ZendCon 2011 UnCon Domain-Driven Design

Domain-Driven Design A Collaboration Between Domain Experts and Software Practitioners

The Book http://domaindrivendesign.org/books/evans_2003

do·maindōˈmān n.a sphere of knowledge,in uence, or activity "domain." Merriam-Webster.com. 2011. http://www.merriam-webster.com/dictionary/domain (17 October 2011).

Core Domain

Core Domain• complexity is in the domain, not the technology

Core Domain• complexity is in the domain, not the technology• let technology play a supporting role

Core Domain• complexity is in the domain, not the technology• let technology play a supporting role• uncover your core domain and distill this core

The Model* *In domain-driven design, the model is not the classes or objects representing concepts within the model.

The Model*• a model is a tool for solving speci c problems within a domain *In domain-driven design, the model is not the classes or objects representing concepts within the model.

The Model*• a model is a tool for solving speci c problems within a domain• “the map is not the territory” —Alfred Korzybski *In domain-driven design, the model is not the classes or objects representing concepts within the model.

The Model*• a model is a tool for solving speci c problems within a domain• “the map is not the territory” —Alfred Korzybski• collaborative explore the model with both domain experts and software practitioners *In domain-driven design, the model is not the classes or objects representing concepts within the model.

Bounded Context

Bounded Context• there are always multiple models

Bounded Context• there are always multiple models• explicitly de ne a bounded context to which a model applies

Bounded Context• there are always multiple models• explicitly de ne a bounded context to which a model applies• speak a ubiquitous language within a bounded context

Building Blocks

Entity

Entity• de ned by a thread of continuity and identity

Entity• de ned by a thread of continuity and identity• only responsibilities should be around identity and life cycle

Entity• 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

Value Object

Value Object• de ned by its encapsulated attributes

Value Object• de ned by its encapsulated attributes• treat value objects as immutable

Value Object• de ned by its encapsulated attributes• treat value objects as immutable• delegate business logic to value objects

De ning an object as an entity or avalue object is context-dependent.

Aggregate

Aggregate• a group of related entities and value objects

Aggregate• a group of related entities and value objects• useful when de ning transaction, distribution and concurrency boundaries

Aggregate• a group of related entities and value objects• useful when de ning transaction, distribution and concurrency boundaries• designate one entity as the aggregate root

Aggregate• a group of related entities and value objects• useful when de ning transaction, distribution and concurrency boundaries• designate one entity as the aggregate root• allow external references to only the aggregate root

Aggregate• a group of related entities and value objects• useful when de ning transaction, distribution and concurrency boundaries• designate one entity as the aggregate root• allow external references to only the aggregate root• delegate persistence of an aggregate root to a repository

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

Domain Event• something important that happens within the domain that may lead to a state change in a domain object 1. http://martinfowler.com/eaaDev/EventSourcing.html

Domain Event• something important that happens within the domain that may lead to a state change in a domain object• current state can be computed by looking at all currently known domain events (a.k.a. event sourcing[1]) 1. http://martinfowler.com/eaaDev/EventSourcing.html

Domain Event• something important that happens within the domain that may lead to a state change in a domain object• current state can be computed by looking at all currently known domain events (a.k.a. event sourcing[1])• domain events can trigger other domain events (e.g. three strikes triggers an out) 1. http://martinfowler.com/eaaDev/EventSourcing.html

Supple Design

Closure of Operations

Closure of Operations• have a method on a value object that returns an instance of the same type of value object

Closure of Operations• 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

Closure of Operations• 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

Closure of Operations• 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 number

Closure of Operations• 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 number • number has an add method

Closure of Operations• 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 number • number has an add method • add method accepts an argument of type number

Closure of Operations• 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 number • number has an add method • add method accepts an argument of type number • add method returns a number

Other Techniques• intention-revealing interfaces• side-eﬀect free functions• assertions

Strategic Design

Context Map

Context Map• draw a context map of the current bounded contexts

Context Map• draw a context map of the current bounded contexts• map what actually exists—not what you wish existed!

Context Map• draw a context map of the current bounded contexts• map what actually exists—not what you wish existed!• identify relationships between contexts

Relationship Patterns customer/ anticorruption partnership supplier layershared kernel big ball of separate ways mud open host conformist published service language

Distillation

Multiple Models

Multiple Models• there are always multiple models

Multiple Models• there are always multiple models• a model my represent:

Multiple Models• there are always multiple models• a model my represent: • your core domain

Multiple Models• there are always multiple models• a model my represent: • your core domain • a supporting domain

Multiple Models• there are always multiple models• a model my represent: • your core domain • a supporting domain • a generic subdomain

Multiple Models• there are always multiple models• a model my represent: • your core domain • a supporting domain • a generic subdomain• focus your modeling eﬀorts on the core domain

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 havingaccess to a domain expert!

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ZendCon 2011 UnCon Domain-Driven Design

by Bradley Holt on Oct 18, 2011

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ZendCon 2011 UnCon Domain-Driven Design Presentation Transcript