Assignment 5 Prep Exercise: Designing a Service Layer for LibTrack

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This exercise is designed to prepare you for the design task in Assignment 5. We will determine service boundaries using the actor heuristic and at least one of the other 3 service boundary heuristics from L18:

1. Actor📌: Things used by different actors should be separate.
2. Rate of Change: Things that change at different speeds should be separate.
3. Interface Segregation: Clients that need different things should get different interfaces.
4. Testability: Things that need independent testing should be separable.

LibTrack is a small command-line application for managing a public library. The following classes are already implemented and available for your services to use:

Class	Responsibility
BookRepository	Stores and retrieves books by ID, title, or author
PatronRepository	Stores and retrieves patron accounts
HoldRepository	Manages the ordered queue of holds on a given book
LoanRepository	Records active and historical loans (checkouts/checkins)
FineRepository	Stores and retrieves fines associated with patrons and loans

Data model notes:

• Patron records in PatronRepository contain only basic account information (name, contact details, library card number). Fines, loans, and holds are stored exclusively in their respective repositories, linked to a patron by ID.
• Patron account format is stable and unlikely to change. Loan policies (loan period, renewal limits) change every few years. The library plans to actively experiment with hold and fine policies (grace periods, fine rates, hold expiry, waiver rules).
• Checking in a book includes assessing any overdue fine based on current fine policy.

LibTrack serves two distinct groups of users:

Actor	Goals	Key Commands
The Patron	Browses the catalog, manages their holds, reviews their borrowing history, and manages their fines	search, hold, holds, history, my account, my fines, pay fine
The Librarian	Handles checkouts and returns, manages the catalog, looks up patron accounts, and manages fines	search, checkout, checkin, catalog add, catalog remove, patron lookup, patron fines, waive fine

Commands

Patron Commands

Command	Description
search <title>	Search the catalog by title or author (case-insensitive substring)
hold <book>	Place a hold on a book for the current patron
holds	List the current patron's active holds
history	Show the current patron's borrowing history
my account	Show the current patron's account details and borrowing status
my fines	Show all outstanding fines for the current patron
pay fine <fine>	Pay an outstanding fine

Librarian Commands

Command	Description
search <title>	Search the catalog by title or author (case-insensitive substring)
checkout <book> <patron>	Check a book out to a patron and record the loan
checkin <book>	Check a returned book back in, assess any overdue fine per current fine policy, and notify the next patron with a hold
catalog add <title> <author> <isbn>	Add a new title to the catalog
catalog remove <book>	Remove a title from the catalog
patron lookup <name>	Look up a patron's account and current borrowing status
patron fines <name>	View all outstanding fines for a patron
waive fine <fine>	Waive an outstanding fine for a patron

Your Task

Write ADR-001: Service Boundaries documenting how you would decompose LibTrack's service layer:

# ADR-001: Service Boundaries

## Context
[What is the situation? What forces are at play?]

## Decision
[What did you decide, and why? Reference specific heuristics.]

## Consequences
[What are the benefits and drawbacks of this decomposition?]

Your ADR should:

• Name your services and state which commands each one owns
• Justify each boundary using the actor heuristic and at least one other heuristic (rate of change, interface segregation, testability)
• Discuss at least one tradeoff — something your decomposition handles well and something it makes harder

Actor-Aligned Services Required

Your service layer must have separate services aligned with the two actors, plus a separate Search service.