503 Service Unavailable

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.
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