503 Service Unavailable is an HTTP gateway error occurring when the server is unavailable, either because it is overloaded or down for maintenance. It is mostly a temporary condition.
From RFC 2616 by the World Wide Web Consortium (W3C):
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.
In the context of Snapt One, what that means is that the server could not dispatch your request to a valid Upstream (server:port pair).
In Snapt One this could be triggered by one of these two scenarios:
- the Web Accelerator is pointed to an IP address that is down
- a Load Balancer Group, Frontend or Backend has no online servers
Debugging Error 503
To debug this program try to
telnet to your Upstreams from the Snapt One console, making sure you match both the IP address and port that you’ve set for the Upstream.
If you can’t get there with
telnet you most surely found the problem, as Snapt One won’t be able to get there either with the virtual Load Balancer or the Web Accelerator.
In that case, you will need to individually manage those servers to see what’s going on—maybe they simply need a reset, or maybe you entered IP address incorrectly, or the IP has been automatically changed by the DHCP server.
In any case, these problems are out of the scope for the Snapt One instance, as you first need to make sure that servers you’ve set on the Upstreams are all up and running.