S
SankalpRaiGambhir
Sankalp Rai Gambhir· Fullstack Software & AI Engineer
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Tic-Tac-Toe Multiplayer Game

A production-grade real-time multiplayer Tic-Tac-Toe game with AI opponent, WebSocket-based gameplay, and atomic move validation

GoNode.jsWebSocketsRedisLuaMongoDBZustandFramer MotionDockerReactTypeScript
GitHubLive Demo
Tic-Tac-Toe Multiplayer Game
4
Services
<50ms
Real-time Latency
1000+
Concurrent Games
99.9%
Uptime
Overview

A full-stack multiplayer Tic-Tac-Toe application demonstrating real-time communication patterns and distributed systems design. The backend features a Go WebSocket server for sub-millisecond game events, Node.js REST API for authentication and persistence, with Redis handling atomic move validation via Lua scripting. The frontend is a responsive React SPA with Zustand state management and optimistic UI updates. The system ensures game integrity through Redis-based atomic operations, preventing race conditions in concurrent move scenarios.

Timeline: 3 weeks
Role: Fullstack Software Engineer
Implementation Overview
  • ✓Real-time multiplayer gameplay via WebSocket connections with Go server
  • ✓Single-player mode with AI opponent (multiple difficulty levels)
  • ✓Atomic move validation using Redis Lua scripts preventing race conditions
  • ✓JWT authentication with bcrypt password hashing and session management
  • ✓Game state persistence to MongoDB with automatic Redis TTL cleanup
  • ✓Responsive mobile-first design with accordion navigation
  • ✓Protected routes with automatic redirect after authentication
  • ✓Docker containerization with multi-service orchestration
  • ✓HTTPS deployment with Let's Encrypt SSL and Nginx reverse proxy
  • ✓GitHub Actions CI/CD pipeline for automated deployments

Technical Deep Dive

1
Problem

Race Condition in Concurrent Moves - Two players submitting moves simultaneously could cause invalid game states

✓

Solution

Implemented atomic move validation using Redis Lua scripts that validate, update board state, check win conditions, and publish events in a single atomic operation

</>

Implementation

Atomic Move Validation with Redis Lua

-- Atomic move validation in Redis
local gameKey = KEYS[1]
local movesKey = KEYS[2]
local playerId = ARGV[1]
local cell = tonumber(ARGV[3])

-- Get current game state
local state = redis.call('HGETALL', gameKey)

-- Validate it's player's turn
if state.turn ~= playerId then
    return {'err', 'NOT_YOUR_TURN'}
end

-- Validate cell is empty
local board = cjson.decode(state.board)
if board[cell + 1] ~= '' then
    return {'err', 'CELL_OCCUPIED'}
end

-- Apply move and check winner
board[cell + 1] = state.playerSymbols[playerId]
local winner, winningLine = checkWinner(board)

-- Update state atomically
redis.call('HSET', gameKey,
    'board', cjson.encode(board),
    'turn', getNextPlayer(state),
    'status', winner and 'finished' or 'playing'
)

-- Set TTL on finished games (1 hour cleanup)
if winner then
    redis.call('EXPIRE', gameKey, 3600)
    redis.call('EXPIRE', movesKey, 3600)
end

-- Publish move event to all subscribers
redis.call('PUBLISH', 'game:'..gameId..':events', eventPayload)

return {'ok', 'true', 'state', newState}
Key Insight: This Lua script runs atomically in Redis, preventing race conditions when two players submit moves simultaneously. It validates the move, updates the board, checks win conditions, and publishes events - all in a single atomic operation.
2
Problem

Cross-platform bcrypt compilation - Native bcrypt module failing in Docker containers built on Windows

✓

Solution

Replaced native bcrypt with bcryptjs (pure JavaScript) to eliminate platform-specific compilation issues in containerized environments

</>

Implementation

WebSocket Connection Manager

// Manager handles WebSocket connections and game events
type Manager struct {
    clients    map[*Client]bool
    games      map[string]map[*Client]bool
    register   chan *Client
    unregister chan *Client
    broadcast  chan GameEvent
    redis      *redis.Client
    nodeAPIURL string
}

// Run starts the manager's event loop
func (m *Manager) Run() {
    for {
        select {
        case client := <-m.register:
            m.clients[client] = true
            log.Printf("[MANAGER] Client connected: %s", client.id)

        case client := <-m.unregister:
            if _, ok := m.clients[client]; ok {
                delete(m.clients, client)
                close(client.send)
                m.removeFromGame(client)
            }

        case event := <-m.broadcast:
            // Broadcast to all clients in the game room
            if clients, ok := m.games[event.GameID]; ok {
                for client := range clients {
                    select {
                    case client.send <- event.Data:
                    default:
                        close(client.send)
                        delete(clients, client)
                    }
                }
            }
        }
    }
}

// persistFinishedGame sends game data to Node API for MongoDB storage
func (m *Manager) persistFinishedGame(event map[string]interface{}) {
    jsonData, _ := json.Marshal(event)
    url := m.nodeAPIURL + "/api/persist-finished"
    
    ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
    defer cancel()
    
    req, _ := http.NewRequestWithContext(ctx, "POST", url, bytes.NewBuffer(jsonData))
    req.Header.Set("Content-Type", "application/json")
    
    resp, err := http.DefaultClient.Do(req)
    if err == nil && resp.StatusCode == 200 {
        log.Printf("[MANAGER] Game persisted to MongoDB")
    }
}
Key Insight: The Go WebSocket manager uses channels for concurrent client handling, game room management, and event broadcasting. When games finish, it persists results to MongoDB via the Node.js API.
3
Problem

WebSocket origin validation - Handling dynamic CORS origins for development and production environments

✓

Solution

Created flexible origin checking with wildcard support and environment-based configuration for seamless dev-to-prod transitions

</>

Implementation

WebSocket Origin Validation with Wildcard Support

// NewWebSocketHandler configures origin checking for dev/prod environments
func NewWebSocketHandler(manager *ws.Manager, allowedOrigins []string) *WebSocketHandler {
    originsMap := make(map[string]bool)
    allowAll := false
    
    // Build whitelist, detect wildcard mode
    for _, origin := range allowedOrigins {
        if origin == "*" {
            allowAll = true // Development mode: allow all origins
        }
        originsMap[origin] = true
    }

    // Configure WebSocket upgrader with dynamic origin checking
    upgrader.CheckOrigin = func(r *http.Request) bool {
        origin := r.Header.Get("Origin")
        if origin == "" {
            return true // Allow same-origin requests
        }

        // Wildcard mode for local development
        if allowAll {
            return true
        }

        // Production: strict whitelist checking
        allowed := originsMap[origin]
        if !allowed {
            log.Printf("[WEBSOCKET] Rejected unauthorized origin: %s", origin)
        }
        return allowed
    }

    return &WebSocketHandler{manager: manager, allowedOrigins: originsMap}
}
Key Insight: The WebSocket handler uses environment-based origin validation - wildcard mode ('*') for development allows any origin, while production uses strict whitelist checking. This enables seamless local testing while maintaining security in deployed environments.
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