Application "freezes" and requests queue is full

Question

I have an asp.net application running on IIS 7.5 , Windows Server 2008 R2 – using an application pool in classic mode, framework version 4.

Sometimes I am running into the following problem: The application can work for a few days, but all of a sudden I receive an http error 503 – server unavailable.

When I look at the application pool it seems to be running (I see it started), but it actually FREEZES – every request to it is responded with 503. At the worker processes list (in the IIS manager) I see a lot of requests unhandled.

It's important to mention that other asp.net application running under other application pools are working just fine which means the IIS is working fine and the problem is only in this specific application pool.

When I researched the http error logs I saw the following error logs in the Windows\System32\LogFiles\HTTPERR folder:

• In regular settings when everything works fine I noticed records of "Timer_ConnectionIdle" (a normal thing from what I have read).
• At a certain times I notice an appearance of "Client_Reset" records.
• 15 minutes after the "Client_Reset" errors started->records of "QueueFull appear".

In order to work with the application I am doing an iisreset (I guess a recycle for the pool will be enough also).

I will be happy to receive any help or suggestions.

EDIT:

It's important to mention that nothing related gets written to the IIS logs, or the System and Application logs. This error occurs before.

Answer 1

Without more information about your problem, most quick fix will be to Configure Recycling Settings for an Application Pool. Since your problem is about request queues, you can choose option After reaching a number of requests.

Answer 2

When I encounter HTTP 500 errors, I find that enabling IIS Tracing is very helpful. Below is a link that describes the process of enabling tracing and then reviewing the trace. The first section of the site describes how to install IIS, so you probably want to skip to the section labeled "Enable Failed-Request Tracing."

Troubleshooting Failed Requests Using Tracing in IIS 7

Edit:

Since you're getting a QueueFull error, you may want to monitor the request queues. The easiest way to do this is using Perfmon. On the server with IIS, open Performance Monitor and add the appropriate counters under "HTTP Service Request Queue." In your case, "Current Request Queue" for the ailing Application Pool would likely be of value.