PayNest

A simplified fintech/ecommerce platform for teaching Java. This project simulates a commerce backend where merchants can create products, accept customer orders, and process payments.

Project Overview

PayNest is a fictional platform that allows merchants to:

• Create products with names and prices
• Accept customer orders
• Process payments via multiple payment methods (card, EFT, wallet)

The codebase is designed for beginner–intermediate Java students and will be extended through multiple capstones. Later capstones add persistence and optional AI-assisted monitoring; early capstones use plain Java with clear packages.

Company Background

PayNest is a fictional fintech company providing a simplified commerce backend. The platform enables merchants to manage products, handle customer orders, and accept various payment types. This project represents a teaching simulation of such a system.

How to Run the Project

Prerequisites

• Java 21
• Maven 3.6+
• Capstone 5 only: Ollama installed locally (ollama serve), with a small model pulled (for example ollama pull llama3.2)

Build and Run

# Compile the project
mvn compile

# Run unit tests
mvn test

# Run the application
mvn exec:java

Alternatively:

mvn compile exec:java -Dexec.mainClass="com.paynestsystem.app.PayNestApplication"

Expected Output

Order Summary
Customer: John Smith

Items:
Laptop x1 - R12000
Mouse x2 - R400

Total: R12400

Payment successful via CARD
Amount: R12400
Order completed successfully.

The default PayNestApplication demonstrates Capstones 1–2 only. Capstones 3–5 are exercised from tests or your own main methods.

Capstone assessments (authoritative briefs)

Learner-facing project briefs, deliverables, and rubrics live under docs/assessments/. Use them as the assignment specification; this README stays focused on build, layout, and quick orientation.

Capstone	Brief
1	Merchant order desk and catalogue engine
2	Unified checkout across payment rails
3	Smart rails selection and transaction risk
4	Durable payment attempts and operations-grade reliability
5	Near-real-time risk monitoring with local LLM assistance

Scaffolding note: Capstones 3–5 ship with interfaces, skeleton implementations, and TODOs in code—complete behaviour to the brief, not only the stubs.

Architecture (capstones 3–5)

flowchart LR
  subgraph cap4 [Capstone4]
    Pipeline[ReliablePipeline]
    Store[TransactionRecordStore]
    Idem[IdempotencyRegistry]
    Reports[ReportGenerator]
  end
  subgraph cap3 [Capstone3]
    Router[RoutingEngine]
    Risk[RiskEvaluator]
  end
  subgraph cap5 [Capstone5]
    Queue[EventQueue]
    Ollama[OllamaClient]
    Hybrid[HybridRisk]
    AiLog[AiDecisionStore]
  end
  Queue --> Pipeline
  Pipeline --> Idem
  Pipeline --> Router
  Pipeline --> Risk
  Hybrid --> Risk
  Hybrid --> Ollama
  Pipeline --> Store
  Hybrid --> AiLog
  Store --> Reports

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Learning Objectives

Objectives are summarized here; each assessment brief defines outcomes and grading in detail.

• Capstone 1: Classes, objects, constructors, encapsulation, collections (List), basic business logic
• Capstone 2: Interfaces, inheritance, polymorphism, dependency design, basic architecture
• Capstone 3: Layered design, strategy-style routing, risk scoring, configuration placeholders, extending interfaces without a finished rules engine
• Capstone 4: Persistence, idempotency, lifecycle state, operational reporting, failure handling
• Capstone 5: Hybrid risk assessment, external HTTP integration, defensive parsing, queued monitoring, audit trails

Project Structure

src/main/java/com/paynestsystem/
├── domain/           # Product, Customer, OrderItem, Order, Transaction, TransactionStatus,
│                     # TransactionRecord, AiDecisionRecord
├── service/          # OrderService
├── payment/          # PaymentMethod, implementations, PaymentProcessor
├── routing/          # RoutingEngine, DefaultRoutingEngine, RouteDecision, DecisionLogger
├── rules/            # RoutingRule, AbstractRoutingRule, example rules
├── providers/        # PaymentProvider, BasePaymentProvider, ProviderA, ProviderB
├── risk/             # RiskLevel, RiskEvaluator, BasicRiskEvaluator, AiResponseParser, HybridRiskEvaluator
├── config/           # RoutingConfig
├── persistence/      # TransactionRecordStore, IdempotencyRegistry, AiDecisionStore; InMemory* stubs
├── persistence/jdbc/ # H2Schema DDL placeholders
├── reliability/      # ReliableTransactionPipeline, PipelineResult
├── reporting/        # ReportGenerator, OperationsReport, StubReportGenerator
├── ollama/           # OllamaConfig, OllamaClient, HttpOllamaClient, UnavailableOllamaClient
├── monitoring/       # RiskMonitoringService
└── app/              # PayNestApplication (Capstones 1–2 demo)