Redis从入门到放弃系列(五) ZSet

本文例子基于：5.0.4 ZSet是Redis中一种比较复杂的数据结构,当存储大小在128之内且member得长度在64以下,其实现为zipList,超过为SkipList

忽然发现,到现在第五篇文章,还没有讲到zipList,然而前面例如Hash,List的篇章都涉及到了zipList的,后面会单独写一篇zipList的实现的~立Flag 请期待 【Redis从入门到放弃系列(外传) ZipList】

言归正传，首先让我们来看一下该如何在redis里面使用ZSet类型

//将一个或多个元素及其分数加入到有序集合里面
ZADD key [NX|XX] [CH] [INCR] score member [score member ...]

代码示例：

//添加元素
>zadd store 1000 xiaoming 2000 xiaoqiang 3000 xiaoyue
(integer) 3
//返回指定区间内的有序集合列表                        
> zrange store 0 -1 withscores    
1) "xiaoming"                                   
2) "1000"                                       
3) "xiaoqiang"                                  
4) "2000"                                       
5) "xiaoyue"                                    
6) "3000"                  
//返回有序集合的数量
>zcard store
(integer) 3  
//查看处于1000到2000的存款的人数
>zcount store 1000 2000 
(integer) 2  
//查询处于1000到2000的存款的人群
> ZRANGEBYSCORE store 1000 2000
1) "xiaoming"
2) "xiaoqiang"
//根据member查看当前排名
>zrank store xiaoming
(integer) 0 

至此,redis zset的用法先告一段落.

源码解析

按照惯例,先来一波zset的数据结构

/* ZSETs use a specialized version of Skiplists */
typedef struct zskiplistNode {
    sds ele;
    double score;
    struct zskiplistNode *backward;
    struct zskiplistLevel {
        struct zskiplistNode *forward;
        unsigned long span;
    } level[];
} zskiplistNode;

typedef struct zskiplist {
    struct zskiplistNode *header, *tail;
    unsigned long length;
    int level;
} zskiplist;

typedef struct zset {
    dict *dict;
    zskiplist *zsl;
} zset;

SkipList编码的有序集合底层是使用一个命为zset的结构体构成的,该结构体拥有两种数据类型,dict跟zskiplist。zskiplist按照score从小到大保存所有集合元素,dict则保存着member到score的映射关系,两个数据结构共用着相同元素的ele和score的内存。 zskiplist是一个双向链表,这是为了方便倒序方式获取一个范围内的元素。 关于跳跃链表的讲解请参考漫画算法：什么是跳跃表？

当我们在使用zadd key member的时候，redis是如何实现的呢？让我们来看一下源码:

/* Insert a new node in the skiplist. Assumes the element does not already
 * exist (up to the caller to enforce that). The skiplist takes ownership
 * of the passed SDS string 'ele'. */
zskiplistNode *zslInsert(zskiplist *zsl, double score, sds ele) {
    zskiplistNode *update[ZSKIPLIST_MAXLEVEL], *x;
    unsigned int rank[ZSKIPLIST_MAXLEVEL];
    int i, level;

    serverAssert(!isnan(score));
    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) {
        /* store rank that is crossed to reach the insert position */
        rank[i] = i == (zsl->level-1) ? 0 : rank[i+1];
        while (x->level[i].forward &&
                (x->level[i].forward->score < score ||
                    (x->level[i].forward->score == score &&
                    sdscmp(x->level[i].forward->ele,ele) < 0)))
        {
            rank[i] += x->level[i].span;
            x = x->level[i].forward;
        }
        update[i] = x;
    }
    /* we assume the element is not already inside, since we allow duplicated
     * scores, reinserting the same element should never happen since the
     * caller of zslInsert() should test in the hash table if the element is
     * already inside or not. */
    level = zslRandomLevel();
    if (level > zsl->level) {
        for (i = zsl->level; i < level; i++) {
            rank[i] = 0;
            update[i] = zsl->header;
            update[i]->level[i].span = zsl->length;
        }
        zsl->level = level;
    }
    x = zslCreateNode(level,score,ele);
    for (i = 0; i < level; i++) {
        x->level[i].forward = update[i]->level[i].forward;
        update[i]->level[i].forward = x;

        /* update span covered by update[i] as x is inserted here */
        x->level[i].span = update[i]->level[i].span - (rank[0] - rank[i]);
        update[i]->level[i].span = (rank[0] - rank[i]) + 1;
    }

    /* increment span for untouched levels */
    for (i = level; i < zsl->level; i++) {
        update[i]->level[i].span++;
    }

    x->backward = (update[0] == zsl->header) ? NULL : update[0];
    if (x->level[0].forward)
        x->level[0].forward->backward = x;
    else
        zsl->tail = x;
    zsl->length++;
    return x;
}

上面的流程我们用一张图来表示,如下所示:

当我们在使用zrank key member的时候,zset是怎么实现的呢？让我们一起来看一下源码

long zsetRank(robj *zobj, sds ele, int reverse) {
    unsigned long llen;
    unsigned long rank;

    llen = zsetLength(zobj);

    if (zobj->encoding == OBJ_ENCODING_ZIPLIST) {
        //忽略掉 zipList查找过程
    } else if (zobj->encoding == OBJ_ENCODING_SKIPLIST) {
        zset *zs = zobj->ptr;
        zskiplist *zsl = zs->zsl;
        dictEntry *de;
        double score;

        de = dictFind(zs->dict,ele);
        if (de != NULL) {
            score = *(double*)dictGetVal(de);
            rank = zslGetRank(zsl,score,ele);
            /* Existing elements always have a rank. */
            serverAssert(rank != 0);
            if (reverse)
                return llen-rank;
            else
                return rank-1;
        } else {
            return -1;
        }
    } else {
        serverPanic("Unknown sorted set encoding");
    }
}
/* Find the rank for an element by both score and key.
 * Returns 0 when the element cannot be found, rank otherwise.
 * Note that the rank is 1-based due to the span of zsl->header to the
 * first element. */
unsigned long zslGetRank(zskiplist *zsl, double score, sds ele) {
    zskiplistNode *x;
    unsigned long rank = 0;
    int i;

    x = zsl->header;
    for (i = zsl->level-1; i >= 0; i--) {
        while (x->level[i].forward &&
            (x->level[i].forward->score < score ||
                (x->level[i].forward->score == score &&
                sdscmp(x->level[i].forward->ele,ele) <= 0))) {
            rank += x->level[i].span;
            x = x->level[i].forward;
        }

        /* x might be equal to zsl->header, so test if obj is non-NULL */
        if (x->ele && sdscmp(x->ele,ele) == 0) {
            return rank;
        }
    }
    return 0;
}

其实查找的时候跟上面插入流程是有很多地方享受的,获取用户的排名是通过累加的span。

应用场景

1.排行榜

2.存储社交关系

3.滑动窗口应用