Domain-Driven Design (Tutorial)

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Starring With Directed by In the role of Also Starring

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@hschwentner Domain-Driven Design

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@hschwentner #dddesign

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-DRIVEN DESIGN

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Java old! C# ABAP PHP Python

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»Business-Software die Spaß macht«

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@hschwentner Timetable

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Workshop Workshop Workshop Workshop Morning Afternoon Not remote

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Morning Afternoon Wor ksho p Wor ksho p Wor ksho p Wor ksho p Wor ksho p Wor ksho p Wor ksho p

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@hschwentner Contents

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@hschwentner #unabsteigbar

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“Football is a simple gam 22 men chase a ball for 90 minutes and at the e the Germans win.” Gary Lineker Gary Lineker in 2011 by Christophe95 CC BY-SA-3.0

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Workshop Domain-Driven Design concrete wps.de/ddd

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Ubiquitous Language

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Ubiquitous Language Building Blocks Domain Event Aggregate Entity Value Object Bounded Context Strategic Design Context Mapping Collaborative Modeling Domain Expert Event Storming Modeling in Code Domain Storytelling Core Domain Domain Model

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Microservices CQRS Hexagonal Architecture Agile Event Sourcing Extreme Programming Scrum Cloud Self-Contained Systems Verticals Clean Architecture DevOps AWS Azure Onion Architecture

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@hschwentner Structure

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04.03.25 //// Seite 72 WPS – Workplace Solutions DOMÄNE DOMÄNE

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Java old! C# ABAP PHP Python

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@hschwentner You

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Programming Languages My Questions My life as a developer My experiences using DDD My wishes regarding the training family My first computer

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Story Time

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Graﬁk: Robert Gloth/Wikipedia

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Foto: Serge Melki/Wikipedia

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Foto: Public Domain/Wikipedia

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@hschwentner How is that supposed to work?

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Foto: Henning Schwentner

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Detail: Depth

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Detail: Tiefenzahl

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Foto: Afrank99/Wikipedia

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Foto: Henning Schwentner

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PUTS NAVIGATING OFFICER CAPTAIN ASKS FOR 7 SHIP SILHOUETTE ON CARTO- GRAPHER SOUNDING SHIP DEPTH MEASURES 1 SENDS 2 TO CALCULATES 3 DRAW S 4 SENDS 5 DEPTH MAP TO MOVES & TURNS 8 SHIP SILHOUETTE TO FIND 9 DISCUSSES WITH 6 ROUTE ROUTE DEPTH MAP DEPTH MAP DEPTH (RAW) ROUTE CONTOUR LINES

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@hschwentner Is There an App for It?

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CYBER

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@hschwentner So!ware != End in itself

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IMPORTANT!

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So#ware Domain

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Foto: Rosie the Riveter/Wikimedia/CC-PD-Mark

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Domain Knowledg e Foto: Rosie the Riveter/Wikimedia/CC-PD-Mark

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Groß Klein

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Collaborative Modeling

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Domain Expert Developer

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Knowledge crunching

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@hschwentner Methods

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https://domainstorytelling.org

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LANGUAGE

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Fach- sprache Domain Expert Developer

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=> direct communication telephone game to user dev user dev

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Ludwig Wi)genstein “Whereof one cannot speak, thereof one must be silent.”

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Ludwig Wi)genstein »Wovon man nicht sprechen kann, darüber muss man schweigen.«

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Depth Scale Silhouette Marking Depth Map to move to measure run aground neon yellow

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Ubiquitous Language

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Jubikjutuss Lengwitsch

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SMALL

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class Silhouette { //... }

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class Silhouette { public void moveBy(Length length) //... }

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class Silhouette { public void moveBy(Length length) //... public void turnBy(Angle angle) //... }

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Erich Gamma “Program to an interface, not to an implementa@on”

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class Silhouette { public void setPosition(Position p) //... } ✘

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✘

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WARNING SETTERS CAUSE MODEL ANEMIA

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MOVES & TURNS 8 SHIP SILHOUETTE TO FIND ROUTE NAUTICAL OFFICER «En$ty» Silhoue(e move() turn()

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Tactical Design

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Entity

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import org.jmolecules.ddd.annotation.*; @Entity public class DepthMap { //... }

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Value Object

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Example: Depth

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import org.jmolecules.ddd.annotation.*; @ValueObject public record Depth(/* ... */)

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Domain Event

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MANEUVER PLANNING DEPTHS SOUNDING DEPTH MAP COMPLETED

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BIG

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Foto: Rosie the Riveter/Wikimedia/CC-PD-Mark

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Foto: Rosie the Riveter/Wikimedia/CC-PD-Mark

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Bounded Context

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TIDAL FORE- CAST MANEUVER PLANNING DEPTH MEASUREMENT

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Ubiquitous Language

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WPS – Workplace Solutions #dddkonkret www.wps.de COLLABORATIVE MODELING Domain-Driven Design concrete

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04.03.25 //// Seite 242 WPS – Workplace Solutions INHALT DOMÄNE DOMÄNE

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Collaborative Modeling

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Collaborative Modeling

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https://www.domainstorytelling.org

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LEGT NAUTIKER KAPITÄN FRAGT NACH 7 SCHIFFS- SILHOUETTE AUF PEILDIENST PEILSCHIFF TIEFE PEILT 1 SENDET 2 AN BERECHNET 3 ERZ EUGT 4 SENDET 5 PEILPLAN AN VERSCHIEBT & DREHT 8 SCHIFFS- SILHOUETTE UND FINDET 9 MELDET AN 6 ROUTE ROUTE PEILPLAN PEILPLAN TIEFEN- ZAHLEN ROUTE TIEFENLINIEN

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@hschwentner Explained

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@hschwentner Domain Storytelling = Pictographic Language + Workshop Format

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@hschwentner The Workshop Format

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@hschwentner The right people

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Storyteller Listeners ! !

