Friday, November 23, 2018

Review: Web-Based Course Test-Driven Development For Embedded C/C++, James W. Grenning

Full disclosure: I was given a seat in this course by James Grenning.

I took James Grenning's 3-day web-based course Test-Driven Development for Embedded C/C++ September 4-6, 2018. It was organized as a live online delivery, 5 hours each day. The schedule worked out perfectly for me in Boston, starting at 9AM each morning, but he had attendees from as far west as California and as far east as Germany, spanning 9 time zones.

The participants ranged from junior level embedded developers to those with more than 20 years of experience. One worked in a fully MISRA-compliant environment. This was the first introduction to TDD for some of them.

The course was organized as blocks consisting of presentation and discussion, coding demo, then live coding exercises. It used CppUTest as the TDD framework.

The short answer: this is an outstanding course. It will change the way you work. I highly recommend it, well worth the investment in time and money. The remote delivery method worked great.

I had previously read Grenning's book, Test Driven Development for Embedded C, which I reviewed in August. I covered a lot of his technical information on TDD in the review, so I'll only touch on that briefly here. He covers the same material in the course presentation portions.

The course naturally has a lot of overlap with the book, so each can serve as a standalone resource. But I found the combination of the two to be extremely valuable. They complement each other well because the book provides room to delve more deeply into background information, while the course provides guided practice time with an expert.

Reading the book first meant I was fully grounded in the motivations and technical concepts of TDD, so I was ready for them when he covered them in the course. I was also already convinced of its value. What the live course brings to that part is the opportunity to ask questions and discuss things.

You can certainly take the course without first reading the book, which was the case for several of the participants.


For the presentation portions, Grenning covered the issues with development using non-TDD practices, what he calls "debug-later programming" (DLP). This consists of a long phase of debug fire-fighting at the end of development, that often leaves bugs behind.

He introduced the TDD microcycle, comparing the physics of DLP to the physics of TDD. By physics he means the time domain, the time taken from injection of a bug (repeat after me: "I are a ingenuer, I make misteaks, I write bugs") to its removal. This is one of the most compelling arguments for adopting TDD. TDD significantly compresses that time frame.

He covered how to apply the process to embedded code and some of the design considerations. He also talked about the common concerns people have about TDD.

One quote from Kent Beck that I really liked:
TDD is a great excuse to think about the problem before you think about the solution.
He covered the concept of test fakes and the use of "spies" to capture information. He covered mocks as well, including mocking the actual hardware so that you can run your tests off-target.

He covered refactoring to keep tests easy to follow and maintain. He also covered refactoring of "legacy code" (i.e. any production code that was not built using TDD), including "code smells" and "code rot", using TDD to provide a safety harness. This included a great quote from Donald Knuth (bold emphasis mine):
Let us change our traditional attitude to the construction of programs. Instead of imagining that our main task is to instruct a computer what to do, let us concentrate rather on explaining to human beings what we want a computer to do.

Coding Demos

Grenning performed two primary live coding demos. First, he used TDD to build a circular buffer, a common data structure in embedded systems. He used this to demonstrate the stepwise process of applying the TDD microcycle.

Second, he performed a refactoring demo on a set of tests. He used this to show how to apply the refactoring steps to simplify the tests and make the test suite more readable and maintainable.

This was just as valuable as the TDD microcycle, because a clean test suite means it will live a long and useful life. Failing to refactor and keep it clean risks making it a one-off throwaway after getting its initial value.

Coding Exercises

Grenning uses Cyber-Dojo to conduct exercises (as well as his demos). This is a cloud-based, Linux VM, ready-to-use, code-build-run environment that allows each student to work individually, but he can monitor everyone's code as they work. This turned out to be one of the most valuable aspects of the course.

I should also mention that I had read Jeff Langr's book Modern C++ Programming with Test-Driven Development: Code Better, Sleep Better in between reading Grenning's book and taking this course. Langr puts a lot of emphasis on short, easily-readable tests, and that's something that also comes out in Grenning's class.

What was so valuable about doing these exercises in Cyber-Dojo is that Grenning was able to stop someone who was heading off in the wrong direction and quickly bring them back on track, or help them if they weren't sure what to do next. That fast feedback cycle is very much in tune with TDD itself. It works just as well as a teaching method.

So if someone started writing code without a test, or wrote too much code for what the test covered, or had too much duplication in tests, or had too much setup that could be factored out, he let them know and guided them back. In some cases he interrupted the exercise to go through someone's code on the screen with everybody watching, not to put them on the spot, but to cover the issues that we all run into.

That's critical because learning to truly work effectively in TDD style requires a reorientation of your thinking. We all have the coding habits of years that need to be overcome.

That doesn't happen automatically just because you read a book and have decided to follow it. It takes effort; half the effort is just noticing that you're straying from the intended path. That's the value of having a live instructor who can watch over your shoulder. It's like being an apprentice under the watchful eye of a master craftsman.

