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Help for HTTP error 503: “Service Unavailable”

This HTTP error occurs when a server cannot respond to a request because of temporary overload or maintenance. A minimal service is available (to return this status code) but full service is unavailable.

Outline: Symptoms   What you can do about it   Extra information

Symptoms

Microsoft Windows Update reports HTTP 503 status codes with error code 0x80244022 (hexadecimal 80244022).

When Internet Explorer receives a HTTP 503 status code, it displays any error message returned by the server, such as those described below.

Apache web servers report this as:

Service Temporarily Unavailable

The server is temporarily unable to service your request due to maintenance downtime or capacity problems. Please try again later.

Microsoft IIS web servers define extended codes that give a more specific reason of the error:

Extended code Reason Description
503.0 Application pool unavailable “Service unavailable. The request is sent to an application pool that is currently stopped or that is currently disabled.”
503.1   According to Microsoft IIS 5.0 documentation “A custom error for HTTP Error 503.1 will be sent to clients if a Web server has exceeded its CPU process throttling limits. This error will inform the client that the Web server has either been paused or stopped due to exceeding these set limits.”
503.2 Concurrent request limit exceeded The IIS server has a value for the appConcurrentRequestLimit property that is lower than the current number of concurrent requests.

HTTP 503 errors are reported in distinctive ways by particular services:

  • The otherwise excellent Internet Archive Wayback Machine sometimes returns HTTP 503 errors with the message:

    Failed Connection.

    We're sorry. Your request failed to connect to our servers. This may be due to temporary problems in the data center, or difficulty in serving a higher-than-usual volume of traffic.
  • Google sites may give a HTTP 503 error with the following text when they have been temporarily overloaded, e.g. by a flash crowd:

    Page view limit exceeded

    We are sorry, but this site has exceeded its page view limit at this time. Please try again later.
  • Bluecoat proxy caches report all of their errors using 503 status codes, and include HTML text that elaborates on the cause of the error. Often these effectively report that the proxy timed out, and can be dealt with in the same way as HTTP 504 Gateway Timeout errors.
  • McAfee Firewall Enterprise appliances (formerly Secure Computing Sidewinder firewalls) may return HTTP 503 errors with the following text:

    ERROR

    Proxy Error


    The proxy encountered an error.

    Please try again later.

What you can do about it

  1. For immediate access, the solution may be to try viewing a replica of the website by entering the address in The Internet Archive Wayback Machine.
  2. The website may return a Retry-After header to indicate when it expects to return to service, in particular for predictable planned maintenance outages. As these are not usually displayed by web browsers, you could use an online tool to view the HTTP headers.
  3. You could monitor the website to determine when it returns to service.

Extra information

Note that temporary overload can also cause other symptoms, e.g. problems establishing or maintaining TCP connections that may lead to Internet Explorer cannot display the webpage errors.

Updated: 14 June 2012

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