Domain-Driven Design: Tactical Modeling

1 Task , Justify the Map

In pairs, answer the following in writing (3–4 sentences total):

• Do you agree that Fines & Payments is Generic rather than Supporting? Defend your position.
• Suppose the library wants to become known for an excellent personalized recommendation engine, meaning it will use machine learning to suggest books to members based on their borrowing history. Which subdomain would move to Core, and why?

1 Worked Example: Member Management Context

The Member Management context is fully solved below. Study it carefully. You will follow the same reasoning for Loan and Collection. You can find the PlantUML code that draws the UML diagram for the Member Management Context written in Appendix A.

Element	Kind	Reasoning
Member	Entity (Aggregate Root)	Has a persistent identity (memberId) that matters regardless of attribute changes (e.g., name change, tier upgrade).
MemberId	Value Object	Needs generation logic (e.g. UUID) and validation (non-null, correct format). A raw String/Long cannot carry that behavior.
MemberName	Value Object	Composed of firstName/lastName; validated (non-empty), immutable.
Email	Value Object	Defined entirely by its string value; validated (format) and immutable.
MembershipTier	Value Object (Enum-like)	STANDARD or PREMIUM: a constrained classification, not a thing with identity.
activeLoanCount	Simple attribute (int)	No validation rules or independent behavior : just a counter the Entity manages internally.
status	Simple attribute (enum)	A plain classification flag with no behavior of its own; kept as a raw enum rather than a VO wrapper.
registrationDate	Simple attribute (LocalDate)	No validation or behavior beyond being a date : a primitive type is sufficient.

Below is the UML diagram for the Member Management Context. You can find the PlantUML code that draws the UML diagram for the Member Management Context written in Appendix A.

2 Your Task: Collection Management Context

The Collection Management context is more interesting: the library lends more than just books. It also lends audio and video materials, which share some common fields (title, publication year, status) but also have their own unique fields. So, how should we model this in a domain-driven way? Concepts that you have learned in OOP (inheritance, polymorphism) are useful here, but we must also consider DDD tactical modeling rules: identity, immutability, and aggregate boundaries. Instead of modeling three unrelated aggregates (Book, AudioMaterial, VideoMaterial), model a single Aggregate Root supertype called LibraryItem, with three concrete subtypes. We will also make LibraryItem abstract, so that students cannot instantiate a generic library item that is neither a book, an audio item, nor a video. This is a common pattern in DDD: a supertype carries the shared fields and behavior, while each subtype adds only what makes it distinct.

This context is an example that has a mix of Entities and Value Objects. The table below lists the elements, their kind, and reasoning for each.

Element	Kind	Notes
LibraryItem	Entity (abstract Aggregate Root)	Common fields: itemId, title, status, publicationYear.
Book	Entity (concrete subtype)	Adds isbn, author.
AudioMaterial	Entity (concrete subtype)	Adds duration, narrator.
VideoMaterial	Entity (concrete subtype)	Adds duration, format, ageRating.
ISBN, Author, Duration, PublicationYear, MaterialFormat	Value Objects	Shared or subtype-specific, all immutable.

Build the UML diagram for this context using PUML following the example of the member context. Use <|-- for the generalization (inheritance) arrows.

3 Your Task: Loan Management Context

Using the reasoning from the Member Management example, analyze the elements below. Note how business invariants (e.g., maximum renewals allowed, valid date intervals, non-negative monetary amounts) determine whether an attribute is modeled as a simple primitive or encapsulated inside a Value Object.

Element	Kind	Reasoning (fill in or study)
Loan	Entity (Aggregate Root)	Possesses a unique lifecycle and identity (loanId). Enforces critical domain invariants across borrowing, renewals, and returns.
LoanId	Value Object	Needs format validation (e.g., non-null UUID/prefix) and factory generation logic.
borrowerId	Simple attribute (MemberId reference)	Cross-context reference to the borrowing Member. Held by identity only, Loan never holds the full Member object.
iemId	Simple attribute (ItemId reference)	Cross-context reference to the borrowed LibraryItem. Held by identity only, Loan never holds the full Book/LibraryItem object.
LoanPeriod	Value Object	Encapsulates temporal rules: dueDate must be after startDate, and calculates whether an item is currently overdue.
RenewalCount	Value Object	Guards the business invariant: a loan can be renewed at most 2 times. Increments itself and throws a domain exception if the threshold is exceeded.
Money	Value Object	Represents monetary amounts (e.g., daily overdue rates and accrued fines). Enforces non-negative values and safe arithmetic with currency.
status	Simple attribute (enum)	State flag (ACTIVE, RETURNED, OVERDUE, LOST) managed directly by the aggregate's transition methods.

Build the UML diagram using PUML as you did for Member and Collection. Use *-- for composition arrows and <|-- for generalization arrows.

@startuml
title Member Management Context

skinparam class {
  BackgroundColor<> #FFE0B2
  BorderColor<> #E65100

  BackgroundColor<> #C8E6C9
  BorderColor<> #2E7D32

  BackgroundColor<> #B3E5FC
  BorderColor<> #01579B
}

class Member <> {
  - memberId : MemberId
  - name : MemberName
  - email : Email
  - tier : MembershipTier
  - status : MemberStatus
  - activeLoanCount : int
  - registrationDate : LocalDate
  --
  + canBorrow() : boolean
  + incrementLoanCount() : void
  + decrementLoanCount() : void
  + suspend() : void
  + reinstate() : void
}

class MemberId <> {
  - value : UUID
  --
  + generate() : MemberId
}

class MemberName <> {
  - firstName : String
  - lastName : String
  --
  + fullName() : String
}

class Email <> {
  - value : String
}

enum MembershipTier {
  STANDARD
  PREMIUM
}

enum MemberStatus {
  ACTIVE
  SUSPENDED
}

Member *-- MemberId
Member *-- MemberName
Member *-- Email
Member *-- MembershipTier
Member *-- MemberStatus

legend right
  |= Color |= Element |
  |<#FFE0B2> | **Aggregate Root** |
  |<#C8E6C9> | **Entity** |
  |<#B3E5FC> | **Value Object** |
  |<#F5F5F5> | **Enumeration** |
  --
  Aggregate Root:
  The entry point to an aggregate and the only
  object referenced from outside the aggregate.
  --
  Value Object:
  An immutable object defined by its values,
  without an independent identity.
endlegend

@enduml