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@hschwentner DEPTH MAP HANDS OVER NAUTICAL OFFICER CARTOGRAPHE R TO “The cartographer sends the depth map to the nautical oﬃcer” SENDS

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Active Listening

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CONCRETE STORIES VS. ABSTRACT PROCESSES

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@hschwentner The Pictographic Language

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@hschwentner ßDraw here Leave some empty space there à Comments Assump5ons Name of the Domain Story

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@hschwentner actor work object ac5vity 5 sequence number

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@hschwentner SHIP SILHOUETTE CAPTAIN MOVES

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@hschwentner A w works on A v hands over using w B to A collaborates on w B collaborates on

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@hschwentner DRAWS MEASURES DEPTH DEPTH MAP

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@hschwentner SENDS DRAWS DEPTH MAP DEPTH MAP

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@hschwentner person group IT system

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ACTORS ONCE/ WORK OBJECTS SEVERAL TIMES PUTS NAUTICAL OFFICER 7 SHIP SILHOUETTE MOVES & TURNS 8 SHIP SILHOUETTE

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PICTOGRAPHIC LANGUAGE – NO IF/SWITCH/OR actor work object ac5vity 5 sequence number

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Scenario-based modeling

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Scenarios Maneuver Planning – The Happy Path Maneuver Planning – Storm Flooding Expected Maneuver Planning – All Berths Full

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@hschwentner Scope

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Scope Factors

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@hschwentner Granularity Point in time Domain Purity

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Granularity ﬁne- grained coarse- grained x x x x x x x x x x x x x x x x x x

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@hschwentner A Day at the Beach " Sea Level # Kite Level ☁ Cloud Level % Fish Level & Clam Level Foto: Dennis Hamilton/flickr/CC BY 2.0 Alistair Cockburn

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Point in Time as-is to-be t

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Point in Time now optimized t with new system

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Domain Purity pure digitalized

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@hschwentner Tools

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PowerPoint

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@hschwentner

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https://www.wps.de/modeler/ • https://egon.io

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@hschwentner Purposes

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@hschwentner draw boundaries learn language work on requirements implement domain model ﬁnd shadow IT …

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@hschwentner More on Domain Storytelling

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https://hschwentner.io Read on in:

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04.03.25 //// Seite 338 WPS – Workplace Solutions EXERCISE: Domain Storytelling in Groups

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May 6th-9th 2024 Vienna, Austria The Collaborative Modeling Unconference comocamp.org Event Storming User Story Mapping Event Modeling Impact Mapping Domain Storytelling Storystorming Context Mapping Example Mapping etc.

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WPS – Workplace Solutions #dddkonkret www.wps.de STRATEGIC DESIGN Domain-Driven Design concrete

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04.03.25 //// Seite 341 WPS – Workplace Solutions ÜBERSICHT DOMÄNE DOMÄNE

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@hschwentner “Whoever uses a canonical model has lost control over his life.” – Karl Lagerfeld

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@hschwentner »Wer ein unternehmensweites Domänenmodell einsetzt, hat die Kontrolle über sein Leben verloren« – Karl Lagerfeld

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Bounded Context

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Movement Cosme*cs ! Emo*ons

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PUTS NAVIGATING OFFICER CAPTAIN ASKS FOR 7 SHIP SILHOUETTE ON CARTO-GRAPHER SOUNDING SHIP DEPTH MEASURES 1 SENDS 2 TO CALCULATES 3 DRAW S 4 SENDS 5 DEPTH MAP TO MOVES & TURNS 8 SHIP SLHOETTE TO FIND 9 DISCUSSES WITH 6 ROUTE ROUTE DEPTH MAP DEPTH MAP DEPTH (RAW) ROUTE CONTOUR LINES DEPTH MEASURE- MENT MANEUVER PLANNING

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@hschwentner Another Example

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@hschwentner Do I get a car for this?

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04.03.25 //// Seite 371 WPS – Workplace Solutions

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04.03.25 //// Seite 372 WPS – Workplace Solutions

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CUSTOMER TELLS WISH FOR 1 SALES- PERSON SIGNS TO GIVES FOR CONTRACT 3 RISK MANAGER CONTRACT PASSES ON TO 4 CONTRACT VO TES CHECKS CALCU LATES 5 6 7 CALCU- LATES TO 8 2 CAR CREDIT RATING INSTALLMENT CAR RESALE VALUE CONTRACT

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CUSTOMER SIGNS CONTRACT 3 RISK MANAGER CONTRACT VOTES 7 «Entity» Contract sign() vote()

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«Entity» Contract sign() vote()

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«Entity» Contract sign() vote() extend() terminate() sell_to_reﬁnance() …

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BBOM Foto: Benutzer:Summi/Wikipedia/CC-BY-SA-3.0-migrated

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@hschwentner Why is that?

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@hschwentner Too big to be understood as a whole

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04.03.25 //// Seite 386 WPS – Workplace Solutions CUSTOMER TELLS WISH FOR 1 SALES- PERSON SIGNS TO GIVES FOR CONTRACT 3 RISK MANAGER CONTRACT PASSES ON TO 4 CONTRACT VOTES CHECKS CALCULATES 5 6 7 CALCU- LATES TO 8 2 CAR CREDIT RATING INSTALLMENT CAR RESALE VALUE CONTRACT SALES RISK ASSESSMENT

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«En$ty» Contract sign() vote() ✘

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«En$ty» Contract sign() «En$ty» Contract vote()

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«En$ty» Contract sign() vote() ✘

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«En$ty» Contract sign() «En$ty» Contract vote()

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«En$ty» Contract sign() «En$ty» Contract vote() SALES RISK ASSESSMENT

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04.03.25 //// Seite 392 WPS – Workplace Solutions Common heuristics: § Points of no Return (persistent work results) § Boundaries within the process § State changes, that affect the “nature” of a work object (e.g. “contract legally binding,” “shopping cart ordered”) § Departments of the organisation § Contextual language § Different usage of work objects § Triggers at different points in time § NOT: by work objects! HOW TO CUT THE DOMAIN? Foto: Wikipedia/PD-ScottForesman

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04.03.25 //// Seite 393 WPS – Workplace Solutions

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04.03.25 //// Seite 394 WPS – Workplace Solutions Team Head Team Tail Team Body Team Legs

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04.03.25 //// Seite 395 WPS – Workplace Solutions Team Emma Team Berta Team Erna Team Lisl

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@hschwentner Code Database Team

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@hschwentner Drawing Boundaries with Domain Storytelling

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@hschwentner Scope: coarse-grained to-be (optimized) pure

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Which activities belong together (from an actor’s perspective)?

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CUSTOMER TELLS WISH FOR 1 SALESPERSON SIGNS TO GIVES FOR CONTRACT 3 RISK MANAGER CONTRACT PASSES ON TO 4 CONTRACT VO TES CHECKS CALCU LATES 5 6 7 CALCU- LATES TO 8 2 CAR CREDIT RATING INSTALLMENT CAR RESALE VALUE CONTRACT SALES Group the sentences • Boundary around activites and work objects • Keep actors outside boundaries Give a name to the group RISK ASSESSMENT

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@hschwentner Naming Subdomains Express what is done Verbs turned into nouns Often: -ing-form Anti-pattern: name of work object as name for subdomain

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@hschwentner Naming Subdomains— Examples Price calculation Risk assessment Maneuver planning Customer Car Price Payment Car inspection

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Indicators: 1) Actor produces result on their own 2) One-way informa

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@hschwentner Drawing Boundaries in Existing Sofware a.k.a.: How to split the monolith

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@hschwentner Strategic Transformation 1. Step: Domain re-discovery 2. Step: Modeling the to-be 3. Step: Aligning as-is with to-be 4. Step: Do the move

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https://hschwentner.io Read on in: https://hschwentner.io/domain-driven-refactorings

