Introduction of TiDB Architecture
TiDB 架构介绍

Understanding TiDB talks about the architecture of TiDB, the modules it consists of, and the responsibility of each module.
《了解 TiDB》讲述了 TiDB 的架构、模块组成以及每个模块的职责。

TiDB Architecture TiDB 架构

When people refer to TiDB, they usually refer to the entire TiDB distributed database that includes three components: the TiDB stateless server, the Placement Driver (PD) server, and the storage server, TiKV or TiFlash. The TiDB server does not store data; it only computes and processes SQL queries. The PD server is the managing components of the entire cluster. The storage server is responsible for persistently storing data.
当人们提到 TiDB 时，通常指的是整个 TiDB 分布式数据库，它包括三个组件：TiDB 无状态服务器、放置驱动程序（PD）服务器和存储服务器 TiKV 或 TiFlash。TiDB 服务器不存储数据；它只计算和处理 SQL 查询。PD 服务器是整个集群的管理组件。存储服务器负责持久存储数据。

Let's see an architecture graph from TiDB stateless server's perspective.
让我们从 TiDB 无状态服务器的角度来看看架构图。

As you can see, TiDB is a SQL engine that supports the MySQL protocol with some kind of distributed KV storage engine that supports transactions as the underlying storage.
正如你所看到的，TiDB 是一个支持 MySQL 协议的 SQL 引擎，其底层存储是某种支持事务的分布式 KV 存储引擎。

Here come three significant questions.
这里有三个重要问题。

1. How to support MySQL protocol?
   如何支持 MySQL 协议？
2. How to communicate with storage engine, store and load data?
   如何与存储引擎通信、存储和加载数据？
3. How to implement SQL functions?
   如何实现 SQL 函数？

This section will start with a few of brief descriptions of what modules TiDB has and what they do, and then put them together to answer these three questions.
本节将首先简要介绍 TiDB 有哪些模块及其作用，然后将它们放在一起回答这三个问题。

Code Structure 代码结构

TiDB source code is fully hosted on Github, you can see all the information from the repository homepage. The whole repository is developed in Golang and divided into many packages according to functional modules.
TiDB 的源代码完全托管在 Github 上，你可以从 代码库主页查看所有信息。整个版本库使用 Golang 开发，并根据功能模块分为多个包。

Most of the packages export services in the form of interfaces, and most of the functionality is concentrated in one package. But there are packages that provide basic functionality and are dependent on many packages, so these packages need special attention.
大多数软件包以接口的形式输出服务，大部分功能都集中在一个软件包中。但有些软件包提供基本功能，并且依赖于许多软件包，因此这些软件包需要特别关注。

The main method of TiDB locates in tidb-server/main.go, which defines how the service is started.
TiDB 的主方法位于 tidb-server/main.go 中，它定义了服务的启动方式。

The build system of the entire project can be found in the Makefile.
整个项目的构建系统可在 Makefile 中找到。

In addition to the code, there are many test cases, which can be found with suffix _test.go. There is also toolkit under the cmd directory for doing performance tests or constructing test data.
除代码外，还有许多测试用例，可在后缀为 _test.go 的目录下找到。cmd 目录下还有工具包，用于进行性能测试或构建测试数据。

Module Structure 模块结构

TiDB has a number of modules. Table below is an overview that shows what each module does, and if you want to see the code for the relevant function, you can find the corresponding module directly.
TiDB 有许多模块。下表概述了每个模块的功能，如果你想查看相关功能的代码，可以直接找到相应的模块。

At a glance, TiDB has 80 packages, which might let you feel overwhelmed, but not all of them are important, and some features only involve a small number of packages, so where to start to look at the source code depends on the purpose of looking at the source code.
一眼望去，TiDB 有 80 个软件包，这可能会让你感到不知所措，但并非所有软件包都很重要，有些功能只涉及少量软件包，因此从哪里开始查看源代码取决于查看源代码的目的。

If you want to understand the implementation details of a specific feature, then you can refer to the module description above and just find the corresponding module.
如果您想了解某个特定功能的实现细节，可以参考上面的模块描述，找到相应的模块即可。

If you want to have a comprehensive understanding of the source code, then you can start from tidb-server/main.go and see how tidb-server starts and how it waits for and handles user requests. Then follow the code all the way through to see the exact execution of the SQL. There are also some important modules that need to be looked at to know how they are implemented. For the auxiliary modules, you can look at them selectively to get a general impression.
如果您想全面了解源代码，可以从 tidb-server/main.go 开始，查看 tidb-server 如何启动、如何等待和处理用户请求。然后跟随代码一路查看 SQL 的具体执行过程。还需要查看一些重要模块，以了解它们是如何实现的。对于辅助模块，可以有选择性地查看，以获得总体印象。

