Wallet Backend

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Project Overview

This is a Personal Finance Management System built as a class project for COMP 4411 - Programming Languages at Lakehead University. The project demonstrates various programming paradigms including Object-Oriented, Procedural, and Concurrent programming using Kotlin and the Ktor framework.

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🎖️ Important Links

For MLH reviewers: Development Notes → 🎖️ documentation.md

Frontend Repo: wallet-frontend

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System Requirements

• OS: macOS, Linux, or Windows
• RAM: Minimum 4GB (8GB recommended)
• Disk Space: ~500MB for dependencies and Docker images
• Java: JDK 21
• Docker: Latest stable version

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Demo

Wallet.-.A.simple.wallet.app.made.with.Ktor.backend.Framework.and.Kotlin.Frontend.Multiplatform.mp4

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🎖️ System design architecture and flow

Expand to see System design and Architecture

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🎖️ Setup & Installation

Expand to see Setup and Installation

Prerequisites

Before running this project, ensure you have the following installed:

1. Java Development Kit (JDK) 21

   • Download from Oracle or use OpenJDK
   • Verify installation: java -version

2. Docker Desktop

   • Download and install Docker Desktop
   • Ensure Docker is running before proceeding

3. Gradle (Optional - project includes Gradle Wrapper)

   • The project uses Gradle Wrapper (./gradlew), so manual installation is not required

Step 1: Clone the Repository

git clone <repository-url>
cd wallet-backend

Step 2: Start PostgreSQL Database with Docker

This project uses PostgreSQL running in a Docker container for consistent development environments.

Start the PostgreSQL container:

docker run --name my-wallet-db -e POSTGRES_PASSWORD=mysecretpassword -p 5432:5432 -d postgres

Verify the container is running:

docker ps

You should see my-wallet-db in the list of running containers.

Important Database Configuration:

• Host: localhost
• Port: 5432
• Database: postgres
• Username: postgres
• Password: mysecretpassword

Note: These credentials are configured in src/main/kotlin/Databases.kt. Update them if you use different credentials.

Step 3: Configure Database Connection

The database connection is configured in Databases.kt:

object Database {
    fun connect(): Connection {
        val url = "jdbc:postgresql://localhost:5432/postgres"
        val user = "postgres"
        val password = "mysecretpassword"
        return DriverManager.getConnection(url, user, password)
    }
}

If you need to change credentials:

1. Open src/main/kotlin/Databases.kt
2. Update the url, user, and password values
3. Ensure they match your Docker PostgreSQL configuration

Step 4: Build the Project

./gradlew build

This will:

• Download all dependencies
• Compile Kotlin code
• Run tests
• Generate build artifacts

Step 5: Run the Application

./gradlew run

The server will start on http://localhost:8080

Expected console output:

2024-11-XX XX:XX:XX.XXX [main] INFO  Application - Application started in 0.303 seconds.
2024-11-XX XX:XX:XX.XXX [main] INFO  Application - Responding at http://0.0.0.0:8080

Step 6: Database Initialization (Automatic)

On first startup, the application will:

1. Create custom PostgreSQL types:

   • transaction_type ENUM ('expense', 'income')
   • period_type ENUM ('daily', 'weekly', 'monthly', 'yearly')

2. Create tables if they don't exist:

   • users - User accounts with authentication
   • transactions - Financial transaction records
   • budgets - Budget limits and tracking

3. Seed demo data (10 demo users with transactions and budgets)

Verify database tables:

Access PostgreSQL interactive terminal:

docker exec -it my-wallet-db psql -U postgres

List all tables:

\dt

Exit PostgreSQL terminal:

\q

Step 7: Test the API

Health check:

curl http://localhost:8080/

Expected response: Hello World!

