The 503 (Service Unavailable) status code indicates that 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 field
() 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.