Tuesday, January 25, 2011
Once in a while we get asked whether a site's visibility in Google's search results can be impacted in a negative way if it's unavailable when Googlebot tries to crawl it. Sometimes downtime is unavoidable: a site owner might decide to take a site down due to ongoing site maintenance, or legal or cultural requirements. Outages that are not clearly marked as such can negatively affect a site's reputation. While we cannot guarantee any crawling, indexing or ranking, there are methods to deal with planned website downtime in a way that will generally not negatively affect your site's visibility in the search results.
For example, instead of returning an HTTP result code 404
(Not Found) or showing an error page with the status code 200 (OK) when a page is requested,
it's better to return a 503 HTTP result code (Service Unavailable) which tells search engine
crawlers that the downtime is temporary. Moreover, it allows site owners to provide visitors and
bots with an estimated time when the site will be up and running again. If known, the length of
the downtime in seconds or the estimated date and time when the downtime will be complete can be
specified in an optional Retry-After header, which Googlebot may use to determine when to recrawl the URL.
Returning a 503 HTTP result code can be a great solution for a number of other situations. We
encounter a lot of problems with sites that return 200 (OK) result codes for server errors,
downtime, bandwidth-overruns or for temporary placeholder pages ("Under Construction"). The 503
HTTP result code is the site owner's solution of choice for all these situations. As for planned
server downtime like hardware maintenance, it's a good idea to have a separate server available
to actually return the 503 HTTP result code. It is important, however, to not treat 503 as a
permanent solution: lasting 503 HTTP result codes can eventually be seen as a sign that the server is now
permanently unavailable and can result in us removing URLs from Google's index.
If you set up a 503 (Service Unavailable) response, the header information might look like this
when using PHP:
header('HTTP/1.1 503 Service Temporarily Unavailable');
header('Retry-After: Sat, 8 Oct 2011 18:27:00 GMT');
Similar to how you can make
404 pages more useful to users, it's also a good idea to provide a customized 503 message
explaining the situation to users and letting them know when the site will be available again. For
further information regarding HTTP result codes, please see
RFC 2616.