TDD - That’s Design Done

For many years now there have been internet flame wars about TDD in the software development community.

Most people I talk to involved in software development seem to have strong opinions for or against Test Driven Development and some of the heated discussions on forums have become as passionate as arguments over politics or religion.

I am going to take a tongue in cheek look at some of the characterisations of the different sides of this long running argument so I’m guaranteed to offend everyone!

On one side are the proponents of TDD as a way of life for developers.

Test Driven Disciples

The strict adherence to the process and disciplines of failing-test-first/implement-test/pass/re-factor/repeat conjures up images of the developer equivalent of the warrior monks of China. Developers rising before the sun to practice their code kata, lines of practitioner hunched over keyboards applying the steps of TDD as if driven by a metronome, each successful invocation of the ‘green’ genie of a passing test accompanied by a Kiai of triumph.

What they think of as the studied concentration of practised discipline is often perceived by non-TDDers as the slavish adherence to an unthinking cult. Mindless automatons relentlessly ‘mocking’ real design to produce a disconnected, disassociated collection of classes that clog the understanding of the problem space with unnecessary layers of abstraction.

On the other are the haters of TDD. More difficult to categorise, they form their own little bands of rebels.

Anarchy of ‘Testless’ Development

The Code Warriors

This post apocalyptic band of ‘code’ warriors hack code as fast as they can to meet delivery ‘dead’-lines set by the cruel warlords of the project management cult. They have no time for testing of any kind. Testing is for ‘wimps’!

Many don’t survive this brutal and unforgiving landscape, brought to an untimely end by premature release schedules. The hardened veterans carry scars, missing the figurative ‘limbs’ of code quality, professional pride bleeding into the sand of the unforgiving desert of the ‘immaculate schedule’.

The Afterthoughts

Usually made up of outcasts and refugees from the hardened ‘Code Warriors’, the Afterthoughts do write tests, if their evil project management overlords allow. Their tests are added after the code has been implemented and sometimes after it’s been shipped.

This tribe of zombie-like figures are characterised by the harassed and haunted 1000 yard stares as they struggle to reverse engineer tests over code that is frequently tightly coupled, riddled with multiple responsibilities, mutated parameters and static references. To them tests are seen as a necessary evil to clear the razor wire fence of the arbitrary unit test coverage percentage.

The Imagineers

This magic Faye folk are hard to see and even harder to catch. They are almost ephemeral and mythical. If you do see them it will be out of the corner of your eye when they are lounging in a hammock thinking and when you look again they will have vanished in a puff of magic logic. They have the mystical ability to visualise complex designs almost fully formed from thin air.

Although they may write tests, these are most likely to be end to end, system or integration tests. They have no need for unit tests as a means of developing code as their astonishing brains and supernatural vision enable them to see the most complex code paths and data structures as if they were ghostly apparitions realised in the swirling mists of an architectural seance.

So to TDD or not to TDD?

So putting aside the sarcasm and the terrible similes, should you use Test Driven Development or not? Does it fulfil the promise of evolutionary design claimed by it’s adherents or is it the ‘snake oil’ claimed by it’s opponents?

Well, firstly a caveat, the following are the opinions of the author based on 16 years of software design before using TDD and 12 years since using it. Most of my opinions have been formed by the experiences I have had and, more relevantly, by the way my brain discovers and visualises problems and abstract concepts, YMMV.

What has TDD ever done for me?

Most of my professional programming life has been spent writing in either procedural or object-oriented languages. In addition, they have been statically ‘typed’ (typing is a lower level concept in COBOL and FORTRAN but you need to declare primitive variable structures up front so I’ll count it). Given these environments TDD gives me a number of properties.

Test coverage

Although I used to write unit tests fairly thoroughly as a COBOL programmer, and these tests were even written in advance, they were not automated and therefore hard and expensive to reproduce. TDD is not synonymous with automated testing and although you can have the later without the former my personal experience is that ‘test after’ will always be sacrificed to the pressure of delivery.

Therefore, TDD, or at least ‘test first’, tends, in real world projects, to produce better automated test coverage at a fairly fine grained level (more on that later).


Again this opinion is definitely flavoured by the OOP languages I’ve written in (mainly Java, a bit of C++ and C#) but, unlike the ‘Imagineers’, I have trouble visualising detailed designs fully formed without a bit of exploration of the concepts and the data involved.

In a language like Java it’s quite hard to ‘try out’ and visualise how data needs to be transformed or what low level abstractions are present in the problem space. As I tend to explore problems from both the top down and bottom up I find it useful to probe at data for insight.

Without TDD the usual option to discover emergent design in ‘the micro’ is to run code liberally peppered with print statements or add break points and step through with a debugger.

I find TDD gives me quick feedback but has the advantage over print statements or break points in that the tests don’t need to be unpicked from the production implementation and that they act as institutionalised memory and capture the abstractions realised so far so that you don’t have to use cognitive load in holding a mental model of every low level data structure and it’s current state.


This one is definitely open to challenge and is almost the same point as in ‘Test coverage’.

Having a network of tests written up front against the low level design criteria means that you have some level of confidence that the lower level design satisfies your understanding of the problem and therefore you have some measure of the ‘completeness’ of your solution. If you’ve fulfilled the tests for that small part of the design you have completed it to your current understanding.

Of course this presupposes that your tests accurately reflect the problem and that your understanding is accurate but this is a problem regardless of TDD.


One advantage of test first is that in order to write tests up front you have to drive the low level design to be ‘testable’. This means that if you’re lazy and impatient, like me, you will want to get your test to have as little set up as possible and only verify one thing per test.

