5xx SERVER ERROR
502 Service Unavailable
The server is not ready to handle the request.
Common causes are a server that is down for maintenance or that is
overloaded. Note that together with this response, a user-friendly page
explaining the problem should be sent. This response should be used for
temporary conditions and the
Retry-After: HTTP header should, if possible,
contain the estimated time before the recovery of the service. The
webmaster must also take care about the caching-related headers that are
sent along with this response, as these temporary condition responses
should usually not be cached.