Splitwise Backend API 💸

A production-ready expense splitting REST API inspired by Splitwise, built with Java and Spring Boot. Features JWT authentication, 3 split types, and a debt simplification algorithm that minimizes settlement transactions.

🔗 Live API: https://splitwise-api.onrender.com 📖 Swagger Docs: https://splitwise-api.onrender.com/swagger-ui.html

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Tech Stack

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Features

• 🔐 JWT Authentication — stateless auth with BCrypt password hashing
• 👥 Group Management — create groups, add/remove members
• 💰 Expense Tracking — 3 split types: Equal, Exact, Percentage
• 🧠 Debt Simplification Algorithm — greedy O(n log n) algorithm using priority queues that minimizes the number of settlement transactions
• 📊 Balance Tracking — real-time balance calculation per user
• ✅ Settlement Recording — mark debts as paid and track history

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API Endpoints

Auth

Method	Endpoint	Description
POST	/api/auth/register	Register a new user
POST	/api/auth/login	Login and get JWT token

Groups

Method	Endpoint	Description
POST	/api/groups	Create a new group
GET	/api/groups/my	Get all groups you're in
GET	/api/groups/{id}	Get group details + members
POST	/api/groups/{id}/members	Add a member
DELETE	/api/groups/{id}/members/{userId}	Remove a member

Expenses

Method	Endpoint	Description
POST	/api/expenses	Add an expense (EQUAL/EXACT/PERCENTAGE)
GET	/api/expenses/group/{id}	All expenses in a group
GET	/api/expenses/my-balances	Your net balance across all groups

Settlements

Method	Endpoint	Description
GET	/api/settlements/group/{id}/simplify	Get minimum transactions to settle
POST	/api/settlements/settle	Record a payment
GET	/api/settlements/group/{id}/history	Settlement history

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Database Schema

6 tables: users · groups · group_members · expenses · expense_splits · settlements

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The Debt Simplification Algorithm

The /simplify endpoint uses a greedy algorithm with two max-heaps (one for creditors, one for debtors) to produce the minimum number of transactions required to settle all debts in a group.

Example: Without simplification: A→B ₹300, B→C ₹200, C→A ₹100 = 3 transactions After simplification: A→B ₹200, C→B ₹100 = 2 transactions

Time complexity: O(n log n) | Space: O(n)

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Running Locally

# Clone the repo
git clone https://github.com/Adityapal67/splitwise.git
cd splitwise

# Set up MySQL
mysql -u root -p
CREATE DATABASE splitwise_db;

# Configure application.properties with your DB credentials

# Run
mvn spring-boot:run

# API available at
http://localhost:8080/swagger-ui.html

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Author

Aditya — B.Tech CSE student passionate about backend systems LinkedIn · GitHub