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04.03.25 //// Seite 421 WPS – Workplace Solutions EXERCISE: Find Bounded Contexts

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04.03.25 //// Seite 422 WPS – Workplace Solutions BOUNDED CONTEXTS FÜR DAS KINO ABLAUF- PLANUNG KARTEN- VERKAUF BUCH- HALTUNG

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04.03.25 //// Seite 444 WPS – Workplace Solutions AND NOW? § Core vs Supporting § Event Storming § Taktisches Design § Code/LeasingNinja § Hexagonal/Onion Architecture § Context Mapping § How to split the monolith

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@hschwentner How to split the monolith

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1) How should it be? 2) How is it? 3) How to move the “is” to the “ideal”? RISK MANAGE- MENT SALES

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1) How should it be? 1) Domain Re-Discovery 2) “ideal” context map 2) How is it? 1) Architecture Analysis 2) As-is context map 3) How to move the “is” to the “ideal”? 1) Compare 2) Create List of Refactorings 4) Do the move 1) Extract a suppor

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old old old old new new new new “just ﬂip the lever” 1 2 3 4 5 a.k.a. big bang replacement

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old old old new new new new 2 3 4 5 old 1

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Strangler Fig Application

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Strangler Fig Application

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Lesson: is better than Strangler Fig Application Big Bang Replacement

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<> Contract sign() SALES RISK ASSESSMENT <> Contract sign() vote() … BIG BALL OF MUD <> Contract vote()

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https://hschwentner.io Read on in: h#ps://hschwentner.io/domain-driven-refactorings

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Read on: h#ps://hschwentner.io/domain-driven-refactorings

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@hschwentner Always Microservices?

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Simon Brown

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Distributed Big Ball Of Mud Modulith Microservices Monolithic Big Ball Of Mud

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@hschwentner Consulting

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04.03.25 //// Seite 525 WPS – Workplace Solutions § Subdomänen § Kern (Core Domain) § Unterstützende (Supporting Domain) § Allgemeine (Generic Domain) § Context Mapping § Shared Kernel § Customer/Supplier § Open-Host-Service § Published Language § Separate Ways § Anticorruption Layer § Conformist STRATEGISCHES DESIGN – WEITERE BEGRIFFE

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WPS – Workplace Solutions #dddkonkret www.wps.de TACTICAL DESIGN Domain-Driven Design concrete

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04.03.25 //// Seite 579 WPS – Workplace Solutions OVERVIEW DOMÄNE DOMÄNE

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04.03.25 //// Seite 580 WPS – Workplace Solutions ARCHITECTURE WITH BOUNDED CONTEXTS TIDAL FORECAST MANEUVER PLANNING DEPTHS MEASUREMENT

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04.03.25 //// Seite 581 WPS – Workplace Solutions ARCHITECTURE WITHIN A BOUNDED CONTEXT MANEUVER PLANNING User Interface Application Domain Infrastructure

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04.03.25 //// Seite 582 WPS – Workplace Solutions DOMAIN-DRIVEN DESIGN—LAYERED ARCHITECTURE § User interface layer § Receives input and user commands and presents information. § Application layer (Fowler: Service Layer) § Describes and coordinates application processes. § Domain layer § Represents the business domain logic. § Infrastructure layer § Provides technical services such as persistence or communication with other systems. User Interface Application Domain Infrastructure

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04.03.25 //// Seite 583 WPS – Workplace Solutions LAYERED ARCHITECTURE – CRITIQUE MANEUVER PLANNING § Domain layer is linked to the database layer § Technology “bubbles up” into domain layer § No clean separation of domain & technology Alistair Cockburn Foto: Fotograf Dennis Hamilton/Alistair Cockburn/flickr/CC BY 2.0 User Interface Application Domain Infrastructure

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04.03.25 //// Seite 584 WPS – Workplace Solutions below above FROM ABOVE/BELOW TO INSIDE/OUTSIDE outside inside

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04.03.25 //// Seite 585 WPS – Workplace Solutions port port HEXAGONAL ARCHITECTURE Foto: Dennis Hamilton/flickr/CC BY 2.0 http://alistair.cockburn.us/Hexagonal%2Barchitecture adapter adapter Alistair Cockburn

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04.03.25 //// Seite 586 WPS – Workplace Solutions KINDS OF PORTS - For UI etc. - Methods to be called - “from above” - For DB and infrastructure - Interfaces to be implemented - “from below”

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04.03.25 //// Seite 587 WPS – Workplace Solutions ONION ARCHITECTURE U I “application core” domain services domain model infra app ture struc serv lication ices Jeffrey Palermo !

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04.03.25 //// Seite 588 WPS – Workplace Solutions CLEAN ARCHITECTURE Robert C. Martin ”Uncle Bob“

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04.03.25 //// Seite 589 WPS – Workplace Solutions ARCHITECTURE HAMBURGER Application Domain Infrastructure UI coarse grained Henning Schwentner !

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04.03.25 //// Seite 590 WPS – Workplace Solutions THE BASIC IDEA OF OBJECT-ORIENTATION: A SOFTWARE-TECHNICAL MODEL BASED ON DOMAIN OBJECTS

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04.03.25 //// Seite 591 WPS – Workplace Solutions FROM DOMAIN STORY TO DOMAIN MODEL Silhouette MOVES SILHOUETTE NAUTICAL OFFICER LENGTH Length moveBy(:Length) BY 8

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EXERCISE: BUILD DOMAIN MODEL FROM DOMAIN STUDIES IN CASE STUDY

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04.03.25 //// Seite 593 WPS – Workplace Solutions En##es Value Objects Aggregates Services Factories Repositories

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04.03.25 //// Seite 594 WPS – Workplace Solutions DOMAIN TERMS – WHAT IS STRIKING? insurance policy vacation request building activity purchase contract order zip code GPS coordinate IBAN container number IATA code

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04.03.25 //// Seite 596 WPS – Workplace Solutions ENTITIES § Objects of a domain that the user works with. § The results of the work are reflected by their state! § Can be composed of other entities. § Have an (immutable) identity. § Maintain their own consistency and integrity! § Have a clearly defined life cycle. § Have a (mostly mutable) state. § Describe their state by value objects. § Synonyms: Business Object / Domain Object / Work Material § NOT TO BE confused with the term “ENTITY” from Entity-Relationship-Model! Foto: Bundesrepublik Deutschland/Wikipedia/PD Germany Foto: Kaz/pixabay/CC0 Foto: Thomas G./Wikipedia/CC BY-SA 3.0 Foto: OpenClipart-Vectors/pixabay/CC0

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04.03.25 //// Seite 598 WPS – Workplace Solutions ENTITIES «Entity» DepthMap depthMapId mark(Marking)

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04.03.25 //// Seite 600 WPS – Workplace Solutions VALUE OBJECTS § Value objects are symbols for values of a certain type in the domain. § Symbolize the same value if they are equal. § They are not edited by the user and cannot be modified. § Can be calculated (from other value objects), if necessary. § Can consist of other value objects, but never of entities! ValueObject 2.5 ValueObject ValueObject ValueObject ValueObject two and a half

