What information should I be logging in my web app?

Question

I finishing up a web application and I'm trying to implement some logging. I've never seen any good examples of what to log. Is it just exceptions? Are there other things I should be logging? What type of information do you find useful for finding and fixing bugs.

Looking for some specific guidance and best practices.

Thanks

Follow up

If I'm logging exceptions what information specifically should I be logging? Should I be doing something more than _log.Error(ex.Message, ex); ?

Answer 1

Here is my logical breakdown of what can be logged within and application, why you might want to and how you might go about doing it. No matter what I would recommend using a logging framework such as log4net when implementing.

Exception Logging

When everything else has failed, this should not. It is a good idea to have a central means of capturing all unhanded exceptions. This shouldn't be much harder then wrapping your entire application in a giant try/catch unless you are using more than on thread. The work doesn't end here though because if you wait until the exception reaches you a lot of useful information would have gone out of scope. At the very least you should try to collect specific pieces of the application state that are likely to help with debugging as the stack unwinds. Your application should always be prepared to produce this type of log output, especially in production. Make sure to take a look at ELMAH if you haven't already. I haven't tried it but I have heard great things

Application Logging

What I call application logs includes any log that captures information about what your application is doing on a conceptual level such as "Deleted Order" or "A User Signed On". This kind of information can be useful in analyzing trends, auditing the system, locking it down, testing, security and detecting bugs of coarse. It is probably a good idea to plan on leaving these logs on in production as well, perhaps at variable levels of granularity.

Trace Logging

Trace logging, to me, represents the most granular form of logging. At this level you focus less on what the application is doing and more on how it is doing it. This is one step above actually walking through the code line by line. It is probably most helpful in dealing with concurrency issues or anything for that matter which is hard to reproduce. You wouldn't want to always have this running, probably only turning it on when needed.

Lastly, as with so many other things that usually only get addressed at the very end, the best time to think about logging is at the beginning of a project so that the application can be designed with it in mind. Great question though!