MySQL连接数被打满：too many connections的3类来源与5个参数

Error 1040: Too many connections

SHOW GLOBAL STATUS LIKE 'Threads_connected';   -- 当前建立的连接总数
SHOW GLOBAL STATUS LIKE 'Threads_running';     -- 真正在执行语句的连接数
SHOW GLOBAL VARIABLES LIKE 'max_connections';  -- 上限

SELECT
    user,
    host,
    command,
    time AS sleep_seconds,
    COUNT(*) AS cnt
FROM information_schema.processlist
WHERE command = 'Sleep'
GROUP BY user, host, command
HAVING sleep_seconds > 300
ORDER BY cnt DESC
LIMIT 20;

// ❌ 错误：Query 之后没有 Close rows，连接不会归还池子
rows, err := db.Query("SELECT id FROM user LIMIT 10")
if err != nil {
    return err
}
// 忘记 rows.Close()，也没有迭代到底 → 连接一直被占着
for rows.Next() {
    // ...
}

// ✅ 正确：defer 关闭，并且迭代完整
rows, err := db.QueryContext(ctx, "SELECT id FROM user LIMIT 10")
if err != nil {
    return err
}
defer rows.Close() // ⭐ 关键
for rows.Next() {
    // ...
}
if err := rows.Err(); err != nil {   // ⭐ 迭代中途出错也要检查
    return err
}

// ❌ 错误：事务拿了不提交/不回滚
tx, _ := db.Begin()
_, _ = tx.Exec("UPDATE ...")
if someErr != nil {
    return someErr   // 直接 return，事务没回滚 → 连接被占住直到超时
}
tx.Commit()

// ✅ 正确：defer 兜底回滚（Commit 之后再 Rollback 会返回 ErrTxDone，无害）
tx, err := db.BeginTx(ctx, nil)
if err != nil {
    return err
}
defer tx.Rollback() // ⭐ 无论成败都执行，Commit 之后是空操作
if _, err := tx.ExecContext(ctx, "UPDATE ..."); err != nil {
    return err
}
return tx.Commit()

-- 看正在执行、且跑得最久的连接
SELECT id, user, host, db, command, time, state,
       LEFT(info, 120) AS sql_text
FROM information_schema.processlist
WHERE command != 'Sleep' AND time > 10
ORDER BY time DESC
LIMIT 20;

总连接数 ≈ 应用实例数 × 每个实例的连接池上限

应用侧 MaxOpenConns ≤ (max_connections × 0.8) / 应用实例数

package main
import (
	"database/sql"
	"time"
	_ "github.com/go-sql-driver/mysql"
)
func newDB(dsn string, maxOpen int) (*sql.DB, error) {
	db, err := sql.Open("mysql", dsn)
	if err != nil {
		return nil, err
	}
	// ⭐ 三个参数必须配套设置
	db.SetMaxOpenConns(maxOpen)                 // 最大打开连接数（含在用+空闲）
	db.SetMaxIdleConns(maxOpen / 2)             // 最大空闲连接数
	db.SetConnMaxLifetime(30 * time.Minute)     // 连接最长存活时间，要小于 MySQL 的 wait_timeout
	db.SetConnMaxIdleTime(5 * time.Minute)      // 空闲连接最长保留时间（Go 1.15+）
	return db, nil
}

SET GLOBAL max_connections = 1000;
SET GLOBAL max_user_connections = 800;
SET GLOBAL wait_timeout = 900;
SET GLOBAL interactive_timeout = 900;   -- ⭐ 别忘了这一个

[mysqld]
max_connections        = 1000
max_user_connections   = 800
wait_timeout           = 900
interactive_timeout    = 900
thread_cache_size      = 64

-- 生成批量 KILL 语句：杀掉 Sleep 超过 600 秒的连接
SELECT CONCAT('KILL ', id, ';')
FROM information_schema.processlist
WHERE command = 'Sleep' AND time > 600;
-- 把结果复制出来执行

SHOW GLOBAL STATUS LIKE 'Threads_connected';
SHOW GLOBAL STATUS LIKE 'Threads_running';

SELECT user, SUBSTRING_INDEX(host, ':', 1) AS ip, COUNT(*) AS cnt
FROM information_schema.processlist
GROUP BY user, ip
ORDER BY cnt DESC
LIMIT 20;

-- Sleep 超过 5 分钟的连接
SELECT COUNT(*) FROM information_schema.processlist WHERE command='Sleep' AND time>300;
-- 执行超过 30 秒的语句
SELECT id, time, LEFT(info,120) FROM information_schema.processlist
WHERE command != 'Sleep' AND time > 30 ORDER BY time DESC LIMIT 10;