Category: FaultInjection

.Net Fault Injection – It’s not just about exceptions

I had a interesting comment on my previous post about .Net fault injection. ‘Losing Side’ asked if this would work for simulating other faults such as timeouts. It’s a good point and one I didn’t think about yesterday, but there are other faults which are interesting when testing the application. Performance is one of those areas, creating performance problems, such as slow disk IO or a slow server is difficult if you don’t have the setup in place, and even then not always possible. How can you effectively, repeatability test for a slow hard drive (and using a virtual machine doesn’t count). Tools such as ANTS Profiler will help tell you where the problems are, but only if you can reproduce the problem.

First demo of the day, I ask the question – how can you simulate a slow write process when using StreamWriter?  Based on my previous post, I’ve changed the method to this:

private static void MethodFails()
{
    Console.WriteLine(“Writing to a file @ ” + DateTime.Now);

    string path = Path.GetTempFileName();
    StreamWriter sw = new StreamWriter(path);
    sw.WriteLine(“This is a test @ ” + DateTime.Now);
    sw.WriteLine(“This is a test @ ” + DateTime.Now);
    sw.WriteLine(“This is a test @ ” + DateTime.Now);
    sw.WriteLine(“This is a test @ ” + DateTime.Now);
    sw.Close();
    Console.WriteLine(“Done”);

    foreach (var s in File.ReadAllLines(path))
        Console.WriteLine(s);
}

Running my console application normally, I get the following output, notice no delays between each write:

Writing to a file @ 15/11/2008 13:23:48
Done
This is a test @ 15/11/2008 13:23:48
This is a test @ 15/11/2008 13:23:48
This is a test @ 15/11/2008 13:23:48
This is a test @ 15/11/2008 13:23:48

Running this using my injector, I have a five second delay (which I set, easily could have been a random number) between each of my writes to the file.

Writing to a file @ 15/11/2008 13:19:45
Done
This is a test @ 15/11/2008 13:19:51
This is a test @ 15/11/2008 13:19:56
This is a test @ 15/11/2008 13:20:01
This is a test @ 15/11/2008 13:20:06

I now get the same five second delay each time this happens, creating a scenario repeatable.

Disappointingly, I tried to use the SQLConnection object, but I couldn’t get this to work. I don’t know if its a limitation or a bug.  Still a lot more work to do until its even remotely useable, but I’m finding the concepts interesting.

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.Net Fault Injection – Very early proof of concept

I’ve just completed my first proof of concept which I’m very excited about and I just had to write a quick blog about it – plus it breaks my blogging silence.

I find fault injection an interesting concept, I’ll explore it in more detail in later posts, but the aim is to insert exceptions at various points in order to test certain behaviours and how the application copes when an error occurs. However, raising exceptions is not a easy task, you have to create the scenario and environment in order for the exception to occur. With my fault injection application, you don’t need to create the scenario, you simply say when you want the exception to occur. For example, I would be able to throw an IOException when System.IO.File.ReadAllLines() is called without actually having to create the scenario for the exception to be raised – saving me time and effort but also allowing me to test more scenarios and error handling.

Tonight, I created a very simple concept to throw exceptions on method calls. I’ve got a very simple console application which calls two methods – nothing special about this.  When MethodFails() is called, my injector will step in and throw a MethodAccessException (could be any exception) which will be caught by the console application with the details of the exception being outputted.

using System;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(“Should output 1 message then fail”);
            try
            {
                MethodA();
                MethodFails();
            }
            catch (Exception e)
            {
                Console.WriteLine(“Exception caught”);
                Console.WriteLine(“Type: ” + e.GetType());
                Console.WriteLine(“Message: ” + e.Message);
            }
        }

        private static void MethodA()
        {
            Console.WriteLine(“This should work”);
        }

        private static void MethodFails()
        {
            Console.WriteLine(“this should have failed”);
        }
    }
}

The output of the execution is the following information:

Should output 1 message then fail
This should work
Exception caught
Type: System.MethodAccessException
Message: You attempted to call ConsoleApplication2.Program.MethodFails, this has been blocked. Goodbye

 

Technical details to come at a later point, it’s getting late in the UK. However, I would love to hear your comments on this idea.

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