Introduction

Roblox is a user-generated content platform where users can create, publish, and play games and experiences. The platform hosts millions of user-created experiences, supports real-time multiplayer gameplay, provides creator tools (Roblox Studio), and includes social features, monetization, and content moderation. Designing Roblox requires handling massive scale: millions of daily active users, billions of hours of engagement, real-time multiplayer with low latency, and petabytes of user-generated content.

This post provides a detailed walkthrough of designing Roblox, covering key features, scalability challenges, and architectural decisions. This is a common system design interview question that tests your understanding of distributed systems, real-time multiplayer, user-generated content platforms, game engines, social features, and content moderation.

Table of Contents

  1. Problem Statement
  2. Requirements
  3. Capacity Estimation
  4. Core Entities
  5. API
  6. Data Flow
  7. Database Design
  8. High-Level Design
  9. Deep Dive
  10. Summary

Problem Statement

Design Roblox, a user-generated content platform for games and experiences with the following features:

  1. User accounts and authentication
  2. Create and publish games/experiences
  3. Real-time multiplayer gameplay
  4. 3D rendering and physics simulation
  5. Social features (friends, chat, groups)
  6. Creator tools (Roblox Studio)
  7. Asset marketplace (models, clothing, accessories)
  8. Monetization (Robux currency, developer exchange)
  9. Content discovery and recommendation
  10. Content moderation and safety

Scale Requirements:

  • 200 million+ monthly active users
  • 70 million+ daily active users
  • 10 million+ concurrent users
  • Millions of user-created experiences
  • Billions of hours of engagement per month
  • Petabytes of user-generated content
  • Real-time multiplayer with < 100ms latency
  • Must support cross-platform (PC, mobile, console)

Requirements

Functional Requirements

Core Features:

  1. User Management: Registration, authentication, profiles
  2. Experience Creation: Create games using Roblox Studio
  3. Experience Publishing: Publish and host experiences
  4. Real-Time Multiplayer: Support multiple players in same experience
  5. 3D Rendering: Render 3D graphics and environments
  6. Physics Simulation: Real-time physics calculations
  7. Social Features: Friends, chat, groups, follow creators
  8. Asset Marketplace: Buy/sell models, clothing, accessories
  9. Monetization: Robux currency, in-experience purchases, developer exchange
  10. Discovery: Browse, search, and discover experiences
  11. Content Moderation: Automated and manual moderation
  12. Cross-Platform: Support PC, mobile, console

Out of Scope:

  • Mobile app development (focus on backend/platform)
  • Specific game mechanics (focus on platform)
  • Payment processing (assume payment gateway)
  • Video streaming (focus on real-time gameplay)

Non-Functional Requirements

  1. Availability: 99.9% uptime
  2. Reliability: No data loss, consistent gameplay
  3. Performance:
    • Game server latency: < 100ms
    • Experience load time: < 5 seconds
    • Asset download: < 2 seconds
    • Chat message delivery: < 200ms
  4. Scalability: Handle 10M+ concurrent users
  5. Consistency: Strong consistency for transactions, eventual for social
  6. Security: Protect user data, prevent cheating, content moderation
  7. Cross-Platform: Support multiple platforms seamlessly

Capacity Estimation

Traffic Estimates

  • Monthly Active Users (MAU): 200 million
  • Daily Active Users (DAU): 70 million
  • Peak Concurrent Users: 10 million
  • Experiences: 50 million+
  • Daily Experience Launches: 100 million+
  • Chat Messages per Day: 1 billion+
  • Asset Downloads per Day: 500 million+
  • Peak Requests per Second: 500,000+
  • Average Session Duration: 2.5 hours

Storage Estimates

User Data:

  • 200M users × 5KB = 1GB
  • User profiles, preferences, settings

Experience Data:

  • 50M experiences × 10MB average = 500TB
  • Game code, assets, configurations

User-Generated Assets:

  • 1B assets × 1MB average = 1PB
  • Models, clothing, accessories, decals

Chat Messages:

  • 1B messages/day × 200 bytes = 200GB/day
  • 30-day retention: ~6TB
  • 1-year archive: ~73TB

Gameplay Data:

  • 10M concurrent × 1KB/sec × 3600 sec = 36TB/hour
  • Real-time state, not persisted

Total Storage: ~1.5PB

Bandwidth Estimates

Normal Traffic:

  • 10M concurrent × 50KB/s = 500GB/s = 4Tbps
  • Game state updates, asset streaming

Peak Traffic:

  • 10M concurrent × 100KB/s = 1TB/s = 8Tbps

Asset Downloads:

  • 500M downloads/day × 1MB = 500TB/day = ~5.8GB/s = ~46Gbps

Total Peak: ~8Tbps

Core Entities

User

  • user_id (UUID)
  • username (VARCHAR, unique)
  • display_name (VARCHAR)
  • email (VARCHAR)
  • password_hash (VARCHAR)
  • robux_balance (INT)
  • avatar_data (JSON)
  • created_at (TIMESTAMP)
  • updated_at (TIMESTAMP)

Experience (Game)