For me, this was ultimately the greatest value in the class. Having Grenning provide real-time guidance had an immediate effect on my coding, for both the test code and the production code. Whether it was talking about my mistakes or someone else's, I was able to immediately improve my work.

That made a huge difference between the test code I wrote before the class and the test code I wrote by the end of the class.

The coding exercises were building our own circular buffer, building a light controller spy, using TDD with the spy to implement a light scheduler, and implementing a flash chip driver. Note that these exercises are also covered in his book.

I also found that Cyber-Dojo made for an interesting example of pair programming, something I've never done before. Grenning provided initial files to work on, like a pair partner guiding you in the next step, then provided active feedback, like a partner asking questions and making suggestions: "Are you missing something there? What if you tried this? Wait, before you do that...".

The Big Lesson

The big lesson for me from this course was that it finally drove home that TDD is ALL ABOUT DEVELOPMENT! Sometimes I have to be clubbed over the head for something to really sink in, and that's what happened here.

We get so focused on the word "test" in TDD that we jump to the conclusion that it's just a test methodology. We emphasize test, as in TEST-Driven Development.

But really, the emphasis should be reversed, it's Test-Driven DEVELOPMENT. That means you apply design concepts and address the requirements of the product as you engage in a very active development thought process that is driven forward by tests.

Did you ever write some throwaway test code just so you could see how something worked, or to explore some design ideas? Hmmm, well TDD formalizes that.

The fact that you do end up with useful unit tests is almost a side effect of the process. An extremely valuable side effect, but a side effect nonetheless.

The real output of the process is working production code. That's what really matters. That's the real goal.

At some point on the last day of the course, I recognized the change in emphasis deep in my being. Maybe the difference is subtle, but it is critical.

That recognition first started to dawn after I read the book and applied it at work. I was amazed at the cleanliness of the resulting code. It was DRY and DAMP and SOLID, with no further refinement or debugging required.

Yes, I had a unit test suite. But look at the production code! It was breathtaking, right out of the chute. That was motivating.

It was in that receptive frame of mind that I did the coding exercises in the course. That was when the club hit. It was one of those moments of realization where you divide time into what came before, and what came after, the physical moment of grok, providing a whole new lens through which to perceive the work.

Savor that consideration for a moment.

People have been saying for years that TDD is about development, but we tend to focus on the test. Grenning emphasizes development when he talks about developing while "test-driving", meaning he is doing his development driven by tests. I guess it just takes time for the real implications to sink in.

One of Grenning's slides quotes Edsger Dijkstra:
Those who want really reliable software will discover that they must find means of avoiding the majority of bugs to start with, and as a result, the programming process will become cheaper. If you want more effective programmers, you will discover that they should not waste their time debugging, they should not introduce the bugs to start with.
While we all aspire to be like Dijkstra, this seems like a pipe dream. Until you realize that TDD does exactly that. It provides the shortest path to working software. I think he would have liked that.

Now that I've relegated the test aspect of this to second-class citizenship, let me bring it back to prominence.

The testing aspect approaches Dijkstra's ideal, because it finds bugs immediately as part of the code, build, test cycle. So the bugs are squashed before they've had time to scatter and hide in the dark corners. That reduces the dreaded unbounded post-development debug cycle to near zero.

If you don't let bugs get into the code, you won't have to spend time later removing them. Yeah, what Dijkstra said.

This doesn't guarantee bug-free code. There might still be bugs that occur during the integration of parts that are working (for example, one module uses feet, while another uses inches), or the code may not have addressed the requirements properly (the customer wanted a moving average of the last 5 data points, while the code uses the average of all data points), but as a functional unit, each module is internally consistent and working according to its API.

The resulting unit test suite is an extremely valuable resource, just as valuable as the production code. What makes it so valuable? Two things: safety harness, and example usage.

It provides a safety harness to allow you to do additional work on the code, then run the suite as a regression test to prove you haven't broken anything. Or to detect breakage so you can fix it immediately.

Using and extending the suite liberates you to make changes to the code safely. Need to add some functionality? Fix one of those integration or requirements bugs? Refactor for better performance or maintainability? Clean up some tech debt? Have at it.

You can instantly prove to yourself that you haven't screwed anything up, or show that you have, so that you can fix it before it ever gets committed to the codebase. No one will ever see that dirty laundry.

It provides example usage, showing how to use the API: how to call the various functions, in what order, how to setup and drive various behavioral scenarios, how to exercise the interfaces for different functional behaviors, how different parameters affect things, how to interpret return values.

This is real, live code, showing how to use the production code as a client. You can even get creative and add exploratory tests that push the production code in odd directions to see what happens. Grenning calls these characterization tests and learning tests.