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Depth Diameter Position

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04.03.25 //// Seite 603 WPS – Workplace Solutions VALUE OBJECTS «Value Object» Depth centimeters : int equals()

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04.03.25 //// Seite 606 WPS – Workplace Solutions EXERCISE: ENTITES, VALUE OBJECTS

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04.03.25 //// Seite 607 WPS – Workplace Solutions ENTITIES UND VALUE OBJECTS IM KARTENVERKAUF Saalplan • Anzahl Plätze suchen • Verkaufte Plätze mark. • Reserv. Plätze mit RN markieren Vorstellung • Datum • Zeitraum • Film Reservierungs -nummer Kinokarte • Mit Platz beschriften • Vorstellung Saalplanstapel • Saalplan zu Vorst. Holen • Saalplan zurücklegen Liste der Reservierungsnummern • Reservierungsnummer abholen • Name+Vorst. Vermerken • Vorst. Mit RN heraussuchen Film • Titel • Regisseur, Schauspieler • Freigabe • Spieldauer Platz/Sitzplatz • Reihe • Nummer Saal/Kinosaal • Kapazität • Anzahl Reihen VO Entity Entity Entity Entity Entity Entity

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04.03.25 //// Seite 608 WPS – Workplace Solutions ENTITIES UND VALUE OBJECTS IM KARTENVERKAUF Saalplan • Anzahl Plätze suchen • Verkaufte Plätze mark. • Reserv. Plätze mit RN markieren Vorstellung • Datum • Zeitraum • Film Reservierungs -nummer Liste der Reservierungsnummern • Reservierungsnummer abholen • Name+Vorst. Vermerken • Vorst. Mit RN heraussuchen Film • Titel • Regisseur, Schauspieler • Freigabe • Spieldauer Platz/Sitzplatz • Reihe • Nummer Saal/Kinosaal • Kapazität • Anzahl Reihen VO Entity Entity Entity Entity Entity Entity

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WPS – Workplace Solutions #dddkonkret www.wps.de REPOSITORIES AND AGGREGATES

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04.03.25 //// Seite 610 WPS – Workplace Solutions REPOSITORIES § Are the access points to the aggregates! § Encapsulate the technical details of the technical infrastructure layer ... § ... and map external data to entities and value objects. § Store aggregates (e. g. in databases). Repository Aggregate Aggregate Aggregate Aggregate Aggregate

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04.03.25 //// Seite 611 WPS – Workplace Solutions REPOSITORY @ HAMBURG PORT: THE DEPTH MAP CABINET

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04.03.25 //// Seite 612 WPS – Workplace Solutions REPOSITORY: TWO PART IMPLEMENTATION «Repository Interface» DepthMapCabinet getDepthMap(Area, Scale) : DepthMap putBack(DepthMap) «Repository Implementation» MySQLDepthMapCabinet

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04.03.25 //// Seite 613 WPS – Workplace Solutions AGGREGATES § Aggregates are entities that are relevant in themselves (and not just as part of another entity). § Protect the consistency and integrity of their inner entities. § They don't necessarily have to hide them to do this! § Always have a designated entity as an entry point (root). § Usually stored. § As a whole! § Not necessarily in a relational way! Root Entity VO VO VO Aggregate

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04.03.25 //// Seite 614 WPS – Workplace Solutions AGGREGATES § Actually, the value objects are outside as they are immutable and may be reused anywhere. Root Entity VO VO VO Some other object Aggregate

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04.03.25 //// Seite 615 WPS – Workplace Solutions AGGREGATES § An aggregate can consist of many entities. Root Entity Entity Entity Entity Entity VO VO VO VO VO VO VO VO VO VO VO Aggregate

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04.03.25 //// Seite 617 WPS – Workplace Solutions Aggregate «Aggregate Root» DepthMap «Entity» Marking «Value Object» Depth

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04.03.25 //// Seite 618 WPS – Workplace Solutions ENTITIES UND VALUE OBJECTS IM KARTENVERKAUF Saalplan • Anzahl Plätze suchen • Verkaufte Plätze mark. • Reserv. Plätze mit RN markieren Vorstellung • Datum • Zeitraum • Film Reservierungs -nummer Liste der Reservierungsnummern • Reservierungsnummer abholen • Name+Vorst. Vermerken • Vorst. Mit RN heraussuchen Film • Titel • Regisseur, Schauspieler • Freigabe • Spieldauer Platz/Sitzplatz • Reihe • Nummer Saal/Kinosaal • Kapazität • Anzahl Reihen VO Entity Entity Entity Entity Entity Entity

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EXERCISE: DRAW AGGREGATE BOUNDARIES. WHICH REPOSITORIES DO WE NEED?

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04.03.25 //// Seite 620 WPS – Workplace Solutions ENTITIES UND VALUE OBJECTS IM KARTENVERKAUF Saalplan • Anzahl Plätze suchen • Verkaufte Plätze mark. • Reserv. Plätze mit RN markieren Vorstellung • Datum • Zeitraum • Film Reservierungs -nummer Liste der Reservierungsnummern • Reservierungsnummer abholen • Name+Vorst. Vermerken • Vorst. Mit RN heraussuchen Film • Titel • Regisseur, Schauspieler • Freigabe • Spieldauer Platz/Sitzplatz • Reihe • Nummer Saal/Kinosaal • Kapazität • Anzahl Reihen VO Entity Entity Wurzel-Entity Entity Entity Entity Aggregate Aggregate Aggregate Aggregate Aggregate

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04.03.25 //// Seite 637 WPS – Workplace Solutions ARCHITECTURE HAMBURGER ﬁne grained model repository interface controller use case service entity value object view data model repository implementation widget library REST frame- work transaction handling domain library programming language ORM ﬁle system access domain event

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WPS – Workplace Solutions #dddkonkret www.wps.de IMPLEMENTING THE BUILDING BLOCKS IN JAVA Domain-Driven Design concrete

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04.03.25 //// Seite 640 WPS – Workplace Solutions

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04.03.25 //// Seite 641 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT public class Account { }

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04.03.25 //// Seite 642 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT import org.jmolecules.ddd.annotation.Entity; @Entity public class Account { }

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04.03.25 //// Seite 643 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT import org.jmolecules.ddd.annotation.Entity; @Entity public class Account { }

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04.03.25 //// Seite 644 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT @Entity public class Account { private int _balance; public int getBalance() { return _balance; } public void setBalance(int balance) { _balance = balance; } } ✘ Bad: The account balance can be set to any value

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04.03.25 //// Seite 645 WPS – Workplace Solutions A BANK ACCOUNT—SECOND VERSION @Entity public class Account { private int _balance; public int balance() { return _balance; } public void deposit(int amount) { _balance += amount; } public void withdraw(int amount) { _balance -= amount; } } Better: Operations with domain-specific behavior and names

