refer to:
https://www.thegeekdiary.com/mysql-error-too-many-connections-and-how-to-resolve-it/
This error is caused by mysql, but not rails config.
check variable: max_connections:
SHOW VARIABLES LIKE 'max_connections'; +-----------------+-------+ | Variable_name | Value | +-----------------+-------+ | max_connections | 151 | +-----------------+-------+
increase it : SET GLOBAL max_connections = 1024;