Can you use redis? Then come and learn about redis protocol

brief introduction

redis Is a very good software , It can be used as an in memory database or as a cache . Because of his excellent performance ,redis It is used in many occasions .

redis Is a client-side and server-side model , The client and server are through TCP Protocol to connect , The client sends the request data to the server , The server returns the request to the client . Such a request process is completed .

Of course, in the beginning , Because very few people use it , The system is not stable enough , adopt TCP The data transmitted by the protocol is not standardized . But more and more people are using it , In particular, it is hoped to develop software that is suitable for different languages and platforms redis Client time , It is necessary to consider the issue of compatibility .

At this time, the client and server need a unified interaction protocol , about redis For example, this general interaction protocol is called Redis serialization protocol(RESP).

RESP Is in Redis 1.2 Version of , And in Redis 2.0 And Redis Standard way of server communication .

That is to say , from Redis 2.0 after , It can be based on redis protocol The protocol developed its own redis The client .

redis Advanced usage of

Generally speaking ,redis The client side and the server side of the consist of a request - Patterns of response , That is, the client sends a request to the server , Then get the response result from the server .

The request and response are redis The simplest use of . be familiar with redis My friend may think of two redis Advanced usage of , These two usages are not traditional requests - Response patterns .

What are the two usages ?

The first is redis Support pipline, That is, pipeline operation , The advantage of the pipeline is redis The client can send multiple commands to the server at one time , Then wait for the server to return .

The second kind redis And support Pub/Sub, The broadcast model , In this case , It is not a request and response pattern , stay Pub/Sub Next , Switch to the server-side push mode .

Redis Medium pipline

Why use pipline Well ？

because redis Is a typical request response pattern , Let's take a common example incr An example of an order ：

Client: INCR X Server: 1 Client: INCR X Server: 2 Client: INCR X Server: 3 Client: INCR X Server: 4

In fact, the client just wants to get the final result , But every time the client needs to wait for the server to return the results , To send the next command . This will lead to a problem called RTT(Round Trip Time) A waste of time .

Although every time RTT It's not long , But it is also a very objective figure .

Can I send all the client commands to the server together ? This optimization is called Pipeline.

piepline This means that the client can continue to send commands to the server without receiving a return from the server .

The above command can be used pipline Rewrite as follows ：

(printf "INCR X\r\nINCR X\r\nINCR X\r\nINCR X\r\n"; sleep 1) | nc localhost 6379
:1
:2
:3
:4
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because redis Server support TCP Protocol to connect , So we can use nc Connected to the redis Execute the command in the server .

In the use of pipline There is one thing to pay attention to , because redis The server caches the requested results on the server side , wait until pipline After all the commands in the are executed, they will be returned uniformly , So if the server returns more data , Need to consider the problem of memory occupation .

that pipline Just to reduce RTT Do you ？

Friends who are familiar with the operating system may have heard of the concept of user space and operating system space , Read data from user input and then write it to system space , This involves a user space switch , stay IO In operation , Such space switching or copying is time-consuming , If you request and respond frequently , This will cause frequent space switching , Thus, the efficiency of the system is reduced .

Use pipline You can send multiple instructions at once , So as to effectively avoid space switching behavior .

Redis Medium Pub/Sub

and Pub/Sub The relevant order is SUBSCRIBE, UNSUBSCRIBE and PUBLISH.

Why use Pub/Sub Well ？ Its main purpose is decoupling , stay Pub/Sub The sender does not need to know the address of the receiver , Similarly, for the message receiver , There is no need to know the address of the specific message sender . They just need to know the associated topic .

subscribe and publish The command of is relatively simple , Let's give you an example , First, the client subscribe topic：

redis-cli -h 127.0.0.1
127.0.0.1:6379> subscribe topic
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "topic"
3) (integer) 1
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Then at another terminal , call publish command ：

redis-cli -h 127.0.0.1
127.0.0.1:6379> publish topic "what is your name?"
(integer) 1
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You can see that the client will receive the following message ：

1) "message"
2) "topic"
3) "what is your name?"
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RESP protocol

RESP The agreement has 5 Types , Namely imple Strings, Errors, Integers, Bulk Strings and Arrays.

Different types take the first in the message byte Distinguish , As shown below ：

type	first byte
Simple Strings	+
Errors	-
Integers	:
Bulk Strings	$
Arrays	*

protocol The different parts of the are marked with "\r\n" (CRLF) To make a difference .

Simple Strings

Simple Strings Is a simple string .

