kunalsingh-bais/offline-upi-system-fullstack

Purpose and topology

This is an offline-payment/UPI-style system: an Angular client packaged as an Android Capacitor app communicates with Spring Boot microservices for authentication, wallet/profile management, and encrypted transaction processing. Bluetooth Low Energy (BLE) supports device-to-device payment exchange and later synchronization.

Client application and entry points

The Angular application starts at frontend/src/main.ts, with routing and providers assembled in frontend/src/app/app.config.ts and frontend/src/app/app.routes.ts. It exposes login/register, dashboard, payment initiation/completion, pending sync, and history views. HTTP authentication is centralized through the route guard and interceptor:

• frontend/src/app/guards/auth-guard.ts

• frontend/src/app/interceptors/auth-interceptor.ts

• frontend/src/app/services/api.ts

Android/BLE native boundary

Capacitor hosts the web client in Android, entered through frontend/android/app/src/main/java/com/offlineupi/MainActivity.java. Native BLE code owns advertising, discovery/client connections, GATT-server behavior, and connection lifecycle; this is the boundary between browser-facing Angular services and Android Bluetooth APIs:

• frontend/android/app/src/main/java/com/offlineupi/ble/BluetoothConnectionManager.java

• frontend/android/app/src/main/java/com/offlineupi/ble/BluetoothGattServerManager.java

• frontend/android/app/src/main/java/com/offlineupi/ble/BluetoothClient.java

Angular-side Bluetooth, encryption, local IndexedDB, network-state, and BLE-sync services enable offline capture and deferred reconciliation:

• frontend/src/app/services/capacitor-bluetooth.ts

• frontend/src/app/services/indexed-db.ts

• frontend/src/app/services/sync-ble.ts

Gateway and service discovery

The backend is independently built Maven/Spring Boot services. Backend/Api-gateway/src/main/java/com/api/gateway/ApiGatewayApplication.java is the intended external backend entry point, configured by Backend/Api-gateway/src/main/resources/application.yml. Service location/registration is handled separately by:

• Backend/Service-registry/src/main/java/com/service/registry/ServiceRegistryApplication.java

• Backend/Service-registry/src/main/resources/application.properties

This establishes an edge-routing/discovery boundary rather than direct frontend coupling to individual services.

Identity and authorization

Auth Service provides registration, login, and token validation. Its controller delegates credential/user operations to AuthService, persists auth users via UserRepository, and issues/verifies JWTs through JwtTokenProvider:

• Backend/Auth-service/src/main/java/com/auth/service/controller/AuthController.java

• Backend/Auth-service/src/main/java/com/auth/service/service/AuthService.java

• Backend/Auth-service/src/main/java/com/auth/service/security/JwtTokenProvider.java

Security configuration is service-local in Backend/Auth-service/src/main/java/com/auth/service/config/SecurityConfig.java. User Service and Transaction Service each contain security configurations and clients, implying token validation or authenticated service-to-service calls rather than a shared in-process security layer.

User profile and wallet domain

User Service separates profile and wallet persistence/operations. It calls Auth Service through AuthServiceClient, then exposes profile and balance endpoints through distinct controllers:

• Backend/User-service/src/main/java/com/user/service/client/AuthServiceClient.java

• Backend/User-service/src/main/java/com/user/service/controller/UserProfileController.java

• Backend/User-service/src/main/java/com/user/service/controller/WalletController.java

Core state is represented by UserProfile and Wallet, persisted through their repositories. Wallet balance updates are a critical downstream boundary for transaction completion.

Transaction lifecycle and cryptography

Transaction Service manages initiate, BLE sync, and completion flows through TransactionController and TransactionService. Transaction state is persisted independently, while a UserServiceClient links completion to wallet/user operations:

• Backend/Transaction-service/src/main/java/com/transaction/service/controller/TransactionController.java

• Backend/Transaction-service/src/main/java/com/transaction/service/service/TransactionService.java

• Backend/Transaction-service/src/main/java/com/transaction/service/client/UserServiceClient.java

The transaction payload boundary explicitly supports encrypted/decrypted forms and BLE-specific payloads. Encryption is layered: RSA key management/public-key publication, AES encryption, hashing, and orchestration in EncryptionProcessor:

• Backend/Transaction-service/src/main/java/com/transaction/service/encryption/RSAKeyService.java

• Backend/Transaction-service/src/main/java/com/transaction/service/encryption/AESEncryptionService.java

• Backend/Transaction-service/src/main/java/com/transaction/service/encryption/EncryptionProcessor.java>