5×× Server Error
503 Service Unavailable
The server is currently unable to handle the request due to a temporary overload or scheduled maintenance, which will likely be alleviated after some delay.
The server MAY send a Retry-After header field1 to suggest an appropriate amount of time for the client to wait before retrying the request.
Note: The existence of the 503 status code does not imply that a server has to use it when becoming overloaded. Some servers might simply refuse the connection.
503 Code References
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant