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String object

String data type is the most commonly used type in Redis, its keys and values are strings, it is very easy to use. Although the values of string data types are collectively referred to as strings, the appropriate encoding is automatically selected according to the different values in the actual storage. There are three kinds of encoding for string objects: int, raw, and embstr.

Redis object

Redis uses a unified data structure to represent an object, which is defined as follows:

Typedef struct redisObject {unsigned type:4; unsigned encoding:4; / / use the LRU algorithm to clear the object 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) in memory when the memory exceeds the limit. * / / the number of references to the object int refcount; / / the value pointer of the object void * ptr;} robj

The type field represents the type of object, and there are 7 values:

/ * A redis object, that is a type able to hold a string / list / set * / / * The actual Redis Object * / # define OBJ_STRING 0 / * string object. * / # define OBJ_LIST 1 / * list object. * / # define OBJ_SET 2 / * Collection object. * / # define OBJ_ZSET 3 / * ordered collection objects. * / # 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. , /

Then there is the encoding field, which represents the actual encoding type of the object value. There are a total of 11 values:

/ * 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 / * simple dynamic string * / # define OBJ_ENCODING_INT 1 / * long type integer * / # define OBJ_ENCODING_HT 2 / * dictionary * / # define OBJ_ENCODING_ZIPMAP 3 / * compress dictionary * / # define OBJ_ENCODING_LINKEDLIST 4 / * old lists that are no longer used, use double-ended linked lists. * / # define OBJ_ENCODING_ZIPLIST 5 / * compressed list * / # define OBJ_ENCODING_INTSET 6 / * Collection of integers * / # define OBJ_ENCODING_SKIPLIST 7 / * Jump Table and Dictionary * / # define OBJ_ENCODING_EMBSTR 8 / * simple dynamic string encoded by embstr * / # define OBJ_ENCODING_QUICKLIST 9 / * list encoded as ziplist * / # define OBJ_ENCODING_STREAM 10 / * encoded as cardinality tree of listpacks * /

It has been mentioned earlier that string objects use only three kinds of encoding: integers of type long, simple dynamic strings, and simple dynamic strings encoded by embstr.

OBJ_ENCODING_INT

When the value of a string object is an integer and can be represented by long, the encoding of the string object is OBJ_ENCODING_INT encoding.

As you can see, when the value is very large, it is stored in OBJ_ENCODING_RAW.

OBJ_ENCODING_RAW

When the value of a string object is a string and its length is greater than 44 bytes, the encoding of the string object will be OBJ_ENCODING_RAW encoding. The specific structure is shown below.

OBJ_ENCODING_EMBSTR

When the value of a string object is a string and its length is less than or equal to 44 bytes, the encoding of the string object will be OBJ_ENCODING_EMBSTR encoding. The main differences between OBJ_ENCODING_EMBSTR coding and OBJ_ENCODING_RAW coding are as follows:

OBJ_ENCODING_RAW-encoded objects are allocated twice when memory is allocated, creating redisObject objects and SDS objects, respectively. The OBJ_ENCODING_EMBSTR code is assigned at once. Similarly, OBJ_ENCODING_RAW-encoded objects need to be freed twice and OBJ_ENCODING_EMBSTR-encoded once. OBJ_ENCODING_EMBSTR-encoded data is stored in contiguous memory, while OBJ_ENCODING_RAW encoding is not. / * Create a string object with EMBSTR encoding if it is smaller than * OBJ_ENCODING_EMBSTR_SIZE_LIMIT, otherwise the RAW encoding is * used. * * The current limit of 44 is chosen so that the biggest string object * we allocate as EMBSTR will still fit into the 64 byte arena of jemalloc. * / # define OBJ_ENCODING_EMBSTR_SIZE_LIMIT 44robj * createStringObject (const char * ptr, size_t len) {if (len)

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