My Issues with Parallel Test Execution in .Net

In a previous post I described how to get Selenium Grid up and running. The main reason for doing that is to speed up test execution. Well, that is my main reason. You may want easy testing cross browser, OS, device or something. At any rate, to get the speed we have to run the tests in parallel. Getting the Grid up was the easy part, running the tests in parallel is the hard part. How do we run tests in parallel? Warning this may be more of a rant that solution.

If we were using MBUnit, it would be simple. Parallel is built into the DNA of MBUnit, but the MBUnit project is on hiatus right now and direction seems to be up in the air. We currently use NUnit as our test framework at work, but it doesn’t support parallel execution. The next version of NUnit is supposed to support it, but it has been promised for a long time now. I use MSTest a lot, but I have never run it in parallel. I heard it is possible, but I also heard there are issues with it and the fact that Visual Studio 2013 now uses VSTest which doesn’t have parallel support, I am not sure what this means for MSTest. I guess I’m rambling as I am rehashing another post, “NUnit, OK Maybe“. The point is parellel test execution is a problem that should have been solved by now across the test frameworks in the .NET community. Hopefully, there will be changes this year.

Anyway, back to the topic at hand, I decided to look into PNUnit. PNUnit brings parallel execution to NUnit, but the documentation claims that it is a solution for running the same test suite in parallel across multiple instances and I need to run individual test across Nodes to increase speed of test execution, so I am not sure if PNUnit is a viable solution. Another problem with PNUnit is you have to configure the tests you want to run in parallel. This is not a maintainable solution at first glance as having to update configuration files every time I add a test would get old real quick.

So, even though MBUnit may or may not be dead I look more into it. Like I said, at work we currently use NUnit so we would have to convert a lot of tests, but it may be possible to automate a lot of the conversion. The main difference in code between the two would be the dependencies (using’s), class and method attributes, and assertions. So, we could probably convert 90% of the code with just plain find and replace or a quick code hack that does something similar. We would also have to do some work to get reporting to produce that same report on the build server. With that in mind I will look more into MBUnit, even if it is dead the source is available on GitHub.

While I am checking out MBUnit I will also have a look at TPL and the async keyword in .NET 4.5. My test framework already has concepts built in that would allow me to actually create my own test runner. I just have to learn the new tools in .NET for parallel and asynchronous coding. This would be a MAJOR under taking and not one I want to do right now, but I will if its what I have to do to speed these browser tests up.

Another issue I have now is I share a single driver across all tests in a fixture. I create a driver when the fixture is created and I cache it so the individual tests can use it without having to recreate it. This is great when the tests run one after the other. Now I need to change it so that each test has its own driver so they won’t step on each other’s toes. This isn’t that hard as I have a central setup methods for my fixtures and tests so the change would only have to occur in a couple spots (I try to keep it SOLID can’t stress this enough). So, I will move the driver creation from the fixture setup to the test setup and I am ready for parallel browser automation, or am I?

Having to fix the driver setup issue made me look closer at things that I am sharing across tests. Getting parallel tests is going to take a little more than flipping a couple switches to turn it on. Did I mention this is the hard part? The lesson learned from the shared driver is to insure your tests are idempotent during parallel execution. Hell, tests should be idempotent period, regardless of parallel execution. If you share anything across the tests, they can not have an effect on the outcome of other tests regardless of where or how they run. Whether they are in separate assemblies, in the same test fixture or they execute on different Grid Nodes a test can not create side effects that change the results of other tests. If they do produce side effects, you will begin to not trust your tests as they will start failing for reasons that have nothing to do with code changes. When you run tests in parallel it becomes a little tricky to locate where you violate the idempotent rule.

In my case I have to do an in depth review of shared data, functionality, and state. So far my biggest issue is I have a static class that I use to store various values during test execution. I call it TestSession and its basically a cache that allows me to save time by creating certain objects once and it also gives me a way to pass state from one test step to the next. TestSession keeps things separated by prefixing the session keys in a way that tests don’t share data, but there are fixture session keys that are shared amongst tests. Also, at the end of fixture execution we clear the entire TestSession object. So, if the state cached in a fixture session key is changed by one test it may have an effect on another test. I am pretty sure that I don’t do this, but there is nothing that prevents me from doing it so I have to look through the code. Also, if the session is cleared before all of the tests have completed there may be problems. So, I have to rethink TestSession completely. I may create an immutable session for the fixture and mutable session for the tests.

Well that’s it for now. I am starting to bore myself :). This is more of a post on my issues with parallel test execution and less of a solution post. I guess this is what you would call a filler just to keep my blogging going. If you made it this far and you have some ideas on how to solve parallel test execution in .NET, please leave me a comment.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s