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.
Retry-After RFC7231 Section 7.1.3
503 SERVICE UNAVAILABLE Source: RFC7231 Section 6.6.4
503 CODE REFERENCES
Rails HTTP Status Symbol
Go HTTP Status Constant
Symfony HTTP Status Constant
Python2 HTTP Status Constant
Python3+ HTTP Status Constant
Python3.5+ HTTP Status Constant