As per Wikipedia.org: The term downtime is used to refer to periods when a system is unavailable. Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function.
This is usually a result of the system failing to function because of an unplanned event, or because of routine maintenance. Unplanned downtime may be the result of a software bug, human error, equipment failure, malfunction, high bit error rate, power failure, overload due to exceeding the channel capacity, a cascading failure, etc.
Inevitably, web sites will suffer outages and downtime based on common events, including:
- Hardware failures
- Software failures
- Server updates
- Human Error
- Many other potential factors
As per W3.org (https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html)
10.5.4 503 Service Unavailable
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.
Best Practices for handling server outages:
- Utilize a 503 HTTP Response Code: Search engines recognize this as the valid response code for a temporary outage, and it will prevent them from removing pages from the index if the duration of the outage is reasonable.
- DO NOT report a 400-level error, or a 200 OK with an error message: This will cause search engines to think the site has changed significantly, or no longer has many of the deep content pages.
- Specify a realistic Retry-After value in the headers: This should be a reasonable estimate of when the site will be alive again. Don't specify 30 minutes if the site will be down for several hours.
- Never choose to refuse the connection if there is an option to report a 503 Response Code in times of excess usage.