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Software Engineering Consultant

Book Review: A Philosophy of Software Design

December 5, 2022 book

What separates the truly great software developers from the average ones? People have lots of opinions about this, but it’s often hard to describe what makes the code of a great engineer so good - and what part of it novice programmers should really try to learn from. John Ousterhout’s book A philosophy of Software Design aims to answer this question and actually introduces some extraordinarily appropriate vocabulary for your next discussion about software quality.

Book Cover of “A Philosophy of Software Design”

Book & Author

Link to the Amazon Store Page

The book is originally from 2018 and its second edition was released in 2021. It it ~180 pages, so it should be a very quick read.

John Ousterhout is a computer science professor for computer science at Stanford who has worked on all kinds of software related topics, but also spent a lot of time teaching how to design software in general - or more precisely, looking at the design itself of software as the underlying challenge.

In his university lectures, as he also describes in this book, he lets his students exercise the design of different kinds of software solutions for small systems. While doing that, he tried to find ways to explain the pitfalls that novice programmers fall into, and what exactly experienced programmers do differently so that the software they create not only works, but stays maintainable.

The experiences and insights he describes, were not new to me. Most of the time I thought “Yeah I know this situation, and it’s also clear to me how to do it differently - but the problem is always explaining it to programmers who don’t think this way (yet)!” But for all the principles and strategies that experienced developers follow with their gut-feel that they developed over time, John found awesome vocabulary and ways to explain them to programmers who don’t know them yet.

Content and Structure

I experienced the book like a journey over two main topics, which shall get their own subsection each. As the book is already relatively short and every programmer should read it anyway, I won’t go much into detail and instead just state the most important points and principles.

Symptoms and Causes of Complexity, Abstraction, Modules, and Interfaces

The preface of the book begins with an observation that is frustratingly apt:

People have been writing programs for electronic computers for more than 80 years, but there has been surprisingly little conversation about how to design those programs or what good programs should look like.

Apart from all the computer languages with their different specialties, agile or non-agile development methodologies, tooling as debuggers, linters, version control systems, and techniques such as object-oriented/functional programs, schools, universities and developers’ book shelves typically lack material about how software should generally be designed.

The average developer just changes their code until the tooling stops emitting errors/warnings and the code actually does what it is written for. In some cases, the rules from clean code literature are over-applied, resulting in short but entangled code. While doing that, the program’s complexity increases. Complexity is like a currency of programming: For more features, you give up your program’s initial simplicity. This typically ends in programs of huge complexity that cannot be maintained easily any longer.

Complexity never comes with big leaps but always with a stepwise increase. The three main symptoms of complexity are:

Change Amplification

Seemingly simple changes require modifications in many different places.

Cognitive Load

The amount of project-knowledge that a developer needs to know in order to complete a task.

Unknown Unknowns

It is not obvious which pieces of code must be modified to complete a task.

The two main causes of complexity are:


A dependency exists when a given piece of code cannot be understood and modified in isolation. They can’t be eliminated, but designed carefully. (This is a definition for the discussions in the book. Software dependencies like installable libraries mean something different.)


Obscurity occurs when important information is not obvious.

In order to keep complexity under control (i.e. to reach sustainable tradeoffs) developers need the right mindset. John distinguishes between two of them:

Tactical Programmers

The main focus of the tactical programmer is to get something working, such as new features or bug fixes.

Strategic Programmers

The strategic programmer does not think of “working code” as their primary goal (although accepting that delivered code must always be working). Their primary goal is a great design.

Most real companies’ work culture and deadline pressure facilitates the rise of tactical programmers (also called tactical tornados), which typically write code with uncontrollable complexity. Strategical programming requires an investment mindset for trading initial upfront slowdown against long-term improvements. The fight against continuously increasing complexity is fought with continuous investments. Discussions between strategic and tactical programmers are often frustrating.

The next topic is the organization of code into modules (as in classes, subsystems, or services) that communicate via interfaces. Modules encapsulate the complexity of systems into domain-specific units. John coins terms for two different qualities of modules:

Deep Modules

The best modules provide powerful functionality over simple interfaces. They provide good abstraction by providing complex functionality but only exposing a small fraction of their internal complexity. One example are the five basic system calls for I/O in UNIX operating systems.

Shallow Modules

In contrast to deep modules, shallow ones have relatively complex interfaces, compared to the functionality they provide. Many shallow interfaces do not even provide much keystroke saving when used, compared to their reimplementation.

Shallow modules are often a result of classitis. Students at university, or readers of clean code lecture are often advised to break code into small units. If this advise is followed without much strategy, it often results in shallow modules which are not much more than leaking abstractions.

Another interesting word that John introduces in this context is temporal decomposition: Developers often structure code modules according to the order in which operations occur. This does often lead to code that shares knowledge, but at different places in the code, leading to change amplification, higher cognitive load, and unknown unknowns again.

A good rule of thumb in module design that John comes up with is:

It is more important for a module to have a simple interface than a simple implementation.

If a module ends up being too complex, but its interface is very simple, then this means that it can easily be substituted by a better one. Also, changes on such a module do not increase change amplification.

Another great principle is to define errors out of existence: John argues that error handling and exceptions make programs much more complex, and a good way to reduce such complexity is representing data, interfaces, and semantics in ways that make it impossible to encode erratic cases that need special handling.

How to Write Code

The second half is about the act of designing and writing code. John argues that interfaces and modules should be designed twice: Implementing a design for the first time often exposes new insights that would lead to a different design in a second approach. I myself often designed things not only twice but tried three or four different approaches, in order to come up with the one that leads to the best result. It’s a game changer for complexity.

Working hard on the first design attempt, just to throw it away because it was just a vehicle for learning how to do it right: This is clearly not the mindset of the average programmer. At first glance, it also looks like it would waste a lot of time, especially from the point of view of a tactical programmer.

The next chapters are about comments, naming, modifying existing code, consistency (of style/documentation/etc. across the project), performance, and contain a lot of fine-grained advise that should not be unknown to the working programmer. Because it can’t be summarized by few principles, i will just drop some interesting highlights: (These read like rules, but they are not and should never be over-applied in a dogmatic fashion)

John summarizes in his book’s conclusion:

The reward for being a good designer is that you get to spend a larger fraction of your time in the design phase, which is fun. Poor designers spend most of their time chasing bugs in complicated and brittle code.


I really enjoyed this book because it gave me an effective new vocabulary in the epic fight against complexity, which is often more social than technical when arguing with tactical programmers.

Read it - as early as possible - in your developer career.