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Below is a common scenario that many system administrators, DevOps engineers, and developers encounter when users try to browse sites hosted on IIS. HTTP Error 503 Service Unavailable The following message appears:

childhood : This is what the user sees on the page, but there are a number of ways that engineers can troubleshoot and resolve this issue. The “503 Service Unavailable” message usually occurs when an application pool is stopped. But why would an application pool be stopped, especially when it is supposed to be running to support the use of a website? Surprisingly, IIS stops it. Does this sound strange? At first, it might, but there is a reason for this. Let’s take a look at why this happens.

Rapid failure protection : So, what should we do about this? Microsoft DocumentationThe World Wide Web Publishing Service (W3SVC) is configured to take all applications in an application pool offline when the number of worker process conflicts reaches a defined maximum. RapidFailProtectionMaxCrashes Properties within a specified time range RapidFailProtectionInterval properties. By default, these settings are enabled and have a failure limit of 5 crashes within 5 minutes. If you are receiving a 503 – Service Unavailable error and you notice that the application pool is stopped, the first step is to check the System Event Log.

But why is IIS stopping the application pool, and why is this configuration included in the advanced settings of the application pool?

This is because Windows Process Activation Service (WAS) keeps creating processes for application pools, and if crashes keep happening, it is expensive to repeatedly create processes that fail on the system. To prevent this, IIS stops the application pool and marks it as “stopped” until the administrator examines and resolves the underlying problem. Once the cause of the crash is resolved, the administrator can manually restart the application pool.

Troubleshooting : Now, enough talking, let’s get straight to the action.

Event Log : While reviewing the event log, I found five warnings following an error from Windows Process Activation Service (WAS).

The error is as follows:

“Please take a closer look,” it states.Due to a series of failuresThis confirms that Rapid Fail Protection is working. What is even more interesting is that each warning event shows a different process ID, even though there is only one worker running (assuming the default configuration has not changed). This indicates that the application is crashing every time it tries to perform a task.

crash : Okay, so we can conclude that the application crashed. Now it’s time to thoroughly review the event logs and see if any exceptions were logged in the application event log. These logs should contain a call stack that the developer can analyze further.

But what if the process crashes for reasons other than your code? This also depends on the details in the event log. If IIS is causing the process crash, it could be due to a missing module that is referenced in ApplicationHost.config but is not available.

For example, here’s one:

Some of the required IIS components are: no Loading correctly. Example: Module DLLDLL path> Failed to load. Data is in error.). This can happen when the feature is not properly installed on the server.

This is a sample error for reference.

• Module DLL C:\WINDOWS\System32\inetsrv\webdav.dll Load failed. Data is in error.

How to solve these types of problems

Make sure your system sees “Failed to load DLL”.C:\Windows\System32\inetsrv\config\applicationHost.config” file. If so, identify the associated role/service and install it on the server. In this example, “WebDAV publishing” IIS module. Let’s install this module by referring to the screenshot below.

NOTE: Depending on your server configuration, there may be one DLL causing this issue. See other “Failed to load DLL” errors for reference.

• Could not load module DLL C:\WINDOWS\System32\inetsrv\iiswsock.dll. The data is an error.
• How to fix: Please install it Web Socket Protocol IIS Module

• Could not load module DLL C:\WINDOWS\System32\inetsrv\warmup.dll. The data is an error.
• How to fix: Please install it Application Initialization IIS Module

• Could not load module DLL C:\WINDOWS\System32\inetsrv\logcust.dll. The data is an error.
• How to fix: Please install it Custom Logging IIS Module

• Could not load module DLL C:\WINDOWS\System32\inetsrv\authcert.dll. The data is an error.
• How to fix: Please install it Client Certificate Mapping Authentication IIS Module

• Could not load module DLL C:\WINDOWS\System32\inetsrv\webdav.dll. The data is an error.
• How to fix: Please install it WebDAV publishing IIS Module