Domain-Driven DesignA Collaboration Between

Domain Experts and Software Practitioners

The Book

http://domaindrivendesign.org/books/evans_2003

Training

http://www.domainlanguage.com/

do·main\dōˈmān\ n.a sphere of knowledge, in#uence, or activity

"domain." Merriam-Webster.com. 2011. http://www.merriam-webster.com/dictionary/domain (17 October 2011).

Complexity is in the domain, not the technology

Case Study: Election Reporting

Elections and referendums are not as similar as they at $rst appear

Election winners may be determined by plurality and/or winning threshold

Referendums can only be passed by meeting a winning threshold

Elections vs. Referendums

Town Meeting Day (March) elections are very different than statewide elections (November)

During statewide elections, voters may be voting in both local and statewide elections at the same time—each with different district boundaries

District boundaries dictate reporting needs—statewide elections need to be reported one way, local elections another

Local vs. Statewide Elections

DistrictsSome districts may contain only part of a municipality or ward

The Chittenden-3-6 Vermont Representative District contains all of Winooski and part of Burlington’s Ward 1

Citizens in this part of Burlington’s Ward 1 vote in Winooski for statewide elections and in Burlington for local elections

Audience expects both aggregate and detailed reporting

Redistricting can occur between elections

Let technology play a supporting role

Models are tools used to solve domain problems

Ames RoomUsed in The Lord Of The Rings: The Fellowship of the Ring to make the hobbits appear the correct size in relation to Gandalf

We are always using mental models to understand the world around us—we do not perceive an objective reality

By Alex Valavanis (own work) [public domain], via Wikimedia Commons

"Why I prefer Fahrenheit to Celsius [Fixed]." reddit. 2012. http://www.reddit.com/r/funny/comments/wigk1/why_i_prefer_fahrenheit_to_celsius_$xed/ (16 September 2012).

Collaboratively explore the model with both domain experts and software practitioners

Case Study: Three-Dimensional Animation

Studied physics and computer science

Made many notable computer graphics discoveries

Eventually moved from two-dimensional to three-dimensional animation

Hired by Lucas$lm to bring his expertise to the entertainment $eld

Software Practioner: Edwin Catmull

Domain Expert: John LasseterStudied animation and taught by veteran animators from Disney

Realized early-on the potential for computer generated imagery

Worked at, but eventually $red from, Disney

Hired by Edwin Catmull at Lucas$lm as an “Interface Designer” because Catmull’s job didn’t include hiring animators[1]

1. Buckley, A. M. "Chapter 3: Art Meets Science." Pixar: The Company and Its Founders. Edina, MN: ABDO, 2011. 27. Print.

“Throughout the process, Lasseter worked side-by-side with the computer scientists. Lasseter’s requests pushed them to develop new tools, and their feedback helped him learn the digital animation process.”[1]

1. Buckley, A. M. "Chapter 3: Art Meets Science." Pixar: The Company and Its Founders. Edina, MN: ABDO, 2011. 30. Print.

Identify your core domain

Identify your core domain

Distill your core domain

Focus your resources on the core domain

Core Domain

Collaboratively explore domain models

A model is an abstract set of tools that is used to solve problems within a domain

While represented in code, do not think of the model as just code

A “real world model” is a fool’s errand

The model must be explored collaboratively with domain experts and software practitioners

The Model

“The map is not the territory.” —Alfred Korzybski

Magritte, René. The Treachery of Images (La trahision des images). 1929. Oil on canvas. Los Angeles County Museum of Art, Los Angeles, California.

This is not a pipe.

Magritte, René. The Treachery of Images (La trahision des images). 1929. Oil on canvas. Los Angeles County Museum of Art, Los Angeles, California.

"Everything simple is false. Everything which is complex is unusable." —Paul Valéry

There are always multiple models

Delineates the applicability of a particular model

Bounded contexts allow each model to be explored in isolation

Clearly de$ne:• Who is responsible for each bounded context• To which parts of the application is the bounded

context applicable• What manifestations the bounded context will take

(code, database schemas, etc.)

