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Posted 16 Nov 2013


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"Design, Test, Developement, Test" in-stead of "Test Driven Development" (DTDT over TDD)

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27 Nov 2013CPOL17 min read
The only good thing of Test Driven Development is, it ensures the maximum test coverage but TDD puts you in risk of a week design. DTDT tries to solve this while ensuring maximum test coverage.


In modern software industry Test Driven Development or TDD is a big buzz word. In most of the job advertisement about software development, you will see knowledge or experience on TDD is must or a plus. But, in reality how many applications are being developed in a so called TDD model? I am in no way talking against writing tests. I have no doubt that having maximum possible tests can improve the quality of the application in-terms of reducing the number of bugs, reducing the number of bugs to recur and reducing the ultimate time to develop and maintain an application. But, if you practice test driven development according to its definition how much realistic is that? Let’s 1st see what actually TDD means in theory.

What is TDD?

[1] Test-Driven Development (TDD) is a technique for building software that guides software development by writing tests. It was developed by Kent Beck in the late 1990's as part of Extreme Programming. In essence you follow three simple steps repeatedly:

  1. Write a test for the next bit of functionality you want to add.
  2. Write the functional code until the test passes.
  3. Refactor both new and old code to make it well structured.

Let’s try to see the process with a picture:

Here you can see a traditional diagram (Figure 1) of Test Driven Development:

Image 1

Figure 1 : Traditional Diagram of TDD

You can see in Test Driven Development you basically have a 3 step cycle. First you write a test for the functionality you desire to develop. As you have not developed the functionality yet, the test will fail. Then you write minimum code to make the test pass. Then you re-factor your code to have the desired functionality implemented in well-structured format. When you re-factor your code you must ensure, at the end of the re-factoring, the test still passes. In addition you also have to ensure all other tests that may exist in the project also still pass.

Let us try to have a more detail diagram in a flowchart format:

Image 2

Figure 2: TDD A Detail Diagram

In Figure 2 you can see in a TDD cycle you always check if your desired implementation of a unit of function is done. Now, How do you define if it is Done! What is the definition of “Done”? The definition really differs developer to developer. Some lazy developers have attitude to get the job done with no real focus on extensibility, maintainability or scalability. Some average developers have some focus on structured development but perhaps they have lack of skill. Some developers could be too much tempted on design. So, every developer will have his own decision taking pattern for the word “Done”. Yes, at one point every developer will be in the same position. That is, the unit of functionality will be developed and it will work. But the difference arises when the question of “Structured” code comes. If you are not very lucky perhaps you will come out with a BBOM.

Yes, theoretically when you re-factor during a TDD cycle, you will look at the design and architecture, and you will re-factor everything perfectly to make your code extensible and maintainable. But in reality how much of that you can achieve? When you concentrate only to the unit of function and become rigid to “Keep It Simple and Stupid (KISS principle)”, Every possibility is that, you do not get an up-front domain knowledge of the application and never do anything for future extension. In many cases you will develop a function even without knowing who will use it and how will it be used. On the other hand as you are a lover of YAGNI (You aren’t gonna need it) principle, You do not add anything that you don’t need at that specific moment. So, KISS and YAGNI are making your life easy for that moment but as the project continues to grow the amount of re-factor will increase, will become difficult and also will become risky. In many cases during the re-factor phase you also require to re-factor your already written tests that passed previously. This is because you are trying to achieve a good design through re-factoring! If that is the case what is the point of writing test before you code? One argument is, writing test first ensures every line of code is covered by a test. Well, perhaps that is true but should we accept the trade-off of trying to increase the test coverage over sacrificing a good architecture of the application? I would say NO! I don’t want to sacrifice the software’s architecture because, this may kill my software pre-maturely. So, what about test coverage? Yes, I want to achieve maximum test coverage too. I will try to tell my way of thinking to solve this problem in later part of this article.

Why TDD might not output a good design?

Let us summarize the reasons why, in reality, whatever the theory says, TDD is not a good way to achieve a great design or architecture for the application:

  1. I do not see a role named architect in a TDD team. Perhaps it is my ignorance but do you see that?
  2. Achieving structured code through re-factoring is not very practical. Because it largely varies developer to developer and even for the same developer it varies time to time.
  3. TDD does not impose any design goal to the developers. So, violation of standard design principles is most likely to happen.
  4. If you want to achieve a good design following TDD, perhaps you need to re-factor the infrastructure of your application every now and then which is always risky.
  5. When your project grows, re-factoring becomes expensive and your client may not agree to pay for that.

A Soccer Team Analogy:

Think of a soccer team. The soccer team has only two objectives. To score goal and to restrict the opposition from scoring goal. So, from this objective every player can start playing and run behind the ball. Because getting the ball means you can try to score a goal and also the opposition has no way to score a goal. Now, when suddenly the opposition gets the ball and passes it through to a forward you discovered no body is there to defend. Because everybody was running behind the ball. At this point few player starts running and by the time they reach near the forward of the opposition, the goal is already scored. That is why soccer teams always have a plan of playing and they design the strategy keeping the plan as a goal . The most successful soccer teams always have a great coach who design the game and also great players who can execute the design.When you have a great coach all the players become motivated and they also become capable of taking decisions instantly on the field so that the ultimate plan is successful.

