Mark Pearl

These notes are based on Kent Beck’s talk on “Software Design: Why, When & How” done at JavaZone in September 2014.

The What of Software Design is well established. We have a pretty good idea of “what” good software is. For instance, we have design patterns which have been around for well over a decade.

Somehow in focussing so long, so effectively on what, we have forgotten to talk about the why.

When do you know when code should be tidied up?
When do you apply a specific pattern or approach? Why would you apply that approach?

Without talking about why we do software design, we don’t have an opportunity to be able to apply the right “what”.

When does design matter?

Software as its sits doesn’t care how it is designed - it just runs. Software design is not software. Software design is about change - it is only when we want to change software does design play a role.

Drivers of software design

The first driver for software design is economics. We get paid to write programs. When we write programs, somebody expects to make money somehow, sometime.

Economic drivers of design

Economics has a good vocabulary for talking about timing. There are several topics in economics we should consider.

  1. Net Present Value
  2. Opportunity Cost
  3. Options Value of Software

Net Present Value

The first economic topic is NPV (Net Present Value). In essence, NPV says a dollar today is worth more than a dollar tomorrow.

This means we should earn money sooner and spend money later, all other things being equal.

Doing all design upfront flies in the face of NPV. When doing all design upfront, we are spending all the money today, and only earning money from that spend later.

Opportunity Cost

Opportunity cost is the opportunites you are missing from everything else you can be doing at this time instead of what you are actually doing right now.

If you look at the effort you will put into software design at a point in time, what else can you be doing at that point in time?

Options Value of Software

The options value of software goes something like this…

If I have software and I can only make one kind of change to it, that software is less valuable than if I have the same software and can change it in 3 different ways. The future is unpredictable and we don’t know which of those 3 ways will be most valuable.

If I have multiple options for change, I have more valuable software.

NPV vs. Options Value

Looking at NPV and Options Value, we have two competing forces. We would like to defer work, because we want to defer investment AND we want options, which requires us to do more work. NPV & Options Value of Software are economic drivers of software design.

Human drivers of design

We also cannot forget than people make software.

Humans like challenges

Part of the satisfaction of creating software is the sense of satisfaction one gets from doing a good piece of design.

As software engineers, we like the challenge of taming software - of finding better design.

Humans like helping the next generation

As humans, we generally have a desire to help the next generation. We might have a section of code that does not require immediate design improvements, but as a sense of compassion for the next developer working on the system, we will work on improving the design.

Humans worry about their reputation

As people, we often worry about our reputation. We fear that others will think poorly of us. This can lead us to doing unecessary design because we think others will have a higher impression of us.

If I don’t do ‘current best practices’, will this impact my reputation as a developer.

When to do software design?

Should you design now, or should you design later?

To determine when to do software design is tricky.

Economically, the ideal point when software should be defined for change is the instant before it needs to be changed.

However we often do not have enough information at the point of time when a design decision should be made to make the right decision.

Forces pushing us to do design later…

NPV pushes us to do design later.
Opportunity cost of software often pushes design later.
Lack of knowledge pushes design later.

Forces pushing us to do software design sooner…

Options value of software pushes design sooner.
Batching effect pushes design sooner.
Humans aspects pushes design sooner.

So when to do software design?

There are a set of forces that are different depending on who, when and what you are designing. There is no easy answer to this.

How to do software design?

If only the answer to the how was as simple as “Big changes in small safe steps”. While this is usually the case, it is not always the case.

Let’s say that you have software that is such a tangled mess that it would take an incredible amount of time to clean up. Small safe steps can be really expensive. It can take an incredible amount of time.

The alternative is the leap - doing a huge change at once. This can be extremely dangerous, and there are a few occaisons when this is the right approach.

The critical strategy is being able to run the old design and the new design at the same time.

The ability to run the old design and the new design at the same time is the mitigation strategy that allows you to move forward with confidence.

Key skills you need to master for design

For all of this there are three key skills that don’t come naturally to programmers, but that can be learned:

  1. Tolerance for ambiguity - the moment when you defer making a design decision because things are still ambigous.
  2. Being able to wait - part of tolerance for ambiguity, being conscious that there is a problem, but being able to wait for now.
  3. Treat design as a social process - managing design requires a social process, and being aware of the other people in your team that are impacted by the design.

Software design isn’t just a matter of what are the things and how they are related. It is keeping in mind why we are designing. It is mindfully choosing when we design and picking the transition strategy of where we are today and where we want to get while being aware of the people in the team and the social process.

Misc Points of Interest


Coupling, two elements are coupled if a change to one element implies a change to another element. Coupling is important to software design because the costs of changing software are dominated by rippling changes (which means one section of code is coupled to another section). The exponetially rippling changes kills you in software design. Reducing coupling takes effort. The more you try to reduce coupling, the more expensive it gets. There is a trade off curve between effort and coupling.

Premature Abstraction

Premature abstraction makes it hard to get the right type of abstraction. When you abstract, you fix certain aspects while allowing other aspects to change. If you don’t have enough types to look at, you may fix the wrong things.

Simplistic Rules

Software design does not lend itself to simplistic rules.

Batching Effect

There are economies of scale to doing design sooner provided you have the design right. (Shared context can be expensive to recreate later).

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