LemonPay
LemonPay is a mobile-first digital payment platform designed to enable secure, real-time financial transactions through a scalable backend infrastructure.
The platform provides wallet management, peer-to-peer transfers, and merchant payment capabilities while ensuring transaction safety using idempotent processing mechanisms.
LemonPay is built using Kotlin, Spring Boot, MongoDB, and Android SDK, with an architecture designed for high availability, scalability, and financial consistency.
Key Capabilities
Real-Time Transactions
Processes financial transactions instantly while maintaining strong consistency and data integrity.
Idempotent Payment Processing
Implements idempotency keys to prevent duplicate transactions caused by retries or network instability.
Secure Wallet Management
Users can manage wallets, check balances, send funds, and receive payments.
Mobile-First Experience
A fully integrated Android application allows users to perform financial operations securely.
Scalable Architecture
Designed to support high transaction throughput typical of fintech platforms.
System Architecture
LemonPay follows a layered service architecture.
Android Application | | HTTPS v API Layer (Spring Boot + Kotlin) | | Business Logic v Transaction Processing Service | | ORM v Hibernate | v MongoDB Database
Architectural Characteristics
Stateless backend services
Horizontal scalability
Strong transaction safety
Mobile-optimized APIs
Modular service design
Technology Stack
Backend
Kotlin
Spring Boot
Hibernate ORM
MongoDB
Mobile Application
Android SDK
Kotlin
Ktor Client
Transaction Safety
Idempotency Key Processing
Transaction logging
Retry-safe financial operations
Repository Structure
lemonpay │ ├── backend │ ├── controllers │ ├── services │ ├── repositories │ ├── models │ ├── idempotency │ └── configuration │ ├── mobile │ ├── ui │ ├── wallet │ ├── transactions │ └── network │ ├── database │ ├── schemas │ └── migrations │ └── docs
Transaction Processing Flow
The LemonPay backend processes financial operations through a controlled transaction pipeline.
User initiates transfer | v Mobile App sends request | v API receives request | v Idempotency Check | |---- If duplicate request → return previous response | v Transaction Validation | v Wallet Balance Verification | v Execute Transaction | v Persist Transaction Record | v Return Transaction Status
This approach prevents:
duplicate transfers
retry errors
inconsistent financial states
Idempotency Mechanism
To guarantee safe financial retries, LemonPay implements idempotency keys.
Each financial request must include a unique header:
Idempotency-Key: unique-transaction-id
If a request is repeated:
The server does not process the transaction again
Instead it returns the original response
This ensures exactly-once transaction execution.
Database Model (Simplified)
Wallet
id user_id balance currency created_at
Transaction
id sender_wallet receiver_wallet amount status idempotency_key created_at
Security Considerations
Financial systems must maintain strict security standards.
LemonPay implements:
HTTPS encryption
Token-based authentication
Input validation
Transaction audit logs
Secure credential storage
Replay protection using idempotency
Running the Backend
Clone the repository:
git clone https://github.com/narches/lemonpay.git cd lemonpay/backend
Start the application:
./gradlew bootRun
Server will start at:
Running the Mobile App
-
Open Android Studio
-
Import project from:
/mobile
- Run the application on an emulator or device.
Deployment Architecture
Production deployment typically includes:
Mobile Apps | v API Gateway | v Spring Boot Service Cluster | v MongoDB Cluster
Infrastructure can be deployed on:
AWS
Oracle Cloud
Google Cloud
Kubernetes environments
Future Enhancements
Planned improvements include:
Merchant payment gateway
QR-code payments
Multi-currency wallets
Fraud detection systems
Financial analytics dashboards
Push notifications
Contributing
We welcome contributions from developers interested in fintech systems.
Steps:
-
Fork repository
-
Create feature branch
-
Commit changes
-
Submit pull request
License
No License
Maintainer
Developed by Joseph Nnachi
Mission:
Building technology that empowers humanity.