The Core Four

If you are having trouble with your agile software development process, here’s something to consider. I would like to introduce The Core Four. These are the four most basic practices of an agile software development team. These practices have proven to work well together to form a solid agile software engineering practice. It has also become evident that it is difficult to maintain a sustainable agile practice without them. Having these four practices in place gives you a return on your investment in agile software development and a solid foundation for other valuable and important agile practices.

I think that I first saw these practices together in a graphical representation of Extreme Programming by Ron Jefferies in his article: “What is Extreme Programming?” I didn’t realize how important it would become later, though. Extreme Programming recommends a lot more practices than four and they are all great, but I could sense from my own experience that I didn’t have to start with all of the practices in order to get some of the benefits. It was also pretty obvious that I couldn’t be successful if I left some of them behind. It wasn’t until Uncle Bob Martin explained in his video: “Agile Programming, Clean Code, Episode 49” that Jefferies had documented an inner circle of practices in his illustration Martin called: “The Agile Circle of Life”. What we, as agile practitioners are discovering is that without these central practices, it’s very difficult, if not impossible, to sustainably support the others, yet these are the very practices that are being overlooked by many teams that call themselves “agile.”

What are the Core Four? They are:

  • Test Driven Development
  • Refactoring
  • Simple Design
  • Pair Programming

Let’s take a quick tour of the benefits of these practices.

Test Driven Development

There has been a great deal of debate about Test Driven Development over the years. Some very smart people are actually against the practice. Even so, the practice is still the basis for good software engineering to many experienced developers like myself. I have not been able to find an adequate replacement for it. It solves so many of software development’s most challenging problems.

One of software’s biggest problems is ambiguous requirements. Test driven development helps to remove this ambiguity early by forcing the requirements to be testable. Is your team having trouble getting the requirements? Is your team building the wrong thing? TDD could create a huge improvement in this area.

TDD is also a way of permanently documenting your software requirements. Unlike classical written requirements documents, tests are executable and must stay accurate. Are your requirements documents useful? Do they represent the system that is actually in place? TDD makes a big difference here.

Test Driven Development also stops your software from leaking requirements. That’s when an “upgrade” removes or corrupts previously enjoyed features. How many times have you upgraded an app only to find that you lost a feature you really liked. It’s happened a lot to me and it really makes you feel like the developers don’t care. It also makes it seem like someone lied to you when they said they had an “upgrade” available. We don’t want to be doing that to our users and a complete, working set of tests helps to stop feature leaks between releases.

If you want to learn more about TDD from the one who brought the practice to light, check out my affiliate link to: “Test-Driven Development By Example” by Kent Beck.


I think of refactoring as “agile architecture” that’s because I think that the most productive architect is one that makes architecture for a product that will be used. When you refactor, you are developing architecture based on code that is already being used.

In my practice, I discovered that about 90% of refactoring is merely an attempt to remove code duplication. It was one of the biggest surprises in my career. By merely trying to remove duplication, I forced myself into using well-known and loved patterns. The main difference between just employing patterns and refactoring to them, was that these patterns were actually proven to be necessary. Back in the 1990’s, during the days of patternmania, we tended to use patterns whether we needed them or not. The move to agile development, primarily by way of Extreme Programming, lead to a more practical use of patterns and refactoring was an underlying hero.

Refactoring can also be considered to be the software fountain of youth. Teams that are vigilant at the practice, have code that looks very new. It appears that the code was just typed out in a few minutes yesterday, even if it is many years old! This is something you almost need to experience to believe. A big contributor to this, other than the removal of duplication, is the basic effort to rename things. As understanding of the system matures, old names are no longer relevant and can be replaced with the more obviously fitting ones. Another big benefit is that refactoring makes the next core practice possible.

If you want to learn more about refactoring, make sure you check out the definitive work by Martin Fowler. Here’s my affiliate link to it: “Refactoring” by Martin Fowler.

Simple Design

A simple design is one that developers who read the code think is obvious. Simple designs like these are not easy to achieve, though. Rarely are our first designs adequate because we have so much to learn about what the system will have to do. A fundamental element of agile software design is to make the system as simple as possible and that usually means that we don’t do things like add code that we think we might need in the future. Because we refactor the system frequently, we don’t need to do that. We can just add the code we need when we need it and not sooner.

The opposite is also true. When code is no longer needed, we should just remove it. We shouldn’t leave code hanging around just because we might need to add it back in again. We need to rely on our version control systems in order to retrieve old code and keep our code simple.

