MySQL基礎知識：MySQL Connection和Session

在connection的生命裏，會一直有一個user thread（以及user thread對應的THD）陪伴它。

Connection和Session概念

來自Stackoverflow的一個回答：

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.

來自官方團隊的描述：

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創建鏈接的過程

Connection Phase

• Connection Requests: 是一個簡單的TCP-IP鏈接消息，發送到MySQL Server的端口（如：3306）;
• Receiver Thread：惟一職責是建立 user thread；要麼新建一個OS thread，要麼重用 thread cache裏的可用thread；
• User Thread: client-server protocol 處理器，好比返回 handshake packet，接收查詢、返回結果等等；

THD

• THD： 表示connection上下文的數據結構；鏈接創建後被建立，斷開鏈接後被銷燬；
• 用戶的connection和THD是一一對應的，THD不會被connection共用；
• THD數據結構的大小約爲 ~10KB，注意用來跟蹤query執行狀態各個方面；

注意：THD 一直沒查到是什麼的簡寫。從查閱的資料看，THD應該也能夠被認爲是 Session 或者 connection的狀態/上下文。

Command Phase

• 當connection phase一切安好後， user thread 會進入 command phase；開始忙碌的一輩子。

斷開鏈接

Client發送COM_QUIT命令開始斷開鏈接操做。

User Thread開始作清理工做：

• 釋放THD；
• thread cache還有空位置： 把本身 放到 thread cache裏並標記爲 suspended狀態；
• thread cache沒有空位置：結束線程。

查看MySQL Sessions/Active Connections

MySQL的鏈接信息，記錄在information_schema和performance_schema數據庫中。

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    |       |
+---------------------+------------+------+-----+---------+-------+

查看鏈接

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;

查看每一個host的當前鏈接數和總鏈接數

select * FROM performance_schema.hosts;

參考資料

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