Have you ever wondered why the Application Pool sometimes fails to recycle according to the specified conditions, such as regular time intervals or specific schedules? There can be various reasons behind this unexpected behavior. In this article, we will explore the most common causes of this issue.

Problem

For the context of this discussion, we have set the recycling condition to occur every 60 minutes. Ideally, the Application Pool should recycle automatically at these regular intervals.

We can confirm the recycling event in the system event logs. As illustrated in the image below, there is an event from the WAS source indicating that the worker process with the process ID ‘<process id of the w3wp.exe>’, serving the application pool ‘<application pool name>’, has requested a recycle because it reached its allowed processing time limit.

However, in some cases, this event may not appear, or the Application Pool does not recycle at the scheduled time or under the specified conditions. The most common reason for this is that the worker process may have already shut down due to inactivity or other factors. To verify this, check the advanced settings of the Application Pool and review the Idle Time-out setting.

Now, look for events from the WAS source to see if there is an entry stating, “A worker process with process ID ‘<process id>’ serving application pool ‘<application pool name>’ was shut down due to inactivity. Application Pool timeout configuration was set to 20 minutes. A new worker process will be started when needed.”

You should also verify if there were no requests made between the time the application pool shut down and the scheduled recycle time. This will confirm that no worker process was running to service requests during that period. You can check this by reviewing the IIS logs. While I won’t go into detail here, analyzing IIS logs could be a separate topic for discussion.

Cause
In summary, if no worker process is running, the recycling conditions for regular intervals or specific times won’t trigger a recycle of the application pool.

Conclusion
In conclusion, understanding why an Application Pool may not recycle according to its defined conditions requires a comprehensive review of several factors. Various issues can prevent this from happening as expected. The most common cause is the shutdown of the worker process due to inactivity, which can result in the application pool not recycling at the scheduled time.

By checking the system event logs for relevant events and verifying the Idle Time-out settings in the Application Pool’s advanced settings, you can identify whether inactivity or other issues are affecting the recycling process. Additionally, reviewing IIS logs to confirm that no requests were made between the shutdown and scheduled recycle time will help ensure that no worker process was available to handle requests.

Ultimately, if no worker process is running, the application pool’s recycling conditions will not be triggered, thus preventing the recycle from occurring. Addressing these issues and ensuring proper configuration can help maintain the reliability and performance of your application pool.

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