  • experience_id (UUID)
  • creator_id (user_id)
  • title (VARCHAR)
  • description (TEXT)
  • thumbnail_url (VARCHAR)
  • genre (VARCHAR)
  • max_players (INT)
  • is_public (BOOLEAN)
  • visits (BIGINT)
  • likes (BIGINT)
  • favorites (BIGINT)
  • created_at (TIMESTAMP)
  • updated_at (TIMESTAMP)

Experience Asset

  • asset_id (UUID)
  • experience_id (UUID)
  • asset_type (model, script, sound, texture, etc.)
  • asset_url (VARCHAR)
  • file_size (BIGINT)
  • version (INT)
  • created_at (TIMESTAMP)

Game Server Instance

  • server_id (UUID)
  • experience_id (UUID)
  • server_ip (VARCHAR)
  • server_port (INT)
  • current_players (INT)
  • max_players (INT)
  • region (VARCHAR)
  • status (active, full, closing)
  • created_at (TIMESTAMP)

Player Session

  • session_id (UUID)
  • user_id (UUID)
  • experience_id (UUID)
  • server_id (UUID)
  • joined_at (TIMESTAMP)
  • left_at (TIMESTAMP)
  • play_duration (INT, seconds)

Friend Relationship

  • friendship_id (UUID)
  • user_id (UUID)
  • friend_id (UUID)
  • status (pending, accepted, blocked)
  • created_at (TIMESTAMP)

Chat Message

  • message_id (UUID)
  • user_id (UUID)
  • experience_id (UUID)
  • server_id (UUID)
  • message_text (TEXT)
  • message_type (chat, whisper, system)
  • moderation_status (pending, approved, rejected)
  • created_at (TIMESTAMP)

Asset Marketplace Item

  • item_id (UUID)
  • creator_id (user_id)
  • item_type (model, clothing, accessory, gamepass)
  • title (VARCHAR)
  • description (TEXT)
  • price_robux (INT)
  • sales_count (INT)
  • is_approved (BOOLEAN)
  • created_at (TIMESTAMP)

Transaction

  • transaction_id (UUID)
  • user_id (UUID)
  • transaction_type (purchase, sale, refund)
  • item_id (UUID)
  • robux_amount (INT)
  • status (pending, completed, failed)
  • created_at (TIMESTAMP)

API

1. Launch Experience

POST /api/v1/experiences/{experience_id}/launch
Request:
{
  "user_id": "uuid",
  "region": "us-west"
}

Response:
{
  "server_id": "uuid",
  "server_ip": "192.168.1.100",
  "server_port": 53640,
  "join_token": "encrypted_token",
  "asset_urls": [
    "https://cdn.roblox.com/assets/..."
  ]
}

2. Join Game Server

POST /api/v1/servers/{server_id}/join
Request:
{
  "user_id": "uuid",
  "join_token": "encrypted_token"
}

Response:
{
  "session_id": "uuid",
  "server_connection_info": {
    "ip": "192.168.1.100",
    "port": 53640,
    "encryption_key": "..."
  },
  "current_players": 5,
  "max_players": 20
}

3. Send Chat Message

POST /api/v1/chat/messages
Request:
{
  "user_id": "uuid",
  "experience_id": "uuid",
  "server_id": "uuid",
  "message_text": "Hello, everyone!",
  "message_type": "chat"
}

Response:
{
  "message_id": "uuid",
  "status": "pending_moderation",
  "created_at": "2025-11-14T10:00:00Z"
}

4. Get Experience Details

GET /api/v1/experiences/{experience_id}
Response:
{
  "experience_id": "uuid",
  "title": "Adopt Me!",
  "creator": {
    "user_id": "uuid",
    "username": "DreamCraft"
  },
  "description": "...",
  "thumbnail_url": "...",
  "visits": 50000000,
  "likes": 1000000,
  "favorites": 500000,
  "max_players": 20,
  "genre": "simulation",
  "created_at": "2020-01-01T00:00:00Z"
}

5. Search Experiences

GET /api/v1/experiences/search?q=adopt&genre=simulation&limit=20&offset=0
Response:
{
  "experiences": [
    {
      "experience_id": "uuid",
      "title": "Adopt Me!",
      "creator": {...},
      "visits": 50000000,
      "thumbnail_url": "..."
    }
  ],
  "total": 100,
  "limit": 20,
  "offset": 0
}

6. Purchase Asset

POST /api/v1/marketplace/purchase
Request:
{
  "user_id": "uuid",
  "item_id": "uuid",
  "price_robux": 100
}

Response:
{
  "transaction_id": "uuid",
  "status": "completed",
  "new_robux_balance": 900,
  "purchased_item": {...}
}

7. Upload Experience Asset

POST /api/v1/experiences/{experience_id}/assets
Request:
{
  "asset_type": "model",
  "asset_data": "base64_encoded_data",
  "version": 1
}

Response:
{
  "asset_id": "uuid",
  "asset_url": "https://cdn.roblox.com/assets/...",
  "file_size": 1024000,
  "status": "uploaded"
}

