Redis data structure (2) dictionary, redis Data Structure dictionary

Redis data structure (2) dictionary, redis Data Structure dictionary

The Redis dictionary is actually a Hash table. Its implementation is similar to the hashmap structure in JAVA. Key-Value pairs are stored in Key-Value mode, but there are some differences.
The hashmap structure in java contains the hash table and implements rehash self-expansion;
The redis dictionary implements the hash table through the dictht structure and the rehash through the dictionary (dict) (the dictionary contains a dictht array dictht ht [2]).

Implementation of Redis dictionary
Redis dictionaryHash table usedDefined by the dict. h/dictht structure:

typedef struct dictht{dictEntry **table;unsigned long size;unsigned long sizemask;unsigned long used;}dictht;

Table is an array of dictEntry structures. Each dictEntry structure stores a key-value pair. Size is the size of the table Array (note that it is not the number of key-value pairs); used is the number of key-value pairs; sizemask is size-1, the purpose will be mentioned later;
The dictEntry structure is defined as follows:

typedef struct dictEntry{void *key;union{void *val;uint64_t u64;int64_t s64;} v;struct dictEntry *next;}

The key is the key, and the v is the value. It can be defined as a pointer, A uint64_t integer, or an int64_t integer.
The next attribute points to the next dictEntry to form a linked list structure. In the dictionary structure, the hash value of each key-value is mapped to the subscript of the table. If the hash value of multiple keys is mapped to the same subscript of the table, then, these key-value pairs form a linked list through the next pointer and store it in the current subscript of the table.

In RedisDictionaryRepresented by the dict. h/dict structure:

typedef struct dict{dictType *type;void *privdata;dictht ht[2];int rehashidx;} dict;

Note:

• Dictht is the hash table structure described above;
• The type pointer is a dictType structure that stores some functions used to operate on key-value pairs of specific types. Redis sets different types of specific functions for different dictionaries;
• Privdata stores the optional parameters that need to be passed to the specific function of type;
  The dictType structure is as follows:

Typedef struct dictType {// calculate the hash value unsigned int (* hashFunction) (const void * key); // copy key void * (* keyDup) (void * privdata, const void * key); // copy the value void * (* valDup) (void * privdata, const void * obj); // compare the key int * (* keyCOmpare) (void * privdata, const void * key1, const void * key2); // destroy the key void (* keyDestructor) (void * privdata, void * key ); // destroy value void (* valDestructor) (void * privdata, void * obj);} dictType;

• The ht array contains two dictht structures. at ordinary times, only ht [0] is used, and ht [1] is used for rehash.
• Rehashidx is a flag. If rehash is not currently performed, the value is-1. redis performs rehash progressively. During rehash, each operation is performed, add 1 to the rehashidx value;

Redis Hash Algorithm

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