Judge0 Setup

Judge0 is the code execution engine for acodeaday. It runs user code in isolated Docker containers for security.

What is Judge0?

Judge0 CE (Community Edition) is an open-source online code execution system. It:

• Executes code in 60+ programming languages
• Runs code in isolated Docker containers
• Provides CPU and memory limits
• Returns execution results (stdout, stderr, runtime, memory)

Prerequisites

• Docker installed
• Docker Compose installed

See Prerequisites for installation instructions.

Setup via Docker Compose

Judge0 is configured in the root docker-compose.yml file along with other services.

1. Start Judge0 Services

From the project root directory:

# Start all Judge0 services
docker compose up -d judge0-server judge0-workers judge0-db judge0-redis

# Or start all services including backend/frontend
docker compose up -d

This will:

• Pull necessary Docker images (first time only, ~2-3 GB)
• Start all Judge0 services in detached mode
• Expose the API on http://localhost:2358

2. Wait for Initialization

Judge0 workers need ~30 seconds to initialize. Check status:

docker compose ps

All services should show Up status:

NAME                        STATUS
acodeaday-judge0-server     Up
acodeaday-judge0-workers    Up
acodeaday-judge0-db         Up (healthy)
acodeaday-judge0-redis      Up (healthy)

3. Verify Judge0 is Running

Test the API:

curl http://localhost:2358/about

You should see JSON response with Judge0 version info:

{
  "version": "1.13.1",
  "homepage": "https://judge0.com",
  ...
}

Configuration

Docker Compose Configuration

Judge0 configuration is in docker-compose.yml at the project root. Key settings:

judge0-server:
  image: judge0/judge0:1.13.1
  environment:
    # Resource limits
    - MAX_CPU_TIME_LIMIT=15        # seconds
    - MAX_WALL_TIME_LIMIT=30       # seconds
    - MAX_MEMORY_LIMIT=512000      # KB (512 MB)
    - MAX_STACK_LIMIT=128000       # KB (128 MB)
    - MAX_PROCESSES_AND_OR_THREADS=60

    # Queue settings
    - MAX_QUEUE_SIZE=1000
    - ENABLE_WAIT_RESULT=true

Environment Variable

Set the Judge0 URL in your .env file:

JUDGE0_URL=http://localhost:2358

For Docker deployment, the backend connects via the Docker network:

JUDGE0_URL=http://judge0-server:2358

Testing Judge0

Test Python Execution

Submit a test Python program:

curl -X POST "http://localhost:2358/submissions?base64_encoded=false&wait=true" \
  -H "Content-Type: application/json" \
  -d '{
    "source_code": "print(\"Hello from Judge0!\")",
    "language_id": 71
  }'

You should get a response like:

{
  "stdout": "Hello from Judge0!\n",
  "status": {"id": 3, "description": "Accepted"},
  "time": "0.02",
  "memory": 1024
}

Language IDs

Common languages supported:

Language	ID
Python 3	71
JavaScript (Node.js)	63
Java	62
C++	54
Go	60
Rust	73

Get all supported languages:

curl http://localhost:2358/languages

Integration with Backend

The backend communicates with Judge0 via synchronous HTTP requests:

# backend/app/services/judge0.py
class Judge0Service:
    def __init__(self):
        self.base_url = settings.judge0_url

    def execute_code(self, source_code: str, language: str, stdin: str = "") -> dict:
        response = httpx.post(
            f"{self.base_url}/submissions?base64_encoded=false&wait=true",
            json={
                "source_code": source_code,
                "language_id": LANGUAGE_MAP[language],
                "stdin": stdin,
            },
            timeout=30.0,
        )
        return response.json()

Common Commands

# Start Judge0 services
docker compose up -d judge0-server judge0-workers judge0-db judge0-redis

# Stop Judge0 services
docker compose stop judge0-server judge0-workers judge0-db judge0-redis

# View logs
docker compose logs judge0-server
docker compose logs judge0-workers

# Restart services
docker compose restart judge0-server judge0-workers

# Remove all data (WARNING: deletes all Judge0 data)
docker compose down -v

Troubleshooting

Services not starting

# Check logs for errors
docker compose logs judge0-server

# Ensure ports are not in use
lsof -i :2358

"Connection refused" errors

1. Wait 30 seconds after docker compose up for initialization
2. Check services are running: docker compose ps
3. Verify firewall isn't blocking port 2358

Submissions stuck in "Processing"

# Check worker logs
docker compose logs judge0-workers

# Restart workers
docker compose restart judge0-workers

High memory usage

Judge0 workers cache language environments. This is normal. To reduce memory:

1. Reduce worker processes in the worker command
2. Set stricter memory limits in environment variables

Permission errors

# Fix volume permissions
sudo chown -R $USER:$USER ./judge0-data

Coming Soon

Support for hosted Judge0 services (like RapidAPI) is planned for a future release. This will allow using Judge0 without self-hosting.

Next Steps

• Backend Setup - Configure FastAPI to use Judge0
• Adding Problems - Create problems with test cases
• Environment Variables - Configure all env vars