Data Flow

Experience Launch Flow

  1. User Launches Experience:
    • User clicks “Play” on an experience
    • Client sends launch request to API Gateway
    • API Gateway routes to Experience Service
  2. Server Selection:
    • Experience Service:
      • Gets user’s region preference
      • Queries Game Server Manager for available servers
      • Selects server with capacity and lowest latency
      • Creates player session
  3. Asset Loading:
    • Experience Service:
      • Gets experience asset list
      • Generates CDN URLs for assets
      • Returns asset URLs to client
  4. Server Connection:
    • Client:
      • Downloads assets from CDN
      • Connects to game server
      • Establishes WebSocket connection
      • Starts gameplay
  5. Gameplay:
    • Game Server:
      • Handles game logic
      • Manages player state
      • Broadcasts updates to all players
      • Handles physics simulation

Real-Time Multiplayer Flow

  1. Player Action:
    • Player performs action (move, interact)
    • Client sends action to Game Server
  2. Server Processing:
    • Game Server:
      • Validates action
      • Updates game state
      • Runs physics simulation
      • Determines state changes
  3. State Broadcast:
    • Game Server:
      • Broadcasts state updates to all players
      • Uses efficient delta compression
      • Prioritizes critical updates
  4. Client Update:
    • Client receives state update
    • Updates local game state
    • Renders changes
    • Maintains smooth gameplay

Chat Message Flow

  1. User Sends Message:
    • User types message in chat
    • Client sends to Chat Service
  2. Moderation Check:
    • Chat Service:
      • Checks message against moderation filters
      • If flagged, queues for review
      • If safe, proceeds to delivery
  3. Message Delivery:
    • Chat Service:
      • Stores message in database
      • Broadcasts to all players in server
      • Updates chat history
  4. Real-Time Delivery:
    • WebSocket Service:
      • Pushes message to connected clients
      • Handles delivery failures
      • Maintains message order

Database Design

Schema Design

Users Table:

CREATE TABLE users (
    user_id UUID PRIMARY KEY,
    username VARCHAR(50) UNIQUE NOT NULL,
    display_name VARCHAR(100),
    email VARCHAR(100) UNIQUE NOT NULL,
    password_hash VARCHAR(255) NOT NULL,
    robux_balance INT DEFAULT 0,
    avatar_data JSON,
    created_at TIMESTAMP DEFAULT NOW(),
    updated_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_username (username),
    INDEX idx_email (email)
);

Experiences Table:

CREATE TABLE experiences (
    experience_id UUID PRIMARY KEY,
    creator_id UUID NOT NULL,
    title VARCHAR(200) NOT NULL,
    description TEXT,
    thumbnail_url VARCHAR(500),
    genre VARCHAR(50),
    max_players INT DEFAULT 20,
    is_public BOOLEAN DEFAULT TRUE,
    visits BIGINT DEFAULT 0,
    likes BIGINT DEFAULT 0,
    favorites BIGINT DEFAULT 0,
    created_at TIMESTAMP DEFAULT NOW(),
    updated_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_creator_id (creator_id),
    INDEX idx_genre (genre),
    INDEX idx_visits (visits DESC),
    FULLTEXT INDEX idx_title_description (title, description)
);

Game Server Instances Table:

CREATE TABLE game_server_instances (
    server_id UUID PRIMARY KEY,
    experience_id UUID NOT NULL,
    server_ip VARCHAR(50) NOT NULL,
    server_port INT NOT NULL,
    current_players INT DEFAULT 0,
    max_players INT DEFAULT 20,
    region VARCHAR(50) NOT NULL,
    status VARCHAR(20) DEFAULT 'active',
    created_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_experience_id (experience_id),
    INDEX idx_status (status),
    INDEX idx_region (region),
    INDEX idx_current_players (current_players)
);

Player Sessions Table (Sharded by user_id):

CREATE TABLE player_sessions_0 (
    session_id UUID PRIMARY KEY,
    user_id UUID NOT NULL,
    experience_id UUID NOT NULL,
    server_id UUID NOT NULL,
    joined_at TIMESTAMP DEFAULT NOW(),
    left_at TIMESTAMP NULL,
    play_duration INT DEFAULT 0,
    INDEX idx_user_id (user_id),
    INDEX idx_experience_id (experience_id),
    INDEX idx_server_id (server_id),
    INDEX idx_joined_at (joined_at DESC)
);
-- Similar tables: player_sessions_1, ..., player_sessions_N

Chat Messages Table (Sharded by experience_id):

CREATE TABLE chat_messages_0 (
    message_id UUID PRIMARY KEY,
    user_id UUID NOT NULL,
    experience_id UUID NOT NULL,
    server_id UUID NOT NULL,
    message_text TEXT NOT NULL,
    message_type VARCHAR(20) DEFAULT 'chat',
    moderation_status VARCHAR(20) DEFAULT 'pending',
    created_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_experience_id (experience_id),
    INDEX idx_server_id (server_id),
    INDEX idx_user_id (user_id),
    INDEX idx_created_at (created_at DESC),
    INDEX idx_moderation_status (moderation_status)
);
-- Similar tables: chat_messages_1, ..., chat_messages_N