Get transactions for user 1:

curl http://localhost:8080/transactions/1

Using Postman or any API client:

• Import the OpenAPI documentation from: http://localhost:8080/openapi
• Test all CRUD endpoints for Users, Transactions, and Budgets

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Docker Commands Reference

Command	Description
docker ps	List running containers
docker stop my-wallet-db	Stop the database container
docker start my-wallet-db	Start the database container
docker rm my-wallet-db	Remove the container (must be stopped first)
docker exec -it my-wallet-db psql -U postgres	Access PostgreSQL CLI
docker logs my-wallet-db	View database logs

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Troubleshooting

Port 5432 Already in Use

If you have PostgreSQL already running locally:

# Stop local PostgreSQL service (macOS)
brew services stop postgresql

# Or use a different port for Docker
docker run --name my-wallet-db -e POSTGRES_PASSWORD=mysecretpassword -p 5433:5432 -d postgres

Then update the port in Databases.kt to 5433.

Database Connection Failed

1. Verify Docker container is running: docker ps
2. Check container logs: docker logs my-wallet-db
3. Ensure credentials in Databases.kt match your Docker setup
4. Try restarting the container: docker restart my-wallet-db

Expected Errors on First Run

You may see error messages about types already existing:

Error executing SQL statement: CREATE TYPE transaction_type...
Error executing SQL statement: CREATE TYPE period_type...

This is normal! The application uses CREATE TYPE IF NOT EXISTS logic. On subsequent runs, these types already exist, causing harmless errors that are caught and logged.

Gradle Build Issues

# Clean and rebuild
./gradlew clean build

# If Gradle wrapper fails to download
chmod +x gradlew
./gradlew wrapper --gradle-version=8.5

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Technology Stack

Backend Framework & Language

• Kotlin - Primary programming language
• Ktor 3.0+ - Asynchronous web framework for building REST APIs
• Gradle - Build automation and dependency management

Database

• PostgreSQL - Relational database management system
• JDBC - Direct database connectivity without ORM

Security & Utilities

• BCrypt (at.favre.lib:bcrypt:0.10.2) - Password hashing
• kotlinx.serialization - JSON serialization/deserialization
• Logback - Logging framework

Server

• Netty - High-performance asynchronous event-driven network application framework

🎖️ Programming Paradigms Demonstrated

Expand to see Programming Paradigms demonstrated in this Project

1. Object-Oriented Programming (OOP)

The project uses OOP principles extensively with data classes, singleton objects, and encapsulation.

Example - Data Models:

// User.kt - Data class with properties
@Serializable
data class User(
    val userId: Int? = null,
    val firstName: String,
    val lastName: String,
    val email: String,
    val password: String,
    val createdAt: String? = null,
    val updatedAt: String? = null,
)

// Transaction.kt - Domain model
@Serializable
data class Transaction(
    val transactionId: Int? = null,
    val userId: Int,
    val title: String,
    val category: String,
    val transactionType: String,
    val amount: String,
    val date: String
)

Example - Repository Pattern:

// UserRepository.kt - Encapsulation of data access logic
class UserRepository {
    fun createUser(first: String, last: String, email: String, hashedPassword: String): Int {
        return DbHelper.query(
            sql = SqlBlueprints.INSERT_USER,
            prepare = { ps ->
                ps.setString(1, first)
                ps.setString(2, last)
                ps.setString(3, email)
                ps.setString(4, hashedPassword)
            },
            map = { rs ->
                rs.next()
                rs.getInt("user_id")
            }
        )
    }
}

Example - Singleton Object Pattern:

// Database.kt - Singleton object for database connection
object Database {
    fun connect(): Connection {
        val url = "jdbc:postgresql://localhost:5432/postgres"
        val user = "postgres"
        val password = "mysecretpassword"
        return DriverManager.getConnection(url, user, password)
    }

    fun init() {
        // Initialize database tables
    }
}

2. Procedural Programming

The project uses procedural approaches for sequential operations and data processing.

Example - Routing Configuration:

// Routing.kt - Procedural route setup
fun Application.configureRouting() {
    routing {
        userRouting()
        transactionRouting()
        budgetRouting()

        get("/") {
            call.respondText("Hello World!")
        }
    }
}

Example - Database Operations:

// TransactionRoutes.kt - Procedural database insert
post {
    val transaction = call.receive<Transaction>()
    val connection = Database.connect()

    val sql = """
        INSERT INTO transactions (user_id, title, category, transaction_type, amount, date)
        VALUES (?, ?, ?, ?::transaction_type, ?, ?::timestamp with time zone)
    """.trimIndent()

    connection.use { conn ->
        val statement = conn.prepareStatement(sql)
        statement.setInt(1, transaction.userId)
        statement.setString(2, transaction.title)
        statement.setString(3, transaction.category)
        statement.setString(5, transaction.transactionType)
        statement.setBigDecimal(6, BigDecimal(transaction.amount))
        statement.executeUpdate()
    }

    call.respond(HttpStatusCode.Created, "Transaction stored successfully")
}

3. Concurrent Programming

The project demonstrates thread-based concurrency with proper synchronization mechanisms.

