Failed to start MariADB Database Server

Background: It's strange, mysql has been running well, suddenly can not connect up, check the process, found mysql processes are gone!

[root@host-39-108-217-12 mariadb]# systemctl status mariadb.service
mariadb.service - MariaDB database server
Loaded: loaded (/usr/lib/systemd/system/mariadb.service; disabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Fri 2018-07-06 08:54:32 CST; 1min 18s ago
Process: 29269 ExecStartPost=/usr/libexec/mariadb-wait-ready $MAINPID (code=exited, status=1/FAILURE)
Process: 29268 ExecStart=/usr/bin/mysqld_safe --basedir=/usr (code=exited, status=0/SUCCESS)
Process: 29238 ExecStartPre=/usr/libexec/mariadb-prepare-db-dir %n (code=exited, status=0/SUCCESS)
Main PID: 29268 (code=exited, status=0/SUCCESS)

Jul 06 08:54:31 host-39-108-217-12 systemd[1]: Starting MariaDB database server...
Jul 06 08:54:31 host-39-108-217-12 mariadb-prepare-db-dir[29238]: Database MariaDB is probably initialized in /var/lib/mysql/data already, nothing is done.
Jul 06 08:54:31 host-39-108-217-12 mysqld_safe[29268]: 180706 08:54:31 mysqld_safe Logging to '/var/lib/mysql/logs/mariadb.log'.
Jul 06 08:54:31 host-39-108-217-12 mysqld_safe[29268]: 180706 08:54:31 mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql/data
Jul 06 08:54:32 host-39-108-217-12 systemd[1]: mariadb.service: control process exited, code=exited status=1
Jul 06 08:54:32 host-39-108-217-12 systemd[1]: Failed to start MariaDB database server.
Jul 06 08:54:32 host-39-108-217-12 systemd[1]: Unit mariadb.service entered failed state.
Jul 06 08:54:32 host-39-108-217-12 systemd[1]: mariadb.service failed.

View mariadb error messages
[root@host-39-108-217-12 mariadb]#  grep 'ERROR' /var/lib/mysql/logs/mariadb.log 
180706  8:47:23 [ERROR] Plugin 'InnoDB' init function returned error.
180706  8:47:23 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
180706  8:47:23 [ERROR] Unknown/unsupported storage engine: InnoDB
180706  8:47:23 [ERROR] Aborting
180706  8:48:48 [ERROR] Plugin 'InnoDB' init function returned error.
180706  8:48:48 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
180706  8:48:48 [ERROR] Unknown/unsupported storage engine: InnoDB
180706  8:48:48 [ERROR] Aborting
180706  8:49:38 [ERROR] Plugin 'InnoDB' init function returned error.
180706  8:49:38 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
180706  8:49:38 [ERROR] Unknown/unsupported storage engine: InnoDB
180706  8:49:38 [ERROR] Aborting
180706  8:54:31 [ERROR] Plugin 'InnoDB' init function returned error.
180706  8:54:31 [ERROR] Plugin 'InnoDB' registration as a STORAGE ENGINE failed.
180706  8:54:31 [ERROR] mysqld: Out of memory (Needed 128917504 bytes)
180706  8:54:31 [ERROR] Unknown/unsupported storage engine: InnoDB
180706  8:54:31 [ERROR] Aborting

The analysis found insufficient memory, just restart the server!