Introduction to Domain Driven Design

Frank Levering• PHP Developer • DevMob

Quality software• Writing tests • Using design patterns • Manual testing • SOLID

Quality software doesn’t give an insurance for a quality

software model

Code example

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It saves a customer no matter what, awesome right?

This model is suffering from Anemia

We have no idea under what business situations this

method is used

And it even gets worse

Code example

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Can we really test this?

DDD helps you creating a more high-quality software

model

It helps you create software that makes sense to the

business

You would be making software that is as close as possible to what the

business leaders and experts would create if they were the developers

Disclaimer• A domain is not DNS or

anything internet related • Domain is the area your

business relies

So how does DDD work?• Strategic design • Tactical design

We will mostly be talking about strategic design today

Key concepts of strategic design• Domain experts • Ubiquitous language • Core Domains, Subdomains

and Domain Models • Bounded Contexts

Not doing any strategic design results in doing DDD-

Lite

Domain experts

Bringing domain experts and developers to the same playing field which produces software

that makes sense to the business

Introducing Ubiquitous language

It’s not the language of the business

It must not adopt industry standard terminology

It’s also not the language used by domain experts

It’s a shared language developed by a team, a team composed of both domain

experts and developers

Ubiquitous language develops on the way

No translations between the domain experts, software

developers and the software

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Ubiquitous language is important when designing the behaviour of objects

Code example

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Code example

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This was kind of an extreme example

Code example

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Code example

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Domains

A domain, in the broad sense, is what an organisation does

But the term domain is a bit overloaded

So let’s use Core Domain and Subdomains

The whole Domain of the organisation is composed of

Subdomains

Retailer example

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But what is the Core Domain?

Generic vs Supporting Subdomains

Why is this so important?

Developers like to focus on the technical part of a

product

We focus on Entities, Value Objects, Services,

Aggregates

By doing that, we blend core concepts with generic ones

Causing the creation of two models into one

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Agile PM tool example

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Focus on Core Domain

The Domain is your problem space

Domain Model

What is the Domain Model?

The Domain Model is your organisedand structured knowledge of the problem.• It should represent the

vocabulary and key concepts of the problem domain • It should identify the

relationships among all of the entities

It could be a diagram

Or written documentation

But in our case..

It’s a software model of the very specific domain you are working in

It should be accessible and understandable by everyone

involved in the project

Bounded Contexts

Bounded Context is an explicit boundary in which a

domain model exists

The boundary is created because each of the model’s concepts inside, with its properties and

operations, has a special meaning

Example with Bounded Contexts

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Inside a Subdomain, you can have multiple Bounded

Contexts

A Bounded Context should be as big as it needs to be in order

to fully express its complete Ubiquitous Language

Concepts that are not truly part of the domain should be

factored out

Naming a Bounded Context

So why should you do DDD?

Bringing domain experts and developers to the same playing field which produces software

that makes sense to the business

You would be making software that is as close as possible to what the

business leaders and experts would create if they were the developers

Learning more about the business, both domain experts and developers

No translations between the domain experts, software

developers and the software

Centralizing knowledge is key

Thank you