MySQL/MariaDB service cannot be started during Plesk update: ERROR 2002 (HY000): Can't connect to local MySQL server through socket

Symptoms

• MySQL/MariaDB server is not started during Plesk update. The following errors can be found in /var/log/plesk/install/autoinstaller3.log :

  CONFIG_TEXT: Failed to start mysql.service: Unit not found.
  Trying to start service mysql... inactive
  WARNING!
  Some problems are found during start service mysql(see log file: /var/log/plesk/install/plesk_18.0.28_installation.log)
  Continue...
  Trying to start service mysql... inactive
  Trying to establish test connection...ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
  ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)

• Checking mysql service status with systemctl utility results in:

  # systemctl status mariadb.service
  Unit mariadb.service could not be found.
  # systemctl status mysql.service
  Unit mariadb.service could not be found.
  # systemctl status mysqld.service
  Unit mariadb.service could not be found.

• mysql service can be started manually with the command:

  # service mysql start

• SystemD-based OS is used

Cause

MySQL or MariaDB service misconfiguration.

Symbolic link for mysql service in /etc/systemd/system/ directory points to non-existing file:

# ls -la /etc/systemd/system/mariadb.service
lrwxrwxrwx 1 root root 38 Jul 29 2016 mariadb.service -> /usr/lib/systemd/system/mysqld.service
# ls -la /usr/lib/systemd/system/mysqld.service
ls: cannot access /usr/lib/systemd/system/mysqld.service: No such file or directory

Resolution

1. Log in to Plesk server using SSH.

2. Make sure that service definition is installed in /usr/lib/systemd/system/mariadb.service.

   # ls -la /usr/lib/systemd/system/mariadb.service
   -rw-r--r-- 1 root root 1697 Aug 16 2018 /usr/lib/systemd/system/mariadb.service

3. Make sure that there is no startup script for mysql service in /etc/init.d/ directory and remove if it exists:

   # rm -f /etc/init.d/mysql

4. Remove broken symbolic link:

   # rm -f /etc/systemd/system/mariadb.service

5. Reload systemd daemon:

   # systemctl daemon-reload