The HyperText Transfer Protocol (HTTP) 503 Service Unavailable server error response code indicates that the server is not ready to handle the request.
Common causes are a server that is down for maintenance or that is overloaded. This response should be used for temporary conditions and the Retry-After HTTP header should, if possible, contain the estimated time for the recovery of the service.
Note: together with this response, a user-friendly page explaining the problem should be sent.
Caching-related headers that are sent along with this response should be taken care of, as a 503 status is often a temporary condition and responses shouldn't usually be cached.