Building Real-Time 1v1 Code Battle Arenas with WebSockets & Judge0

Competitive coding platforms have traditionally relied on asynchronous submission queues where users submit code and wait seconds for test suite execution. At CosmoDex, we wanted to create an adrenaline-fueled, real-time 1v1 battle experience where two developers duel head-to-head on identical algorithmic challenges with live opponent progress.

In this deep dive, we break down our real-time WebSocket architecture, low-latency code execution engine, and anti-cheat state verification pipelines.

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⚡ The Architecture at a Glance

When two players click "Find Opponent", our matchmaking engine pairs them based on Elo combat rank and establishes a persistent bi-directional WebSocket pipeline:

[ Client Player 1 ] <---> [ Node WebSocket Relay ] <---> [ Client Player 2 ]
                                 │
                                 ▼
                     [ Judge0 Isolated Execution Cluster ]

Key Architectural Requirements:

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🔒 Sandboxing Submissions with Judge0

When a player hits Run Tests or Submit Solution, the payload is dispatched to our Judge0 engine cluster over high-speed RPC:

{
  "source_code": "def solve(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        diff = target - num
        if diff in seen:
            return [seen[diff], i]
        seen[num] = i
    return []",
  "language_id": 71,
  "stdin": "[2, 7, 11, 15]
9",
  "expected_output": "[0, 1]"
}

Execution Rules Enforced:

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🚀 Optimized WebSocket Broadcast Engine

To ensure seamless 60 FPS visual feedback during battles without overloading network bandwidth, we implement delta compression for live cursor position and milestone events:

export function broadcastMilestone(roomId: string, player: string, milestoneIndex: number) {
  const payload = JSON.stringify({
    type: 'OPPONENT_PROGRESS',
    player,
    milestoneIndex,
    timestamp: Date.now(),
  });
  
  wsServer.to(roomId).emit('game_event', payload);
}

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🎯 What's Next?

We are currently testing Multi-Player Squad Battles (4v4 Team Arenas) and expanding our Judge0 cluster nodes to support Rust and C++20.

Stay tuned for our upcoming developer blog on Designing Anti-Cheat Algorithmic Verification Systems!