Usually, removing duplication makes code simpler, but it’s also good to remember that there is a time to duplicate. You can imagine how ridiculous it would be to try to limit the letter “s” to as few places in a sentence as possible. You could try to write your sentences using a crossword style, but that would be harder to read, not easier. When it comes to communication, duplication can actually be a helpful thing. If duplication makes the system easier to read and understand, then it overrides the “don’t repeat yourself” rule.

Simple design is pretty subjective. What seems simple to me may seem pretty difficult to you and vice versa. I may also miss an opportunity to refactor. In fact, it’s pretty easy to make an important and costly mistake anywhere in the agile process. That’s one thing that makes the next core practice so important.

Pair Programming

This may be one of the most difficult agile software development practices to start but without it, it’s difficult to consider yourself to be a truly agile development team. How do you know you really are an agile development team if you don’t know whether or not your team members are using agile practices? Worse that that, how can you really be a team if you don’t ever work together on things? How can you teach a new member of the team what they need to know in order to be a good contributor to the project?

I think that the main reason that Extreme Programming became so popular so quickly, was because pair programming was a fundamental practice of the method. It tends to spread and balance out the knowledge on the team. There will still be experts, but there will also be those who will learn from those experts.

An important part of having an agile process is the ability to be predictable. The stakeholders begin to rely on the team’s velocity. It’s pretty obvious that this consistency depends on developers, but what if the developer of a key part of the code is sick or on vacation? An agile team should be able to continue to go at the same velocity even when a developer isn’t available for part of the iteration or sprint. When the developers work together, expertise is spread around. Even if the best person for the job isn’t available, someone else knows enough to get through it. Pair programming makes the team stronger by reducing the single points of failure caused by programmers working alone.

Pair programming ensures that all code is reviewed. Studies have shown that peer review processes reduce error. Other studies have shown that error produces late projects. Even though it seems like you will go slower when two programmers work on the same thing, it is logical to assume that this is not the case. When errors are reduced, software gets done faster. Even non-agile developers have learned to do peer-reviews. Pair programming makes the practice efficient by keeping the team involved in the code on a day-to-day basis. Errors are found quickly and are less likely to be missed and found later when they are more expensive to fix.

If you want to do a deep dive into this important practice, here’s my affiliate link to a book I highly recommend: “Pair Programming Illuminated” by Laurie Williams and Robert Kessler.

Returning to the Core

It’s one thing to write and talk about these practices, and quite another to actually do them. That’s my challenge to you as a professing agile practitioner. These practices are far from new. They’ve been proven over and over in my experience and in the experience of many other developers over the last 20 years or so. Don’t be satisfied with an inadequate trial of agile software development. Make sure that you give The Core Four a try before assuming that you have failed. I think you will be very happy you did.

Why Should We Do Test-Driven Development? Part Four

When a team practices the discipline of test-driven development, it gives them a new power.  When all of the code’s intentions are preserved by the tests, it allows anyone to change any of the code at any time without breaking it.  As far as I know, there is no other way to do this.  What this means to a developer, is that it is possible to revise existing code whenever it needs to be revised.

The practice of revising existing code without changing its intended behavior is called “Refactoring.”  Over the last 15 years or so, I have witnessed the power of refactoring.  One of the most obvious things that refactoring does is that it helps us remove duplicated code from the system.  This not only makes future change easier, it reduces the lines of code that must be maintained.  By way of refactoring, TDD makes the code itself more agile.  Refactoring has become such a major tool in my practice, that I now wonder if I have written more lines of code than I have deleted in the last five years.  When I come to a system that is completely covered by tests, I am able to remove duplicated code.  This allows me to remove duplicated tests as well. If a system hasn’t been refactored well, I may end up removing more lines of code because of duplication, than I add when I make a new features.

Refactoring improves the internal design by allowing us to do simple things like renaming variables and methods.  I do realize that some of this can be done using modern refactoring tools without TDD, but there are many cases where dynamic types, strings and reflection cannot be determined by a refactoring tool.  You can break a system by renaming when you don’t have tests.

