redis数据结构之RedisObject详解

Posted 2022-05-24 _瞳孔

Redis中的任意数据类型的键和值都会被封装为一个RedisObject，也叫做Redis对象，源码如下:

typedef struct redisObject 
    unsigned type:4;
    unsigned encoding:4;
    unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
                            * LFU data (least significant 8 bits frequency
                            * and most significant 16 bits access time). */
    int refcount;
    void *ptr;
 robj;

各成员介绍如下：

• type：对象类型，分别是string、hash、list、set和zset，占4个bit位
• encoding: 底层编码方式，共有11种，占4个bit位
• lru：lru表示该对象最后一次被访问的时间，其占用24个bit位。便于判断空闲时间太久的key
• refcount：对象引用计数器，redis对象被创建完后，每被引用一次，则refcount加一，每释放一次引用则减一，计数器为0则说明对象无人引用，可以被回收
• ptr：指针，指向存放实际数据的空间

其中redis的对象类型如下：

/* A redis object, that is a type able to hold a string / list / set */

#define OBJ_STRING 0    /* String object. */
#define OBJ_LIST 1      /* List object. */
#define OBJ_SET 2       /* Set object. */
#define OBJ_ZSET 3      /* Sorted set object. */
#define OBJ_HASH 4      /* Hash object. */

/* The "module" object type is a special one that signals that the object
 * is one directly managed by a Redis module. In this case the value points
 * to a moduleValue struct, which contains the object value (which is only
 * handled by the module itself) and the RedisModuleType struct which lists
 * function pointers in order to serialize, deserialize, AOF-rewrite and
 * free the object.
 *
 * Inside the RDB file, module types are encoded as OBJ_MODULE followed
 * by a 64 bit module type ID, which has a 54 bits module-specific signature
 * in order to dispatch the loading to the right module, plus a 10 bits
 * encoding version. */
#define OBJ_MODULE 5    /* Module object. */
#define OBJ_STREAM 6    /* Stream object. */

/* Extract encver / signature from a module type ID. */
#define REDISMODULE_TYPE_ENCVER_BITS 10
#define REDISMODULE_TYPE_ENCVER_MASK ((1<<REDISMODULE_TYPE_ENCVER_BITS)-1)
#define REDISMODULE_TYPE_ENCVER(id) (id & REDISMODULE_TYPE_ENCVER_MASK)
#define REDISMODULE_TYPE_SIGN(id) ((id & ~((uint64_t)REDISMODULE_TYPE_ENCVER_MASK)) >>REDISMODULE_TYPE_ENCVER_BITS)

其中encoding的编码方式具体为如下：

/* Objects encoding. Some kind of objects like Strings and Hashes can be
 * internally represented in multiple ways. The 'encoding' field of the object
 * is set to one of this fields for this object. */
#define OBJ_ENCODING_RAW 0     /* Raw representation */
#define OBJ_ENCODING_INT 1     /* Encoded as integer */
#define OBJ_ENCODING_HT 2      /* Encoded as hash table */
#define OBJ_ENCODING_ZIPMAP 3  /* Encoded as zipmap */
#define OBJ_ENCODING_LINKEDLIST 4 /* No longer used: old list encoding. */
#define OBJ_ENCODING_ZIPLIST 5 /* Encoded as ziplist */
#define OBJ_ENCODING_INTSET 6  /* Encoded as intset */
#define OBJ_ENCODING_SKIPLIST 7  /* Encoded as skiplist */
#define OBJ_ENCODING_EMBSTR 8  /* Embedded sds string encoding */
#define OBJ_ENCODING_QUICKLIST 9 /* Encoded as linked list of ziplists */
#define OBJ_ENCODING_STREAM 10 /* Encoded as a radix tree of listpacks */

翻译成中文就是这样：

一个type占4bit，encoding占4bit，lru占24bit，refcount占4B，ptr占8B，因此redis对象的头部需要占16B

因此，加入redis存入十个字符串，那么需要存入十个redis对象头和十个SDS，160B就被浪费掉了，但如果十个字符串弄成list存进去，就可以节省144B空间，因此有大量数据存储的时候，往往推荐使用list等类型保存而不是字符串。

redis会根据存储的数据类型不同，选择不同的编码方式，每种数据类型使用的编码方式如下：

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