MySQL基础知识:MySQL Connection和Session

在connection的生命里,会一直有一个user thread(以及user thread对应的THD)陪伴它。

Connection和Session概念

来自Stackoverflow的一个回答:

A session is just a result of a successful connection. 
Any MySQL client requires some connection settings to establish a connection, 
and after the connection has been established,
it acquires a connection id (thread id) and some context which is called session.

来自官方团队的描述:

Connections correspond to Sessions in SQL standard terminology. 
A client connects to the MySQL Server and stays connected until it does a disconnect. 

MySQL Client和MySQL Server建立连接的过程

Connection Phase

mysql connection

  • Connection Requests: 是一个简单的TCP-IP连接消息,发送到MySQL Server的端口(如:3306);
  • Receiver Thread:唯一职责是创建 user thread;要么新建一个OS thread,要么重用 thread cache里的可用thread;
  • User Thread: client-server protocol 处理器,比如返回 handshake packet,接收查询、返回结果等等;

THD

  • THD: 表示connection上下文的数据结构;连接建立后被创建,断开连接后被销毁;
  • 用户的connection和THD是一一对应的,THD不会被connection共用;
  • THD数据结构的大小约为 ~10KB,注意用来跟踪query执行状态各个方面;

注意:THD 一直没查到是什么的简写。从查阅的资料看,THD应该也可以被认为是 Session 或者 connection的状态/上下文

Command Phase

mysql Active Connection

  • 当connection phase一切安好后, user thread 会进入 command phase;开始忙碌的一生。

断开连接

mysql disconnect

Client发送COM_QUIT命令开始断开连接操作。

User Thread开始做清理工作:

  • 释放THD;
  • thread cache还有空位置: 把自己 放到 thread cache里并标记为 suspended状态;
  • thread cache没有空位置:结束线程。

查看MySQL Sessions/Active Connections

MySQL的连接信息,记录在information_schemaperformance_schema数据库中。

 desc information_schema.processlist;
+---------+---------------------+------+-----+---------+-------+
| Field   | Type                | Null | Key | Default | Extra |
+---------+---------------------+------+-----+---------+-------+
| ID      | bigint(21) unsigned | NO   |     |         |       |
| USER    | varchar(32)         | NO   |     |         |       |
| HOST    | varchar(64)         | NO   |     |         |       |
| DB      | varchar(64)         | YES  |     |         |       |
| COMMAND | varchar(16)         | NO   |     |         |       |
| TIME    | int(7)              | NO   |     |         |       |
| STATE   | varchar(64)         | YES  |     |         |       |
| INFO    | varchar(65535)      | YES  |     |         |       |
+---------+---------------------+------+-----+---------+-------+
desc performance_schema.hosts;
+---------------------+------------+------+-----+---------+-------+
| Field               | Type       | Null | Key | Default | Extra |
+---------------------+------------+------+-----+---------+-------+
| HOST                | char(60)   | YES  | UNI | NULL    |       |
| CURRENT_CONNECTIONS | bigint(20) | NO   |     | NULL    |       |
| TOTAL_CONNECTIONS   | bigint(20) | NO   |     | NULL    |       |
+---------------------+------------+------+-----+---------+-------+

查看连接

方法1:

show status where variable_name = 'threads_connected';

方法2:

show processlist;

方法3:

select id,
       user,
       host,
       db,
       command,
       time,
       state,
       info
from information_schema.processlist;

查看每个host的当前连接数和总连接数

select * FROM performance_schema.hosts;

参考资料

  1. MySQL show status - active or total connections?
  2. MySQL concepts: session vs connection
  3. 推荐: MySQL Connection Handling and Scaling
  4. Connection Phase
  5. Command Phase
  6. MySQL Error: Too many connections
  7. 5.1.10 Server Status Variables

posted on 2021-03-15 22:02  AI应用技术  阅读(4716)  评论(6编辑  收藏  举报

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