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04.03.25 //// Seite 646 WPS – Workplace Solutions A BANK ACCOUNT—THIRD VERSION @Entity public class Account { private int _balance; public int balance() { return _balance; } public void deposit(int amount) { _balance += amount; } public void withdraw(int amount) { if (amount > balance()) { throw new IllegalArgumentException("Amount too big"); } _balance -= amount; } Even better: Preconditions can be checked

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04.03.25 //// Seite 648 WPS – Workplace Solutions DESIGN BY CONTRACT Foto: Urheber Fuchsias/Bertrand Meyer/Wikipedia/CC BY-SA 4.0

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04.03.25 //// Seite 649 WPS – Workplace Solutions A BANK ACCOUNT—DESIGN BY CONTRACT USING “ASSERT” @Entity public class Account { // ... public void withdraw(int amount) { assert amount <= balance(); _balance -= amount; } } Assertion using keyword assert

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04.03.25 //// Seite 650 WPS – Workplace Solutions A BANK ACCOUNT—DESIGN BY CONTRACT USING “VALID4J” import static org.valid4j.Assertive.*; @Entity public class Account { // ... public void withdraw(int amount) { require(amount <= balance()); _balance -= amount; } } Hamcrest matchers can be used

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04.03.25 //// Seite 654 WPS – Workplace Solutions GREAT, BUT… @Entity public class Account { // ... public void withdraw(int amount) { assert amount <= balance(); _balance -= amount; } } Can I withdraw a negative amount? In EUR or GBP or…?

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04.03.25 //// Seite 656 WPS – Workplace Solutions AN AMOUNT TYPE public class Amount { }

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04.03.25 //// Seite 657 WPS – Workplace Solutions AN AMOUNT TYPE import org.jmolecules.ddd.annotation.ValueObject; @ValueObject public class Amount { }

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04.03.25 //// Seite 658 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private int _amount; private Currency _currency; }

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04.03.25 //// Seite 659 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private final int _amount; private final Currency _currency; }

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04.03.25 //// Seite 660 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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04.03.25 //// Seite 661 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private final int _amount; private final Currency _currency; private Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } public static Amount of(int amount, Currency currency) { return new Amount(amount, currency); } }

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04.03.25 //// Seite 662 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } @Override public boolean equals(Object other) { return _amount == ((Amount) other)._amount && _currency.equals(((Amount) other)._currency); } }

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04.03.25 //// Seite 663 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public class Amount { private final int _amount; private final Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } @Override public boolean equals(Object other) { return _amount == ((Amount) other)._amount && _currency.equals(((Amount) other)._currency); } // hashCode() }

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04.03.25 //// Seite 664 WPS – Workplace Solutions AN AMOUNT TYPE—JAVA 14 AND HIGHER @ValueObject public record Amount(int amount, Currency currency);

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04.03.25 //// Seite 665 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public record Amount(int amount, Currency currency) { public Amount add(Amount otherAmount) { return Amount.of(amount + otherAmount.amount, currency); } } The new type has domain behavior

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04.03.25 //// Seite 666 WPS – Workplace Solutions AN AMOUNT TYPE @ValueObject public record Amount(int amount, Currency currency) { public Amount add(Amount otherAmount) { assert hasSameCurrency(otherAmount); return Amount.of(amount + otherAmount.amount, currency); } public boolean hasSameCurrency(Amount otherAmount) { return otherAmount.currency == currency; } } ... and contracts ensure the correct currency

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04.03.25 //// Seite 670 WPS – Workplace Solutions A BANK ACCOUNT—FOURTH VERSION @Entity public class Account { private Amount _balance; public Amount balance() { return _balance; } public void deposit(Amount amount) { _balance = _balance.add(amount); } public void withdraw(Amount amount) { assert amount.isLessOrEqual(balance()); _balance = _balance.subtract(amount); } } Now we can use the amount type

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04.03.25 //// Seite 671 WPS – Workplace Solutions A BANK ACCOUNT—FOURTH VERSION @Entity public class Account { private Amount _balance; public Amount balance() { return _balance; } public void deposit(Amount amount) { _balance = _balance.add(amount); } public void withdraw(Amount amount) { assert amount.hasSameCurrency(balance()); assert amount.isLessOrEqual(balance()); _balance = _balance.subtract(amount); } } ... and define new contracts

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04.03.25 //// Seite 672 WPS – Workplace Solutions A BANK ACCOUNT—EVENT SOURCED @Entity public class Account { private List _transactions; public Amount balance() { // sum up _transactions } public void deposit(Amount amount) { _transactions.append(new Deposition(amount)); } public void withdraw(Amount amount) { assert amount.hasSameCurrency(balance()); assert amount.isLessOrEqual(balance()); _transactions.append(new Withdrawal(amount)); } } History instead of current status

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04.03.25 //// Seite 675 WPS – Workplace Solutions IDENTITY OF ENTITIES § In Java, every object has an identity § Namely its reference. § It’s a technical identity. § An entity has a identity from a domain perspective § Different technical objects can represent the same entity § In different processes: e.g. on the client / on the server § When they are stored and later read from the database. § The identity can be implemented in two ways: § explicitly (e.g. IBAN) by using a unique key the user is aware of § implicitly (e.g. UUID / GUID) by using a technical key hidden from the user ?

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04.03.25 //// Seite 676 WPS – Workplace Solutions A BANK ACCOUNT—WITH EXPLICIT IDENTITY @Entity public class Account { @Identity private final IBAN _iban; public Account(IBAN iban) { _iban = iban; } @Override public boolean equals(Object other) { return _iban.equals(((Account)other)._iban); } // ... } The identity is immutable The key is of course a value object The implementation of equals() is based on the identity

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04.03.25 //// Seite 677 WPS – Workplace Solutions HOW CAN A NEW IDENTITY BE CREATED? § Database § Pro: Central § Can sometimes be created “lazily” § A system-wide value type factory § Domain-specific keys known to the user § Often implemented by the repository of a corresponding aggregate type the key is used for § A UUID / GUID generator § Pro: can be generated independently on any server or client § Con: String in a special format

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04.03.25 //// Seite 678 WPS – Workplace Solutions BANK TRANSFER—BY DOMAIN SERVICE @Service public class CreditTransferService { public void transfer(Account source, Account destination, Amount amount) { source.withdraw(amount); destination.deposit(amount); } } No fields à stateless

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EXERCISE: MOBPROGRAMMING @ THE CINEMA

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04.03.25 //// Seite 681 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN GLOSSARY—REVISITED DEPTH DEPTH AT A SPECIFIC GEO POSITION CONSIDERING THE CURRENT TIDAL LEVEL DEPTH DEPTH BELOW SEA LEVEL MEASURED BY ECHO SOUNDER MANEUVER PLANNING DEPTH MEASUREMENT