Asset Marketplace Items Table:

CREATE TABLE marketplace_items (
    item_id UUID PRIMARY KEY,
    creator_id UUID NOT NULL,
    item_type VARCHAR(50) NOT NULL,
    title VARCHAR(200) NOT NULL,
    description TEXT,
    price_robux INT NOT NULL,
    sales_count INT DEFAULT 0,
    is_approved BOOLEAN DEFAULT FALSE,
    created_at TIMESTAMP DEFAULT NOW(),
    updated_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_creator_id (creator_id),
    INDEX idx_item_type (item_type),
    INDEX idx_price (price_robux),
    INDEX idx_sales_count (sales_count DESC),
    FULLTEXT INDEX idx_title_description (title, description)
);

Transactions Table (Sharded by user_id):

CREATE TABLE transactions_0 (
    transaction_id UUID PRIMARY KEY,
    user_id UUID NOT NULL,
    transaction_type VARCHAR(50) NOT NULL,
    item_id UUID,
    robux_amount INT NOT NULL,
    status VARCHAR(20) DEFAULT 'pending',
    created_at TIMESTAMP DEFAULT NOW(),
    INDEX idx_user_id (user_id),
    INDEX idx_status (status),
    INDEX idx_created_at (created_at DESC)
);
-- Similar tables: transactions_1, ..., transactions_N

Database Sharding Strategy

Player Sessions Table Sharding:

  • Shard by user_id using consistent hashing
  • 1000 shards: shard_id = hash(user_id) % 1000
  • All sessions for a user in same shard
  • Enables efficient user session queries

Chat Messages Table Sharding:

  • Shard by experience_id using consistent hashing
  • 1000 shards: shard_id = hash(experience_id) % 1000
  • All messages for an experience in same shard
  • Enables efficient experience chat queries

Transactions Table Sharding:

  • Shard by user_id using consistent hashing
  • 1000 shards: shard_id = hash(user_id) % 1000
  • All transactions for a user in same shard
  • Enables efficient user transaction queries

Shard Key Selection:

  • Ensures related data is co-located
  • Enables efficient queries
  • Prevents cross-shard queries for single entity

Replication:

  • Each shard replicated 3x for high availability
  • Master-replica setup for read scaling
  • Writes go to master, reads can go to replicas

High-Level Design

┌─────────────┐
│   Client    │
│ (PC/Mobile) │
└──────┬──────┘
       │
       │ HTTP/WebSocket
       │
┌──────▼──────────────────────────────────────────────┐
│        API Gateway / Load Balancer                   │
│        - Rate Limiting                               │
│        - Request Routing                             │
└──────┬──────────────────────────────────────────────┘
       │
       ├──────────────┬──────────────┬──────────────┬──────────────┐
       │              │              │              │              │
┌──────▼──────┐ ┌─────▼──────┐ ┌─────▼──────┐ ┌─────▼──────┐ ┌─────▼──────┐
│ Experience  │ │ Game Server │ │ Chat       │ │ Social     │ │ Marketplace│
│ Service     │ │ Manager     │ │ Service    │ │ Service    │ │ Service    │
└──────┬──────┘ └─────┬──────┘ └─────┬──────┘ └─────┬──────┘ └─────┬──────┘
       │              │              │              │              │
       │              │              │              │              │
┌──────▼──────────────▼──────────────▼──────────────▼──────────────▼──────┐
│              Message Queue (Kafka)                                        │
│              - Chat events                                                 │
│              - Game events                                                 │
│              - Transaction events                                          │
└──────┬───────────────────────────────────────────────────────────────────┘
       │
       │
┌──────▼───────────────────────────────────────────────────────────────────┐
│         Game Server Cluster                                               │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐                              │
│  │ Server   │  │ Server   │  │ Server   │                              │
│  │ Instance │  │ Instance │  │ Instance │                              │
│  │ 1        │  │ 2        │  │ N        │                              │
│  └──────────┘  └──────────┘  └──────────┘                              │
│  - Game logic                                                             │
│  - Physics simulation                                                     │
│  - State management                                                       │
│  - Real-time updates                                                      │
└──────┬───────────────────────────────────────────────────────────────────┘
       │
       │
┌──────▼───────────────────────────────────────────────────────────────────┐
│         Database Cluster (Sharded)                                         │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐                              │
│  │ Users    │  │ Experiences│ │ Sessions │                              │
│  │ DB       │  │ DB        │ │ (Sharded) │                              │
│  └──────────┘  └──────────┘  └──────────┘                              │
│                                                                           │
│  ┌──────────┐  ┌──────────┐  ┌──────────┐                              │
│  │ Chat     │  │ Marketplace│ │ Transactions│                            │
│  │ (Sharded)│  │ DB        │ │ (Sharded) │                              │
│  └──────────┘  └──────────┘  └──────────┘                              │
└───────────────────────────────────────────────────────────────────────────┘

