Redis数据结构
dict 字典（dict）是redis里最核心的数据结构，正如其全称Remote Dictionary Service所说，redis其实就是一个字典服务，字典以key、value的形式呈现给用户，key是简单的字符串，而value可以是各种数据结构，比如字符串（string）、链表（list）、集合（set）
dict
字典（dict）是redis里最核心的数据结构，正如其全称Remote Dictionary Service所说，redis其实就是一个字典服务，字典以key、value的形式呈现给用户，key是简单的字符串，而value可以是各种数据结构，比如字符串（string）、链表（list）、集合（set）、排序集合（zset）、哈希表（hash）等。
redis里dict的实现也比较简单，通过链表来解决哈希冲突，值得一提的是dict的动态扩展能力，当dict里元素不断增加时，其会扩大哈希桶数，然后对现有元素进行rehash，重新分布到对应的桶上，但dict的rehash并不是一次性完成的，这样会导致当dict里元素较多时，rehash耗时较长，导致服务阻塞。
typedef struct dict {
dictType *type;
void *privdata;
dictht ht[2];
int rehashidx; /* rehashing not in progress if rehashidx == -1 */
int iterators; /* number of iterators currently running */
} dict;
dict创建时，会初始化ht[0]，插入和访问都通过ht[0]来完成，当需要rehash时，创建一个新的dict ht[1]，并以桶为单位将ht[0]里的元素逐步迁移到新的ht[1]上（迁移会在访问dict时触发，也可以配置redis主动在后台周期性的迁移）。所以当访问dict时，如果正在进行rehash，就必须先后查询ht[0], ht[1]，因为被访问的元素可能已经迁到新的ht[1]上；当所有的元素都迁移到ht[1]后，用ht[1]代替ht[0]，并释放掉原来的ht[0]。
dict是通用的数据结构，采用void*来存储任意类型的对象，由于需求多变，比如有的场景需要在插入元素时重新分配内存，而有的场景直接存储指针，有的场景需要定制对象比较的方式，redis为解决这个问题，定义了一系列的接口，用户可以根据需求在创建dict时指定。
typedef struct dictType {
unsigned int (*hashFunction)(const void *key); // key的hash方法
void *(*keyDup)(void *privdata, const void *key); // 插入元素时，复制key对象的方法
void *(*valDup)(void *privdata, const void *obj); // 插入元素时，复制value对象的方法
int (*keyCompare)(void *privdata, const void *key1, const void *key2); // key比较的方法
void (*keyDestructor)(void *privdata, void *key); // 删除元素时，释放key的方法
void (*valDestructor)(void *privdata, void *obj); // 删除元素时，释放value的方法
} dictType;
string
redis所有的key都是string类型，redis的是基于c string的一个封装，在字符串的头部存储了长度信息（strlen的复杂度为O(1)）,并且支持动态扩展等特性。
struct sdshdr {
unsigned int len;
unsigned int free;
char buf[];
};
typedef char *sds;
redis的sds类型其实就是char*，redis创建一个string时，返回的是buf的指针，通过这个指针，就可以计算出sdshdr的位置，来访问到len、free等字段。
list
redis的list实现是一个双向链表，与dict类似，list也可以定制对象对比、析构等方法；list在头结点会存储链表的长度，使得获取list长度的复杂度为O(1)；头结点还存储了list头部、尾部节点的指针，使得可以从两个方向遍历list。
typedef struct listNode {
struct listNode *prev;
struct listNode *next;
void *value;
} listNode;
typedef struct list {
listNode *head;
listNode *tail;
void *(*dup)(void *ptr); // 复制链表节点value的方法
void (*free)(void *ptr); // 释放链表节点value的方法
int (*match)(void *ptr, void *key); // 链表节点对比方法
unsigned long len;
} list;
ziplist
双向链表的的最大问题在于头尾指针的开销，64bit OS上，每个节点有16bytes的额外开销，为了节省内存，当list里的元素较少并且大小较小时，redis采用ziplist的格式来实现链表。
ziplist实际上是二进制的形式组织链表，"二进制数据"的存储链表头部，记录总长度，头尾的位置等信息，然后每个节点除了存储节点内容外，还记录自身的长度，以及上一个节点的长度，这样通过遍历"二进制数据"就能访问到ziplist里所有的元素。另外，ziplist节点的长度、以及上一个节点的长度通过变长整数编码，进一步节省内存。
set
set代表一个集合（元素不重复），集合在redis里实现为字典（字典的value为NULL）；如果set里所有的元素都是整数时，redis会以intset的形式存储，intset是一个排序的整形数组，为节省内存，当intset里元素值在[INT16_MIN, INT16_MAX]之间时，每个数组元素占2个字节；当inset里元素值都在[INT32_MIN, INT32_MAX]之间时，每个数组元素占4个字节；否则每个元素占8个字节。
zset
zset是排序集合，与set不同的时，zset里每个元素有一个score（double类型），作为排序的标准；redis里zset默认通过一个dict和一个ziplist来实现，dict用于维护set元素到score的映射关系，所有的元素都追加到一个skiplist来保证其排序。当zset里元素较少并且大小较小时，zset也可以ziplist的形式存储，zset的元素以及对应的score会作为ziplist的两个连续节点存储。
hash
redis的hash是维护filed==>value的映射表，hash的默认实现也是dict，以通过field快速找到value；当hahs里元素较少并且大小较小时，hash也以ziplist的形式存储以节省内存。
The post Redis数据结构 appeared first on Yun Notes.
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