This inherently tends to lead to a design that favours small methods and objects with few dependencies and one responsibility

‘Change net’

Another side-effect of writing a lot of automated tests is that when you want to make changes to one part of your system the tests for the part of the system you have not changed will inform you if you’ve made any breaking changes. I call this the ‘change net’, a bit like the concept of a safety net.

This kind of failure on change of a different part of the system will flag coupling that you may not have been aware of and give a trigger to reconsider the design.

Although this effect can be achieved with good test coverage written after the implementation, as discussed, it is not typical in practice to get high levels of test coverage in the real world if tests are written after the fact.

In addition, the practice of writing new tests or modifying existing tests to reflect the change about to be made can give a guide as to when the change has been completed as the tests will fail until this is so.


A few years ago I employed an experienced developer with a lot of TDD experience. She came into a team where the first version of the software had been released and we were working on version 2. I was the lone web developer on the team and had written all the code to serve the dynamic web application.

As, at the time, I was the CTO of the consultancy concerned, I was constantly involved in short term consultancy work and a lot of presales activity and therefore out of the office on client sites a lot of the time.

I had about 3 hours to explain the project on Monday morning before I had to leave and I wasn’t back in the office until Wednesday afternoon.

I got back on Wednesday and asked said developer how she was getting on (apologising profusely for leaving her with such little support). She replied:

Initially I was a bit lost, then I started to read your tests and I’ve already implemented the first enhancement for version 2

Again, this is not the exclusive territory of TDD as any well written test harness can provide this ‘living’ document, and BSD style tests are even more effective, but in the real world TDD grown tests provide a useful starting point.

Communication through tests - nuff said.

So TDD is a ‘no brainer’ then?

So TDD is a ‘no brainer’ then? Well, yes, sometimes it is implemented with no brains.

Lets look at the potential negatives:

You’re mocking me

A frequent criticism of TDD is that it leads to a lot of the objects that interact with the ‘system under test’ (probably a class or method in OOP, more on that later) being replaced with mocks that are ‘primed’ with a canned response for each test case. The argument is that this can lead to:

  1. False positives as mocked ‘objects’ behave as expected but actual ones don’t.
  2. Designs that introduce layers and/or abstractions that are simply there to support mocking.

Tackling the second point first, I’ve never seen this in the wild myself but I’ve seen people cite examples. My own feelings are that this is rare and that the abstractions introduced in most cases are improvements that reduce coupling rather than introduce more.

The first point I have more sympathy for. I’ve definitely seen this ‘mocking masking behaviour’ issue in codebases including my own code. I think the antidote to this is:

  1. Try and use the TDD ‘classicist’ approach and only mock external interactions with your system if possible.
  2. Try to design your code to use less side effects1 and pass most arguments as immutable values as this makes each method/function testable without using mocks at all.
  3. Pick your ‘unit to test’ carefully. Try and test a single behaviour from end to end rather defaulting to testing methods and classes as units.

[1] A side effect is when a method, function or procedure acts upon something other than it’s inputs and outputs. For example, it may write to some output device, read input from some external source or change some stateful value.

Tests, tests everywhere and not a drop to think

TDD, in it’s purist form, focuses mainly on lower level abstractions and the ‘fine detail’ of your application design. This tends to drive a “can’t see the wood for the trees” perspective where attention to detail means you lose sight of the bigger picture and therefore miss opportunities to employ alternative approaches.

An antidote to this is to practice TDD ‘in the large’ as well, otherwise known as behaviour driven development (BDD) a specialisation of specification driven development2. By considering the acceptance criteria of each ‘feature’, ‘property’ or ‘story’ first and codifying them in, preferably automated, tests you will be drawn back up to the level of considering the larger components and the system as a whole whenever you run and amend these BDD tests.

[2] I’m thinking of trademarking Geek Driven Development.

Your tests are a big (re)factor

Again, this is not exclusive to TDD, but a side effect of using TDD is that you frequently have a lot of tests. Many of these tests may be covered by higher level acceptance, system and integration tests.

This is fine until you have to introduce a change to the design that involves refactoring. This kind of change may involve rewriting tests at unit (traditionally where TDD sits), component and system level resulting in three times the effort.

There are several ways to mitigate this.

Following the techniques mentioned in “You’re mocking me” will reduce the amount of ‘white box’ testing you do and therefore reduce the amount of low level changes required.

Using your test coverage tool (you do instrument your tests with a test coverage tool right?) will help identify when your code is already exercised by higher level tests. Given this information it may then be possible to use TDD at the fine grained level to act as feedback when writing methods/functions/procedures but delete some of those tests once the implementation has been developed as they’re already covered at the higher level.


So where does that leave TDD?

In my opinion, TDD is a useful tool for providing feedback and a level of confidence when developing low level design constructs.

It’s a fine grained tool and is useful in the micro.

However, it’s of limited use in the design of high level concepts, like:

  • what are your components?
  • what are the public APIs?
  • how should you delineate responsibilities within your components (i.e. packaging, namespaces, etc)

All of these can use TDD in a supporting role but they need other tools like thought about architecture/design and context (possibly documented in the form of diagrams and documents, wikis, etc.). TDD is useful in some languages to provide a feedback loop as described in Feedback but there are other tools that can provide this in certain languages, for example a REPL3.

TDD has it’s place and should be used where useful. I think of it as a watch makers screwdriver, a fine grained tool, and as such should be used where appropriate but it’s not a religion.

[3] Read Eval Print Loop

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