Ever have to take down your site for an extended period of time? By extended period of time, I am talking about more than 5 hours or so.
Those who have, might also worry about how Google or other search engine spiders may treat the long down time.
A Google Groups thread has a response from Google's Berghausen, a Google Webmaster Central representative. Berghausen recommends that you serve up "a '503 Service Unavailable' with a 'Retry-After' header indicating when you expect your site to be back up." That is, if you can serve up that server message while your site is down.
A 503 status is defined as:
The server is currently unable to handle the request due to a temporary overloading or maintenance of the server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header. If no Retry-After is given, the client SHOULD handle the response as it would for a 500 response.
Note: The existence of the 503 status code does not imply that a server must use it when becoming overloaded. Some servers may wish to simply refuse the connection.
Berghausen says that "Googlebot will not index your error page, and will come back looking for updates some time after the date specified in the 'Return-After' header." Very good to know that Google supports this specific header.
Forum discussion at Google Groups.