Domain Driven Design

💚 Alan Tech Talk #23 💚

The Blue Book

Tackling Complexity in the Heart of Software

Eric Evans, 2003

DDD Quickly

😅 ~100 pages vs 560 pages https://www.infoq.com/fr/minibooks/domain-driven-design-quickly

What’s DDD ?

👥 Domain model oriented approach from the domain expert inputs
🛠 Toolbox of tactical and strategic patterns
🔫 Not a silver bullet

DDD vs CRUD

Why DDD ?

🤝 Better understand and communicate with your business experts
🤯 Decrease accidental complexity to focus on business complexity
😎 Master the complexity and improve maintenability
✂️ Separate code that changes frequently from code that is stable

Complexity

https://www.lilobase.me/certaines-complexites-sont-plus-utiles-que-dautres/

Strategic & Tactical patterns

Ubiquitous Language

🤝 Build a common and rigorous language between developers and business experts
🥸 As the team better understands their business, it could evolve
😇 You should align your code naming (class, variables, …) with the ubiquitous Language

Domain model

🤯 The domain is the problem
😎 The model is the solution
❌ Not a database model, not a UML model
📚 Object oriented model based on Entity, ValueObject and Aggregate

Entity

Is an object defined by its identity
Has a lifecycle (created, updated, deleted)
Has attributes that change over time

Entity - Example

Let’s define a Contract

Value Object

Is an object with no identity
Is defined by its immutable attributes
Can be shared between identities
🤫 It’s an immutable dataclass

Value Object - Example

Let’s introduce a Period and use it in Contract

Entity vs Value Object

Aggregate

Is a cluster of domain objects that can be treated as a single unit (= same transaction)
Has one root entity
All references from other aggregates should reference the root, not the sub entities.

Aggregates

Value Aggregates - Example 1

Value Aggregates - Example 2

Repository

Represents a persistent collection of entities
One repository per root aggregate
Is not a DAO (Data Access Object)

Repository - Example

class Companies:
    def get_company(company_id) -> Optional[Company]:
        pass

    def add_company(company) -> uuid:
        pass

    def remove_company(company_id):
        pass

Show me some code

One Aggregate

def add_contract_to_company(
        company_id, reference, start_date) -> Contract:
    company = companies.get_company(company_id)
    contract = company.add_contract(reference, start_date)
    return contract

Two Aggregates

def create_contract(
        company_id, reference, start_date) -> Contract:
    contract = Contract(company_id, reference, start_date)
    contracts.add_contract(contract)
    return contract

Scaling DDD - #15430

DDD & Architecture

Hexagonal Architecture

Alistair Cockburn, 2005

Hexagonal dependencies

I can talk DDD for hours…🤣

🗣 Ubiquitous Langage
👨‍🏫 Domain Model: Entity, Value Object organized by Aggregates
🧳 Repository to handle collection of root Aggregates
⬢ Hexagonal architecture to isolate domain model from technical stuff

Alan & DDD

🍿 DDD and 13280 - Moving towards a product architecture

❓ Do you think we can use some DDD patterns for Alan codebase?