💳 Payment System

Overview

This project is a backend payment system inspired by eSewa/Khalti, built with a focus on:

• Financial correctness
• Distributed systems design
• Backend interview readiness

It implements real-world payment system patterns such as:

• Double-entry ledger
• Idempotent transactions
• Saga-based workflows
• Event-driven architecture

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🚀 Tech Stack

Core Backend

• Node.js (TypeScript)
• Fastify (high-performance HTTP framework)
• Prisma ORM
• PostgreSQL (ACID-compliant financial storage)

Infrastructure

• Docker & Docker Compose
• Redis (caching, idempotency, rate limiting)
• Kafka (event-driven communication)
• MinIO (object storage for KYC)

Observability

• Pino (structured logging)
• Prometheus (metrics)
• OpenTelemetry (tracing)

Testing

• Jest (unit + integration)
• Testcontainers (real DB in tests)
• k6 (load testing)

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🧠 Key Architecture Decisions

1. Modular Monolith

• Clear domain boundaries
• Easier to develop within 1 month
• Future-ready for microservices

2. Double-Entry Ledger

• Every transaction = debit + credit
• Immutable ledger entries
• Financial correctness guaranteed

3. Idempotency

• Prevents duplicate payments
• Redis + DB-level enforcement

4. Integer Money Storage

• All money stored as BIGINT (paisa)
• Avoids floating point errors

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📦 Project Structure

• apps/api → Main backend service
• apps/worker → Kafka consumers
• packages/shared-types → Shared DTOs
• infra/ → Docker, Terraform, K8s
• docs/ → Architecture & ADRs

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⚙️ Getting Started

1. Clone Repo

git clone https://github.com/your-team/esewa-clone
cd esewa-clone

2. Install Dependencies

pnpm install

3. Start Infrastructure

docker-compose up -d

4. Run Migrations

cd apps/api
pnpm prisma migrate dev

5. Seed Data

pnpm run seed

6. Start Server

pnpm run dev

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🧪 Testing

pnpm run test                # Unit tests
pnpm run test:integration    # Integration tests
pnpm run test:coverage       # Coverage report

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📊 Load Testing

pnpm run load-test

Target:

• p99 latency < 500ms
• zero double charges

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🔐 Security

• JWT RS256 authentication
• bcrypt password + PIN hashing
• HMAC request signing (merchant APIs)
• Rate limiting (Redis)
• TLS enforced

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📡 Core Features

• User registration + KYC
• Wallet management
• Payment processing
• Merchant integration
• Webhooks with retry
• Refund system
• Ledger reconciliation

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🧠 Interview Topics Covered

You should be able to explain:

• Idempotency
• Optimistic locking
• Saga pattern
• Double-entry accounting
• Event-driven architecture
• ACID vs eventual consistency

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🚧 Scope Limitations

• No frontend (API-first)
• Single currency (NPR)
• No real banking integration

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👥 Team Roles

• Identity & Auth
• Wallet & Balance
• Transactions & Idempotency
• Ledger & Reconciliation

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📄 License

MIT