┌───────────────────────────────────────────────────────────────────────────┐
│              CDN (CloudFront/Cloudflare)                                   │
│              - Experience assets                                           │
│              - User-generated content                                       │
│              - Static assets                                                │
└───────────────────────────────────────────────────────────────────────────┘

┌───────────────────────────────────────────────────────────────────────────┐
│              Object Storage (S3)                                           │
│              - Experience assets                                           │
│              - User-generated content                                       │
│              - Backup and archival                                          │
└───────────────────────────────────────────────────────────────────────────┘

┌───────────────────────────────────────────────────────────────────────────┐
│         Content Moderation Service                                         │
│         - Automated filtering                                              │
│         - Manual review queue                                              │
│         - Safety and compliance                                            │
└───────────────────────────────────────────────────────────────────────────┘

┌───────────────────────────────────────────────────────────────────────────┐
│         Search & Discovery Service                                         │
│         - Experience search                                                │
│         - Recommendation engine                                             │
│         - Trending and popular                                              │
└───────────────────────────────────────────────────────────────────────────┘

Deep Dive

Component Design

1. Game Server Manager

Responsibilities:

  • Manage game server instances
  • Allocate servers for experiences
  • Monitor server health and capacity
  • Handle server scaling

Key Design Decisions:

  • Dynamic Server Allocation: Allocate servers on demand
  • Regional Distribution: Servers in multiple regions
  • Auto-Scaling: Scale servers based on demand
  • Health Monitoring: Monitor server health and capacity

Implementation:

class GameServerManager:
    def __init__(self):
        self.server_pool = ServerPool()
        self.region_manager = RegionManager()
        self.load_balancer = LoadBalancer()
    
    def allocate_server(self, experience_id, region, max_players):
        """Allocate a game server for an experience"""
        # Check available servers in region
        available_servers = self.server_pool.get_available_servers(
            region=region,
            min_capacity=max_players
        )
        
        if not available_servers:
            # Scale up new server
            server = self.scale_up_server(experience_id, region, max_players)
        else:
            # Select server with lowest load
            server = self.select_best_server(available_servers)
        
        # Allocate server to experience
        server.allocate(experience_id, max_players)
        
        return server
    
    def select_best_server(self, servers):
        """Select server with lowest load"""
        return min(servers, key=lambda s: s.current_load)
    
    def scale_up_server(self, experience_id, region, max_players):
        """Scale up a new game server"""
        # Get server configuration
        config = self.get_server_config(experience_id)
        
        # Launch server instance
        server = self.server_pool.launch_server(
            region=region,
            config=config,
            max_players=max_players
        )
        
        # Wait for server to be ready
        server.wait_for_ready()
        
        return server

2. Real-Time Game Server

Responsibilities:

  • Handle game logic and state
  • Manage player connections
  • Broadcast state updates
  • Run physics simulation

Key Design Decisions:

  • State Management: Centralized game state on server
  • Delta Compression: Send only state changes
  • Update Frequency: 30-60 updates per second
  • Client Prediction: Allow client-side prediction

Implementation:

class GameServer:
    def __init__(self, experience_id, max_players):
        self.experience_id = experience_id
        self.max_players = max_players
        self.players = {}  # user_id -> Player
        self.game_state = GameState()
        self.physics_engine = PhysicsEngine()
        self.update_rate = 60  # updates per second
    
    def handle_player_action(self, user_id, action):
        """Handle player action"""
        player = self.players.get(user_id)
        if not player:
            return
        
        # Validate action
        if not self.validate_action(player, action):
            return
        
        # Update game state
        self.game_state.apply_action(player, action)
        
        # Run physics simulation
        self.physics_engine.step(self.game_state)
        
        # Broadcast update to all players
        self.broadcast_state_update()
    
    def broadcast_state_update(self):
        """Broadcast state update to all players"""
        # Get delta (changes since last update)
        delta = self.game_state.get_delta()
        
        # Compress delta
        compressed_delta = self.compress_delta(delta)
        
        # Send to all players
        for player in self.players.values():
            player.send_update(compressed_delta)
    
    def compress_delta(self, delta):
        """Compress state delta"""
        # Use delta compression
        # Only send changed values
        # Use efficient serialization
        return json.dumps(delta, separators=(',', ':'))

3. Chat Service

Responsibilities:

  • Handle chat messages
  • Content moderation
  • Real-time message delivery
  • Chat history management

Key Design Decisions:

  • Moderation Pipeline: Automated + manual review
  • Real-Time Delivery: WebSocket for instant delivery
  • Message Storage: Store for moderation and history
  • Rate Limiting: Prevent spam

Implementation:

class ChatService:
    def __init__(self):
        self.moderation_service = ModerationService()
        self.websocket_manager = WebSocketManager()
        self.message_queue = MessageQueue()
    
    def send_message(self, user_id, experience_id, server_id, message_text):
        """Send chat message"""
        # Rate limiting
        if not self.check_rate_limit(user_id):
            raise RateLimitError("Too many messages")
        
        # Content moderation
        moderation_result = self.moderation_service.check_message(message_text)
        
        if moderation_result.is_blocked:
            # Block message
            return {
                'status': 'blocked',
                'reason': moderation_result.reason
            }
        