When I first switched to TDD, this power was a game changer.  For the first part of my career, I had to be very, very careful not to change things.  I looked for ways to not touch things in order to improve my ability to finish a new feature faster.  When a design is good, this can still be a good thing to do, but the fear of change usually leads to system deterioration.  Doesn’t it make sense that we learn more as we spend more time in a problem domain?  Don’t you think that if we know more about a problem domain that it would help us design code that matches that domain better?  Then it only makes sense that we will have better ideas for the code’s design after some of the code has already been developed.  The more experience we gain, the more we see how the design could be improved.  TDD gives you the power to make those improvements.

As I mentioned in the previous article in this series, TDD also helps us design better interfaces because it usually forces us to build something that we can imagine.  This helps us to make small, single-responsibility classes.  That’s the “S” in the SOLID principles. In fact, TDD tends to naturally help us to follow most of those principles.  We tend to make small classes, so they tend to be closely related to a single feature and that makes them more closed to change.  That’s the Open Closed Principle.  We also tend to inject the objects because they are much easier to test that way.  That causes us to reverse the dependencies naturally.  That’s the Dependency Inversion Principle.  When we near a boundary, it becomes much harder to test, so we tend to use interfaces that represent the difficult boundaries to test.  That’s the Interface Segregation Principle.  Interesting isn’t it?

Have you ever considered how you might make a spider web?  I’m going to guess that it wouldn’t be trivial, especially if I gave you constraints that actual spiders have, such as where certain objects are placed, how much wind there is and so on.  Now what if I was able to reduce your brain to the size of a spider’s and then ask you to make those same webs?

The reason I ask such a strange question, is that it seems possible that some things we do can be done with less mental energy if we use simple, repeatable disciplines.  TDD appears to be a discipline like that.  When we practice it, it helps us to naturally produce beautiful code in a variety of unusual circumstances, much like a spider is able to make a beautiful and functional web in many different places and conditions.

Why Should We Do Test-Driven Development? Part Three

In part one of this series I discussed what I believe is the most important reason to do TDD and that is for programmers to have a way to preserve their intentions.  In the second part, I explain the benefit that TDD gives a development team in the area of documentation.  In this part, I would like to discuss the effect that TDD tends to have on software design.

When we discipline ourselves to consider the end result of our programming efforts, it focuses our minds on the reason that the software is being made.  Our thinking shifts from the effect, to the cause.  By forcing ourselves to document the test first, we must do enough investigation and thinking to determine what is being requested.  One way that this kind of thinking improves code design, is that it ensures that the developer is the first user of it.  It requires that we experience what using our software is really like.  Since we don’t like complicated interfaces, and since we can only imagine a limited amount of complication in our minds, we tend to naturally test against a simple interface and this makes the system easier to use for others too.

When we force ourselves to consider our tests first, it makes us expend more mental energy than we would if we were to just make the code.  This also challenges us to consider if what we are doing is really worth doing at all.  It has long been documented that 80% of our software is rarely if ever used.  Could it be that some of this is because software is easy to write but not easy to use? I haven’t done a scientific study to determine whether or not TDD would change the 80/20 rule, but my experiences as a test-driven developer have demonstrated to me that I have avoided unnecessary code when doing TDD because of the extra effort required to test it.

Another thing that TDD does is that it amplifies bad code designs.  That probably doesn’t sound good at first, but it really is.  It has been obvious as I have practiced TDD that when code is hard to test, it’s often because the design of the code itself is overly complicated. This is especially obvious when attempting to “back fill” tests by writing them last instead of first.  One of the common mistakes that new test-driven developers make is to assume that if a test takes a lot of mocking, it must mean that TDD is not very helpful.  Actually, when TDD is hard, it’s often pointing to a design that could use some work.  There are other reasons that TDD seems hard to new practitioners and I hope to share that in another post.  The point here is that when TDD creates ugly tests, it’s actually a benefit to us because it shows us the bad design earlier rather than later.  Good designs have clean and understandable interfaces.  Tests use interfaces and when those interfaces are not very intuitive, it tends to become obvious.

Another design benefit of TDD is the effect it tends to have on names.  How we name our classes, methods, functions and variables changes the readability of the code.  If code is easier to read, it’s much easier to maintain in the future.  That’s an important measure of software quality because software usually spends most of its life in maintenance mode.  The way that TDD improves names, is that it causes developers to think in terms of the problem domain.  When we test first, we focus on exposing a problem, so we tend to use wording that is a part of that problem’s domain.  When we write the code first, we often focus on our solutions.  We may use words like “loop” or “string”.  Those types of words don’t express the problem we are solving but the way that we are solving it.  I have found that when I use TDD, words from the problem domain find their way into public methods.  They may go deeper than that as well.  I don’t believe that TDD automatically generates a system with good names, but it appears to help us start off with better names than we would otherwise.

