Resolve MySQL Connection Errors

MySQL connection errors are among the most common application issues. This guide covers the most frequent errors and their resolutions.

Error: Too Many Connections

Symptoms: ERROR 1040 (HY000): Too many connections

Cause: The number of simultaneous connections has reached max_connections.

Check current connections:

$mysql -u root -p -e "SHOW STATUS LIKE 'Threads_connected';"

+-------------------+-------+
| Variable_name     | Value |
+-------------------+-------+
| Threads_connected | 151   |
+-------------------+-------+

$mysql -u root -p -e "SHOW VARIABLES LIKE 'max_connections';"

+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections | 151   |
+-----------------+-------+

Resolution: Increase max_connections:

$mysql -u root -p -e "SET GLOBAL max_connections = 300;"

To make it permanent, edit /etc/mysql/mysql.conf.d/mysqld.cnf:

[mysqld]
max_connections = 300

[!NOTE] Increasing max_connections requires more RAM. Each connection uses approximately 1 MB. Consider using a connection pooler (ProxySQL, PgBouncer) for high-traffic applications.

Error: Access Denied

Symptoms: ERROR 1045 (28000): Access denied for user 'user'@'host'

Cause: Wrong password, user does not exist, or the user is not allowed to connect from that host.

Check user grants:

$mysql -u root -p -e "SELECT user, host FROM mysql.user WHERE user='myapp_user';"

+------------+-----------+
| user       | host      |
+------------+-----------+
| myapp_user | localhost |
+------------+-----------+

If the application connects from 10.0.1.10 but the user is only allowed from localhost, add the correct host:

CREATE USER 'myapp_user'@'10.0.1.10' IDENTIFIED BY 'password';
GRANT ALL PRIVILEGES ON myapp.* TO 'myapp_user'@'10.0.1.10';
FLUSH PRIVILEGES;

Error: Lost Connection

Symptoms: ERROR 2013 (HY000): Lost connection to MySQL server during query

Cause: Query timeout or packet size exceeded.

Check and increase timeouts:

$mysql -u root -p -e "SHOW VARIABLES LIKE 'wait_timeout';"

[mysqld]
wait_timeout = 600
interactive_timeout = 600
max_allowed_packet = 64M

Check MySQL Error Log

$tail -50 /var/log/mysql/error.log

Monitor Slow Queries

Enable the slow query log to find performance bottlenecks:

[mysqld]
slow_query_log = 1
slow_query_log_file = /var/log/mysql/slow.log
long_query_time = 2

$tail -f /var/log/mysql/slow.log