Simple understanding of RPC

This article mainly wants to make a record , Mainly reprinted from ：

RPC The implementation of the -Aquester-ChinaUnix Blog

1.  Preface

RPC Its full name is Remote Procedure Call, That is, far procedure call . without RPC, Then the process communication between machines usually takes the form of message （ Network communication tcp perhaps udp）, This reduces development efficiency , It will also increase the coupling between the network layer and the upper layer ,RPC Can help us solve these problems .

In fact, it encapsulates the implementation details of network communication , Not coupled with specific communication , No need to write the code of network communication , Naturally, development efficiency will be improved . There are two main ways I use in my work rpc Namely    Zeroc  Of ice, as well as google Of grpc.

2.  Basic concepts

Before the formal explanation , Let's introduce the relationship between RPC Basic concepts about ：

2.1. IDL

IDL The full name is Interface Definition Language, Interface definition language （ Sometimes called interface description language ）. because RPC Usually cross process 、 Cross machine 、 Cross system and cross language ,IDL To solve this problem , It has nothing to do with language , Translate it into different programming languages with the help of compiler .

Google Open source ProtoBuf Medium “.proto” A document is one IDL file ：

message Person {
  required int32 id = 1;
  required string name = 2;
  optional string email = 3;
}

stay .proto In file message Be similar to C In language struct Of , convert to C++ After language , It corresponds to C++ A class in . of ProtoBuf For more information , May refer to ：.

Please note that ,IDL Data types in （ Such as ProtoBuf Medium int32） Is independent of any language , But it usually has a mapping relationship with the data types in the target language , Otherwise, you will not be able to IDL Compile the file into the target language file .

2.2.  agent （Proxy）

agent （Proxy） yes RPC The client implementation of , Client code always communicates with... Through a proxy RPC Server communication .Proxy The code is completely composed of IDL Compiler generation .

2.3.  stub （Stub）

stub （Stub） yes RPC Server implementation of . On the server , Need to achieve IDL The interface defined in the file ; On the client side, you can use . The relationship between proxy and stub is shown in the figure below ：

  Here I'll explain a little bit ,Proxy, Agency is generally what we call positive agency , It proxy server . And that stub Stub, Generally speaking, it means Left manuscript , The manuscript must be on the server , Then we need to realize on the server side Stub The method in , because Stub The framework is also written according to you IDL Language generated , And users should realize Implement This class , We usually use this when we use it   Implement Pointer form of , Because we may not necessarily achieve Stub All interfaces of , So maybe we can't instantiate objects . Then no matter which framework , We all have to put our   Implement The pointer of is registered to this   Stub Or the bottom , Because if the bottom layer receives data , You must trigger polymorphism through pointers to call the methods you implement .

3.  Three elements

To achieve a complete RPC, Three things need to be done , Here we call these three things three elements ：

 

3.1.  Network communication

Be responsible for sending the client's request to the server , And sending the response of the server back to the client . This is a familiar piece , It is mainly the implementation of high-performance network program .

3.2.  Message codec

IDL Interface defined in 、 Functions and data, etc , It needs to be encoded into a byte stream before sending , Decode after receiving . For example, the function name 、 The parameter type and parameter value are encoded into byte stream , Then send it to the opposite end , Then decode the end , Revert to function call .ProtoBuf Is a very good codec tool .

Please note that IDL All data types supported are required to be codec ,IDL The compiler needs to know how to encode it into a byte stream , And decoding and restoring from the byte stream .

3.3. IDL compiler

For most people , This job is strange , Because there is not much contact with daily development . And because of that , Most people will find it difficult to be profound . In fact, as long as we overcome psychological obstacles , Learning it is much easier than expected . To make a long story short , It's not hard to get started , Especially after reading this article . But if you need to implement something like Hive or GCC East east , That's quite difficult , Among them, optimizing the syntax tree is a very big challenge .

In order to be able to use RPC, Need to put IDL Compile the file into the specified language code .ProtoBuf In fact, this function has been realized . If based on ProtoBuf Achieve one RPC, Then this IDL The compilation step can be skipped , Will only need to realize network communication , And realize google::protobuf::RpcController and google::protobuf::RpcChannel Two interfaces .

This article is to introduce RPC The implementation of the , The goal is to enable readers to achieve a set of RPC, For the three elements of network communication and message encoding and decoding , It's easy for ordinary people to understand and get started , But for the IDL Compiling this piece is relatively strange . So , The rest of this article will focus on IDL Implementation of compilation , All implementations will be based on Flex and Bison Two open source tools , Of course, you can also use JavaCC、SableCC and Antlr（ANother Tool for Language Recognition） etc. .