[BE] - Nguyen Chi Thuc

[chithuchcmus]

Description

This PR adds solutions for three coding challenge problems, covering algorithmic implementation, REST API development, and system design.

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Problem 4: Sum to N (TypeScript)

Three unique implementations of a sum_to_n function that computes the summation from 1 to n for any integer input.

Approach	Time Complexity	Space Complexity
Iterative (for loop)	O(n)	O(1)
Recursive (trampoline)	O(n)	O(1)
Mathematical (Gauss's formula with BigInt)	O(1)	O(1)

• Supports positive, zero, and negative integers
• Trampoline-based recursion avoids stack overflow for large inputs
• BigInt used in formula approach to prevent intermediate overflow
• 21 test cases covering edge cases and large numbers

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Problem 5: Product CRUD REST API

A production-ready RESTful API for managing products, built with Express.js, TypeScript, and PostgreSQL.

Features:

• Full CRUD: Create, Read (with filters), Update (partial), Delete (soft delete)
• Layered architecture: Router → Middleware → Controller → Service → Repository
• Zod validation with centralized middleware
• Dependency injection with interfaces (IProductService, IProductRepository)
• Docker + Docker Compose setup (app + PostgreSQL)
• pino logging with request ID correlation
• Graceful shutdown handling
• Pagination with separate COUNT query for performance
• Optimized PostgreSQL indexes (partial indexes, trigram search)

API Endpoints:

• POST /v1/products — Create product
• GET /v1/products — List with filters (category, price range, search, pagination)
• GET /v1/products/:id — Get by ID
• PATCH /v1/products/:id — Partial update
• DELETE /v1/products/:id — Soft delete

Testing: Unit tests (service layer with mocked repo) + integration tests (controller layer with Supertest)

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Problem 6: Real-Time Leaderboard System Design

A comprehensive system design document for a real-time leaderboard service supporting 1M MAU, 10K TPS, and sub-100ms query latency.

Documents included:

• SYSTEM_DESIGN.md — Complete architecture, data models, API specs, and design decisions
• SEQUENCES.md — 7 Mermaid sequence diagrams for key flows
• SERVICES.md — Microservices overview and scaling strategy

Architecture (3 microservices):

1. Score Update Service — Validates actions, publishes to Kafka (202 Accepted)
2. Event Processor Service — Consumes Kafka events, updates Redis + ClickHouse
3. Leaderboard Service — Serves queries (HTTP) and live updates (SSE)

Key design decisions:

• Kafka for durable event streaming with replay capability
• Redis Sorted Sets for sub-5ms leaderboard queries
• ClickHouse for long-term analytics with materialized views
• SSE with selective broadcasting (90% bandwidth reduction)
• Horizontal scaling with Redis pub/sub fan-out

Future optimizations documented: Event batching, priority-based tiers, distributed tracing, graceful degradation