MySQL Error 1040

Error number

1040

SQLSTATE

08004

Symbol

ER_CON_COUNT_ERROR

Product

MySQL Server

Versions

MySQL 5.7, 8.0 and 8.4 (default max_connections 151)

Category

Connections & Network

Last reviewed

24 October 2026

What it means

Error 1040 means the server already has as many client connections as max_connections allows, so it refused a new one.

Every client connection uses a slot on the server, whether it is running a query or sitting idle. max_connections (151 by default) caps the number of ordinary client connections; when they are all taken, the next connection attempt fails with 1040 and the server increments the Connection_errors_max_connections status counter. The error says nothing about the client that receives it: the cause is the connections that are already open, very often idle ones that an application, a pool or a script opened and never closed.

MySQL keeps one extra connection for administrators: the server actually permits max_connections + 1 client connections, and the extra one is reserved for accounts with the CONNECTION_ADMIN privilege (or the deprecated SUPER). From MySQL 8.0.14 the server can also accept administrative connections on a separate interface (admin_address and admin_port). Either way, an administrator can still get in to run SHOW PROCESSLIST and find out who holds the connections, which is the first step. A per-account limit (max_user_connections) gives a different error, 1203.

Common causes

• Connection leaks: code paths that open a connection and do not close it (an exception before close, persistent connections in many web server processes).
• Connection pools that together allow more connections than the server: for example 20 application instances with a pool of 20 each against max_connections 151.
• Slow queries or lock waits keep connections busy for longer, so a normal traffic peak needs more connections at the same time.
• Many idle connections that the server keeps for wait_timeout seconds (eight hours by default) before closing them.
• A traffic spike, a retry storm after an outage, or a batch job that opens one connection per task.

Diagnose it

Read-only These statements only read metadata (information_schema, performance_schema and SHOW statements) or your own data. They do not change data, privileges or configuration, so they are safe to run first.

SHOW VARIABLES LIKE 'max_connections';
SHOW GLOBAL STATUS LIKE 'Threads_connected';
SHOW GLOBAL STATUS LIKE 'Max_used_connections%';
SHOW GLOBAL STATUS LIKE 'Connection_errors_max_connections';

SELECT user, SUBSTRING_INDEX(host, ':', 1) AS client_host, db, command,
       COUNT(*) AS connections, MAX(time) AS longest_seconds
FROM performance_schema.processlist
GROUP BY user, client_host, db, command
ORDER BY connections DESC
LIMIT 20;

SELECT user, host, current_connections, total_connections
FROM performance_schema.accounts
WHERE user IS NOT NULL
ORDER BY current_connections DESC
LIMIT 20;

SHOW VARIABLES LIKE 'wait_timeout';
SHOW VARIABLES LIKE 'max_user_connections';
SELECT user, host, max_user_connections
FROM mysql.user
WHERE max_user_connections > 0;

How to fix it

Each fix is labelled with what it changes. Try read-only checks and the least invasive fix first, and test changes outside production.

KILL 5389;  -- the id of an idle connection from performance_schema.processlist

SET SESSION wait_timeout = 600;  -- this connection only, in seconds

SET PERSIST max_connections = 300;

# or in my.cnf
[mysqld]
max_connections = 300

Examples

Every connection slot is in use

Shown as documented in the MySQL reference manual (Too many connections); not reproduced here, because filling every connection slot would mean lowering max_connections on a shared server.

-- On the server (as an administrator):
SHOW VARIABLES LIKE 'max_connections';     -- 151, the default
SHOW GLOBAL STATUS LIKE 'Threads_connected'; -- at the limit

-- A new client without CONNECTION_ADMIN then connects:
-- mysql -h db.example.com -u app -p

ERROR 1040 (08004): Too many connections

A per-account limit is a different error (1203)

Shown as documented in the MySQL server error message reference (ER_TOO_MANY_USER_CONNECTIONS); not reproduced here, because it needs an account with a connection limit and the lab may not create accounts. The fixes are the same as for 1040, applied to that one account.

-- The account may hold at most 10 connections:
SELECT user, host, max_user_connections FROM mysql.user WHERE user = 'app';  -- app | % | 10

-- An eleventh connection as app:
-- mysql -h db.example.com -u app -p

ERROR 1203 (42000): User app already has more than 'max_user_connections' active connections

How to prevent it

Budget connections: list every application instance, worker, cron job and tool with its maximum pool size, keep the total below max_connections, and revisit it whenever you scale out. Close connections reliably, give pools an idle limit, and keep queries and transactions short so connections are returned quickly.

Monitor Threads_connected against max_connections and alert well before the limit (for example at 80 percent), and alert on any increase in Connection_errors_max_connections. Keep CONNECTION_ADMIN for administrators only, so the reserved connection is free in an emergency.

Common questions

Learn more on CodeWithSQL

• Best MySQL monitoring tools
• Managed MySQL services

Official documentation

• Too many connections
• Connection interfaces (max_connections + 1, administrative interface)
• Server system variables (max_connections, wait_timeout)
• Persisted system variables (SET PERSIST)

Last technically reviewed 24 October 2026; examples and diagnostics run on MySQL 8.0.46. Message templates checked against the server's error catalogue (perror) for that version.