在connection的生命裡，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建立連線的過程 #### Connection Phase  - Connection Requests: 是一個簡單的TCP-IP連線訊息，傳送到MySQL Server的埠（如：3306）; - Receiver Thread：唯一職責是建立 ```user thread```；要麼新建一個OS thread，要麼重用 thread cache裡的可用thread； - User Thread: [client-server protocol](https://dev.mysql.com/doc/dev/mysql-server/latest/PAGE_PROTOCOL.html) 處理器，比如返回 ```handshake packet```，接收查詢、返回結果等等； #### THD - THD： 表示connection上下文的資料結構；連線建立後被建立，斷開連線後被銷燬； - 使用者的connection和THD是一一對應的，THD不會被connection共用； - THD資料結構的大小約為 ~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應該也可以被認為是 ```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```；開始忙碌的一生。 #### 斷開連線  Client傳送```COM_QUIT```命令開始斷開連線操作。 User Thread開始做清理工作： - 釋放THD； - ```thread cache```還有空位置： 把自己 放到 ```thread cache```裡並標記為 ```suspended```狀態； - ```thread cache```沒有空位置：結束執行緒。 ## 檢視MySQL Sessions/Active Connections MySQL的連線資訊，記錄在```information_schema```和```performance_schema```資料庫中。 ```sql 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 | | | | +---------+---------------------+------+-----+---------+-------+ ``` ```sql 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 | | +---------------------+------------+------+-----+---------+-------+ ``` #### 檢視連線 方法1： ```sql show status where variable_name = 'threads_connected'; ``` 方法2： ```sql show processlist; ``` 方法3： ```sql select id, user, host, db, command, time, state, info from information_schema.processlist; ``` #### 檢視每個host的當前連線數和總連線數 ```sql select * FROM performance_schema.hosts; ``` ## 參考資料 1. [MySQL show status - active or total connections?](https://stackoverflow.com/questions/7432241/mysql-show-status-active-or-total-connections) 2. [MySQL concepts: session vs connection](https://stackoverflow.com/questions/8797724/mysql-concepts-session-vs-connection) 3. **推薦：** [MySQL Connection Handling and Scaling](https://mysqlserverteam.com/mysql-connection-handling-and-scaling/) 4. [Connection Phase](https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_connection_phase.html) 5. [Command Phase](https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_command_phase.html) 6. [MySQL Error: Too many connections](https://www.percona.com/blog/2013/11/28/mysql-error-too-many-connections) 7. [5.1.10 Server Status Variables](https://dev.mysql.com/doc/refman/8.0/en/server-status-variabl