {
                if (!fired || fe->wfileProc != fe->rfileProc) {
                    fe->rfileProc(eventLoop,fd,fe->clientData,mask);
                    fired++;
                }
            }

            processed++;
        }
    }

    /*处理时间事件*/
    /* Check time events */
    if (flags & AE_TIME_EVENTS)
        processed += processTimeEvents(eventLoop);

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

1. aeApiPool函数最大的堵塞事件是由最近的「时间事件」来决定的；这样可以避免服务器对时间事件的频繁轮询；
2. 如果处理完一次「文件事件」后，还没有「时间事件」到达，服务器则再次执行「文件事件」直到「时间事件」的触发；

---

aePeocessEvents函数位于一个循环里；加上服务器初始化函数和清理函数，就构成redis服务器的主函数；

//servce.c/main
int main(int argc, char **argv) {
    /*
    ....
    读取配置
    ....
    */
    //初始化服务
    initServer();
    
    /*
    ........
    */

    //事件处理函数
    aeMain(server.el);
    
    /*
    .....
    */
    //清理
    aeDeleteEventLoop(server.el);

}