Example - Thread-Safe Demo Data Generation:

// DemoDataSeeder.kt - Concurrent data seeding with thread safety
object DemoDataSeeder {
    // Concurrency primitives
    private val userInsertLock = ReentrantLock()
    private val userIdListRWLock = ReentrantReadWriteLock()
    private val sharedUserIdList = mutableListOf<Int>()

    // Thread-safe write operation
    private fun addUserIdThreadSafely(newId: Int) {
        userIdListRWLock.writeLock().withLock {
            sharedUserIdList.add(newId)
        }
    }

    // Thread-safe read operation
    private fun safelyGetAllUserIdsSnapshot(): List<Int> {
        userIdListRWLock.readLock().withLock {
            return sharedUserIdList.toList()
        }
    }

    // Creating concurrent worker threads
    private fun createConcurrentWorkerThreadsForAllUsers(userIds: List<Int>) {
        val allThreads = mutableListOf<Thread>()

        for (userId in userIds) {
            // One transaction thread per category
            for (category in categories) {
                val t = Thread({
                    generateTransactions(userId, category)
                }, "Tx-User$userId-$category")
                t.start()
                allThreads.add(t)
            }

            // One budget thread per user
            val b = Thread({
                generateBudgets(userId)
            }, "Budget-User$userId")
            b.start()
            allThreads.add(b)
        }

        // Wait for all threads to complete
        allThreads.forEach { it.join() }
    }
}

4. Functional Programming Elements

Kotlin's functional programming features are used throughout the project.

Example - Higher-Order Functions:

// DbHelper.kt - Generic query function using lambdas
fun <T> query(
    sql: String,
    prepare: (PreparedStatement) -> Unit,
    map: (ResultSet) -> T
): T {
    Database.connect().use { connection ->
        connection.prepareStatement(sql).use { ps ->
            prepare(ps)
            ps.executeQuery().use { rs ->
                return map(rs)
            }
        }
    }
}

Key Features

• User Management: Registration, login with BCrypt password hashing, profile updates
• Transaction Tracking: Create, read, update, and delete financial transactions
• Budget Management: Set and monitor spending budgets by category
• RESTful API: Clean REST endpoints for all operations
• Thread-Safe Demo Data: Concurrent data seeding with proper synchronization
• PostgreSQL Integration: Direct JDBC usage with custom SQL queries
• CORS Support: Cross-origin resource sharing for frontend integration

API Endpoints

Users

• POST /users - Create new user
• GET /users/login - Authenticate user
• PUT /users/{id} - Update user profile

Transactions

• POST /transactions - Create transaction
• GET /transactions/{userId} - Get all transactions for a user
• PUT /transactions/{id} - Update transaction
• DELETE /transactions/{id} - Delete transaction

Budgets

• POST /budgets - Create budget
• GET /budgets/{userId} - Get all budgets for a user
• PUT /budgets/{id} - Update budget
• DELETE /budgets/{id} - Delete budget

Database Schema

The application uses PostgreSQL with the following custom types and tables:

Custom ENUM Types

• transaction_type: 'expense' | 'income'
• period_type: 'daily' | 'weekly' | 'monthly' | 'yearly'

Tables

• users: User accounts with authentication
• transactions: Financial transaction records
• budgets: Budget limits and tracking

Building & Running

To build or run the project, use one of the following tasks:

Task	Description
./gradlew test	Run the tests
./gradlew build	Build everything
./gradlew run	Run the server

If the server starts successfully, you'll see the following output:

2024-12-04 14:32:45.584 [main] INFO  Application - Application started in 0.303 seconds.
2024-12-04 14:32:45.682 [main] INFO  Application - Responding at http://0.0.0.0:8080