        # Create message
        message = ChatMessage(
            user_id=user_id,
            experience_id=experience_id,
            server_id=server_id,
            message_text=message_text,
            moderation_status='approved' if moderation_result.is_safe else 'pending'
        )
        
        # Store message
        message.save()
        
        # If pending moderation, queue for review
        if message.moderation_status == 'pending':
            self.message_queue.publish('moderation_review', message)
        
        # Broadcast to server players
        self.broadcast_message(server_id, message)
        
        return {
            'message_id': message.message_id,
            'status': message.moderation_status
        }
    
    def broadcast_message(self, server_id, message):
        """Broadcast message to all players in server"""
        # Get all players in server
        players = self.get_server_players(server_id)
        
        # Send via WebSocket
        for player in players:
            self.websocket_manager.send_message(
                player.user_id,
                {
                    'type': 'chat_message',
                    'message': message.to_dict()
                }
            )

4. Content Moderation Service

Responsibilities:

  • Automated content filtering
  • Manual review queue
  • Safety and compliance
  • Ban management

Key Design Decisions:

  • Multi-Layer Filtering: Keyword, ML, manual review
  • Real-Time Filtering: Fast automated checks
  • Manual Review: Human review for edge cases
  • Learning System: Improve filters over time

Implementation:

class ModerationService:
    def __init__(self):
        self.keyword_filter = KeywordFilter()
        self.ml_classifier = MLClassifier()
        self.review_queue = ReviewQueue()
    
    def check_message(self, message_text):
        """Check message for moderation"""
        # Keyword filtering
        keyword_result = self.keyword_filter.check(message_text)
        if keyword_result.is_blocked:
            return ModerationResult(
                is_blocked=True,
                reason='keyword_filter',
                confidence=1.0
            )
        
        # ML classification
        ml_result = self.ml_classifier.classify(message_text)
        
        if ml_result.confidence > 0.9:
            # High confidence, auto-decision
            return ModerationResult(
                is_blocked=ml_result.is_toxic,
                reason='ml_classifier',
                confidence=ml_result.confidence
            )
        else:
            # Low confidence, queue for manual review
            return ModerationResult(
                is_blocked=False,
                reason='pending_review',
                confidence=ml_result.confidence,
                needs_review=True
            )
    
    def check_asset(self, asset_data, asset_type):
        """Check asset for moderation"""
        # Image/text analysis
        # 3D model analysis
        # File content scanning
        # Similar to message checking
        pass

5. Experience Discovery Service

Responsibilities:

  • Search experiences
  • Recommend experiences
  • Trending and popular
  • Personalized recommendations

Key Design Decisions:

  • Search Engine: Elasticsearch for full-text search
  • Recommendation Engine: ML-based recommendations
  • Caching: Cache popular searches
  • Personalization: User-based recommendations

Implementation:

class DiscoveryService:
    def __init__(self):
        self.search_engine = ElasticsearchClient()
        self.recommendation_engine = RecommendationEngine()
        self.cache = RedisCache()
    
    def search_experiences(self, query, filters, limit=20, offset=0):
        """Search experiences"""
        cache_key = f"search:{query}:{filters}:{limit}:{offset}"
        
        # Check cache
        cached = self.cache.get(cache_key)
        if cached:
            return cached
        
        # Search
        results = self.search_engine.search(
            index='experiences',
            query=query,
            filters=filters,
            limit=limit,
            offset=offset
        )
        
        # Cache results
        self.cache.setex(cache_key, 300, results)  # 5 min cache
        
        return results
    
    def get_recommendations(self, user_id, limit=20):
        """Get personalized recommendations"""
        # Get user preferences
        user_preferences = self.get_user_preferences(user_id)
        
        # Get recommendations
        recommendations = self.recommendation_engine.recommend(
            user_id=user_id,
            preferences=user_preferences,
            limit=limit
        )
        
        return recommendations
    
    def get_trending(self, limit=20):
        """Get trending experiences"""
        # Calculate trending score
        # Based on visits, likes, recent activity
        trending = self.calculate_trending_score(limit)
        
        return trending

Detailed Design

Real-Time State Synchronization

Challenge: Synchronize game state across multiple clients with low latency

Solution:

  • Server Authority: Server is source of truth
  • Delta Compression: Send only changes
  • Client Prediction: Allow client-side prediction
  • Interpolation: Smooth client-side interpolation

Implementation:

class StateSynchronizer:
    def __init__(self):
        self.server_state = {}
        self.client_states = {}
        self.update_rate = 60  # Hz
    
    def sync_state(self, server_state, client_id):
        """Synchronize state with client"""
        # Get client's last known state
        client_last_state = self.client_states.get(client_id, {})
        
        # Calculate delta
        delta = self.calculate_delta(server_state, client_last_state)
        
        # Compress delta
        compressed_delta = self.compress(delta)
        
        # Send to client
        self.send_to_client(client_id, compressed_delta)
        
        # Update client state
        self.client_states[client_id] = server_state
    
    def calculate_delta(self, new_state, old_state):
        """Calculate state delta"""
        delta = {}
        for key, value in new_state.items():
            if key not in old_state or old_state[key] != value:
                delta[key] = value
        return delta

