Library Management System | Spring Backend

CoolLib — Distributed Library Core API

Spring Boot Docker Nginx PostgreSQL JWT Security Kotlin

A high-performance backend ecosystem engineered with Clean Architecture. It implements a strict separation between JPA Entities and DTOs, serving as the central orchestration hub for both Android (Kotlin) and iOS (SwiftUI) clients.

Source Code
Android CI/CD

Project Overview

CoolLib Backend is a robust RESTful service built with Spring Boot and Kotlin. Beyond standard CRUD operations, it manages complex business logic layers to ensure data consistency, security, and seamless synchronization across the mobile ecosystem.

Key Technical Wins

Layered Architecture

Strictly follows the Controller-Service-Repository pattern, ensuring complete decoupling between business logic and data access layers.

Secure Sessions

Integrated Spring Security with JWT (JSON Web Tokens) to provide a stateless authentication mechanism tailored for mobile clients.

Persistence Layer

Leverages PostgreSQL and Spring Data JPA, ensuring the atomicity of borrowing operations through @Transactional services.

Cloud-Native Deployment

Containerized with Docker Compose and secured via Cloudflare Tunnel, utilizing Nginx as a reverse proxy for robust, Zero-Trust external access.

System Architecture

Architecture Design
graph TD A[CoolLib Android Jetpack Compose] B[CoolLib iOS SwiftUI + SwiftData] C{Restful API Spring MVC + JWT} D[Business Logic Book Service Layer] E[Data Access Spring Data JPA] F[(Persistence PostgreSQL)] A --> C B --> C C --> D D --> E E --> F style A fill:#3DDC84,stroke:#2eb26a,stroke-width:2px,color:#fff style B fill:#5AC8FA,stroke:#3fa7d6,stroke-width:2px,color:#fff style C fill:#6DB33F,stroke:#5a9234,stroke-width:2px,color:#fff style D fill:#4CAF50,stroke:#3d8b40,stroke-width:1px,color:#fff style E fill:#388E3C,stroke:#2e7031,stroke-width:1px,color:#fff style F fill:#336791,stroke:#264e6e,stroke-width:2px,color:#fff linkStyle default stroke:#8B4513,stroke-width:1.5px

*The backend follows a strict Stateless pattern.*

Authentication Flow

JWT Authentication Flow
sequenceDiagram autonumber participant Mobile as Mobile App participant Spring as Spring Boot API participant JWT as JwtProvider participant DB as PostgreSQL Note over Mobile, DB: 1. Login & Token Generation Mobile->>Spring: POST /api/auth/login Spring->>DB: Validate Credentials (BCrypt) DB-->>Spring: User Found & Matches Spring->>JWT: Generate Claims (UserID, Role) JWT-->>Spring: Return Signed JWT Spring-->>Mobile: 200 OK (Access Token) Note over Mobile, DB: 2. Authorized Request (Stateless) Mobile->>Spring: POST /api/loan (Header: Bearer <token>) Spring->>JWT: Validate Signature & Claims JWT-->>Spring: Token Valid (User: Ryan) Spring->>DB: Execute Loan Books Business DB-->>Spring: Return Data

*The security layer uses a OncePerRequestFilter to intercept and validate JWTs.*

Tech Stack

  • Core Framework: Spring Boot, Kotlin, Gradle
  • Security: JWT, Spring Security, BCrypt Password Hashing
  • Data Management: PostgreSQL, Spring Data JPA, Hibernate ORM
  • Standards & DevOps: RESTful Design, Docker, Docker Compose, Nginx, Cloudflare Tunnel

Core Features

Advanced Search

Transaction Logic

Deployment Stack

Method Endpoint Description Auth
POST /api/v1/auth/login Verify credentials and issue a JWT Access Token.
POST /api/v1/borrow/scan Process borrowing via Barcode/ISBN scan. In Dev
GET /api/v1/stats/loans Fetch aggregate data of active loans and trends. Planned
GET /api/v1/users/profile Retrieve identity claims from the decrypted JWT payload.
GET /api/v1/auth/validate Check if the current token is expired or blacklisted.
Method Endpoint Description Auth
GET /api/v1/books/search Multi-criteria filtering by Title, Author, Publisher, and Year.
GET /api/v1/books/fuzzy Fuzzy search using Levenshtein Distance or partial matching logic. In Dev
GET /api/v1/books/recommend Content-based filtering using User Borrowing History. Planned
GET /api/v1/books/isbn/{isbn} Direct lookup via 10 or 13-digit International Standard Book Number.
Transaction Processing Pipeline
ACID SECURE
Identity Authorization
Extract userId & roles from JWT claims.
Concurrency Locking
Execute SELECT FOR UPDATE to prevent race conditions.
Business Validation
Verify stock availability and user borrow limits.
Atomic Commit
Persist LoanRecord & update Inventory.

Managed by Spring @Transactional logic.

Production-Ready Infrastructure
DOCKERIZED
Zero-Trust Tunnel
Integrated Cloudflare Tunnel to expose the API securely without opening public ports or managing SSH certs.
Nginx Gateway
Reverse proxy handling SSL termination and request rate-limiting before reaching the Spring Boot instance.
User → Cloudflare → [Cloudflared] → Nginx → Spring Boot → PostgreSQL

Mobile Connectivity

Both Android and iOS clients interact with this backend through standard REST endpoints.

Required Header: Authorization: Bearer <token>

Developed with passion by Ryan Su © 2026