Look at HTTP API Logs in IIS 7

In this article, we will look into the HTTP API error log feature of IIS 7/ 7.5, when to use it and how to analyze it. When a connection times out, dropped or a request is orphaned on your website, it will be logged in the HTTP API error log file by IIS under "c:\Windows\System32\LogFiles\HTTPERR\httperr#.log". This file helps us to identify the following kinds of errors:

1. Connection time-outs for a pending request.
2. Orphaned requests when w3wp.exe crashes or stops unexpectedly, while there are pending queued requests.
3. Error responses to clients like parser errors.

Let's look at the following sample HTTPERR#.LOG file:



It shows date, time, client's IP, port as well the s-reason field that shows specific error types, like BadRequest, Connection_Dropped and the following is the explanation for that field's value:

s-reason	Description
AppOffline	A service unavailable error occurred (an HTTP error 503). The service is not available because application errors caused the application to be taken offline.
AppPoolTimer	A service unavailable error occurred (an HTTP error 503). The service is not available because the application pool process is too busy to handle the request.
AppShutdown	A service unavailable error occurred (an HTTP error 503). The service is not available because the application shut down automatically in response to administrator policy.
BadRequest	A parse error occurred while processing a request.
Client_Reset	The connection between the client and the server was closed before the request could be assigned to a worker process. The most common cause of this behavior is that the client prematurely closes its connection to the server.

Refer here for a complete list of values for the s-reason field. This field will help us to understand what exactly caused the problem like Application Pool is down or is it a Bad request or a connection timed out.

We can use the Log Parser to better analyze HTTPERR#.LOG files. Open the Log Parser and point to the log file as shown below:



Go to the library and filter it by httperr as shown below:



Let's find abandoned requests by selecting "HTTPERR: Abandoned Requests" and hitting F5 to execute the query:



Analyzing HTTPERR#.LOG with the Log parser will help us to identify connection time-outs, orphaned requests, bad requests and so on quickly.