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04.03.25 //// Seite 682 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN IMPLEMENTATION public struct Depth { public Depth( int depthBelowSeaLevel, int tideLevel) { // ... } } MANEUVER PLANNING DEPTH MEASUREMENT public class Depth { public Depth( int centimeter, LocalDate soundingDate) { // ... } }

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WPS – Workplace Solutions #dddkonkret www.wps.de IMPLEMENTING THE BUILDING BLOCKS IN C# Domain-Driven Design concrete

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04.03.25 //// Seite 684 WPS – Workplace Solutions

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04.03.25 //// Seite 685 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT public class Account { }

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04.03.25 //// Seite 686 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT using NMolecules.DDD; [Entity] public class Account { }

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04.03.25 //// Seite 687 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT [Entity] public class Account { public int Balance { get; set; } } ✘ Bad: The account balance can be set to any value

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04.03.25 //// Seite 688 WPS – Workplace Solutions A BANK ACCOUNT—SECOND VERSION [Entity] public class Account { public int Balance { get; private set; } public void Deposit(int amount) { Balance += amount; } public void Withdraw(int amount) { Balance -= amount; } } Better: Operations with domain-specific behavior and names

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04.03.25 //// Seite 689 WPS – Workplace Solutions A BANK ACCOUNT—THIRD VERSION [Entity] public class Account { public int Balance { get; private set; } public void Deposit(int amount) { Balance += amount; } public void Withdraw(int amount) { if (amount > balance()) { throw new ArgumentException("Amount too big"); } Balance -= amount; } Even better: Preconditions can be checked

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04.03.25 //// Seite 691 WPS – Workplace Solutions DESIGN BY CONTRACT Foto: Urheber Fuchsias/Bertrand Meyer/Wikipedia/CC BY-SA 4.0

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04.03.25 //// Seite 694 WPS – Workplace Solutions A BANK ACCOUNT—DESIGN BY CONTRACT USING “ASSERT” using static System.Diagnostics.Debug; [Entity] public class Account { // ... public void Withdraw(int amount) { Assert(amount <= balance()); Balance -= amount; } } Assertion using Debug.Assert

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04.03.25 //// Seite 695 WPS – Workplace Solutions GREAT, BUT… using static System.Diagnostics.Debug; [Entity] public class Account { // ... public void Withdraw(int amount) { Assert(amount <= balance()); Balance -= amount; } } Can I withdraw a negative amount? In EUR or GBP or…?

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04.03.25 //// Seite 697 WPS – Workplace Solutions AN AMOUNT TYPE public class Amount { }

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04.03.25 //// Seite 698 WPS – Workplace Solutions AN AMOUNT TYPE using NMolecules.DDD; [ValueObject] public class Amount { }

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04.03.25 //// Seite 699 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private int _amount; private Currency _currency; }

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04.03.25 //// Seite 700 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; }

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04.03.25 //// Seite 701 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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04.03.25 //// Seite 702 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; private Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } public static Amount Of(int amount, Currency currency) { return new Amount(amount, currency); } }

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04.03.25 //// Seite 703 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public override bool Equals(object other) { return other is Amount && _amount == ((Amount) other)._amount && _currency.Equals(((Amount) other)._currency); } }

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04.03.25 //// Seite 704 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public override bool Equals(object other) => other is Amount && _amount == ((Amount) other)._amount && _currency.Equals(((Amount) other)._currency); }

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04.03.25 //// Seite 705 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public override bool Equals(object other) => other is Amount && _amount == ((Amount) other)._amount && _currency.Equals(((Amount) other)._currency); // GetHashCode() }

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04.03.25 //// Seite 706 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public static bool operator ==(Amount a, Amount b) => a.Equals(b); }

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04.03.25 //// Seite 707 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public class Amount { private readonly int _amount; private readonly Currency _currency; // ... public static bool operator ==(Amount a, Amount b) => a.Equals(b); public static bool operator !=(Amount a, Amount b) => !a.Equals(b); }

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04.03.25 //// Seite 709 WPS – Workplace Solutions 3 CLASSES DEFINE REFERENCE TYPES

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04.03.25 //// Seite 710 WPS – Workplace Solutions 3 STRUCTS DEFINE VALUE TYPES

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04.03.25 //// Seite 711 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public struct Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } }

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04.03.25 //// Seite 712 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public struct Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } // Equals() does not have to be overridden }

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04.03.25 //// Seite 713 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public struct Amount { private readonly int _amount; private readonly Currency _currency; public Amount(int amount, Currency currency) { _amount = amount; _currency = currency; } // Equals() does not have to be overridden // but the operators have to be overridden }

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@hschwentner equals() ! == "

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04.03.25 //// Seite 715 WPS – Workplace Solutions AN AMOUNT TYPE—C# 9 AND HIGHER [ValueObject] public record Amount(int amount, Currency currency);

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04.03.25 //// Seite 716 WPS – Workplace Solutions AN AMOUNT TYPE—C# 9 AND HIGHER [ValueObject] public record Amount(int amount, Currency currency); // automatically readonly // Equals(), GetHashCode(), ToString(), operators // do not have to be overloaded and work as expected

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04.03.25 //// Seite 717 WPS – Workplace Solutions AN AMOUNT TYPE—C# 9 AND HIGHER [ValueObject] public record struct Amount(int amount, Currency currency);

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04.03.25 //// Seite 718 WPS – Workplace Solutions AN AMOUNT TYPE—C# 10 AND HIGHER [ValueObject] public readonly record struct Amount(int amount, Currency currency);

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04.03.25 //// Seite 719 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public readonly record struct Amount(int amount, Currency currency) { public Amount Add(Amount otherAmount) { return Amount.of(this.amount + otherAmount.amount, this.currency); } } The new value object type behaves correctly

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04.03.25 //// Seite 720 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public readonly record struct Amount(int amount, Currency currency) { public Amount Add(Amount otherAmount) { Assert(HasSameCurrency(otherAmount)); return Amount.of(this.amount + otherAmount.amount, this.currency); } public boolean HasSameCurrency(Amount otherAmount) => otherAmount.currency == this.currency; } ... and contracts ensure the correct currency

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04.03.25 //// Seite 721 WPS – Workplace Solutions AN AMOUNT TYPE [ValueObject] public readonly record struct Amount(int amount, Currency currency) { public Amount operator +(Amount otherAmount) { Assert(HasSameCurrency(otherAmount)); return Amount.Of(this.amount + otherAmount.amount, this.currency); } public boolean HasSameCurrency(Amount otherAmount) => otherAmount.currency == this.currency; }

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04.03.25 //// Seite 722 WPS – Workplace Solutions A BANK ACCOUNT—FOURTH VERSION [Entity] public class Account { public Amount Balance { get; private set; } public void Deposit(Amount amount) { Balance += amount; } public void Withdraw(Amount amount) { Assert(amount <= Balance); Balance -= amount; } } Now we can use the amount type

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04.03.25 //// Seite 723 WPS – Workplace Solutions A BANK ACCOUNT—FOURTH VERSION [Entity] public class Account { public Amount Balance { get; private set; } public void Deposit(Amount amount) { Balance += amount; } public void Withdraw(Amount amount) { Assert(amount.HasSameCurrencyAs(Balance); Assert(amount <= Balance); Balance -= amount; } } ... and define new contracts

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04.03.25 //// Seite 724 WPS – Workplace Solutions A BANK ACCOUNT—EVENT SOURCED [Entity] public class Account { private IList _transactions; public Amount Balance { get { // sum up _transactions } } public void Deposit(Amount amount) { _transactions.Add(new Deposition(amount)); } public void Withdraw(Amount amount) { Assert(amount.HasSameCurrencyAs(Balance); Assert(amount <= Balance); _transactions.Add(new Withdrawal(amount)); } History instead of current status

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04.03.25 //// Seite 727 WPS – Workplace Solutions IDENTITY OF ENTITIES § In Java, every object has an identity § Namely its reference. § It's a technical identity. § An entity has a identity from a domain perspective § Different technical objects can represent the same entity § In different processes: e.g. on the client / on the server § When they are stored and later read from the database. § The identity can be implemented in two ways: § explicitly (e.g. IBAN) by using a unique key the user is aware of § implicitly (e.g. UUID / GUID) by using a technical key hidden from the user ?

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04.03.25 //// Seite 728 WPS – Workplace Solutions A BANK ACCOUNT—WITH EXPLICIT IDENTITY [Entity] public class Account { [Identity] private readonly IBAN _iban; public Account(IBAN iban) { _iban = iban; } public override bool Equals(Object other) { return _iban.Equals(((Account)other)._iban); } // ... } The identity is immutable The identity is a value object The implementation of Equals() is based on the identity

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04.03.25 //// Seite 729 WPS – Workplace Solutions HOW CAN A NEW IDENTITY BE CREATED? § Database § Pro: Central § Can sometimes be created “lazily” § A system-wide value type factory § Domain-specific keys known to the user § Often implemented by the repository of a corresponding aggregate type the key is used for § A UUID / GUID generator § Pro: can be generated independently on any server or client § Con: String in a special format

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04.03.25 //// Seite 730 WPS – Workplace Solutions BANK TRANSFER—BY DOMAIN SERVICE [Service] public class CreditTransferService { public void Transfer(Account source, Account destination, Amount amount) { source.Withdraw(amount); destination.Deposit(amount); } } No fields à stateless

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EXERCISE: MOBPROGRAMMING @ THE CINEMA

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04.03.25 //// Seite 733 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN GLOSSARY—REVISITED DEPTH DEPTH AT A SPECIFIC GEO POSITION CONSIDERING THE CURRENT TIDAL LEVEL DEPTH DEPTH BELOW SEA LEVEL MEASURED BY ECHO SOUNDER MANEUVER PLANNING DEPTH MEASUREMENT

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04.03.25 //// Seite 734 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN IMPLEMENTATION public struct Depth { public Depth( int depthBelowSeaLevel, int tideLevel) { // ... } } MANEUVER PLANNING DEPTH MEASUREMENT public class Depth { public Depth( int centimeter, LocalDate soundingDate) { // ... } }

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WPS – Workplace Solutions #dddkonkret www.wps.de IMPLEMENTING THE BUILDING BLOCKS IN PHP Domain-Driven Design concrete

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04.03.25 //// Seite 736 WPS – Workplace Solutions

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04.03.25 //// Seite 737 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT class Account { }

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04.03.25 //// Seite 738 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT use PHPMolecules\DDD\Attribute\Entity; #[Entity] class Account { }

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04.03.25 //// Seite 739 WPS – Workplace Solutions A BANK ACCOUNT—FIRST DRAFT #[Entity] class Account { private int $balance; public function getBalance(): int { return $this->balance; } public function setBalance(int $balance): void { $this->balance = $balance; } } ✘ Bad: The account balance can be set to any value

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04.03.25 //// Seite 740 WPS – Workplace Solutions A BANK ACCOUNT—SECOND VERSION #[Entity] class Account { private int $balance; public funtion balance(): int { return $this->balance; } public function deposit(int amount): void { $this->balance += amount; } public function withdraw(int amount): void { $this->balance -= amount; } } Better: Operations with domain-specific behavior and names

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04.03.25 //// Seite 741 WPS – Workplace Solutions A BANK ACCOUNT—THIRD VERSION Even better: Preconditions can be checked #[Entity] class Account { private int $balance; public funtion balance(): int { return $this->balance; } public function deposit(int amount): void { $this->balance += amount; } public function withdraw(int amount): void { assert(amount <= $this->balance()); $this->balance -= amount; }

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04.03.25 //// Seite 743 WPS – Workplace Solutions DESIGN BY CONTRACT Foto: Urheber Fuchsias/Bertrand Meyer/Wikipedia/CC BY-SA 4.0

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04.03.25 //// Seite 744 WPS – Workplace Solutions DESIGN BY CONTRACT IN PHP § https://github.com/php-deal § http://www.terminally-incoherent.com/blog/2008/06/05/design-by-contract-in-php-with-assertions/

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04.03.25 //// Seite 745 WPS – Workplace Solutions GREAT, BUT… #[Entity] class Account { // ... public function withdraw(int $amount): void { assert($amount <= $this->balance()); $this->balance -= $amount; } } Can I withdraw a negative amount? In EUR or GBP or…?

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04.03.25 //// Seite 747 WPS – Workplace Solutions AN AMOUNT TYPE class Amount { }

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04.03.25 //// Seite 748 WPS – Workplace Solutions AN AMOUNT TYPE use PHPMolecules\DDD\Attribute\ValueObject; #[ValueObject] class Amount { }

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04.03.25 //// Seite 749 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] class Amount { private int $amount; private Currency $currency; }

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04.03.25 //// Seite 750 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] class Amount { private readonly int $amount; private readonly Currency $currency; }

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04.03.25 //// Seite 752 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] class Amount { private readonly int $amount; private readonly Currency $currency; public function __construct(int $amount, Currency $currency) { $this->amount = $amount; $this->currency = $currency; } }

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04.03.25 //// Seite 753 WPS – Workplace Solutions AN AMOUNT TYPE—PHP 8.1 AND HIGHER #[ValueObject] class Amount { public function __construct( private readonly int $amount, private readonly Currency $currency ) {} }

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04.03.25 //// Seite 754 WPS – Workplace Solutions AN AMOUNT TYPE—PHP 8.2 AND HIGHER #[ValueObject] readonly class Amount { public function __construct( private int $amount; private Currency $currency; ) {} }