The test suite is actually something quite magical: self-updating documentation! Since you need to invest the time to maintain the tests in order to get the development done, you are also automatically updating the example usage documentation for free.

You might argue that tools like Doxygen offer similar self-updating capability, but they still require updating textual explanations along with the code. They are subject to the same staleness that can happen with any comments, where the comments (or Doxygen annotations) aren't kept up to date with code changes (see Tim Ottinger's Rules for Commenting for advice to help avoid stale comments).

But if you want to really know how to use the production code, go read the tests! If you've truly followed the TDD process as Grenning shows you in this course, they will tell you how to produce every bit of behavior that it is capable of, because every bit of behavior implemented will have been driven by the tests.

That's the full-circle, closed-loop feedback power of test-driven DEVELOPMENT.

Doxygen still has its place. I think of the Doxygen docs as API reference, while the test suite is API tutorial, showing actual usage.

Another Lesson

I've already alluded to the other interesting lesson that I drew from this course: it takes practice! We're not used to working like this, so it takes practice and self-awareness to learn how to do it.

That was particularly driven home by the coding exercises. Even though I had just read his book and followed through the exact same exercises, and read Langr's book, and applied the knowledge at work, I still had trouble getting rolling on the first couple of exercises. It was a matter of instilling the new habits.

It took a few times having Grenning redirect me (or listen to the advice he gave someone else). By the final exercise, after the benefit of his live feedback, I was able to catch myself in time and start applying the habits on my own.

It's still going to take some time. I'll know I've gotten there when I start thinking of the tests automatically as the first step of coding.

Third Time's A Charm

At one point in the discussion I mentioned that Grenning's book and this course represented my third attempt at using TDD, and one of the participant said he would be interested in hearing about my previous attempts.

My first attempt was in 2007, when I was introduced to TDD by a coworker. I read Kent Beck's Test Driven Development: By Example and used it to develop the playback control module for a large video-on-demand server intended for use in cable provider head ends.

This was both a great success and a classic failure. It was a great success in that it accelerated my work on the module, avoiding many bugs and shortening the debug cycle. In that respect it lived up to the promise of TDD completely.

It was a classic failure in that I made the tests far too brittle. I put too much internal knowledge of the module in them, with many internal details that were useful when I was first developing the module, but that became a severe impediment to ongoing maintenance.

The classic symptom of this problem was that a minor change in implementation would cause a cascade of test failures. The production code was fine, but some internal detail such as a counter value that was being checked by the tests had changed. Otherwise the test code itself was also fine. But I had overburdened it with details that should have been hidden by encapsulation.

The result was that ultimately I had to abandon the test suite. It had provided good initial value, but failed to deliver on-going value because it became a severe maintenance burden.

This is exactly the type of situation that Grenning's course seeks to prevent. During coding exercises, he watches out for cases of inappropriate information exposure. Thus another benefit of this is improved encapsulation and information hiding.

My second attempt was in 2013, when I wanted to refactor some of the code in an IP acceleration server as part of improvements to one of its features. I had read Michael Feathers' Working Effectively with Legacy Code, and found that many of the things he covered applied to the codebase I was working on.

This was a revenue-generating service product, so I needed to be sure I didn't break it.

The main strategy the book covers is to use TDD to provide that safety harness I mentioned above, in order to verify that the legacy code behaves the same after modification as it did before.

I began building a set of test fakes that could be used with Google Test. One issue was that the code relied heavily on the singleton pattern, so there always had to be some implementation of each class that would satisfy the linker. And of course there were chains of such dependencies interlocked in a web.

My first task was to replace that bit by bit with dependency injection. I focused just on the parts necessary to allow me to test the area I was modifying. Part of Feathers' strategy is to tackle just enough of the system at a time to be able to make progress, rather than a wholesale break-everything-down-and-rebuild approach.

I had enough success with this that once I finished my primary work on the feature changes, I embarked on a background project to put the entire codebase into 100% dependency injection. That would allow me to build unit tests for any arbitrary component, in combination with any set of faked dependencies, with the longer-term goal of building out near-100% unit test coverage incrementally.

However, not too long after starting this, I ended up changing jobs. So once again I got the short-term benefit from TDD, but didn't reap the long-term benefit. It was a useful exercise to go through, though, providing good experience on how to migrate such a codebase to TDD.

This is another area that Grenning's course covers.

Related Links

For the perspective of another class participant, see Phillip Johnston's post What I Learned from James Grenning's Remote TDD Course.

There are things about the TDD process that make people suspicious. Is it just hacking? In this interview with Grenning, embedded systems expert Jack Ganssle raises some of those concerns. Grenning explains how the process works to reach the goal of well-designed, working production code that meets customer requirements.

Elecia and Christopher White have a great interview podcast with Grenning. Best joke: how many Scrum masters does it take to manage one developer? Also good Shakespeare and Bradbury quotes that are much ado about programming.

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