Lean Architecture: for Agile Software Development [Englisch] [Taschenbuch]

This is a different book. Where most books expound a single theme such as Agile, Lean, or Scrum, "Lean Architecture for Agile Software Development" paints on a much broader canvas: Working with the end user, end user's mental model, user requirements, system architecture, and right down to actual code.

This is neither a beginner's "how to do it in ten easy lessons" nor is it a design method. It is a book written for the mature professional by two authors whose long experience has given them a deep understanding of what really matters in practical programming.

At a first glance, many methodologies appear as mere fads, but Coplien and Bjørnvig see through the fads and build on their real worth to create a thought-provoking and eminently practical book.

Three random jottings from my first reading:

* Architecture: "No matter how we care to define it, software architecture should support the enterprise value stream even to the extent that the source code itself should reflect the end user's mental model of the world."

* Lean secret: "...unite specialists together in one room: everybody, all together, from early on."

* Form and functionality: "System architecture should reflect the end user's mental model of the world. The model has two parts: The first part relates to the user's thought process when viewing the screen, and to what the system is: its form. The second part relates to what end users do - interacting with the system - and how the system should respond to user input. This is the system functionality. We work with users to elicit and develop these models and to capture them in code as early as possible."

The authors claim that an end user should have a picture in his head that enables him to see the commands that are meaningful in a given situation and to understand what they will do for him. This picture, Jim calls it the end user's mental model, it will be reflected into the actual code in well-built systems.

A few years ago, this reviewer introduced a new programming paradigm that he called Data, Context, and Interaction (DCI). The main feature of this paradigm is that it splits the code into two distinct parts. One part specifies system state; the other part specifies system behavior. Coplien and Bjørnvig use this paradigm to fill in the gap between architecture and code execution. To quote from the book:

* Key building blocks of object-oriented design: "Objects, which are end users' conceptualization of things in their business world; Classes, which provide simple, encapsulated access to the data that represents business information; Roles, which interact in a use case to achieve some business goal."

This book is a MUST read for all who want to understand the true nature of systems development.

When I was a C++ programmer in the early 90's Coplien's Advanced C++ Programming Styles and Idioms was a source of interview material when looking for programmers. It's a good bet that this book may fill the same role for those looking to see if candidates for architect roles understand what it means to be an architect in a Lean or Agile Organization. This book dispels the myth that Agile and Architecture don't go together and explains the balance between Agile architecture and too much Big Up Front Design. This book emphasizes the importance of frequent collaboration between stakeholders in defining a good architecture and helps you to understand the importance of architecture to the success of agile projects. With code examples throughout, this book emphasizes that architecture and coding must go together. After describing some general principles of how architecture can add value to an agile project, the authors explain the Data Context, Interaction (DCI) architecture, which provides an framework for building lean architectures. My one minor complaint is that the transition between the general discussions of lean architecture and the focused discussion of DCI was a bit abrupt. But this was a minor distraction from an enjoyable and informative read. Rich with citations, places to go for more information, and historical context, this book will be useful for anyone who is struggling with how to build systems that need to support complicated user interactions (which could describe most non-trivial systems).

This first book covering the new DCI paradigm of programming is a must read for anyone interested in separating the more often changing behavioral parts of code related to use cases from the more slowly evolving structural parts of domain data.

A great practical description of how the use case evolves and translates directly into code, of how to reflect the end users mental model in code - making it much more readable for both programmers and domain experts. It goes into detail about how use case roles translates to Object Roles playing out their part of a use case algorithm and how they get injected into the domain objects to use their state. And we are presented with how the Context can set up the mapping of Roles to domain Objects in different flexible ways before firing off the trigger Interaction of the use case.

Apart from example code in C++ and Ruby through out the last chapters of the book, it also have a great appendix with coded DCI examples in Scala, Python, C#, Ruby and Squeak.

For anyone interested I can also recommend visiting the Google "object-composition" group where all the concepts are discussed and explored.