Expense Sharing Backend System (Splitwise-like)

Engineering Design Assignment

This project implements a backend system for expense sharing, inspired by applications like Splitwise.

This README is intentionally detailed to clearly explain:

• How the problem was broken down
• Why specific design decisions were taken
• How correctness, scalability, and clarity were prioritized

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Objective

Design and implement a backend system that allows users to:

• Create groups
• Add shared expenses
• Track balances
• Settle dues

Supported split types

1. Equal split
2. Exact amount split
3. Percentage split

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Problem-Solving & Design Thinking

1️. Problem Decomposition

The system was divided into four core domains, each with a single responsibility:

Domain	Responsibility
Users	Identity & ownership
Groups	Context for shared expenses
Expenses	Recording shared spending
Balances	Net money flow between users

This separation ensures:

• Low coupling
• High readability
• Easier testing and debugging

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2️. Key Design Decisions (With Reasoning)

Use of BigDecimal for monetary values

• Why: Floating point (double) leads to rounding errors
• Trade-off: Slightly more verbose code
• Benefit: Financial correctness and precision

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Balance Simplification Strategy

Instead of storing raw transactions, the system maintains net balances only.

Example:

A owes B ₹500
B owes A ₹300
→ Net: A owes B ₹200

This keeps balances:

• Minimal
• Easy to understand
• Efficient to query

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Centralized Balance Logic

All balance-related operations are handled in BalanceService:

• Expense balance updates
• Reverse balance cancellation
• Settlements

This avoids duplicated logic and ensures consistency.

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DTO-Based API Design

API requests use DTOs instead of entities to:

• Prevent over-posting
• Keep APIs stable if database models change
• Improve clarity and validation

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Architecture Overview

Layer	Responsibility
Controller	Handles REST APIs
Service	Business logic
Repository	Database access
Model	Domain entities
DTO	API contracts

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Tech Stack

Category	Technology
Language	Java
Framework	Spring Boot
ORM	Hibernate / JPA
Database	MySQL
API Style	REST
Build Tool	Maven
Testing	Postman

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Project Structure

src/main/java/com/expensesplit
│
├── Controllers
│ ├── UserController.java
│ ├── GroupController.java
│ ├── ExpenseController.java
│ └── BalanceController.java
│
├── Services
│ ├── UserService.java
│ ├── GroupService.java
│ ├── ExpenseService.java
│ └── BalanceService.java
│
├── Repositories
│ ├── UserRepository.java
│ ├── GroupRepository.java
│ ├── ExpenseRepository.java
│ ├── ExpenseSplitRepository.java
│ └── BalanceRepository.java
│
├── DTO
│ ├── CreateExpenseRequest.java
│ ├── ExactSplitRequest.java
│ ├── PercentageSplitRequest.java
│ ├── SettleRequest.java
│ └── BalanceSummary.java
│
├── model
│ ├── User.java
│ ├── Group.java
│ ├── Expense.java
│ ├── ExpenseSplit.java
│ ├── Balance.java
│ └── SplitType.java
│
└── ExpenseApplication.java

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Setup Instructions

1. Create Database

CREATE DATABASE expensesplit;

2. Configure application.yaml

spring:
  datasource:
    url: jdbc:mysql://localhost:3306/expensesplit
    username: root
    password: YOUR_PASSWORD
  jpa:
    hibernate:
      ddl-auto: update
    show-sql: true

3. Run application

server starts at http://localhost:8080

Postman Testing Flow

1. Create Users

POST http://localhost:8080/user

{
  "name": "Neha",
  "email": "neha@gmail.com"
}

Output:

2. Create Group

POST http://localhost:8080/groups

{
  group name: "group_name",
  userIds: 
  [
  "USER_ID_1",
  "USER_ID_2",
  "USER_ID_3"
  ]
}

Output:

3. Add Equal Expense

POST http://localhost:8080/expenses

{
  "groupId": "GROUP_ID",
  "paidBy": "USER_ID",
  "amount": 3000,
  "description": "Hotel"
}

Output:

4. Add Exact Split Expense

POST http://localhost:8080/expenses/exact

{
  "groupId": "GROUP_ID",
  "paidById": "USER_ID",
  "description": "Taxi",
  "splits": {
    "USER_ID_1": 500,
    "USER_ID_2": 300
  }
}

Output:

5. Add Percentage Split Expense

POST http://localhost:8080/expenses/percentage

{
  "groupId": "GROUP_ID",
  "paidById": "USER_ID",
  "amount": 3000,
  "description": "Resort",
  "splits": {
    "USER_ID_1": 50,
    "USER_ID_2": 30,
    "USER_ID_3": 20
  }
}

6. View User Balances

GET http://localhost:8080/balances/user/{userId}

Output:

7. User Balance Summary

GET http://localhost:8080/balances/summary/{userId}

Output:

8. View Group Balances

GET http://localhost:8080/balances/group/{groupId}

9. Settle Dues

POST http://localhost:8080/balances/settle

{
  "fromUserId": "USER_ID",
  "toUserId": "USER_ID",
  "amount": 500
}

Output: