Build Complex Domains in Rails

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BUILD ! COMPLEX DOMAINS! IN RAILS Mike AbiEzzi

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BUILD ! COMPLEX DOMAINS! IN RAILS Mike AbiEzzi

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WHAT’S A DOMAIN?

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WHAT’S A DOMAIN?

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WHAT’S A DOMAIN?

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WHAT’S A DOMAIN?

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WHY DDD?

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WHY DDD?

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WHY DDD?

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WHY DDD?

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WHY DDD?

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WHY DDD?

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WHY DDD?

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WHY DDD? software becomes complex quickly

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AIM OF THIS TALK ✘ Custom DDD architectures! ✘ DDD design patterns! ✘ Advanced DDD topics (e.g. Repository Pattern) (e.g. Bounded Context)

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AIM OF THIS TALK ✘ Custom DDD architectures! ✘ DDD design patterns! ✘ Advanced DDD topics (e.g. Repository Pattern) (e.g. Bounded Context) custom is expensive!

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AIM OF THIS TALK ✓ DDD principles ✓ DDD+Rails w/o a heavy investment

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AIM OF THIS TALK ✓ DDD principles ✓ DDD+Rails w/o a heavy investment we want to hit the ground running!

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▾ app/! ▸ assets/! ▸ controllers/! ▸ helpers/! ▸ mailers/! ▸ models/! ▸ views/

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1. Deﬁning the domain! 2. Communicating the domain! 3. Relationships between models! 4. Aggregates! 5. Data access! 6. Value Objects! 7. Domain Services

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the domain 1

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App Store

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App Store brainstorm

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App Store App brainstorm

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Developer 1 n App Store App brainstorm

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Developer 1 n App Store App Version 1 n brainstorm

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Developer 1 n App Store App Version 1 n 1..6 Screenshot 1 brainstorm

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Developer 1 n App Store Customer App Version 1 n 1..6 Screenshot 1 brainstorm

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Developer 1 n App Store Customer App n n Purchase Version 1 n 1..6 Screenshot 1 brainstorm

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Developer 1 n App Store Customer App Install n n n n Purchase Version 1 n 1..6 Screenshot 1 brainstorm

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Developer 1 n App Store Customer App Install n n n n Purchase Comment 1 n n 1 Version 1 n 1..6 Screenshot 1 brainstorm

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Version 1 n 1..6 Screenshot 1 reﬁne

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Version 1 n 1..6 Screenshot 1 Developer/ Company reﬁne

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Version 1 n 1..6 Screenshot 1 Developer/ Company Seller reﬁne

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller reﬁne

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller reﬁne

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communication 2

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COMMUNICATION

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COMMUNICATION Domain! Expert

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COMMUNICATION Domain! Expert Software! Expert

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COMMUNICATION Domain! Expert Software! Expert brainstorm → draw diagrams → ! speak out assumptions → let them correct you → reﬁne

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RESPONSIBILITIES -- Eric Evans

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RESPONSIBILITIES “Domain experts should object to terms or structures that are awkward or inadequate to convey domain understanding” -- Eric Evans

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RESPONSIBILITIES “Domain experts should object to terms or structures that are awkward or inadequate to convey domain understanding” “Developers should watch for ambiguity or inconsistency that will trip up design.” -- Eric Evans

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UBIQUITOUS LANGUAGE “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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UBIQUITOUS LANGUAGE User “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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UBIQUITOUS LANGUAGE Product! Owner User “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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UBIQUITOUS LANGUAGE Domain! Expert Product! Owner User “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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Tester UBIQUITOUS LANGUAGE Domain! Expert Product! Owner User “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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Tester UBIQUITOUS LANGUAGE Domain! Expert Product! Owner User Designer “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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Developer Tester UBIQUITOUS LANGUAGE Domain! Expert Product! Owner User Designer “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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Code Developer Tester UBIQUITOUS LANGUAGE Domain! Expert Product! Owner User Designer “a common, rigorous language between developers and users […] the need for it to be rigorous, since software doesn't cope well with ambiguity” — Martin Fowler

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relationships 3

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller relationships

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller class App < ActiveRecord::Base has_many :customers, through: :purchases ... end ! ! class Customer < ActiveRecord::Base has_many :apps, through: :purchases ... end relationships

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App Store Developer 1 n Customer App Install n n n n Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller relationships