Asset Management and CDN

Challenge: Efficiently serve petabytes of user-generated content

Solution:

  • CDN Distribution: Distribute assets globally
  • Asset Versioning: Version assets for updates
  • Lazy Loading: Load assets on demand
  • Compression: Compress assets for faster delivery

Implementation:

class AssetManager:
    def __init__(self):
        self.s3_storage = S3Client()
        self.cdn = CDNClient()
        self.asset_cache = RedisCache()
    
    def upload_asset(self, experience_id, asset_data, asset_type):
        """Upload asset"""
        # Generate asset ID
        asset_id = generate_asset_id()
        
        # Compress asset
        compressed_data = self.compress_asset(asset_data, asset_type)
        
        # Upload to S3
        s3_key = f"experiences/{experience_id}/assets/{asset_id}"
        self.s3_storage.upload(s3_key, compressed_data)
        
        # Invalidate CDN cache
        self.cdn.invalidate(f"assets/{asset_id}")
        
        # Store metadata
        asset_metadata = AssetMetadata(
            asset_id=asset_id,
            experience_id=experience_id,
            asset_type=asset_type,
            s3_key=s3_key,
            file_size=len(compressed_data)
        )
        asset_metadata.save()
        
        return asset_id
    
    def get_asset_url(self, asset_id):
        """Get CDN URL for asset"""
        # Check cache
        cached_url = self.asset_cache.get(f"asset_url:{asset_id}")
        if cached_url:
            return cached_url
        
        # Get metadata
        metadata = self.get_asset_metadata(asset_id)
        
        # Generate CDN URL
        cdn_url = self.cdn.get_url(metadata.s3_key)
        
        # Cache URL
        self.asset_cache.setex(f"asset_url:{asset_id}", 3600, cdn_url)
        
        return cdn_url

Monetization System

Challenge: Handle Robux transactions securely and at scale

Solution:

  • Transaction Logging: Log all transactions
  • Idempotency: Prevent duplicate transactions
  • Atomic Operations: Ensure consistency
  • Fraud Detection: Detect suspicious transactions

Implementation:

class MonetizationService:
    def __init__(self):
        self.transaction_log = TransactionLog()
        self.fraud_detector = FraudDetector()
    
    def process_purchase(self, user_id, item_id, price_robux):
        """Process purchase transaction"""
        # Check user balance
        user = self.get_user(user_id)
        if user.robux_balance < price_robux:
            raise InsufficientFundsError("Not enough Robux")
        
        # Fraud detection
        fraud_check = self.fraud_detector.check_transaction(
            user_id, item_id, price_robux
        )
        if fraud_check.is_suspicious:
            # Queue for manual review
            self.queue_for_review(user_id, item_id, price_robux)
            return {'status': 'pending_review'}
        
        # Create transaction
        transaction = Transaction(
            user_id=user_id,
            transaction_type='purchase',
            item_id=item_id,
            robux_amount=price_robux,
            status='pending'
        )
        transaction.save()
        
        # Process transaction atomically
        try:
            # Deduct Robux
            user.robux_balance -= price_robux
            user.save()
            
            # Grant item to user
            self.grant_item(user_id, item_id)
            
            # Update transaction status
            transaction.status = 'completed'
            transaction.save()
            
            # Log transaction
            self.transaction_log.log(transaction)
            
            return {
                'transaction_id': transaction.transaction_id,
                'status': 'completed',
                'new_balance': user.robux_balance
            }
        except Exception as e:
            # Rollback
            transaction.status = 'failed'
            transaction.save()
            raise

Scalability Considerations

Horizontal Scaling

Game Servers:

  • Stateless game logic, horizontally scalable
  • Auto-scaling based on demand
  • Regional distribution for low latency
  • Load balancing across servers

Backend Services:

  • Stateless services, horizontally scalable
  • Multiple instances behind load balancer
  • Shared database and cache
  • Message queue for async processing

Caching Strategy

Redis Cache:

  • User Data: TTL 1 hour
  • Experience Metadata: TTL 30 minutes
  • Server List: TTL 1 minute
  • Search Results: TTL 5 minutes
  • Asset URLs: TTL 1 hour

CDN Cache:

  • Static Assets: Long TTL (1 year)
  • Dynamic Content: Short TTL (5 minutes)
  • Edge Caching: Cache at edge locations

Security Considerations

Content Moderation

  • Automated Filtering: Keyword and ML-based filtering
  • Manual Review: Human review for edge cases
  • Reporting System: User reporting mechanism
  • Ban Management: Temporary and permanent bans

Anti-Cheat

  • Server Authority: Server validates all actions
  • Rate Limiting: Limit action frequency
  • Behavioral Analysis: Detect suspicious patterns
  • Client Validation: Validate client inputs

Data Protection

  • Encryption: Encrypt sensitive data
  • Access Control: Role-based access control
  • Audit Logging: Log all sensitive operations
  • Privacy: Protect user privacy