Image 3

Figure 3: 4-4-2 format for soccer

The soccer analogy is just an example to convince you for an upfront design. You see if you do not have any plan in the game and suddenly you lose the ball even by running as Usain Bolt will not save you. In the same way I believe if you do not have a planned architecture of your project whatever re-factor effort you give, at some-point you are in risk to fail.

Software Development Analogy:

Now I will try to present an analogy from software development world. Say you are going to work in a HR management application. The specification is written and the whole application is broken down in some user stories. You start the project and you are assigned to develop an user story where you need to save an employee. So according to the extremists of TDD you start writing the test first. Note that, you have nothing in your project upfront. You just have a blank solution. So what will you do now? As you need to write a test first. You must create a Test project. Now the question comes what will you test? Will you test the "Employee" object creation to check if any business rule or validation is violated? Will you check if a valid employee can be saved from the business layer? Well, You do not have any business layer infrastructure for your project yet. In your test first you will check the object creation. So, write the test, make it fail and then create the employee class that make the test eventually pass. In the same way you will create another test for save method of the employee and thus you create an EmployeeManager class, implement the save method and it will make your test pass. During creating the test for Save method you also decided you need a data-access method to save the object to database. And at this point you found you need to Fake the DataAccess method. So, you created an EmployeeDataAccess class that is derived from an interface named IEmployeeDataAccess.

Now I have some questions:

  1. Where did you write your test? Did you write both the test in the same class?
  2. Where did you create the Employee class?
  3. Where did you create the EmployeeManager class?
  4. Where did you create the IEmployeeDataAccess interface and EmployeeDataAccess class?

If your answer shows the following project structure (Figure 4) I don’t see any reason to blame you. Because you are keeping it simple (KISS) and you are not doing anything that you don’t need (YAGNI) at this moment.

Image 4

Figure 4: HR Management Project Structure

But yes, perhaps you will not come up with this because you already have the concept of multilayer project architecture and you know you should have different layers for your Models, DataAccess and Business. So, when you start developing following extreme TDD, that means, without any up-front architecture, the structure of your code depends on your skill and thinking capability. If you have no idea about layered architecture you may come out with the above project structure. If you have no idea of software design principles whatever re-factor you do you never come out with an optimum design. Do you see the reason? The reason is you are not obliged to follow a pattern of development as no up-front architecture is created.

As it is natural that in a team there will be different kind of developers. Fresher or Experienced, Talented or Un-Smart, Expert or Novice, Hard Working or Lazy and so on. Should you really take the risk by allowing every developer to re-factor as TDD suggests?

See, If 5 person starts running from different place and they all know the single destination, with whatever speed they run and in whatever path they choose all of them will reach the destination. And if there are milestones defined before the destination that they must touch, all of them will eventually run with correct path from the beginning. But if they had no destination defined, no milestone defined, where they will reach? At no-where. So, isn’t it better to set your destination and milestones first?

Let us find a solution

The solution I will be talking about is nothing new. Whatever slogan people give in favor of TDD I don’t believe any project can be successful with the extreme practice of Test Driven Development. So , we must do something where we can build a great architecture for the project and still we have great deal of unit test coverage.

In summary you need to follow the following steps:

  1. You Do the Design!
  2. You Write the minimum Test Code! You make it fail forcefully!
  3. You Develop!
  4. You implement the test completely. Now, if you had developed correctly, your test will pass.

So, we can call it as DTDT (Design àTestàDevelopmentàTest) in short. It should be a simple cycle as the following picture:

Image 5

Figure 5: Design > Test > Development > Test (DTDT)

Now, I will try to elaborate the process.

Design, Test, Develop, Test (DTDT)

According to my opinion your 1st sprint should never implement even a single user story. It should be dedicated towards designing the architecture of the project. Yes, I agree architecture evolves. I agree you should not create anything based on an assumption that is not supported by evidence. I also agree in an agile framework you cannot create a big infrastructure for your project in the 1st sprint. But, you must agree, the project definitely does not start from the 1st sprint of development. It starts way back. It goes through information analysis, business analysis, requirement analysis etc. So, at the first sprint when you start designing and start questioning yourself about the project you definitely will have lot of knowledge’s from the engagement phase. So, you can off-course create a minimum domain model for the application and also you can create a minimum infrastructure of the application. Yes you will keep it simple but I don’t agree to keep it stupid. So, better you remove one “S” from the KISS principle. Yes, you follow YAGNI but do not become too rigid. Because, the interface that you know you must need in few days, off-course you create that. There is no point of leaving it behind because “You aren’t Gonna Need It” just now.

