LetsCode Backend

Microservices backend for LetsSolve — a coding practice platform where users solve problems, submit code, and receive real-time evaluation results.

Architecture

flowchart LR
  User([User / Frontend])

  subgraph Backend
    Problem[Problem Service<br/>:3000]
    Submission[Submission Service<br/>:3001]
    Evaluator[Evaluator Service<br/>:3002]
    Socket[Socket Service<br/>:3003]
  end

  Mongo[(MongoDB Atlas)]
  Redis[(Redis / BullMQ)]
  Docker[[Docker containers<br/>Python · Java · C++]]

  User -->|REST| Problem
  User -->|POST submission| Submission
  User <-->|Socket.IO| Socket

  Submission -->|fetch problem + tests| Problem
  Problem --> Mongo
  Submission --> Mongo

  Submission -->|enqueue job| Redis
  Redis -->|consume job| Evaluator
  Evaluator -->|run code| Docker
  Evaluator -->|enqueue result| Redis
  Redis -->|result worker| Submission
  Submission -->|sendPayload| Socket
  Socket -->|submissionPayloadResponse| User

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Services

Service	Stack	Description
lets-solve-problem-service	Express, MongoDB	Admin API to create and manage coding problems
letsSolve-submission-service	Fastify, MongoDB, BullMQ	Accepts code submissions and orchestrates evaluation
LetsCode-Evaluator-Service	TypeScript, Express, Docker, BullMQ	Runs submitted code in isolated containers
letsCode-Socket-Service	Express, Socket.IO, Redis	Pushes evaluation results to clients in real time

Prerequisites

• Node.js 18+
• MongoDB (local or Atlas)
• Redis
• Docker (required for the Evaluator Service)

Getting Started

Each service runs independently. Clone the repo, then set up and start each service in its own directory.

1. Problem Service

cd lets-solve-problem-service
npm install

Create a .env file:

PORT=3000
ATLAS_DB_URL=<mongodb-connection-string>
LOG_DB_URL=<mongodb-log-db-url>   # optional

npm run dev

Optionally seed starter problems:

npm run seed

Health check: GET /ping

2. Submission Service

cd letsSolve-submission-service
npm install

Create a .env file:

PORT=3001
ATLAS_DB_URL=<mongodb-connection-string>
REDIS_HOST=127.0.0.1
REDIS_PORT=6379
PROBLEM_ADMIN_SERVICE_URL=http://localhost:3000

npm run dev

API: POST /api/v1/submissions

3. Evaluator Service

cd LetsCode-Evaluator-Service
npm install

Create a .env file:

PORT=3002
REDIS_HOST=127.0.0.1
REDIS_PORT=6379

npm run dev

API: POST /api/v1/submission

4. Socket Service

cd letsCode-Socket-Service
npm install
npm run dev   # listens on port 3003

API Overview

Problem Service (/api/v1/problems)

Method	Endpoint	Description
GET	/	List all problems
GET	/:id	Get a problem by ID
POST	/	Create a problem
PUT	/:id	Update a problem
DELETE	/:id	Delete a problem

Submission Service (/api/v1/submissions)

Method	Endpoint	Description
POST	/	Submit code for evaluation

Evaluator Service (/api/v1)

Method	Endpoint	Description
GET	/ping	Health check
POST	/submission	Queue a submission for evaluation

Supported Languages

• Python
• Java
• C++ (partial support)

Project Structure

LetsCodeBackend/
├── lets-solve-problem-service/   # Problem CRUD & test cases
├── letsSolve-submission-service/ # Submission handling & queueing
├── LetsCode-Evaluator-Service/   # Code execution via Docker
└── letsCode-Socket-Service/      # Real-time result delivery

License

ISC