Why Should We Do Test-Driven Development? Part Two

The first reason that I gave for doing test-driven development in part one of the series, is that it enforces communication between developers about their intent for the software they have written.  It ensures that those intentions stay in place as new software is added to the system over time.  Another important benefit of test-driven development is that it helps us document our intentions.

It’s important for professional developers to understand that they have a responsibility to document the system’s behavior.  Software isn’t just for computers to read.  It’s for other programmers and stakeholders too.  One of the benefits of a disciplined agile approach to software engineering, is that it naturally relaxes the need for some of the documentation.  One of the reasons that this is true, is that it is assumed that a full set of tests are going to be produced that form the specification.

Without TDD, where will the design be documented for future developers to read and understand?  It is true that the code is there, but the code doesn’t preserve the intent of the original developer.  It merely satisfies that intent.  I can see how it would be possible to preserve the intent in declarative code, but object-oriented and functional code is far more difficult to be sure about.  What if a future developer considers part of the existing code to be a side-effect?  That future developer will preserve what they perceive to be the original intent as they refactor or port the code to a new design.  I have actually seen this cause a production error in an area where tests were lacking, even on an a team that was trying to do TDD.

Another issue is that without a complete set of requirements documents, the team eventually forgets how the system works.  This seems to happen a few short years.  Eventually, even the most brilliant minds on the team start to forget what the system does in certain important state transitions.  The system gets more complicated as new features are added and, at the same time, we start to forget the things we did in the first few iterations.

If we are going to take hold of the privilege of agile software engineering, then we must also embrace the responsibility of it.  If we decide not to do TDD, then we had better plan on going back to written design documentation.  We must also keep that documentation current as changes are made or else it won’t help.  As you probably imagine, that is going to slow the team down.  It’s difficult to say that a team is “agile” when it chooses to go slower because it doesn’t want to perform the discipline of TDD.  If it decides to not do TDD and not document the design, I would question whether the team is even professional.  If we are to be both professional and agile, then I know of no other way to deal as efficiently with requirements documentation, than to maintain the discipline of test-driven development.

Why Should We Do Test-Driven Development? Part One

I want to explain why I find Test-Driven Development so important to modern software development teams.  There are many reasons, and it will take a few articles for me to go through them.  I’d like to start with a reason that may be the most important and is fairly easy to understand.

I have found that one of the most important things that TDD does, is that it protects an individual developer’s ability to create new code without harming the existing code.  When TDD is a team discipline, that team must run all of the unit tests and see them pass before they merge their code in with the rest of the team’s code.  When a developer knows that they may have broken something by what they add, it creates a high level of anxiety for a truly professional developer.  We often take pride in our work as developers and when our code breaks everything, it is very shameful to us.  Obviously, it is also very unproductive.  It is one of those hidden costs that makes software take longer to make and causes it to be less reliable for those who use it.  It can actually harm people when our new code breaks some of the old code that we didn’t realize would be affected.

It may be hard for those who aren’t software developers to understand the impossibility of merely reading the code in order to figure out if what we add will break something.  Even the most confident developer makes a typo.  All it takes is one letter or even a missing semi-colon in to harm the code.  I recently broke code by having the wrong case.  The letter was right but its case was not.  Trying to write code without a way to check it, is like having a job at which you must walk a tight-rope all day.  It’s clearly not sustainable.

The discipline of test-driven development, causes all team members to write their tests before they write their code.  This isn’t just a random and cruel requirement.  It’s a practice that ensures that every programmer’s intentions are both stated and preserved.  This may be the fundamental benefit of TDD.  It’s a way to communicate a programmer’s intentions to all future programmers.  That way, when a future developer makes a change, they add their new intentions and make sure all of the existing intentions are still being met.  In this light, TDD is fundamentally a discipline that enforces communication between developers.

It’s important to note that even when a programmer works alone, they often forget their own previous intentions for the software!  Our work requires that we focus.  It doesn’t take long for us to exceed our own ability to remember the details.  By practicing TDD, the intentions we preserve may be our own.

This is just one of the benefits I have witnessed over the years as I have practiced TDD.  It turns out that it also can improve our requirements process, serve as documentation, improve software design and promote the software’s internal quality, but this one reason alone would probably be worth the effort to me.