Monitoring & Observability

Key Metrics

Platform Metrics:

  • Concurrent users
  • Experience launches per second
  • Game server count
  • Average session duration
  • Chat messages per second

Performance Metrics:

  • Game server latency (p50, p95, p99)
  • Asset download latency
  • Chat message delivery latency
  • API response time

Business Metrics:

  • Daily active users
  • Experience visits
  • Robux transactions
  • Marketplace sales
  • Creator earnings

Trade-offs and Optimizations

Trade-offs

1. State Synchronization: Full vs Delta

  • Full: Simpler, higher bandwidth
  • Delta: Complex, lower bandwidth
  • Decision: Delta compression for efficiency

2. Server Authority: Strict vs Lenient

  • Strict: More secure, higher latency
  • Lenient: Lower latency, less secure
  • Decision: Strict with client prediction

3. Content Moderation: Automated vs Manual

  • Automated: Fast, may have false positives
  • Manual: Accurate, slower
  • Decision: Hybrid approach

4. Asset Storage: Centralized vs Distributed

  • Centralized: Simpler, higher latency
  • Distributed: Complex, lower latency
  • Decision: CDN distribution

Optimizations

1. Client Prediction

  • Predict actions client-side
  • Reduce perceived latency
  • Improve user experience

2. Delta Compression

  • Send only state changes
  • Reduce bandwidth
  • Improve performance

3. Asset Streaming

  • Stream assets on demand
  • Reduce initial load time
  • Improve experience launch

4. Regional Distribution

  • Servers in multiple regions
  • Reduce latency
  • Improve user experience

What Interviewers Look For

Real-Time Multiplayer Skills

  1. Game Server Architecture
    • Low-latency game servers
    • State synchronization
    • Delta compression
    • Red Flags: High latency, no compression, poor sync
  2. State Synchronization
    • Efficient state updates
    • Client prediction
    • Red Flags: No prediction, high latency, poor UX
  3. Connection Management
    • Millions of concurrent connections
    • Efficient handling
    • Red Flags: Connection limits, poor scaling, bottlenecks

Distributed Systems Skills

  1. Content Delivery
    • CDN distribution
    • Asset streaming
    • Efficient delivery
    • Red Flags: No CDN, slow delivery, poor UX
  2. User-Generated Content
    • Petabytes of content
    • Efficient storage
    • Red Flags: Inefficient storage, high costs, slow access
  3. Scalability Design
    • Horizontal scaling
    • Regional distribution
    • Red Flags: Vertical scaling, single region, bottlenecks

Problem-Solving Approach

  1. Cross-Platform Support
    • PC, mobile, console
    • Platform-specific optimizations
    • Red Flags: Single platform, no optimization, poor UX
  2. Edge Cases
    • Network failures
    • High latency
    • Content moderation
    • Red Flags: Ignoring edge cases, no handling
  3. Trade-off Analysis
    • Latency vs quality
    • Consistency vs performance
    • Red Flags: No trade-offs, dogmatic choices

System Design Skills

  1. Component Design
    • Game server cluster
    • Content service
    • Social service
    • Red Flags: Monolithic, unclear boundaries
  2. Content Moderation
    • Automated moderation
    • Manual review
    • Red Flags: No moderation, post-processing, slow
  3. Monetization System
    • Robux currency
    • Secure transactions
    • Red Flags: Insecure, no validation, fraud possible

Communication Skills

  1. Real-Time Architecture Explanation
    • Can explain game server design
    • Understands state sync
    • Red Flags: No understanding, vague
  2. Content Delivery Explanation
    • Can explain CDN strategy
    • Understands streaming
    • Red Flags: No understanding, vague

Meta-Specific Focus

  1. Real-Time Systems Expertise
    • Game server knowledge
    • Low-latency design
    • Key: Show real-time systems knowledge
  2. Scale Thinking
    • Millions of users
    • Petabytes of content
    • Key: Demonstrate scale expertise

Summary

Designing Roblox requires careful consideration of:

  1. Real-Time Multiplayer: Low-latency game servers with state synchronization
  2. User-Generated Content: Petabytes of content with efficient storage and delivery
  3. Creator Tools: Roblox Studio for experience creation
  4. Social Features: Friends, chat, groups with real-time delivery
  5. Monetization: Robux currency with secure transactions
  6. Content Moderation: Automated and manual moderation at scale
  7. Discovery: Search and recommendation for experiences
  8. Scalability: Handle 10M+ concurrent users
  9. Cross-Platform: Support PC, mobile, console
  10. Performance: Sub-100ms game server latency

Key architectural decisions:

  • Game Server Cluster for real-time multiplayer
  • CDN Distribution for asset delivery
  • Delta Compression for state synchronization
  • Hybrid Moderation for content safety
  • Sharded Database for scalability
  • Regional Distribution for low latency
  • Client Prediction for better UX
  • Horizontal Scaling for all services

The system handles 10 million concurrent users, serves petabytes of content, processes billions of chat messages, and maintains sub-100ms game server latency while ensuring content safety and supporting millions of creators.