ERROR 1040 (HY000): Too many connections

MySQL has a maximum number of simultaneous connections (default: 151). All slots are in use.

Fix 1: Increase max_connections

-- Check current limit
SHOW VARIABLES LIKE 'max_connections';

-- Increase temporarily
SET GLOBAL max_connections = 300;

-- Permanent: add to my.cnf
-- [mysqld]
-- max_connections = 300

Fix 2: Close Idle Connections

-- See all connections
SHOW PROCESSLIST;

-- Kill idle ones
KILL <process_id>;

-- Reduce wait timeout (close idle connections faster)
SET GLOBAL wait_timeout = 300;  -- 5 minutes instead of 8 hours
SET GLOBAL interactive_timeout = 300;

Fix 3: Fix Connection Leaks in Code

// ❌ Opening connections without closing
const conn = await mysql.createConnection(config);
const data = await conn.query('SELECT * FROM users');
// Forgot conn.end()

// ✅ Always close connections
try {
    const data = await conn.query('SELECT * FROM users');
    return data;
} finally {
    await conn.end();
}

Fix 4: Use Connection Pooling

// ✅ Pool manages connections automatically
const pool = mysql.createPool({
    host: 'localhost',
    user: 'root',
    database: 'mydb',
    connectionLimit: 10,  // Max connections from this app
    waitForConnections: true,
});

const [rows] = await pool.query('SELECT * FROM users');
// Connection automatically returned to pool

Fix 5: Check What’s Using Connections

-- Group by user/host
SELECT user, host, COUNT(*) as connections
FROM information_schema.processlist
GROUP BY user, host
ORDER BY connections DESC;

Related resources

• MySQL access denied fix
• MongoDB connection failed fix
• Connection timeout fix

Related: PostgreSQL Cheat Sheet