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func Open(driverName, dataSourceName string) (*DB, error) {
	// 查找已注册的驱动 由_ "github.com/go-sql-driver/mysql"提供
  // 驱动是真正和数据库打交道的
  driversMu.RLock()
	driveri, ok := drivers[driverName]
	driversMu.RUnlock()
	if !ok {
		return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
	}
  
  

	if driverCtx, ok := driveri.(driver.DriverContext); ok {
    // 获取连接器
		connector, err := driverCtx.OpenConnector(dataSourceName)
		if err != nil {
			return nil, err
		}
		return OpenDB(connector), nil
	}

	return OpenDB(dsnConnector{dsn: dataSourceName, driver: driveri}), nil
}


// OpenDB may just validate its arguments without creating a connection
// to the database. To verify that the data source name is valid, call
// Ping.
func OpenDB(c driver.Connector) *DB {
  // 初始化数据，为后续维护连接池做准备
	ctx, cancel := context.WithCancel(context.Background())
	db := &DB{
		connector:    c,
		openerCh:     make(chan struct{}, connectionRequestQueueSize),
		lastPut:      make(map[*driverConn]string),
		connRequests: make(map[uint64]chan connRequest),
		stop:         cancel,
	}

	go db.connectionOpener(ctx)

	return db
}

// Runs in a separate goroutine, opens new connections when requested.
func (db *DB) connectionOpener(ctx context.Context) {
	for {
		select {
		case <-ctx.Done():
			return
		case <-db.openerCh:
      // 收到创建新的连接的请求
			db.openNewConnection(ctx)
		}
	}
}

// gorm 连接数据库
func Open(dialector Dialector, opts ...Option) (db *DB, err error) {
  // 全局配置
  config := &Config{}

	...

  // 加载配置
	for _, opt := range opts {
		if opt != nil {
			if applyErr := opt.Apply(config); applyErr != nil {
				return nil, applyErr
			}
			defer func(opt Option) {
				if errr := opt.AfterInitialize(db); errr != nil {
					err = errr
				}
			}(opt)
		}
	}

  // 时间精度配置 默认毫秒
	if d, ok := dialector.(interface{ Apply(*Config) error }); ok {
		if err = d.Apply(config); err != nil {
			return
		}
	}


  // 注册query/create..等操作执行时所需要调用的函数
  // gorm:query -> gorm:preload -> gorm:after_query
  // 对不同版本的数据库做相应的约束配置
  // 调用sql.Open()初始化连接池
	db.callbacks = initializeCallbacks(db)
	if config.Dialector != nil {
		err = config.Dialector.Initialize(db)
	}

  ...

  // clone=1
	db = &DB{Config: config, clone: 1}

	// 在执行sql前后，相关的信息都会存在其中
	db.Statement = &Statement{
		DB:       db,
		ConnPool: db.ConnPool,
		Context:  context.Background(),
		Clauses:  map[string]clause.Clause{},
	}

  // 连接时，自动会进行ping操作
	if err == nil && !config.DisableAutomaticPing {
		if pinger, ok := db.ConnPool.(interface{ Ping() error }); ok {
			err = pinger.Ping()
		}
	}

	if err != nil {
		config.Logger.Error(context.Background(), "failed to initialize database, got error %v", err)
	}

	return
}