So, in the start of your project you create the minimum infrastructure and create a minimum model of the domain. That means, the 1st sprint is used for architecture and you already have a minimum design to start developing the user stories.

Now, you start working in the 1st user story using DTDT in the following way:

Step 1 - Design:

  1. You make sure you understand the user story perfectly.
  2. You make sure you know how and why this functionally will be used.
  3. You find all the objects that you need to implement the functionality.
  4. You look at your solution if any of the object is already created.
  5. You try to find if the objects that you will work with have any relation with other objects in the system.
  6. You decide what are the interfaces you need to create. And also what are the methods and attributes those interfaces should have.
  7. You ensure you honor SOLID design principle when you create your interfaces, classes and method definitions.
  8. Remember, In this step you do not implement any method. You just find or create the required interfaces, classes and just define the signature of the methods.

So, you are done with the design step. Now you are more confident and you know:

  1. What exactly you are going to do.
  2. What classes and interfaces you need to work in.
  3. Which code you should write in which place.
  4. What extension point you are keeping open for future.
  5. How the functionality fits with the overall system.

As you already had created the infrastructure in the 1st iteration, you are now forced to keep yourself with-in a minimum boundary and you no more can develop in your own pattern.

For example if now you want to build the “Employee Save” user story you will do the following:

  1. You check if an entity is already created for Employee.
  2. If not, you think if there is any other type of human resource that can exist.
  3. Perhaps you may come up with another type of human resource called "Consultant".
  4. So, You realize you need one interface named "IHumanResource".
  5. As you already have a minimum up-front infrastructure in place you will be forced to derive this IHumanResource interface from something like IEntity interface.
  6. Now, You create the IEmployee interface derived from IHumanResource.
  7. In future when you need IConsultant interface you can derive it from IHumanResource too.
  8. So, when you create the Employee class derived from IEmployee interface it will have its hierarchy correct.
  9. Later You create the IEmployeeManager (Business Layer) interface for Employee.
  10. But when you create the IEmployeemanager interface, your infrastructure will enforce you to derive it from IManager interface.
  11. In the same way when you will create IEmployeeRepository interface you must derive it from IRepository.
  12. You have no way to violate the design. Because, your up-front infrastructure will ensure a Manager interface derived from IManager only can work with a repository that is derived from IRepository.
  13. In the same way, the Repository that is derived from IRepository only can work with entities that are derived from IEntity.
  14. So, the up-front design is forcing you to design and develop your work in standard way.

Next, DTDT has two steps to write the tests. Now, I will talk about the 1st Test writing step:

Step 2 - Test:

  1. You already did a good amount of analysis of your user story during design. So you already know the maximum detail of your work.
  2. In this step you find out the atomic units of works that some up to the full user story.
  3. You write unit tests for all the atomic units.
  4. No. I am not telling that you have to write the full functional tests at this step. You at-least create the definition of the tests and make all the test methods fail forcefully by possibly creating a false assertion.

So, now you have your design ready. You also have defined the tests that you must pass after you finish your work. Currently all the tests are failing. That means, you can do your development now, as you are not going with any risk of missing any test.

Step 3 - Development

  1. At this step you start implementing your user story. You have the design in place and you also have the atomic unit tests defined.
  2. The design and the unit tests that you already have defined, will guide you to develop your functionality in proper way.
  3. Once you are done with the development, you enter in the 2nd Test writing step.

Step 4 - Test

  1. In this step you will implement all the test functions one by one. As now you already have all the methods for your user story implemented, Once you finish the implementation of a test method perfectly, it will pass, unless your implementation is not wrong.
  2. Once you finish coding all the test functions and all of them passes you are done with your implementation.

So, here I have tried to find a solution where you can find the following benefits:

  1. You do the analysis to come up with a good design. So, most likely you will be able to raise important questions on your work that you may miss when you analyze to find unit of works for the sake of writing tests.
  2. When you analyze from broader perspective before you go for finding atomic units of work, the chance of missing is less.
  3. In this approach you 1st write all the tests that fails initially. So, there is no chance of missing any test.
  4. When you develop you give more concentration on the design and requirements, so your code looks way better.
  5. At the end when you implement all the test methods and after they pass you achieve 100% test coverage.

That means, in DTDT you achieve all of the following:

  1. Good Design!
  2. Great Code!!
  3. Test Coverage!!!


As I said before, in this article I actually did not tell anything new. What I tried, is to formalize the practice what most of the software projects go through if the project targets to include unit test. Unit Test is actually a great weapon to fight against bugs and recurring bugs. So, there is no question that the more coverage of unit test you have in your project, the quality of the output is greater. But really the extreme TDD might not be the best way to proceed. Because your development has to be design driven, not anything else driven. The unit test will exist in your code, so that it confirms your work never breaks. We should not be extreme TDD practitioner. We should find a compromising point, so that the design never suffers. It could be DTDT or it could be any other way that you think is right. 


1 : Test Driven Development by Martin Fowler


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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