log4net FallbackAppender – under the hood

Some Background

I recently put together log4net contrib project up on google code the only item currently up there (though hopefully there should be more stuff from the community up there) is a custom appender I wrote to address the need for having an appender that is clever enough to fallback to other appenders if one fails, this originally came from a forum post of someone wondering how to do it and after some investigation I was that NLog has this built-in.

Investigating how to add it

So my first step was to grab the log4net source and see how I could integrate this appender in, one thing I really didn’t want to do was to change the log4net code and instead allow the appender to be plugged in like you would do normally.

First step was to see how to create an appender that holds other appenders and stumbled across IAppenderAttachable and then the ForwardingAppender which implements it, this interface is what allows you to user the following xml:


And the configuration makes sure that the appender is populated with the referenced appenders. Looking at the behaviour of the existing ForwardingAppender it gave me a lot of what I was after so I inherited from this.

Hitting the main problem

Now I had the FallbackAppender which got most of it’s behaviour from the ForwardingAppender the next step was to setup the following logic:

  1. Get first appender in the collection
  2. Try and append to it
  3. If succeeded exit
  4. Otherwise try and append to the next appender in the collection
  5. Rinse repeat until one succeeds or all are exhausted

On the face of it this seems to be quite simple, loop the appenders and put a try…catch around the call to Append then have some logic in the catch to use the next appender and repeat, something like this:

while (apppenderQueue.Count > 0)   
        var appender = appenderQueue.Dequeue();   
    catch (Exception thrownException)  
        // log to internal log about failure 

When I tried this and tested against log4net it didn’t work only the first appender was being appended to if I setup the config incorrectly to simulate an issue.

After some digging the issue became obvious the appenders don’t let exceptions bubble up through the stack and instead log to an IErrorHandler that they get via a property from the AppenderSkeleton object they inherit from and with this being the only way to track errors from appenders I had to then impose a limitation.

Adding the AppenderSkeleton limitation to go forward

With the ErrorHandler property coming from AppenderSkeleton being the only way to discover errors I came to the conclusion that I needed to hook into this property luckily the property can be set, this also imposed the limitation that any of the appenders being referenced had to inherit from AppenderSkeleton in the inheritance tree at some point in order to get access to the ErrorHandler property.

Putting in the hooks

The next step I did was to create my own IErrorHandler that simply recorded if an error occurred, and at appender activation I replace each of the referenced appenders ErrorHandler with that one, now the code can see if an error has been raised for a particular appender and move onto the next one.


I guess this post demonstrates that even if a third party component doesn’t necessarily provide you with an API that makes extending behaviour easy in this example if log4net appenders let the exception bubble up or the Append method returned a bool whether it succeeded or not, there are still tricks we can use to try and achieve it.