SQL Layer Architecture SQL 层架构

This is a detailed SQL layer architecture graph. You can read it from left to right.
这是一张详细的 SQL 层架构图。您可以从左到右阅读。

Protocol Layer 协议层

The leftmost is the Protocol Layer of TiDB, this is the interface to interact with Client, currently TiDB only supports MySQL protocol, the related code is in the server package.
最左边的是 TiDB 的协议层，这是与客户端交互的接口，目前 TiDB 只支持 MySQL 协议，相关代码位于 server 包中。

The purpose of this layer is to manage the client connection, parse MySQL commands and return the execution result. The specific implementation is according to MySQL protocol, you can refer to MySQL Client/Server Protocol document. If you need to use MySQL protocol parsing and processing functions in your project, you can refer to this module.
该层的目的是管理客户端连接、解析 MySQL 命令并返回执行结果。具体实现根据 MySQL 协议，可参考 MySQL 客户端/服务器协议文档。如果你需要在你的项目中使用 MySQL 协议解析和处理函数，你可以参考这个模块。

The logic for connection establishment is in the Run() method of server.go, mainly in the following two lines.
建立连接的逻辑在 server.go 的 Run() 方法中，主要在以下两行。

conn, err := s.listener.Accept()
clientConn := s.newConn(conn)
go s.onConn(clientConn)

The entry method for a single session processing command is to call the dispatch method of the clientConn class, where the protocol is parsed and passed to a different handler.
单个会话处理命令的入口方法是调用 clientConn 类的 dispatch 方法，在该方法中，协议将被解析并传递给不同的处理程序。

SQL Layer SQL 层

Generally speaking, a SQL statement needs to go through a series of processes:
一般来说，SQL 语句需要经过一系列过程：

1. syntax parsing 语法分析
2. validity verification 有效性验证
3. building query plan 建立查询计划
4. optimizing query plan 优化查询计划
5. generating executor according to plan
   按计划产生执行人
6. executing and returning results
   执行和返回结果

These processes locate at the following modules:
这些流程位于以下模块：

KV API Layer KV 应用程序接口层

TiDB relies on the underlying storage engine to store and load data. It does not rely on a specific storage engine (such as TiKV), but has some requirements for the storage engine, and any engine that meets these requirements can be used (TiKV is the most suitable one).
TiDB 依靠底层存储引擎来存储和加载数据。它不依赖于特定的存储引擎（如 TiKV），但对存储引擎有一些要求，任何满足这些要求的引擎都可以使用（TiKV 是最合适的引擎）。

The most basic requirement is "Key-Value engine with transactions and Golang driver". The more advanced requirement is "support for distributed computation interface", so that TiDB can push some computation requests down to the storage engine.
最基本的要求是 "带有事务和 Golang 驱动程序的键值引擎"。更高级的要求是 "支持分布式计算接口"，这样 TiDB 就能将一些计算请求推送给存储引擎。

These requirements can be found in the interfaces of the kv package, and the storage engine needs to provide a Golang driver that implements these interfaces, which TiDB then uses to manipulate the underlying data.
这些要求可以在 kv 包的接口中找到，存储引擎需要提供一个实现这些接口的 Golang 驱动程序，然后 TiDB 将使用它来操作底层数据。

As for the most basic requirement, these interfaces are related:
就最基本的要求而言，这些接口是相关的：

• Transaction: Basic manipulation of transaction
  Transaction: 交易的基本操作
• Receiver: Interface for reading data
  接收器：读取数据的接口
• Mutator: Interface for mutating data
  Mutator: 用于更改数据的接口
• Storage: Basic functionality provided by the driver
  存储：驱动程序提供的基本功能
• Snapshot: Basic manipulation of data snapshot
  Snapshot: 数据快照的基本操作
• Iterator: Result of Seek, used to iterate data
  迭代器：Seek的结果，用于迭代数据

With the above interfaces, you are able to do all the required operations on the data and complete all the SQL functions. However, for more efficient computing, we have also defined an advanced computing interface, which can focus on these three interfaces or structures:
通过上述界面，您可以对数据进行所有必要的操作，并完成所有 SQL 函数。不过，为了提高计算效率，我们还定义了高级计算界面，它可以集中使用这三个界面或结构：

• Client: Send request to storage engine
  客户端：向存储引擎发送请求
• Request: Payload of the request
  Request：请求的有效载荷
• Response: Abstraction of result
  Response：结果抽象

Summary 摘要

This section talks about the source structure of TiDB and the architecture of three significant components. More details will be described in the later sections.
本节将介绍 TiDB 的源结构和三个重要组件的架构。更多细节将在后面的章节中介绍。