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1 n App Store Developer 1 n Customer App Install n n Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller relationships

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1 n App Store Developer 1 n Customer App Purchase Comment 1 n n 1 Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller relationships

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1 n App Store Developer 1 n Customer App Purchase Comment 1 n Review! comment! rating Version 1 n 1..6 Screenshot 1 Release! version_number Developer/ Company Seller relationships

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aggregates 4

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aggregates 4

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release = Release.new( version_major: 1, version_minor: 1, ...) release.screenshots << [ Screenshot.new(file: "shot1.png"), Screenshot.new(file: "shot2.png")] release.status = :submitted ! app.releases << release Submit a new release of an app

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app.submit_release(“1.1.0”, screenshots: ["shot1.png", "shot2.png"]) release = Release.new( version_major: 1, version_minor: 1, ...) release.screenshots << [ Screenshot.new(file: "shot1.png"), Screenshot.new(file: "shot2.png")] release.status = :submitted ! app.releases << release A BETTER WAY? Submit a new release of an app

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 AGGREGATE Release! version_number

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class App < ActiveRecord::Base ... ! def submit_release ... def approve_release ... def flag_for_abuse ... ! def mark_as_staff_favorite ... ! end

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class App < ActiveRecord::Base ... ! def submit_release ... def approve_release ... def flag_for_abuse ... ! def mark_as_staff_favorite ... ! end Tells a story of how the domain works

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data access 5

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Data Access App.where( "create_at > ? and purchase_count > ?", 1.week.ago, 10000).all

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Data Access App.where( "create_at > ? and purchase_count > ?", 1.week.ago, 10000).all

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Data Access App.where( "create_at > ? and purchase_count > ?", 1.week.ago, 10000).all require 'fig_leaf' ! class App < ActiveRecord::Base scope :new_and_noteworthy, -> { where("create_at > ? and purchases > ?", 1.week.ago, 10000) } end Use scopes

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class App < ActiveRecord::Base scope :new_and_noteworthy, ... scope :staff_picks, ... scope :most_popular, ... ... ! def submit_release ... def approve_release ... ... end

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class App < ActiveRecord::Base scope :new_and_noteworthy, ... scope :staff_picks, ... scope :most_popular, ... ... ! def submit_release ... def approve_release ... ... end One expressive point of entry

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▾ app/! ▾ models/! ▾ apps/! app.rb! release.rb! review.rb! screenshot.rb! version_number.rb! ▸ customers/! ▸ sellers/

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▾ app/! ▾ models/! ▾ apps/! app.rb! release.rb! review.rb! screenshot.rb! version_number.rb! ▸ customers/! ▸ sellers/ Aggregate Aggregate Aggregate

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value objects 6

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ENTITY VALUE OBJECT

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ENTITY VALUE OBJECT a thing

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ENTITY VALUE OBJECT a thing describes a thing

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ENTITY VALUE OBJECT a thing describes a thing class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing class Customer class Name Me  “Mike AbiEzzi”

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ENTITY VALUE OBJECT a thing describes a thing class Customer class Name Me  “Mike AbiEzzi” My little cousin  “Mike AbiEzzi”

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ENTITY VALUE OBJECT a thing describes a thing unique independent! of attributes class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing unique independent! of attributes has a lifecycle class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing can change state unique independent! of attributes has a lifecycle class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing can change state unique independent! of attributes has a lifecycle class Customer class Name “Mike AbiEzzi” “Mike AbiEzzi”

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ENTITY VALUE OBJECT a thing describes a thing can change state unique independent! of attributes immutable has a lifecycle class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing can change state doesn’t reference anything unique independent! of attributes immutable has a lifecycle class Customer class Name

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ENTITY VALUE OBJECT a thing describes a thing can change state doesn’t reference anything unique independent! of attributes avoids design complexities immutable has a lifecycle class Customer class Name

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Release! version_number What’s what?

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Entity Release! version_number What’s what?

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Entity Release! version_number Entity What’s what?

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Value Entity Release! version_number Entity What’s what?

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Value Entity Release! version_number Entity What’s what? ?

