golang go-sql-driver/mysql基本原理

亦凉 2022-11-21 14:41 193阅读 0赞

### 目录 ###

*  1.驱动注册
 *  2.打开一个database
 *  3.查询
 *  4.参考

上篇文章[关于golang database\_sql 包][golang database_sql]讲述了 `database/sql` 的整体设计框架。

本文简要介绍`go-sql-driver/mysql`的调用关系，是如何与`database/sql`关联起来的，包括从驱动注册到具体查询，每个步骤的底层调用。

我们查询MySQL的大体代码demo如下：

package main
    import (
            "database/sql"
            "log"
            _ "github.com/go-sql-driver/mysql"
    )
    
    var DB *sql.DB
    var dataBase = "root:Aa123456@tcp(127.0.0.1:3306)/?loc=Local&parseTime=true"
    
    func main() {
            var err error
            DB, err = sql.Open("mysql", dataBase)
            if err != nil {
                    log.Fatalln("open db fail:", err)
            }
    
            var connection_id int
            err := DB.QueryRow("select CONNECTION_ID()").Scan(&connection_id)
            if err != nil {
                    log.Println("query connection id failed:", err)
                    return
            }
    
            log.Println("connection id:", connection_id)
    }

从上面的代码可以看到，  
要连接MySQL，首先是注册MySQL驱动 `go-sql-driver/mysql`。

# 1.驱动注册 #

驱动注册代码：

func init() { 
        	sql.Register("mysql", &MySQLDriver{ })
    }

位置：`github.com/go-sql-driver/mysql/driver.go`。

上面的demo中，却没有看到这个操作。  
那是什么时候做的呢？

答案是import 的时候：

import (
    	_ "github.com/go-sql-driver/mysql"
    )

接着看下Register 函数的实现：

// Register makes a database driver available by the provided name.
    // If Register is called twice with the same name or if driver is nil,
    // it panics.
    func Register(name string, driver driver.Driver) { 
    	driversMu.Lock()
    	defer driversMu.Unlock()
    	if driver == nil { 
    		panic("sql: Register driver is nil")
    	}
    	if _, dup := drivers[name]; dup { 
    		panic("sql: Register called twice for driver " + name)
    	}
    	drivers[name] = driver
    }

其中，drivers是一个map，定义为：

var (
    	drivers   = make(map[string]driver.Driver)
    )

也就是说，  
`Register`只是将`MySQLDriver`放到一个map中，key为`mysql`。

`Register` 的第二个参数类型是`driver.Driver`，是一个接口类型。

接口定义如下：

// Driver is the interface that must be implemented by a database
    // driver.
    //
    // Database drivers may implement DriverContext for access
    // to contexts and to parse the name only once for a pool of connections,
    // instead of once per connection.
    type Driver interface { 
    	// Open returns a new connection to the database.
    	// The name is a string in a driver-specific format.
    	//
    	// Open may return a cached connection (one previously
    	// closed), but doing so is unnecessary; the sql package
    	// maintains a pool of idle connections for efficient re-use.
    	//
    	// The returned connection is only used by one goroutine at a
    	// time.
    	Open(name string) (Conn, error)
    }

接口中定义了一个`Open`函数，因此`MySQLDriver`必须实现这个函数。

`go-sql-driver/mysql`中已在文件`github.com/go-sql-driver/mysql/driver.go`中实现了该函数：

// Open new Connection.
    // See https://github.com/go-sql-driver/mysql#dsn-data-source-name for how
    // the DSN string is formatted
    func (d MySQLDriver) Open(dsn string) (driver.Conn, error) { 
        ... ...
    	// New mysqlConn
    	mc := &mysqlConn{ 
    		maxAllowedPacket: maxPacketSize,
    		maxWriteSize:     maxPacketSize - 1,
    		closech:          make(chan struct{ }),
    	}
        ... ...
        return mc, nil
    }

# 2.打开一个database #

DB, err = sql.Open("mysql", dataBase)

第一个参数是driver名称，第二个参数是dsn(DataSourceName)，指数据库地址。

该函数从`drivers`map中找到`mysql`的driver，将其作为参数初始化`DB`结构体，并返回。

`DB`接口定义：

type DB struct { 
    	connector driver.Connector
    	... ...
    }
    
    type dsnConnector struct { 
    	dsn    string
    	driver driver.Driver
    }
    
    func (t dsnConnector) Connect(_ context.Context) (driver.Conn, error) { 
    	return t.driver.Open(t.dsn)
    }
    
    func (t dsnConnector) Driver() driver.Driver { 
    	return t.driver
    }

# 3.查询 #

var connection_id int
    err := DB.QueryRow("select CONNECTION_ID()").Scan(&connection_id)
    if err != nil { 
            log.Println("query connection id failed:", err)
            return
    }
    
    log.Println("connection id:", connection_id)

底层调用关系是这样的：

func (db *DB) QueryRow()
      -->db.QueryRowContext() 
        -->db.QueryContext()
            --> db.conn() --> 最后调用的是MySQLDriver.Open()
            --> db.queryDC() --> 最后调用的是mysqlConn.Query()

具体调用关系如下图所示。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbnlhbmcxMjM0NTY_size_16_color_FFFFFF_t_70_pic_center]

其他的查询也是类似的调用过程。

`MySQLDriver`代码关系如下图所示。  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbnlhbmcxMjM0NTY_size_16_color_FFFFFF_t_70_pic_center 1]

# 4.参考 #

[ go-sql-driver /mysql ][go-sql-driver _mysql]

[golang package sql][]

[golang database_sql]: https://blog.csdn.net/lanyang123456/article/details/108425687
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbnlhbmcxMjM0NTY_size_16_color_FFFFFF_t_70_pic_center]: /images/20221120/174d0a5cd99046a290a50264d662ec29.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2xhbnlhbmcxMjM0NTY_size_16_color_FFFFFF_t_70_pic_center 1]: /images/20221120/cf7df523159945dcbface0c7e314b0c6.png
[go-sql-driver _mysql]: https://github.com/go-sql-driver/mysql
[golang package sql]: https://golang.org/pkg/database/sql/