摘要

Redis是一款開源的高性能key-value數(shù)據(jù)庫(kù)，廣泛應(yīng)用于各種場(chǎng)景。在Redis中，數(shù)據(jù)類型（type）和編碼（encoding） 是非常重要的概念。本篇博客將詳細(xì)介紹Redis支持的數(shù)據(jù)類型以及相應(yīng)的編碼方式和底層實(shí)現(xiàn)原理。

要查看Redis某個(gè)key的內(nèi)部編碼，可以使用Redis命令OBJECT ENCODING key。其中，key是你想要查詢的鍵名。例如，如果你想要查詢名為mykey的鍵的內(nèi)部編碼，可以執(zhí)行以下命令：

127.0.0.1:6379> object encoding mykey  // 查看某個(gè)Redis鍵值的編碼

redisObject

在 Redis 中，redisObject 是 Redis 中最基本的數(shù)據(jù)結(jié)構(gòu)之一。redisObject 用于表示 Redis 中的鍵值對(duì)中的值，它可以是字符串、整數(shù)、列表、哈希表等任意一種 Redis 數(shù)據(jù)類型。

redisObject 的定義如下：

typedef struct redisObject {

    // 類型
    unsigned type:4;

    // 編碼方式
    unsigned encoding:4;

    // 引用計(jì)數(shù)
    int refcount;

    // 指向?qū)嶋H值的指針
    void *ptr;

} robj;

• type：表示 redisObject 的類型。

• encoding：表示 redisObject 的編碼方式。

• refcount：表示當(dāng)前 redisObject 被引用的次數(shù)。

• ptr： ptr字段則是一個(gè)指針，指向?qū)嶋H的 Redis 對(duì)象。

Redis源碼encoding取值有如下幾種：

#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 */

類型與編碼介紹

Redis支持五種主要的數(shù)據(jù)類型：字符串(string)、列表(list)、集合(set)、有序集合(sorted set)和哈希(hash)。每種數(shù)據(jù)類型都有對(duì)應(yīng)的編碼方式。

數(shù)據(jù)類型與編碼方式總覽如下：

String

String is the most basic data type in Redis, usually used to store text or binary data. Redis supports two encoding methods:

• In Redis, when a string can be represented as an integer, it will be converted to an integer and stored using int encoding. The advantages of int encoding are small storage space and high operating efficiency. The disadvantage is that it can only store integers and does not support string operations.

• embstr(embstr-encoded string): Save a string with a length less than 44 bytes. When a string is relatively short, use this encoding method to store it. Reduce memory usage.

• raw(raw-encoded string): Save strings longer than 44 bytes. When a string is relatively long, this encoding method is used to store it.

List

A list is an ordered collection of strings in which elements can be added, modified, and deleted. Redis supports three encoding methods:

• ziplist: Before Redis3.2 version, When the length of each string in the List is less than 64 bytes and the length of the string in the List is When the number of elements is less than 512, the List object uses ziplist encoding, and in other cases, linkedlist encoding is used. ziplist is a compact, compressed list structure that saves memory. Good for small lists.

• Linkedlist is a type of data structure that supports lists of any size through a linked list structure.. But its memory footprint will increase as the length of the list increases.

• quicklist: Introduced in Redis 3.2 version, quicklist is a list structure composed of multiple ziplists, which can not only ensure performance, but also save memory. Suitable for large lists.

Collection

A collection is a series of unordered string collections that support adding, deleting and querying elements. Redis supports two encoding methods:

• intset: When the elements in the collection are all integers, Redis will use intset encoding to store . The advantages of intset encoding are small storage space and high operating efficiency.

• hashtable: When the elements in the collection contain strings, Redis will use hashtable encoding to store . The advantage of hashtable encoding is that it can store elements of any type and supports string operations. The disadvantage is that the storage space is relatively large and the operating efficiency is relatively low.

Ordered Set

Unordered collection of strings will be assigned a score, so that they can be sorted according to the score and form an ordered set. Redis supports two encoding methods:

• ziplist: Save less than 128 elements and the size of all elements is less than 64 bytes. Use ziplist encoding , ziplist is a A compact, compressed list structure suitable for small ordered collections.

• skiplist: skiplist is a skip list structure that supports fast query and sorting. Suitable for large ordered collections.

Hash table

A hash table is a collection of key-value pairs, with each key associated with a value. Redis supports two encoding methods:

• ziplist: The string length of all key values ??stored in the hash object is less than 64 bytes and the number of key-value pairs is less than 512, Redis will use ziplist encoding to store. The advantages of the ziplist encoding method are small storage space and high operating efficiency. The disadvantage is that fast key lookup operations are not supported.

• In addition to the above conditions, Redis will use hash table encoding to store. The advantage of hashtable encoding is that it supports fast key lookup operations. The disadvantage is that the storage space is relatively large and the operating efficiency is relatively low.

The underlying principles of types and encoding

After understanding the data types and encoding methods supported by Redis, let's take a look at their underlying implementation principles.

Encoding conversion

In Redis, each key-value pair has a type identifier, which is used to indicate the data type stored in the key-value pair. When we operate on a key, Redis will encode the key-value pair based on the current encoding of the key and the encoding required for the operation.

For example, when we append content to a string, if the current encoding method of the string is raw, but the new content can be stored using embstr encoding, then Redis will encode the string The method is converted from raw to embstr.

Data structure

Redis uses a variety of classic data structures to implement various data types, not just limited to encoding methods. For example, Redis lists and hash tables are implemented using linked list structures. Skip List is an efficient data structure used in ordered collections.

These data structures have been carefully designed and optimized to meet application needs in various scenarios. For example, the linked list structure is suitable for situations where elements need to be frequently added and deleted, while the skip list structure is more suitable for sorting and search operations.

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