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App 1 Comment 1 n Review! comment! rating n 1..6 Screenshot 1 Value Entity Release! version_number Entity What’s what? ? Value

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class Screenshot attr_reader :file, :position ! def initialize(file, position) @file, @position = file, position end ! def ==(other) file == other.file && position == other.position end alias_method :eql?, :== def hash; file.hash ^ position.hash; end end VALUE OBJECT immutable / equality

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class VersionNumber attr_reader :major, :minor, :build ... def next_major_version new VersionNumber( major + 1, minor, build) end ! def next_minor_version ... def next_build_version ... end VALUE OBJECT factory methods

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3 ways to persist value objects

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3 ways to persist value objects A. Inline on the Entity’s table

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3 ways to persist value objects A. Inline on the Entity’s table B. Serialized on the Entity’s table

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3 ways to persist value objects A. Inline on the Entity’s table B. Serialized on the Entity’s table C. In its own table

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Release! VersionNumber! Entity Value 1 1 A. Inline on the Entity’s table releases version_major version_minor version_build …

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A. Inline on the Entity’s table class Release < ActiveRecord::Base def version_number=(vn) @version_number = vn self[:version_major] = vn.major self[:version_minor] = vn.minor self[:version_build] = vn.build end ! def version_number @version_number ||= new VersionNumber( self[:version_major], ... ) end ! private attr_accessor :version_major, ... ! end

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Release! Screenshot! 1 1..6 B. Serialized on the Entity’s table releases … screenshots …

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require 'oj' ! class Release < ActiveRecord::Base ! def screenshots=(screenshots) @screenshots = screenshots self[:screenshots] = Oj.dump(screenshots) end ! def screenshots @screenshots ||= Oj.load(self[:screenshots]) end ! end B. Serialized on the Entity’s table

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Release! Review! 1 n C. In its own table releases … reviews release_id …

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C. In its own table class Review < ActiveRecord::Base after_save { |record| immutable!(record) } after_find { |record| immutable!(record) } ! def immutable!(record) record.readonly! attributes.each do |attr, _| record.define_singleton_method :"#{attr}=" do |_| raise "readonly" end end end ! def ==(other) ... alias_method :eql?, :== def hash ... end

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domain services 7

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Gift an App SERVICE

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Gift an App SERVICE 1. Charge the gifter that’s purchasing the app.

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Gift an App SERVICE 1. Charge the gifter that’s purchasing the app. 2. Assign access rights to the giftee receiving the app.

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Gift an App SERVICE 1. Charge the gifter that’s purchasing the app. 2. Assign access rights to the giftee receiving the app. Facilitates a transaction between two aggregates

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Purchase 1 n Customer

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Purchase 1 n Customer AccessRight 1 n

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class GiftApp def self.execute(app, gifter, giftee) Purchase.create( app: app, customer: gifter, ...) AccessRight.create( app: app, customer: giftee, purchase: purchase, ...) end end Purchase 1 n Customer AccessRight 1 n

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▾ app/! ▸ models/! ▾ services/! gift_app.rb! refund_purchase.rb! purchase_app.rb! ...

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IN SUMMARY

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Continuously reﬁne the domain with a domain expert 1

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Insist on having a ubiquitous language for seamless communication 2

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Constrain relationships to only what the domain needs 3

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Create aggregates that express domain concepts and manage complexity 4

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Express the domain’s intent through! well deﬁned data access 5

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Create value objects to eliminate unnecessary complexity 6

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Create domain services to express transactions ! between aggregates 7

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▾ app/! ▸ assets/! ▸ controllers/! ▸ helpers/! ▸ mailers/! ▸ models/! ▸ views/

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▾ app/! ▾ models/! ▸ apps/! app.rb! ...! ▸ customers/! ▸ sellers/! ▸ ...! ▸ services/! gift_app.rb! refund_purchase.rb! ...

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▾ app/! ▾ models/! ▸ apps/! app.rb! ...! ▸ customers/! ▸ sellers/! ▸ ...! ▸ services/! gift_app.rb! refund_purchase.rb! ... aggregates

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▾ app/! ▾ models/! ▸ apps/! app.rb! ...! ▸ customers/! ▸ sellers/! ▸ ...! ▸ services/! gift_app.rb! refund_purchase.rb! ... aggregates entry point to domain behaviors & ! data retrieval

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▾ app/! ▾ models/! ▸ apps/! app.rb! ...! ▸ customers/! ▸ sellers/! ▸ ...! ▸ services/! gift_app.rb! refund_purchase.rb! ... aggregates cross aggregate transactions entry point to domain behaviors & ! data retrieval