Usually used in server-side returns , The format of this message is "+" Plus text messages , Finally "\r\n" ending .

For example, the server returns OK, Then the corresponding message is ：

"+OK\r\n"
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The above message is a non binary secure message , If you want to send binary secure messages , You can use Bulk Strings.

What is a non binary secure message ？ about Simple Strings Come on , Because the message is "\r\n" ending , So the message cannot contain "\r\n" These two special characters , Otherwise, it will have the wrong meaning .

Bulk Strings

Bulk Strings It's binary safe . This is because Bulk Strings Contains a character length field , Because the character length is judged according to the length , Therefore, there is no disadvantage of judging whether the character ends according to a specific character in the character .

To be specific Bulk Strings The structure of is "$"+ String length +"\r\n"+ character string +"\r\n".

With OK For example , If the Bulk Strings To express , As follows :

"$2\r\nok\r\n"
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Bulk Strings It can also contain empty strings ：

"$0\r\n\r\n"
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Of course, it can also mean nonexistent Null value ：

"$-1\r\n"
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RESP Integers

This is a redis An integer in represents , The specific format is ":"+ Integers +"\r\n".

such as 18 This integer can be expressed in the following format ：

":18\r\n"
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RESP Arrays

redis Multiple commands of can be in the form of array To express , Multiple values returned by the server can also be used arrays To express .

RESP Arrays The format is "*"+ Number of elements in the array + Other similar data .

therefore RESP Arrays Is a composite structure of data . For example, an array contains two Bulk Strings:"redis","server" You can use the following format to represent ：

"*2\r\n$5\r\nredis\r\n$6\r\nserver\r\n"
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RESP Arrays Primitives in can not only use different types , It can also contain RESP Arrays, That is to say array Nesting of ：

"*3\r\n$5\r\nredis\r\n$6\r\nserver\r\n*1\r\n$4\r\ngood\r\n"
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To facilitate observation , Let's format the above message ：

"*3\r\n
$5\r\nredis\r\n
$6\r\nserver\r\n
*1\r\n
$4\r\ngood\r\n"
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The above message is an array of three elements , The first two elements are Bulk Strings, The last one is an array containing one element .

RESP Errors

Last ,RESP It can also represent an error message .RESP Errors The message format of is "-"+ character string , As shown below ：

"-Err something wrong\r\n"
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In general ,"-" The following first word indicates the error type , But this is just a conventional rule , Not at all RESP Mandatory requirements in the agreement .

in addition , After comparison , You may find that RESP Errors and Simple Strings Yes, the message format is similar .

This processing of different message types is differentiated on the client side .

Inline commands

If you press RESP Requirements of the agreement , When we connect to the server, we need to include RESP Define all the formats of messages in , But these messages contain additional message types and carriage return line breaks , So it would be confusing to use the protocol directly .

therefore redis Some inline commands are also provided , That is, a condensed version of the protocol command , This compact version removes message types and carriage return line breaks .

We use "get world" For example, this order . Let's take a look at the connections in different ways .

The first is to use redis-cli Connect ：

redis-cli -h 127.0.0.1
127.0.0.1:6379> get world
"hello"
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because redis-cli yes redis The client of , So you can use it directly inline command To execute an order .

If you use telnet, We can also use the same command to get the results ：

telnet 127.0.0.1 6379
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
get world
$5
hello
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You can see that the return result is "$5\r\nhello\r\n".

If you want to use protocol messages to request redis What should the server do ？

The order we want to ask is "get world", Turn it into RESP The message for is ：

"*2\r\n$3\r\nget\r\n$5\r\nworld\r\n"
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Let's try using the above command nc Pass on to redis server On ：

(printf "*2\r\n$3\r\nget\r\n$5\r\nworld\r\n"; sleep 1) |  nc localhost 6379
-ERR Protocol error: expected '$', got ' '
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I'm sorry we got ERR, Can't you use it directly RESP Message format for transmission ？ Of course not. , The problem is $ A symbol is a special character , We need to escape ：

(printf "*2\r\n\$3\r\nget\r\n\$5\r\nworld\r\n"; sleep 1) |  nc localhost 6379
$5
hello
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You can see the output and use it directly redis-cli Agreement .

summary

That's all RESP Basic contents of the protocol and examples of manual use , With RESP, We can then create the..., according to the format defined in the protocol redis client .

Maybe you will ask again , Why is it just redis The client ？ Is there an agreement redis The server side can also be created ？ The answer, of course, is yes , You only need to transmit messages according to the protocol . The main problem is redis The implementation of the server side is complex , It's not that easy .