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04.03.25 //// Seite 755 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] readonly class Amount { private function __construct( private int $amount, private Currency $currency ) {} public static function of(int $amount, Currency $currency): Amount { return new Amount($amount, $currency); } }

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04.03.25 //// Seite 756 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] readonly class Amount { public function __construct( private int $amount; private Currency $currency; ) {} public function equals(Amount $other): bool { return $this->amount === $other->amount && $this->currency->equals($other->currency); } }

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04.03.25 //// Seite 759 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] readonly class Amount { public function __construct( private int $amount, private Currency $currency ) {} public function add(Amount $otherAmount): Amount { return Amount.of($this->amount + $otherAmount->amount, $this->currency); } } The new value object type behaves correctly

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04.03.25 //// Seite 760 WPS – Workplace Solutions AN AMOUNT TYPE #[ValueObject] readonly class Amount { public function __construct( private int $amount, private Currency $currency ) {} public function add(Amount $otherAmount): Amount { assert($this->hasSameCurrencyAs($otherAmount)); return Amount.of($this->amount + $otherAmount->amount, $this->currency); } public function hasSameCurrencyAs(Amount $otherAmount): bool { return $this->currency === $otherAmount->currency; } } ... and contracts ensure the correct currency

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04.03.25 //// Seite 761 WPS – Workplace Solutions A BANK ACCOUNT—FOURTH VERSION Now we can use the amount type #[Entity] class Account { private Amount $balance; public function balance(): Amount { return $this->balance; } public function deposit(Amount $amount): void { $this->balance = $this->balance->add($amount); } public function withdraw(Amount $amount): void { assert($amount.isLessOrEqual($this->balance()); $this->balance = $this->balance->subtract($amount); }

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04.03.25 //// Seite 762 WPS – Workplace Solutions #[Entity] class Account { private Amount $balance; public function balance(): Amount { return $this->balance; } public function deposit(Amount $amount): void { $this->balance = $this->balance->add($amount); } public function withdraw(Amount $amount): void { assert $amount.hasSameCurrency($this->balance()); assert($amount.isLessOrEqual($this->balance()); $this->balance = $this->balance->subtract($amount); A BANK ACCOUNT—FOURTH VERSION ... and define new contracts

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04.03.25 //// Seite 763 WPS – Workplace Solutions A BANK ACCOUNT—EVENT SOURCED class Account { /** @var FinancialTransaction[] */ private $transactions; public function balance(): Amount { // sum up $transactions } public function deposit(Amount $amount): void { $this->transactions[] = new Deposition($amount); } public function withdraw(Amount $amount): void { assert($amount->hasSameCurrencyAs(balance()); assert($amount->isLessOrEqualTo(balance()); $this->transactions[] = new Withdrawal(amount); History instead of current status

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04.03.25 //// Seite 766 WPS – Workplace Solutions IDENTITY OF ENTITIES § In Java, every object has an identity § Namely its reference. § It's a technical identity. § An entity has a identity from a domain perspective § Different technical objects can represent the same entity § In different processes: e.g. on the client / on the server § When they are stored and later read from the database. § The identity can be implemented in two ways: § explicitly (e.g. IBAN) by using a unique key the user is aware of § implicitly (e.g. UUID / GUID) by using a technical key hidden from the user ?

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04.03.25 //// Seite 767 WPS – Workplace Solutions A BANK ACCOUNT—USING A UNIQUE KEY #[Entity] public class Account { #[Identity] private readonly IBAN $iban; public __construct(IBAN $iban) { $this->iban = $iban; } public function equals(Account $other): bool { return $this->iban.equals($other->iban); } // ... } The identity is immutable The key is of course a value object The implementation of equals () is based on the unique key

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04.03.25 //// Seite 768 WPS – Workplace Solutions HOW CAN A NEW UNIQUE KEY BE CREATED? § Database § Pro: Central § Can sometimes be created “lazily” § A system-wide value type factory § Domain-specific keys known to the user § Often implemented by the repository of a corresponding aggregate type the key is used for § A UUID / GUID generator § Pro: can be generated independently on any server or client § Con: String in a special format

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04.03.25 //// Seite 769 WPS – Workplace Solutions BANK TRANSFER—BY DOMAIN SERVICE #[Service] public class CreditTransferService { public function transfer(Account $source, Account $destination, Amount $amount): void { $source->withdraw(amount); $destination->deposit(amount); } } No fields à stateless

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EXERCISE: MOBPROGRAMMING @ THE CINEMA

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04.03.25 //// Seite 772 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN GLOSSARY—REVISITED DEPTH DEPTH AT A SPECIFIC GEO POSITION CONSIDERING THE CURRENT TIDAL LEVEL DEPTH DEPTH BELOW SEA LEVEL MEASURED BY ECHO SOUNDER MANEUVER- PLANNING SOUNDING SERVICE

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04.03.25 //// Seite 773 WPS – Workplace Solutions EVERY BOUNDED CONTEXT HAS ITS OWN IMPLEMENTATION public struct Depth { public Depth( int depthBelowSeaLevel, int tideLevel) { // ... } } MANEUVER- PLANNING SOUNDING SERVICE public class Depth { public Depth( int centimeter, LocalDate soundingDate) { // ... } }

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@hschwentner What now?

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Architecture Hamburger Application Domain Infrastructure UI coarse grained Henning Schwentner !

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Architecture Hamburger ﬁne grained model repository interface controller use case service entity value object view data model repository implementation widget library REST frame- work transaction handling domain library programming language ORM ﬁle system access domain event

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@hschwentner Further topics: Ubiquitous Language Tactical Design (Entity, Value Object, Aggregate, Repository, Service)

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Workshop Domain-Driven Design konkret wps.de/ddd

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@hschwentner LeasingNinja.io

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@hschwentner Literature

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https://hschwentner.io

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@hschwentner FEEDBACK

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@hschwentner Bibliography Brandolini, Alberto. Introducing EventStorming. Self-published, Leanpub. Cockburn, Alistair. “Hexagonal Architecture.” January 4, 2005. https://alistair.cockburn.us/hexagonal-architecture/. Conway, Melvin E. “How Do Committees Invent?” Datamation 14, no. 5 (April 1968): 28–31. Evans, Eric. Domain-Driven Design: Tackling Complexity in the Heart of Software. Boston: Addison-Wesley, 2004. Foote, Brian and Joseph Yoder. “Big Ball of Mud.” PLoP ’97, Monticello, IL, September 1997. Fowler, Martin. Patterns of Enterprise Application Architecture. Boston: Addison- Wesley, 2003. Fowler, Martin. “Strangler Fig Application.” Bliki, June 29, 2004. Hofer, Stefan and Henning Schwentner. Domain Storytelling: a Collaborative, Visual, and Agile Way to Develop Domain-Driven Software. Boston: Addison-Wesley, 2022.

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Henning Schwentner  https://hschwentner.io  in/henningschwentner ✉ [email protected] Kolleg:in gesucht (Deutschlandweit)