redis client原理分析

type Pool struct {
   // Dial is an application supplied function for creating and configuring a
   // connection.
   //
   // The connection returned from Dial must not be in a special state
   // (subscribed to pubsub channel, transaction started, ...).
   Dial func() (Conn, error)  //生成網絡連接對象

   // TestOnBorrow is an optional application supplied function for checking
   // the health of an idle connection before the connection is used again by
   // the application. Argument t is the time that the connection was returned
   // to the pool. If the function returns an error, then the connection is
   // closed.
   TestOnBorrow func(c Conn, t time.Time) error  //測試連接是否通暢

   // Maximum number of idle connections in the pool.
   MaxIdle int  //最大空閑連接數

   // Maximum number of connections allocated by the pool at a given time.
   // When zero, there is no limit on the number of connections in the pool.
   MaxActive int //最大活動（正在執行任務）連接數

   // Close connections after remaining idle for this duration. If the value
   // is zero, then idle connections are not closed. Applications should set
   // the timeout to a value less than the server's timeout.
   IdleTimeout time.Duration //空閑連接超時時間，超時會釋放

   // If Wait is true and the pool is at the MaxActive limit, then Get() waits
   // for a connection to be returned to the pool before returning.
   Wait bool //當到達最大活動連接時，是否阻塞

   chInitialized uint32 // set to 1 when field ch is initialized 初始化標記

   mu     sync.Mutex    // mu protects the following fields 鎖
   closed bool          // set to true when the pool is closed. 連接池關閉標記
   active int           // the number of open connections in the pool 連接總數
   ch     chan struct{} // limits open connections when p.Wait is true 用于實現阻塞邏輯
   idle   idleList      // idle connections 雙向鏈表，存放空閑連接
}

type idleList struct { //空閑連接鏈表
   count       int //空閑連接數
   front, back *idleConn //空閑連接信息
}

type idleConn struct {  //空閑連接信息
   c          Conn      //連接接口
   t          time.Time //加入空閑隊列的時間，用于判斷空閑超時
   next, prev *idleConn //雙向鏈表指針
}

func (c *conn) Do(cmd string, args ...interface{}) (interface{}, error) {
   return c.DoWithTimeout(c.readTimeout, cmd, args...)
}

func (c *conn) DoWithTimeout(readTimeout time.Duration, cmd string, args ...interface{}) (interface{}, error) {
   c.mu.Lock()
   pending := c.pending
   c.pending = 0
   c.mu.Unlock()

   if cmd == "" && pending == 0 {
      return nil, nil
   }

   //設置寫超時時間
   if c.writeTimeout != 0 {
      c.conn.SetWriteDeadline(time.Now().Add(c.writeTimeout))
   }
   //發送命令內容
   if cmd != "" {
      if err := c.writeCommand(cmd, args); err != nil {
         return nil, c.fatal(err)
      }
   }

   if err := c.bw.Flush(); err != nil {
      return nil, c.fatal(err)
   }

   var deadline time.Time
   if readTimeout != 0 {
      deadline = time.Now().Add(readTimeout)
   }
   //設置讀超時時間
   c.conn.SetReadDeadline(deadline)

   if cmd == "” {
      //獲取server回復信息并解析
      reply := make([]interface{}, pending)
      for i := range reply {
         r, e := c.readReply()
         if e != nil {
            return nil, c.fatal(e)
         }
         reply[i] = r
      }
      return reply, nil
   }

   var err error
   var reply interface{}
   for i := 0; i <= pending; i++ {
      var e error
      if reply, e = c.readReply(); e != nil {
         return nil, c.fatal(e)
      }
      if e, ok := reply.(Error); ok && err == nil {
         err = e
      }
   }
   return reply, err
}

set命令消息格式：
*3\r\n$3\r\nSET\r\n$4\r\nhlxs\r\n$28\r\nhttps://www.rzrgm.cn/hlxs\r\n

注釋如下：
*3        //參數個數是*開頭，3個參數
$3        //參數長度是$開頭，命令長度
SET       //命令名稱SET
$5        //參數長度是$開頭，key長度
mykey     //key的內容
$28       //參數長度是$開頭，value長度
https://www.rzrgm.cn/hlxs      //value內容

* 狀態回復（status reply）的第一個字節是 “+”，如：+ok\r\n
* 錯誤回復（error reply）的第一個字節是 “-“，如：-ERR unknown command xxx\r\n
  在 "-" 之后，直到遇到第一個空格或新行為止，這中間的內容表示所返回錯誤的類型
* 整數回復（integer reply）的第一個字節是 “:”，如：:1000\r\n
* 批量回復（bulk reply）的第一個字節是 “$”，如：$6\r\nfoobar\r\n，也是長度加內容的風格
* 多條批量回復（multi bulk reply）的第一個字節是 “*”，如：*5\r\n:1\r\n:2\r\n:3\r\n:4\r\n$6\r\nfoobar\r\n，前面多了數量