Talk less and do more .

redis What exactly is the agreement ?
Redis The agreement is called RESP(redis Serialization protocol ) And redis The server communicates . It's for redis A set of serialization protocol designed .
This agreement is actually redis Of 1.2 We have the version , But here it is redis2.0 To become redis Standard of communication protocol .
RESP Characteristics of :

1.RESP You can serialize different data types , For example, integers , character string , array . There is also a specific type of error . The request is sent as an array of strings from the client to Redis The server , These strings represent the parameters of the command to execute .Redis Reply with command specific data types .

2.RESP It's binary secure , And there is no need to process batch data transferred from one process to another , Because it uses prefix length to transmit bulk data . be careful : The protocols outlined here are for clients only - Server communication .Redis
Cluster Use different binary protocols to exchange messages between nodes .

RESP Description of the agreement
RESP Supports serialization protocols for the following data types : Simple string , error , integer , Massive strings and arrays .
RESP stay Redis As a request - The way to respond to the protocol is as follows :
1. The client uses the command as a bulk string RESP Array sent to Redis The server .
2. The server implements the RESP Type one to reply .
stay RESP in , The type of some data depends on the first byte :
For simple strings , The first byte of the reply is “ +”
For errors , The first byte of the reply is “-”
For integers , The first byte of the reply is “:”
For batch strings , The first byte of the reply is “ $”
For arrays , The first byte of the reply is “ ”
stay RESP in , Different parts of the agreement are always *“ \ r \ n”(CRLF)** termination .

redis Communication protocol of ( supplement , Serialization protocol of data type ) A little boring can be ignored
We talked about it earlier resp There are five serialization protocols for data types . Now let's talk about these five meanings carefully .

RESP Simple string

Simple strings are encoded as follows : A plus sign , The following one cannot contain CR or LF Character string ( Line breaks are not allowed ), with CRLF termination ( Namely “ \ r \ n”).
Simple strings are used to transmit non binary secure strings with minimal overhead . for example , many Redis The command only answers when successful “

OK” To send binary secure strings , Please use RESP Batch string . When Redis When replying with a simple string , The client library should return a string to the caller , The string consists of ’+' The first character that follows until the end of the string , Not including the last CRLF byte .

RESP error

RESP Has a specific data type for the error . actually , Errors and RESP Simple strings are exactly the same , But the first character is a minus sign “-” Not a plus sign .RESP The real difference between a simple string in and an error in is , The client treats errors as exceptions , The string that makes up the error type is the error message itself .

The basic format is :
“-Error message\r\n”
Send an error reply only when an error occurs , for example , If you try to perform an operation on the wrong data type , Or the command doesn't exist, etc . When an error reply is received , Library client should throw an exception .
Here is an example of an error reply :
-ERR unknown command ‘foobar’
-WRONGTYPE Operation against a key holding the wrong kind of value
“-” First word after , Until the first space or newline character , Represents the type of error returned . It's just Redis Agreement on use , no RESP Part of the wrong format .

for example ,ERR It's a general error ,WRONGTYPE It's a more specific mistake , Indicates that the client attempted to perform an operation on the wrong data type . This is called the error prefix , Is a way to enable the client to understand the type of error returned by the server , It doesn't have to depend on the exact information given , The message may change over time .
The client implementation can return different kinds of exceptions for different errors , Or you can provide a common way to catch errors by providing the error name directly to the caller as a string .
however , This feature should not be considered critical , Because it's rarely useful , And limited client implementations may simply return common error conditions , for example false.

RESP integer

This type is just a CRLF Terminated string , It represents an integer , with “:” Bytes as prefix . for example ,“:0 \ r \ n” or “:1000 \ r
n” It's an integer reply .

many Redis Command return RESP integer , image INCR,LLEN and LASTSAVE.
The returned integer has no special meaning , It's just INCR Increment number of ,LASTSAVE Of UNIX Time and so on . however , Ensure that the returned integer is in the signed 64 Bit integer range .
In order to return true or false, It is also widely used Integer reply . for example , image EXISTS or SISMEMBER Such a command would be true return 1, by false return 0.
If the operation is actually performed , Other commands ( as SADD,SREM and SETNX) Will return 1, Otherwise return 0.
The following command will reply with an integer :SETNX,DEL,
EXISTS,INCR,INCRBY,DECR,DECRBY,DBSIZE,LASTSAVE,RENAMENX,MOVE,LLEN,SADD,SREM,SISMEMBER,SCARD.

RESP Bulk string

Use a large string to represent a single maximum length of 512 MB Binary security string for . Batch strings are encoded in the following way : “
$” byte , Followed by the number of bytes that make up the string ( Prefixed length ), from CRLF( Carriage return /r/n) termination .

Actual string data . final CRLF.

therefore , character string “ foobar” The code of the is as follows :
“$6\r\nfoobar\r\n”
When an empty string is just :
“KaTeX parse error: Can't use function '\r' in math mode at position 2:
0\̲r̲\n\r\n" You can also use RE…-1\r\n”
This is called Null Bulk String.
When the server uses Null Bulk String When replying , Client Library API An empty string should not be returned , Instead, return nil object . for example ,Ruby Library should return “
nil”, and C Library should return NULL( Or in the reply Set special flag in object ), And so on .

RESP array

The array type can be used by the client to send commands to the server , Similarly, when some commands combine elements back to the client , The array type is also used as the reply type . It uses “*”
start , It is followed by the number of returned elements , Then there was CRLF, Then there is the type of each element in the array .

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