Wednesday 29 April 2009

A Fluent NHibernate update

Since James has announced the latest happenings in the ongoing saga of moving Fluent NHibernate to using a semantic model, I thought a companion post might be appropriate.

My first post on the Fluent NHibernate rewrite is as good a place to start as any. In it I explained the limitations of the existing implementation and outlined my strategy for dissolving those limitations. My branch differs from the trunk by separating the fluent interface from the mapping model. The fluent interface manipulates the underlying model, and a family of visitors build a Hbm object graph that represents that underlying model. That object graph is serialized to xml and then passed to NHibernate. This graph consists of classes that are generated from the NHibernate mapping schema, so the serialization step is all automatic and works flawlessly.

The use of the visitor pattern allows for two things. One, the mapping model is not tied to any particular output format. Sure we pass NHibernate xml today, but tomorrow it would be nice to pass it the Hbm object graph directly so it can skip the xml schema validation step. The day after it would be nice to pass it a prebuilt NHibernate.Mapping graph, which is an idea that Tuna Toksoz was exploring from within my branch. Secondly, the visitor implementation allowed for powerful manipulations of the mapping model that would otherwise be difficult. I talked more about the visitor implementation here.

Sure, passing NHibernate its mappings in a format other than xml would be nice some day, but there was another reason why I chose to use a Hbm graph instead of directly writing xml. Basically, its much harder to go off the rails when working with the Hbm objects. One example is that the NHibernate schema is finicky about element ordering and you simply don’t have to worry about this when working with Hbm objects.

As time went on the code base grew, things changed, but slow and steady progress was made. The problem is that slow and steady doesn’t always win the race – specifically in the case where the goalpost is moving as fast or faster than you! This is the classic rewrite problem. I’m sure you are familiar with it – its why all the well respected software professionals tell you that rewriting your app is a fools errand. And don’t get me wrong, I am in agreement here. I approached my work to rewrite Fluent NHibernate as a prototyping exercise - “one possible future” if you will. As the work continued I would discuss our long term goals with James and for a while we found ourselves hoping that we could replace the existing trunk with my branch, if only we could “catch up” to the trunk functionality. Of course this sort of thinking was dangerous, and the consequences are well described in James’s post. My intentions, while good, have lead to a lack of progress on the Fluent NHibernate trunk and this is simply not a healthy place to be.

James asked me whether I could see a way to merge the two code bases. Basically inject my model into the existing guts of the trunk. I told him I couldn’t see a way how, but I encouraged him to try. The problem, I’m sure you’ve already guessed, is that I was too “caught up” in my code and simply not prepared to make sacrifices. This is why James managed something that I could not – he merged my branch into the trunk code under an “integration”  branch, with all the existing trunk tests passing. Its not pretty, but I suspect that it represents the best direction to work from. We can now do what the experts tell us to do and refactor, rather than rewrite.

So what were the sacrifices? The main one is the Hbm object graph approach. Fluent NHibernate will only be capable of spitting out xml for the near future. Lets face it, its not a huge loss. Plus I say near future because the visitor implementation survived, so there is no reason why we couldn’t later start retrofitting my Hbm code in. I am yet to take full stock of the “casualties”, but I expect that everything that has been lost in the merge can be regained through a gradual process of refactoring and reintroducing code and ideas from my branch. It will be fun!

Finally I want to use this post as an opportunity to thank James for all his amazingly hard work on this great project. We don’t always see eye to eye but the thing that I often remind myself is that time and again he has proven to me that he has his finger on the pulse of our users and can see how this project can best fulfil their needs. Please keep it up James.

Monday 27 April 2009

Legacy code: a follow up

A few weeks back I wrote a post on Working Effectively with Legacy Code, a book by Michael Feathers. At the time I lamented that even though I accepted Michael’s definition of legacy code (code without tests), I wasn’t doing much about it – I hadn’t written any tests at work. Today I wanted to follow up on this because the last two weeks have gone much better – I’ve been writing unit tests. I was given the job of rewriting a particularly hairy module and this was simply too good an opportunity to pass up. I’m not doing TDD yet, and I certainly haven’t written as many tests as I would like, but its a start.

