Case Study: A Library Lending System — Tech Epitome

Modelling Case Studies

Case Study: A Library Lending System

Members, books, copies, loans and reservations — and why the difference between a book and a copy decides the whole design.

Medium13 min readModelling Case Studies

By the end of this chapter

Separate a catalogue title from the physical copies of it
Model a Loan as the object that relates a member to a copy over time
Put rules that vary by member type behind a policy interface

The brief

A library lends physical books. Members search the catalogue, borrow copies, renew them and return them. If every copy of a title is out, a member can reserve it and is notified when one comes back. Late returns earn a fine. Payments, acquisitions and inter-library loans are out of scope.

Assume a member may hold at most five loans at once; staff accounts may hold more.
Assume a 14-day loan period and one renewal, refused if another member is waiting for the title.
Assume fines are charged per day late and capped per loan.
Assume a member with unpaid fines over a threshold cannot borrow.

Entities

Underlining nouns gives book, copy, member, loan, reservation, fine, catalogue. The single most important decision is splitting Book — the title, with an ISBN and an author — from BookCopy — one physical object with a barcode, a condition and a location. Members borrow copies and reserve books.

Before and after

Why the split matters

One Book class with a counter

You’ve reached the end

What this chapter was for:

Book is the title; BookCopy is the thing on the shelf. Conflating them hides every interesting question.
A relationship with its own dates and rules — a Loan — deserves to be an object.
Fines and loan limits vary; put them behind policies and keep the lending flow stable.
Reservations attach to the title, holds attach to a copy.

Entity	Is the only thing that knows
Book	A title's bibliographic details (ISBN, authors, edition)
BookCopy	One physical item's barcode, condition and status
Member	Who someone is and what type of membership they hold
Loan	Which member has which copy, since when, until when, and how often renewed
ReservationQueue	Who is waiting for a title, in order
FinePolicy	How lateness turns into money
LendingService	The checkout, renewal and return workflows that span the above

One sentence each. Anything that fails the sentence is not an entity yet.

java

1final class Loan {2    private final LoanId id;3    private final MemberId member;4    private final CopyId copy;5    private final LocalDate borrowedOn;6    private LocalDate dueOn;7    private LocalDate returnedOn;          // null while the copy is out8    private int renewals;9 10    boolean isOverdue(LocalDate today) {11        return returnedOn == null && today.isAfter(dueOn);12    }13 14    void renew(LoanRules rules, boolean someoneIsWaiting) {15        if (someoneIsWaiting) throw new RenewalRefused("another member is waiting");16        if (renewals >= rules.maxRenewals()) throw new RenewalRefused("limit reached");17        dueOn = dueOn.plus(rules.loanPeriod());18        renewals++;19    }20 21    void close(LocalDate today) { returnedOn = today; }22}

The loan owns its dates and its renewal rule.

java

1final class LendingService {2    Loan checkout(MemberId memberId, CopyId copyId) {3        Member member = members.get(memberId);4        if (fines.unpaidFor(memberId).isGreaterThan(rules.fineBlockThreshold()))5            throw new CheckoutRefused("outstanding fines");6        if (loans.activeCount(memberId) >= rules.maxLoans(member.type()))7            throw new CheckoutRefused("loan limit reached");8 9        BookCopy copy = copies.get(copyId);10        copy.lendTo(memberId);        // refuses unless AVAILABLE, or ON_HOLD for this member11        LocalDate today = LocalDate.now(clock);12        return loans.save(Loan.open(memberId, copyId, today, today.plus(rules.loanPeriod())));13    }14}

The service coordinates; each entity guards its own rule.

java

1enum CopyStatus {2    AVAILABLE, ON_LOAN, ON_HOLD, IN_REPAIR, LOST;3 4    boolean canMoveTo(CopyStatus next) {5        return switch (this) {6            case AVAILABLE -> next == ON_LOAN || next == ON_HOLD || next == IN_REPAIR;7            case ON_LOAN   -> next == AVAILABLE || next == ON_HOLD || next == LOST;8            case ON_HOLD   -> next == ON_LOAN || next == AVAILABLE;   // collected, or hold lapsed9            case IN_REPAIR -> next == AVAILABLE;10            case LOST      -> next == AVAILABLE;                      // found again11        };12    }13}

The legal transitions, in one place the compiler checks for completeness.

java

1interface FinePolicy {2    Money fineFor(Loan loan, LocalDate returnedOn);3}4 5final class PerDayCapped implements FinePolicy {6    private final Money perDay;7    private final Money cap;8 9    public Money fineFor(Loan loan, LocalDate returnedOn) {10        long daysLate = Math.max(0, ChronoUnit.DAYS.between(loan.dueOn(), returnedOn));11        Money owed = perDay.times(daysLate);12        return owed.isGreaterThan(cap) ? cap : owed;13    }14}

A new fine rule is a new class, not an edit to the return flow.

python

1from collections import deque2 3 4class ReservationQueue:5    """Members waiting for one Book (the title, not a copy)."""6 7    def __init__(self) -> None:8        self._waiting: deque[str] = deque()9 10    def join(self, member_id: str) -> int:11        if member_id in self._waiting:12            raise ValueError("already waiting")13        self._waiting.append(member_id)14        return len(self._waiting)            # position in line15 16    def on_copy_returned(self, copy) -> str | None:17        if not self._waiting:18            return None                      # copy goes back on the shelf19        member_id = self._waiting.popleft()20        copy.hold_for(member_id)             # ON_HOLD, with a pick-up deadline21        return member_id

Subscribed to 'copy returned'; the return workflow never calls it directly.

Case Study: A Library Lending System

The brief

Entities

Why the split matters

You’ve reached the end

Relationships: the Loan is an object

Patterns, and why each one is here

Reservations and holds

Trade-offs

Where do the lending rules live?

All rules in LendingService

Entities guard their own state; policies hold variable rules

Rules engine / configuration-driven

Extensions that would change the model

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The brief

Entities

Why the split matters

You’ve reached the end

Relationships: the Loan is an object

Patterns, and why each one is here

Reservations and holds

Trade-offs

Where do the lending rules live?

All rules in LendingService

Entities guard their own state; policies hold variable rules

Rules engine / configuration-driven

Extensions that would change the model