DDD.12 – Relating Design Patterns to the Model

Design Patterns are very technical patterns in code, while Domain Patters are conceptual patters in the model.


Some times, we have different ways in which we can perform an action, a process. In order to decide which strategy we are going to use at run time, we will eventually have a conditional.

In a naive approach we would have a method with as many conditionals as strategies to to perform the action, and all strategies coded directly in that same method. An approach which is a bit better, is to have a method for each different strategy, but still in the same class.

Using the STRATEGY design pattern, we create a class per each strategy algorithm. All of those strategy classes will implement a common interface so that the calling code can use either strategy class without caring which one it is actually executing. This will completely decouple the calling code from the concrete implementation of the different strategies.

The calling code will then use an ABSTRACT FACTORY to instantiate the appropriate strategy class according to a SPECIFICATION.


The composite pattern tries to simplify the usage of a complex structure of an object, by modelling that structure in as a tree where all elements are of the same type.

An example can be the products categories of a webshop. In this case, we have the categories completely decoupled from the products. It doesn’t really matter what products the category has inside. A category can have several categories inside, and those can have other categories in them, and so forth, so on, building the mentioning tree like structure where all elements have the same interface.

The example given by Eric Evans, is about travel routes, where each route is composed by an origin, a destination, and several smaller routes which are composed by even smaller routes and so on.

This post is part of a set of posts with my personal notes about all the chapters in the book “Domain Driven Design” by Eric Evans. I will do this as I read through the book, and take notes on the concepts I personally find more relevant.

DDD.11 – Applying Analysis Patterns

Analysis patterns are specialized, high level patterns who generically model a very well known and wide domain, they are not technical solutions.

These patterns allow us to cut down on the time spent in early stages of development, while doing knowledge crunching and trial and error iterations. They give us a head start, a solid advanced starting point from where to build what is specific to our domain, and anticipate consequences that are expensive if we have to discover them on our own.

Recommended reading:

Analysis patterns: Reusable object models
[Martin Fowler, 1997]


This post is part of a set of posts with my personal notes about all the chapters in the book “Domain Driven Design” by Eric Evans. I will do this as I read through the book, and take notes on the concepts I personally find more relevant.

DDD.10 – Supple Design

Supple, or elegant and flexible, design is a complement to Deep Modelling. While deep modelling is about digging out domain concepts with the domain experts, and reflecting those concepts in code, Supple Design is about building that code in a way that is easy to understand, modify and extend, in sum: code that is maintainable.

If we want to maintain speed of development as the project grows, we must design code that is a pleasure to work with, code that invites change. Otherwise the code will be a mess, where a small change can aggravate or break something: developers will dread to look at it, let alone work with it.

Eric Evans came up with a set of patterns that can help achieve this supple design.

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DDD.9 – Making implicit concepts explicit

Implicit concepts are concepts that exist but , for some reason, the domain experts don’t talk about them explicitly. It takes some time until the developers actually realize the existence of those implicit concepts, by means of hints while discussing the model or at later stages, when several other concepts are already set in place.

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DDD.8 – Refactoring toward deeper insight / Breakthrough

The third part of the book starts in chapter 8 and talks about refactoring.

Refactoring towards deeper insight has the objective of having a domain model that closely reflects the domain. This makes the software more in tune with what the domain experts think and to the users current and future needs.

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DDD.6 – The lifecycle of a domain object

There are three main objects that can control the life-cycle of a domain object:

  • Aggregates: Hold several objects in a root object and can control their creation, manipulation, query and deletion;
  • Factories: Create new objects with a set of data;
  • Repositories: Recreate persisted objects from queried data, possibly delegating the  reconstruction of the actual object instance to a factory.

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DDD.4 – Isolating the domain

In order to easily identify, understand and manipulate the code that reflects the domain, we need to isolate it from all the support code, the code that allows for user interaction (views, controllers) and the code that allows for data persistence and retrieval.

The most widely used and easy to understand approach is to isolate the code into layers:

  • User interface: Views to interact with the user;
  • Application: Controllers to specify use cases;
  • Domain: Business rules that reflect the model;
  • Infrastructure: Persistence, queues, … ;

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