How to fix ERROR 1040: Too many connections in MySQL

Connect to MySQL as an admin user and check how many connections are active:

SHOW PROCESSLIST;
SHOW STATUS LIKE "Threads_connected";
SHOW STATUS LIKE "max_connections";

If Threads_connected equals max_connections, your server is at capacity. Review the PROCESSLIST output to identify sleeping or long-running queries.

If safe to do so, terminate idle or sleeping connections to free up slots:

-- View idle connections
SHOW PROCESSLIST WHERE Command = "Sleep" AND Time > 300;

-- Kill a specific connection (replace CONN_ID with actual ID)
KILL CONNECTION_ID;

-- Or use pt-kill from Percona Toolkit for bulk operations
pt-kill --victims=oldest --busy-time=300 --sleep-time=300

This immediately frees connection slots for new clients.

If you need an immediate fix and have admin access:

SET GLOBAL max_connections = 500;

This takes effect immediately without restarting MySQL, but resets to the configured value on next server restart. Use this to buy time while implementing permanent fixes.

Edit your MySQL configuration file (usually /etc/my.cnf or /etc/mysql/my.cnf) and locate the [mysqld] section:

[mysqld]
max_connections = 500

Use a value appropriate for your server's memory: max_connections = (Available_RAM_MB - Global_Buffers_MB) / Per_Connection_Memory_MB. Most Linux systems handle 500-1000 connections without issues. Then restart MySQL:

sudo systemctl restart mysql
# or
sudo service mysql restart

The wait_timeout setting (default 28800 seconds / 8 hours) determines how long MySQL holds idle connections. Reduce this to automatically free connections:

[mysqld]
wait_timeout = 300
interactive_timeout = 300

This closes idle connections after 5 minutes. Adjust based on your application's needs. Restart MySQL for the changes to take effect.

Use a connection pool to limit application connections to MySQL. This prevents applications from opening excessive connections:

In Node.js (mysql2/promise):

const mysql = require("mysql2/promise");
const pool = mysql.createPool({
  host: "localhost",
  user: "root",
  password: "password",
  database: "mydb",
  waitForConnections: true,
  connectionLimit: 10,
  queueLimit: 0
});

In Python (SQLAlchemy):

from sqlalchemy import create_engine
engine = create_engine(
  "mysql+pymysql://user:password@localhost/db",
  pool_size=10,
  max_overflow=20
)

Connection pooling acts as a intermediary between applications and MySQL, reusing connections efficiently.

For high-traffic applications, use a proxy like ProxySQL to handle connection multiplexing:

# Install ProxySQL
sudo apt install proxysql

# Configure to limit backend connections
# Applications connect to ProxySQL (port 6033) instead of MySQL (port 3306)
# ProxySQL multiplexes many client connections into fewer MySQL connections

ProxySQL can buffer requests, reuse connections, and distribute load across multiple MySQL servers.