Interviewer: what is the internal implementation of strings in redis?

While waiting in the interview room , It feels so warm , My cold rental house has to be covered with two layers of quilt to sleep . I was about to take off my coat , I suddenly heard footsteps outside the door , The door is then opened , The young man with clean clothes and beautiful face came in , A faint fragrance of men's perfume comes to us. .

interviewer ：Redis What are the basic data types in ？

I ：Redis The basic data types of are ： character string （string）、 Hash （hash）、 list （list）、 aggregate （set）、 Ordered set （zset）.

interviewer ： What is the internal implementation of string type ？

I was also immersed in the complacency of the last question , Suddenly, his expression solidified , The palms began to sweat .“ This .. Not much in-depth understanding ”, I hesitated .

interviewer ： Go back and wait for the news .

This sentence is clear , And then there's no then . Failure is the mother of success , I'm not discouraged , Decided to mend it right away .

Type and code

First , Understand what is type ？ What is? code ？ stay Redis Objects are used in to represent keys and values in memory . Each object consists of a redisObject The structure represents , There are three properties ： type （type）、 code （encoding）、 Pointer to specific data （ptr）.

We usually say string 、 Hash 、 list 、 aggregate 、 Ordered sets are redisObject Medium type , In fact, for each data structure Redis There are various internal coding implementations at the bottom of the system , This is to select the appropriate internal code in the appropriate scene , To achieve a balance between memory space and processing efficiency , This may be the golden mean .

In the interview , Often asked about the internal implementation 、 Internal construction 、 internals , Generally speaking, it means redisObject Medium code .

String encoding

There are three types of encoding for string types ：

• int：8 A long integer of bytes .
• embstr： Less than or equal to 44 A byte string .
• raw： Greater than 44 A byte string .

stay 3.2 After the version ,embstr and raw Change into 44 Byte is the boundary , Before it was with 39 Byte is the boundary . The newer version shall prevail here .

To verify and understand , We use object encoding Command to check the internal code .

The effect of integer type is as follows ：

Log in and copy

127.0.0.1:6379> set one-more-num 1
OK
127.0.0.1:6379> object encoding one-more-num
"int"

     

      
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The effect of short string type is as follows ：

Log in and copy

127.0.0.1:6379> set one-more-str  Wanmao society 
OK
127.0.0.1:6379> object encoding one-more-str
"embstr"

     

      
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The effect of long string type is as follows ：

Log in and copy

127.0.0.1:6379> set one-more-str  Wanmao society | Wanmao society | Wanmao society | Wanmao society | Wanmao society | Wanmao society | Wanmao society | Wanmao society 
OK
127.0.0.1:6379> object encoding one-more-str
"raw"

     

      
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Of course , I haven't fully understood the above details yet “ conquer ” interviewer , We need to go deeper :)

Simple dynamic string

stay C In language , A string is an array of characters ending with a null character . stay Redis There is no direct use of C Language string , Instead, a simple dynamic string is defined （Simple Dynamic String,SDS） Structure , And take it as Redis The default string represents .

Simple dynamic strings have three properties ：

• len： Record buf Number of bytes used in the character array
• free： Record buf The number of bytes used in the character array
• buf[]： A character array , To hold strings

In order to understand , Let's take an example ：

Log in and copy

127.0.0.1:6379> set one-more-str OneMore
OK

     

      
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that , The corresponding simple dynamic string is like this ：

among ,len by 7, Indicates that a simple dynamic string is saved 7 A byte string ;free by 0, Indicates that this simple dynamic string does not allocate unused space ;buf It's an array of characters , An array of former 7 Each byte holds O、n、e、M、o、r、e character , The last byte is a null character \0.

be relative to C Language string , What are the benefits of simple dynamic strings ？

• The time complexity of getting the string length is O(1).
• Can save byte array , Support secure binary data storage .
• The pre allocation mechanism of memory space is implemented internally , Reduce the number of memory space allocations .
• The internal implementation of the inert deletion mechanism , Memory is not released after string reduction , As pre allocated space .
• API Is safe , No buffer overflow .

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Interviewer, you'll see , You don't care about me today , Tomorrow I'll keep you up , Ha ha ha ...

reference ：
《Redis Design and implementation 》
《Redis Development and operations 》
《Redis Deep Adventure ： Core principles and application practice 》

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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 .