How to fix ERROR 2006: MySQL server has gone away in MySQL

# Check if MySQL process is running:
ps aux | grep mysqld

# Try a simple connection:
mysql -u root -p -h localhost -e "SELECT 1;"

# Or check the status:
sudo systemctl status mysql

# Check for recent restarts in the MySQL error log:
tail -50 /var/log/mysql/error.log | grep -i "started\|restarted\|ready for connections"

dmesg | tail -20  # kernel OOM killer, segfaults, hardware errors
journalctl -u mysql -n 50  # systemd logs

# Check the current timeout:
mysql -u root -p -e "SHOW VARIABLES LIKE 'wait_timeout';" 

# Temporarily increase it (until restart):
mysql -u root -p -e "SET GLOBAL wait_timeout = 86400;"

# Make it permanent by editing /etc/mysql/mysql.conf.d/mysqld.cnf (or /etc/my.cnf):
[mysqld]
wait_timeout = 86400
interactive_timeout = 86400

# Restart MySQL:
sudo systemctl restart mysql

# In /etc/php/8.0/apache2/php.ini or /etc/php/8.0/cli/php.ini:
default_socket_timeout = 86400

# Check the current setting:
mysql -u root -p -e "SHOW VARIABLES LIKE 'max_allowed_packet';" 

# Temporarily increase it:
mysql -u root -p -e "SET GLOBAL max_allowed_packet = 256M;"

# Make it permanent in /etc/mysql/mysql.conf.d/mysqld.cnf:
[mysqld]
max_allowed_packet = 256M

# Restart MySQL:
sudo systemctl restart mysql

# From the client, test connectivity to the server:
telnet <mysql-host> 3306
nc -zv <mysql-host> 3306

# Check firewall status on the MySQL server:
sudo iptables -L | grep 3306  # Linux iptables
sudo ufw status | grep 3306    # UFW firewall

# Temporarily allow MySQL port (if using UFW):
sudo ufw allow 3306

# Check for remote connection limits in MySQL:
mysql -u root -p -e "SELECT user, host FROM mysql.user WHERE user = 'your-app-user';"

[mysqld]
bind-address = 0.0.0.0  # Listen on all interfaces
# Or specify a specific IP:
bind-address = 192.168.1.100

const pool = mysql.createPool({
  connectionLimit: 10,
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'mydb',
  waitForConnections: true,
  enableKeepAlive: true,
  keepAliveInitialDelayMs: 0
});

$pdo = new PDO('mysql:host=localhost;dbname=mydb', 'root', 'password', [
    PDO::ATTR_PERSISTENT => false,
    PDO::ATTR_TIMEOUT => 5,
    PDO::MYSQL_ATTR_INIT_COMMAND => "SET NAMES utf8mb4"
]);

HikariConfig config = new HikariConfig();
config.setJdbcUrl("jdbc:mysql://localhost:3306/mydb");
config.setMaximumPoolSize(10);
config.setConnectionTimeout(30000);
config.setIdleTimeout(600000);
HikariDataSource ds = new HikariDataSource(config);

# Check if any queries are being killed (monitor in another terminal):
mysql -u root -p -e "SHOW FULL PROCESSLIST;"

# Look for your connections and their status
# If Status shows "Killed", the connection was terminated

# Check server resource limits:
FILE_DESCRIPTORS=`ulimit -n`
echo "Max open files: $FILE_DESCRIPTORS"

# Check for memory pressure:
free -h
htop  # Check for OOM killer

# Review the error log for OOM killer activity:
grep -i "killed\|oom" /var/log/syslog  # Ubuntu/Debian
grep -i "killed\|oom" /var/log/messages  # CentOS/RHEL

mysql -u root -p -e "SET GLOBAL innodb_buffer_pool_size = 512M;"  # Adjust based on available RAM

# Enable slow query logging:
mysql -u root -p -e "SET GLOBAL slow_query_log = 1; SET GLOBAL long_query_time = 10;"

# Make it permanent in /etc/mysql/mysql.conf.d/mysqld.cnf:
[mysqld]
slow_query_log = 1
long_query_time = 10
log_queries_not_using_indexes = 1
log-slow-admin-statements = 1

# Check the slow query log:
tail -100 /var/log/mysql/slow.log

# Analyze a slow query:
mysql -u root -p
EXPLAIN SELECT ... <your slow query>\G

# Look for "Using temporary" or "Using filesort" which indicate inefficiency
# Add indexes to improve performance