Document the interactions between bounded contexts with a context map

Bounded Context

Ubiquitous LanguageSpeak a ubiquitous language within a bounded context

Terms must be clearly de$ned, unambiguous, and consistent

Critically important when communicating between domain experts and software practitioners

The ubiquitous language will (and must) evolve as a progressively richer understanding of the domain and the model are achieved

If the ubiquitous language cannot be used to clearly express complex ideas, then you have more work to do!

Exploring the Model

Ask a domain expert to tell you a story (the scenario)

Propose a model

Code the scenario using unit tests

Repeat

A Model Exploration Process

Use concrete scenarios in discussions with domain experts and in unit tests

Building Blocks

De$ned by a thread of continuity and identity

Only responsibilities should be around identity and life cycle

May be composed of other entities and/or value objects

Entity

Value ObjectDe$ned by its encapsulated attributes

Treat value objects as immutable

Delegate business logic to value objects

De$ning an object as an entity or a value object is context-dependent

A group of related entities and value objects

Useful when de$ning transaction, distribution and concurrency boundaries

A model will likely have multiple aggregates

Aggregate

Aggregate RootDesignate one entity as the aggregate root

Allow external references to only the aggregate root

Use unidirectional references within an aggregate root for less complexity• Example: Reference a line item’s order, or an order’s line items,

but not both

Maintain references on the “many” side of a “one-to-many” relationships for less complexity:• Example: Reference a line item’s order, rather than an order’s

line items

RepositoryDelegate persistence of an aggregate root to a repository

A repository should behave as if it were an in-memory data store

If using an object-relational mapper (ORM): Database -> ORM -> Repository

Use an in-memory strategy for unit tests

Straddles persistence and domain layers, allowing you to stay focused on the domain model

ServiceA place for operations that aren’t naturally part of any domain object

Like value objects, services should be immutable

Operations on services are stateless

Command-Query Responsibility Segregation (CQRS)

Commands are responsible for changing state

Queries are responsible for retrieving state

A commands may delegate the actual state change to a domain event

Commands & Queries

Write Model/Read ModelDe$ne one model for writing data (commands)

De$ne another model for reading data (queries)

Both models will likely share aggregate root entity identi$ers

Event Sourcing[1]

1. http://martinfowler.com/eaaDev/EventSourcing.html

Domain EventSomething important that happens within the domain that may lead to a state change in a domain object

Domain events can trigger other domain events (e.g. three strikes triggers an out)

Domain events are immutable

Typically stored in an event log

Event LogCurrent state can be computed by reading the event log

Retroactive events can be used to “$x” application state

Current state may be cached, if necessary for performance

Can also serve as an audit log

Supple Design

Have a method on a value object that returns an instance of the same type of value object

Any method arguments should also be the same type as the value object

Example: 2 + 3 = 5• “2” is a value object of type integer• integer has an add method• add method accepts an argument of type integer• add method returns an integer• integers are closed under the operation of addition

Closure of Operations

Other TechniquesIntention-revealing interfaces

Side-effect free functions

Assertions

Strategic Design

Draw a context map of the current bounded contexts

Map what actually exists—not what you wish existed!

Identify relationships between contexts

Context Map

customer/supplier

Relationship Patterns

partnership

shared kernelbig ball of

mud

conformist

anticorruption layer

separate ways

open host service

published language

Distillation

A model my represent:• your core domain• a supporting domain• a generic subdomain

Focus your modeling efforts on the core domain

Consider outsourcing work on supporting domains

Consider off-the-shelf software for generic subdomains

Types of Domains

Identifying the Core DomainAsk organizational leaders and domain experts:• What keeps you awake at night?• What makes your system worth writing?• Why not buy it off the shelf?• Why not outsource it?

Don’t settle on not having access to a domain expert!

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Copyright © 2011-2012 Bradley Holt. All rights reserved.