How to debug a bug that can not be reproduced?

Question

Now that the use of one of my asp.net apps has gone up significantly, two odd problems are occurring that are very infrequent, and that I cannot reproduce.

I am at a loss as to how to debug and troubleshoot these problems.

Here are two examples:

One of my aspx pages resets a session state value to 0 on !IsPostBack (is true). However, one of my users at a specific location frequently comes to that page when it is not a postback and the session state value does not get reset on his laptop. (I am basing this statement on how the app subsequently behaves, not on running in debug mode) But the code works and the session state is reset on my laptop when I am sitting next to him running the app on my laptop using the same browser on the same internet connection at the same time. And when this user runs the app on his laptop from home where he has a better internet connection he does not have the problem as frequently.

One of the aspx pages in my app does a server.transfer to itself after running code that saves data to a DB. Almost all the time after the server.transfer the textboxes contain their default value (as they should since !isPostback==True), but about 1% of the time the textboxes contain the previous value. I know that there has been a roundtrip to the server because data has been saved. This problem occurs on the same pcs using the same browsers by the same users doing the same actions. So 99% of the time it works correctly, and 1% of the time they do the exact same thing and it does not work correctly.

How do I even start trying to figure out what is causing these problems if they seem to be occurring randomly?

I suspect that the quality of the internet connection is the issue because it is the one variable that is changing, but how does that info help me?

It's not like I can debug either of these problems by running my app in debug mode.

I am using Asp .Net 3.5, C# 3.5 and the app is run in IE 6-8. (IE 8 in compatibility mode)

Answer 1

It seems like you're having problems with Session State. By default, ASP.NET uses InProc session state mode which uses server memory to store values. In many occasions, this can be lost or reset (app pool recycled). Switching to SQLServer Session State might help you solve the issue.

Answer 2

I would add logging to code where the problem is occurring. Then inform the users who are having the problem to try and note the time when they run into the issue. Once you have the logs and an approximate time, you can go in and pour over these logs to see if anything points you in the right direction. I would also look at your IIS and Event Logs on the server.

Answer 3

You can install Firefox Throttle plugin to simulate slow connections. Lot's of things can happen in ASP.NET with slow page loads. If the page isn't loaded fully but items are clickable ASP.NET can get really upset with event validation...etc

Also, I encourage you to start logging and tracing the problem areas in your application. You can then correlate that with the IIS request logs and get a fairly accurate picture of whats's happening when.