Today we’ll see what causes “IIS HTTP Error 503. The service is unavailable.” error and how we resolved it. Bobcares, as a part of our Server Management Service offers solutions to every query that comes our way.

Overview

1. What is “HTTP Error 503: The service is unavailable” error?
2. What are the causes of the Error?
3. How to Fix the Error?
4. Conclusion

What is “HTTP Error 503: The service is unavailable” error?

When we see HTTP Error 503, it means that either temporary server maintenance or overloading has prevented the web server from handling the request. This error usually appears when there is maintenance going on, there is a brief overload on the server, or there are problems that prohibit the server from completing the request.

To put it another way, imagine if we contact a store and, rather than speaking with a live person, we get an automated message stating that they are now unavailable. Numerous factors, including heavy traffic, server maintenance, as well as technological problems, may cause this.

What are the causes of the Error?

Our Windows server experts noted that all the domains in the server were using the same application pool [ie. ‘plesk(default)(4.0)(pool)’]. In our experience handling such errors, the following are the main causes:

1. Application Pool of the corresponding Wep Application is Stopped – In an IIS server, the domains are assigned to application pools, some dedicated and some shared. If the associated application pool for a domain is in stopped or disabled state, it would cause the website to show “HTTP Error 503. The service is unavailable.” message.
2. Any misconfiguration in the application pool or site settings, such as spawning applications in 32-bit mode, that can tamper with its proper functioning, can cause error in the site. Improper IIS configuration have noted to cause the worker process to fail. Process crashes also happen due to incorrect application logic. In some cases, the web server protocol may not be active in the site settings, and the site can show up ‘HTTP Error 503.’
3. Issues related to the user Identity of application Pool – An application pool is associated with a user identity, which allows you to run that application pool under a unique account. This user account also can get locked at times, can have its password expired, or may not have adequate privileges to run the application pool. Any of these issues can tamper with the website functioning.
4. Application pool running out of RAM or other server resources, causing it to crash, often lead to ‘HTTP Error 503.’
5. Server migrations have also seen to have caused such errors in the website, if the IIS configuration settings of two servers are not compatible with each other, which can affect domain performance.

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How to Fix the Error?

In order to pinpoint the actual cause of the 503 error, we examine the System event logs, which gives a hint towards what is the reason for the error. Based on that, we follow various debugging methods to resolve it.

If the error is due to a stopped application pool, then usually starting it would resolve the issue. From the IIS Manager -> Application Pools node, right click on the DefaultAppPool in order to check the status.

On noticing that the service is stopped, we start it. If the service is running, we try restarting it and the error would be gone. But in most cases, the fix may not be as simple as that.

In cases where the application pool crashes frequently, restarting it all the time is not the right way to fix it. So, we monitor the server and debug further into what is causing the application pool to crash.

For websites that are resource-intense, it is always better to assign a dedicated application pool for that domain. Another aspect we check and fix the settings of application pool and site, such as enabling the relevant protocol.

One of the key issues many fail to notice, is the user identity settings. We check the account settings and password of the identity and confirm that it is working fine. If mismatch noted, we would update the settings.

However, in this specific scenario, the issue was due to inadequate permissions of the application to run, which caused the error. By reconfiguring the application pool to run with a higher privileged identity, we were able to resolve the issue.

To avoid resource crunch leading to application crashes, we also optimized the IIS server and could see improvement in the capacity to handle connections with lesser CPU usage.

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Conclusion

To sum up, our Tech team went over the details of HTTP Error 503: The service is unavailable error, along with its solutions.