Quickly understand what the compressed list in redis is

Introduction to compressed list

Compressed list （ziplist） It is a sequential data structure composed of a continuous memory . A compressed list can contain any number of nodes , A byte array or integer value can be saved on each node . It is Redis Developed to save memory space .

Compressed list （ziplist） It's hash （hash） And ordered set （zset） One of the internal codes of . When hash （hash） When the number of elements in is relatively small and the value of each element takes up less space ,Redis The compressed list will be used as the internal encoding of the hash . When ordered set （zset） When the number of elements in is relatively small and the value of each element takes up less space ,Redis Compressed lists are also used as the internal encoding of ordered sets .

Compressed list structure

Next , Let's look at the internal structure of the following compressed list , The compressed list consists of the following parts ：

• zlbytes： Indicates the number of memory bytes occupied by the whole compressed list .
• xltail： Indicates the number of bytes from the starting address of the compressed list to the last node , You can quickly find the last node .
• zllength： Indicates the number of nodes contained in the compressed list .
• entries： The node list , Compact storage one by one .
• zlend： Special values 0xFF（ Decimal system 255）, Indicates the end of the compressed list .

Compress list node structure

The node of each compressed list consists of three parts ： prevlen、 encoding and content.

prevlen

prevlen： Indicates the byte length of the previous node of the node . prevlen The length of may be 1 Bytes , It could be 5 Bytes .

The length of the current node is less than 254 Bytes , prevlen The length of is 1 Bytes , Directly store the byte length of the previous node ;

The length of the current node is greater than or equal to 254 Bytes , prevlen The length of is 5 Bytes , The first byte is set to 0xFE, The next four bytes save the byte length of the previous node .

Can pass prevlen And... In the compressed list structure xltail Traverse the compressed list in reverse order .

encoding

encoding Indicates the type and length of data saved in this node .

When encoding The highest order of 00 At the beginning , Indicates that the maximum length is 63 Short string of , here encoding The length of is 1 Bytes , Behind it 6 Bits represent the byte length of the string ;

When encoding The highest order of 01 At the beginning , Indicates that the maximum length is 16383 Medium length string , here encoding The length of is 2 Bytes , Behind it 14 Bits represent the byte length of the string ;

When encoding The highest order of 10 At the beginning , Indicates that the maximum length is 4294967295 Special long string , here encoding The length of is 5 Bytes , Behind it 4 Bytes represent the byte length of the string ;

When encoding The highest order of 11 At the beginning , Represents integer value , here encoding The length of is 1 Bytes , Behind it 6 Bits indicate the type and length of the integer value .

content

content The value used for the storage node , The value of a node can be an array of bytes , It can also be a positive number , Its type and length are determined by encoding decision .

summary

Compressed list （ziplist） It is a sequential data structure composed of a continuous memory .

A compressed list can contain any number of nodes , A byte array or integer value can be saved on each node .

Compressed list （ziplist） It's hash （hash） And ordered set （zset） One of the internal codes of .

---

I've seen it here , You and I must be predestined friends , Leave your give the thumbs-up and Focus on , It will become a great thing in the future .