Design Splitwise

Groups, expenses, splits, and balance simplification.

Expert17 min read

Read these first

Design StackOverflow Clone

Why this matters

Users form groups (trip, apartment)
Scope — what "Split strategies" covers inside Splitwise and what it does not
Scope — what "Balance graph" covers inside Splitwise and what it does not
Scope — what "Settlement" covers inside Splitwise and what it does not
Platform problems test entity graphs and derived state (reputation, balance).

Dependency direction

Controller / FacadeHTTP or API entry

Serviceuse cases, orchestration

Domainentities, rules, invariants

Repositorypersistence port

Dependencies point inward — domain never imports infrastructure.

CampusOS context

CampusOS is the fictional smart-campus platform used across this track. Treat this module as a real product slice:

Context

Belongs to CampusOS — same naming and design vocabulary as parking, library, and booking modules
Scope is one JVM — persistence is in-memory or behind a repository interface
Goal is a class diagram + core Java that an interviewer can follow in 35–45 minutes
Extension paragraph at the end links to System Design when scale exceeds one process

Requirements snapshot

Functional requirements

Users form groups (trip, apartment)
Add expense with payer and split among members (equal, exact, percentage)
Show who owes whom net balance per group
Simplify debts to minimum number of settlements
Settlement marks debt as paid

Non-functional requirements

Single JVM, in-process design for this LLD article
Core flows testable with in-memory repositories
Extension points for new rules without editing central god classes
Clear public API on facade/service layer
CampusOS extension links to System Design when scale demands distribution

Overview

This CampusOS module designs Splitwise as a production-ready in-process service. Read requirements point-by-point before drawing classes.

Requirements

Points to cover

Users form groups (trip, apartment)
Add expense with payer and split among members (equal, exact, percentage)
Show who owes whom net balance per group
Simplify debts to minimum number of settlements
Settlement marks debt as paid

Split strategies

Points to cover

Scope — what "Split strategies" covers inside Splitwise and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Balance graph

Points to cover

Scope — what "Balance graph" covers inside Splitwise and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Settlement

Points to cover

Scope — what "Settlement" covers inside Splitwise and what it does not
Classes — name 2–4 classes/interfaces responsible for this slice
Flow — step-by-step: trigger → validation → state change → side effect
Edge cases — invalid input, concurrent access, empty state, timeout
Pattern — Strategy, State, Observer, or Facade if it fits naturally
Test hook — one scenario an interviewer will ask you to walk through

Java

// CampusOS — Splitwise facade (keep thin)
public final class SplitwiseService {
    private final SplitwiseRepository repository;

    public SplitwiseService(SplitwiseRepository repository) {
        this.repository = repository;
    }

    /** Orchestrate: validate → domain rules → persist → return result */
    public Result handle(Request request) {
        request.validate();
        var domain = DomainModel.from(request);
        domain.applyRules();
        return repository.save(domain);
    }
}

Quick recall

Everything you need if you only revisit this box.

Requirements — review this slice before mock interviews.
Split strategies — review this slice before mock interviews.
Balance graph — review this slice before mock interviews.
Settlement — review this slice before mock interviews.

CampusOS extension

When this outgrows one JVM

Shard inventory or state by campus ID or geographic region
Push notifications, payments, and search to dedicated services (System Design track)
Use message queues for async side effects (email, analytics)
The class diagram you drew here becomes one box on the HLD architecture diagram

Test yourself

Answer these before moving on — recall is what makes it stick.