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HTTP Error 503 – Service unavailable

by bernt & torsten

The 503-page error means that the webserver is currently unable to handle the HTTP request due to temporary overloading of the server. It’s a temporary condition which will be alleviated after some delay.

Some servers in this state may also simply refuse the socket connection, in which case a different error may be generated because the socket creation timed out.

The Web server is still functioning minimally because it can respond with a 503 status code, but full response service is impossible i.e. the Web site is simply unavailable. This can be caused by many possible reasons, in general, it is because of some human intervention by the administrators of the Web server. You can usually expect that someone is working on the problem, and normal service will resume as soon as possible.

Any client goes through the following cycle when it communicates with the Web server:

1. Obtain an IP address from the IP name of the site (the site URL without the leading ‘http://’). This lookup (conversion of IP name to IP address) is provided by domain name servers (DNSs).
2. Open an IP socket connection to that IP address.
3. Write an HTTP data stream through that socket.
4. Receive an HTTP data stream back from the Web server in response. This data stream contains status codes whose values are determined by the HTTP protocol. Parse this data stream for status codes and other useful information.

This error occurs in the final step above when the client receives an HTTP status code that it recognizes as ‘503’.