Rather than using a mocking framework, I’ve been hand rolling stubs. I believe that the other developers will have an easier time understanding the tests if I use manual stubs. Mocking frameworks are certainly convenient but as Michael demonstrates in his book, they are not necessary per se. So far I haven’t found this particularly painful and I think its a sound approach for introducing automated unit testing. I must admit that the fact that I am working on a .NET 2.0 project made this decision easier. I learned Rhino Mocks using the .NET 3.5 API and I think I would have a hard time going without the sweetness that lambda expressions provide.

Writing that very first test is a bit of a barrier, but its only a mental one. I was fortunate to have a nice piece of work come along and give me a push. Hopefully it will not be necessary the next time I find myself faced with legacy code – I intend to write that first test without a moment of hesitation. Its all downhill from there.

Monday 13 April 2009

Working Effectively with Legacy Code

Working Effectively with Legacy Code (Robert C. Martin Series)

Today I finished reading Working Effectively with Legacy Code by Michael Feathers. If you are not familiar with this book, you might be wondering if I’ve somehow been roped into maintaining some sort of 10 year old VB6 application -fortunately this is not the case. But I am indeed working on a legacy application – at least, it is legacy by Michael’s rather unique definition. Michael defines legacy code as code that is not under test, specifically unit tests. By Michael’s definition, virtually all of the code I have written during my career was legacy from the day it was born, and when I reflect on it, this seems like a fair statement. Here is a short excerpt from the preface:

Code without tests is bad code. It doesn’t matter how well written it is; it doesn’t matter how pretty or object-oriented or well-encapsulated it is. With tests, we can change the behaviour of our code quickly and verifiably. Without them, we really don’t know if our code is getting better or worse.

It doesn’t matter how clean and refactored my code was – developers that had to maintain it were forced to do so with uncertainty. It is specifically that element of uncertainty that I associate with the legacy code experience.

The book uses a FAQ format, with chapter titles such as:

  • I need to make a change. What methods should I test?
  • I can’t get this class into a test harness.
  • This class is too big and I don’t want it to get any bigger.
  • I need to make a change but I don’t know what tests to write.

It does a great job of illustrating refactorings that let you break dependencies so you can get the code under test. These refactorings are designed to be performed safely without tests so you can get the tests written and then move on to more significant refactorings. The book also does a great job of tackling the mechanics of writing the tests themselves.

I think the book could have been better named. Ayende says that it should be mandatory reading for people that write code for a living. I agree with him, but the title does not communicate this. In general I found it very readable and relatively quick to get through, and the lessons the book offers are profound.

So the obvious question is whether it has changed the way I’ve been writing code. Well unfortunately, I wrote a bunch of legacy code just last week! By night I revel in the freedom that my unit tests give me as I work on Fluent NHibernate, but by day I continue to work in a manner that Uncle Bob would consider unprofessional. Why?

Unfortunately the book doesn’t have a chapter called “I’m all alone, nobody else is convinced that writing unit tests is worthwhile. Is it worth starting if its just me doing it?”. Nor does it have a “This is just a quick fix for someone else’s broken code, I’ll write tests when the –real- work begins” chapter (its a weak argument but I’ve found it an easy trap to fall into). The closest the book comes is the very last FAQ chapter: “We feel overwhelmed. It isn’t going to get any better”. But at less than 2 pages, it didn’t quite deliver what I needed to hear.

I’m finding it difficult to take that first step and add that 55th project to the solution (yeah…) so I can write some tests. Part of it is procrastination, but part of it is fear. I believe these ideas have value and I don’t want to screw it up. But simply “waiting until the time is right” is the refrain of the procrastinator.

This makes me feel that Working with Legacy Code is a great book, but not an entirely complete package. It gives you the tools and techniques but assumes that the reader can simply read it and run with it. Michael developed these techniques as he worked with teams that heeded his advice and were prepared to follow his lead. A piece of the puzzle is missing because Michael has been the consultant for so long.

Maybe this criticism is unfair. People can write books full of advice, but its up to the reader to start putting that advice into practice. A new work week begins tomorrow, perhaps I’ll add that 55th project before I do anything else tomorrow morning.