Upon examining the EventViewer logs, we could see the error “A process serving application pool ‘plesk(default)(4.0)(pool)’ suffered a fatal communication error with the Windows Process Activation Service”.
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Today we’ll see what causes IIS “HTTP Error 503. The service is unavailable.” error and how we resolved it.
What causes IIS “HTTP Error 503. The service is unavailable.” 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 causes have been noted for this error.
- 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.
- 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 enabled in the site settings, and the site can show up ‘HTTP Error 503.’
- 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 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.
- Application pool running out of RAM or other server resources, causing it to crash, often lead to ‘HTTP Error 503.’
- 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 resolve IIS “HTTP Error 503. The service is unavailable.” error
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 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. 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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With our systematic debugging approach for service or other software errors, we have been able to provide an exciting support experience to the customers.
If you would like to know how to avoid downtime for your customers due to errors or other service failures, we would be happy to talk to you.
HTTP Error 503. The service is unavailable.
plz solve the problem
Create a new App pool with default settings…and re-try
Need to start the application pool
tank you very muuuuuuuuuuuuch
My application pool crushes everytime, I think the reason might be that it’s the only application that falls over none of the other ones do, please let me know how can I resolve this?
Something stopped the app pool in my case. Maybe leaving the debugger stopped in VS for 6 days may have done something? Did not realize that if something stops the app pool, restarting the computer is no help … its stopped unless started in IIS manager. Thanks for pointing me in the right direction.
My server shows 503 server error .Please help to fix this issue
Hello Mohanram,
We can take a closer look at your server settings. If you still have the 503 error and need help, we’ll be happy to talk to you on chat (click on the icon at right-bottom).
Hi .. thanks a lot for the useful info. We faced same issue solved by starting the app pool.
application stopped most of the time what might be the issue in IIS.
Mostly, the application will be misconfigured and that results in errors. We can help you debug further. We’ll be happy to talk to you on chat (click on the icon at right-bottom).
1. cmd (run as administrator)
2. input ==> iisreset
3.wait
4.Done.
Yes. IIS reset is recommended when multiple websites report errors. For a single website, recycling the application pool is a wise choice.
Hi this has started to happened suddenly for me… not sure what has changed. Im on windows 10. and installed IIS for development purposes. Everything was one until about 2 weeks ago when I was last using it. I uninstalled and reinstalled IIS. Even emptied the contents of the website and just kept won’t empty index.html in that… very weird…
Hello Nag,
Must be something with the IIS settings.
If you still have errors and need help, we’ll be happy to talk to you on chat (click on the icon at right-bottom).
Hi,
Very good article.
My application pool was stopped.
Many thanks