连接池有两个作用：
1.创建数据库连接是一个很耗时的操作
2.控制连接数

Beego orm连接池分析
1.orm结构中的alias

type orm struct {
    alias *alias
    db    dbQuerier
    isTx  bool
}

type alias struct {
    Name         string
    Driver       DriverType
    DriverName   string
    DataSource   string
    MaxIdleConns int
    MaxOpenConns int
    DB           *sql.DB
    DbBaser      dbBaser
    TZ           *time.Location
    Engine       string
}

再看sql.DB结构

type DB struct {
    // Atomic access only. At top of struct to prevent mis-alignment
    // on 32-bit platforms. Of type time.Duration.
    waitDuration int64 // Total time waited for new connections.

    connector driver.Connector
    // numClosed is an atomic counter which represents a total number of
    // closed connections. Stmt.openStmt checks it before cleaning closed
    // connections in Stmt.css.
    numClosed uint64

    mu           sync.Mutex // protects following fields
    freeConn     []*driverConn

然后看orm的RegisterDataBase方法

db, err = sql.Open(driverName, dataSource)
    if err != nil {
        err = fmt.Errorf("register db `%s`, %s", aliasName, err.Error())
        goto end
    }

    al, err = addAliasWthDB(aliasName, driverName, db)
    if err != nil {
        goto end
    }

    ...

    for i, v := range params {
        switch i {
        case 0:
            SetMaxIdleConns(al.Name, v)
        case 1:
            SetMaxOpenConns(al.Name, v)
        }
    }

然后再到SetMaxIdleConns方法里面看

func SetMaxIdleConns(aliasName string, maxIdleConns int) {
    al := getDbAlias(aliasName)
    al.MaxIdleConns = maxIdleConns
    al.DB.SetMaxIdleConns(maxIdleConns)
}

其实最终还是调用了go的database/sql 里的方法 al.DB.SetMaxIdleConns，其中al是一个sql实例
即调用了sql.DB.SetMaxIdleConns方法

大家都知道，go的sql本来就是用的连接池。
那么我们继续看看go的sql包连接池是怎样实现的。
我们先看sql.DB这个结构体

type DB struct {
    // Atomic access only. At top of struct to prevent mis-alignment
    // on 32-bit platforms. Of type time.Duration.
    waitDuration int64 // Total time waited for new connections.

    connector driver.Connector
    // numClosed is an atomic counter which represents a total number of
    // closed connections. Stmt.openStmt checks it before cleaning closed
    // connections in Stmt.css.
    numClosed uint64

    mu           sync.Mutex // protects following fields
    freeConn     []*driverConn

注意“freeConn []*driverConn”,这个就是存放连接的池子。

db.conn这个方法里面，技术和重新分配db.freeConn

numFree := len(db.freeConn)

那么db.conn这个方法在哪里调用呢？
db.prepare,db.exec,db.query等查询的时候。
到此，我们可以一窥Go在sql连接处理上的大概了。