ARCD

Multiplayer Engines 🌐

Building multiplayer games opens up incredible possibilities for social interaction, competition, and collaboration 🤝. However, it also introduces significant technical challenges. Connecting multiple players over the internet, keeping their game worlds synchronized, and providing a smooth experience despite network imperfections requires careful planning and the right technology.

This guide provides an overview of the core concepts, architectures, and techniques involved in building multiplayer games, helping you choose the right path for your project on ARCD.

Core Concepts: Latency, Sync, State 📊

Understanding these three concepts is fundamental:

  • Latency (Ping): The time it takes for data to travel from a client to the server and back. High latency leads to noticeable delays (lag) between a player's action and its effect in the game 🐢.
  • Synchronization (Sync): The process of ensuring that all players see a consistent and reasonably up-to-date version of the game world. Perfect synchronization is impossible due to latency.
  • State: The collection of all data that defines the current status of the game world—player positions, scores, object properties, timers, etc. Managing and synchronizing this state efficiently is the core challenge.

Networking Models: Client-Server vs. P2P ↔️

There are two primary architectural models for multiplayer games:

  1. Client-Server: Players (clients) connect to a central, authoritative server. The server manages the game state, processes inputs, and sends updates back to the clients. This is the most common model, especially for games requiring cheat prevention and persistent state.
  2. Peer-to-Peer (P2P): Clients connect directly to each other without a central server acting as the game host. One player might act as the "host" peer, or the state might be distributed. P2P can be simpler for small groups and potentially lower latency between nearby players, but it's harder to secure against cheating and more complex to synchronize reliably.

Client-Server Architectures 🖥️ ➡️🧍

This is the dominant model for most online games.

  • Authoritative Server: The server is the ultimate source of truth for the game state. Clients send their inputs (e.g., "move forward", "shoot"), and the server calculates the outcome and updates all clients. This makes cheating much harder, as clients cannot simply manipulate their local game state.
  • Dedicated Server: A server program running on hardware specifically intended for hosting the game (often in data centers). Offers the best performance and reliability.
  • Listen Server: One of the players also acts as the server. Simpler to set up for casual games but relies on the host player's machine and internet connection, and the game ends if the host leaves.

Pros: Better security 🛡️, easier state management, scalability. Cons: Server costs (hosting, maintenance) 💰, potential single point of failure, latency depends on distance to the server.

Peer-to-Peer (P2P) Architectures 🧍 ➡️ 🧍

Clients communicate directly.

  • Synchronization Challenges: Ensuring all peers have a consistent view of the game state without a central authority is complex. Requires careful handling of input timing and state resolution (e.g., lockstep protocols).
  • NAT Traversal: Connecting peers directly often requires techniques like STUN/TURN servers to navigate network address translators (NAT) and firewalls.
  • Security Issues: Since clients communicate directly, it's easier for malicious players to cheat or disrupt the game (e.g., lag switching).
  • Host Migration: If the "host" peer leaves, a mechanism is needed to elect a new host and transfer the game state smoothly.

Pros: Potentially lower latency (if peers are close), no dedicated server costs. Cons: Harder synchronization, security vulnerabilities 🔓, complex implementation, less scalable.

State Synchronization Techniques 🔄

How do we keep the game state consistent across clients?

  • Snapshotting: Periodically, the server sends a complete "snapshot" of the relevant game state to each client. Simple but can consume significant bandwidth.
  • Delta Compression: Send only the changes (deltas) in the game state since the last update. More bandwidth-efficient but requires clients to maintain the previous state accurately.
  • Event/RPC Broadcasting: Send specific events (e.g., "Player A shot") or Remote Procedure Calls (RPCs) to clients, letting them update their local state accordingly.
  • Interest Management: Only send updates about objects/events relevant to a specific client (e.g., things within their view distance). Crucial for large-world games like MMOs.

Handling Latency: Prediction & Interpolation 🪄

Network delay makes objects appear jerky or actions feel unresponsive. We use techniques to hide latency:

  • Client-Side Prediction: The client predicts the result of its own actions immediately (e.g., start moving forward the instant W is pressed) without waiting for server confirmation. If the server later corrects the position, the client adjusts smoothly.
  • Entity Interpolation: Instead of snapping remote entities (other players, objects) to their latest received position, the client smoothly interpolates (moves) them from their previous position to the new one over a short period, hiding packet jitter.
  • Lag Compensation: Techniques used by the server to "rewind time" slightly when processing actions like shooting, accounting for the shooter's latency to make hit detection feel more accurate.

Game Genres & Network Needs 🎮

The best networking approach depends heavily on the type of game:

FPS Games: Low Latency Focus 🎯

  • Examples: Counter-Strike, Valorant, Call of Duty
  • Requirements: Very low latency (sub-100ms ideal), high update rate (tickrate), precise hit detection.
  • Common Approach: Authoritative Client-Server model with sophisticated client-side prediction, interpolation, and lag compensation.

Turn-Based/Card Games: Simpler State 🃏

  • Examples: Hearthstone, Chess, Slay the Spire (with mods)
  • Requirements: Latency is less critical. Focus is on reliable state transmission for actions/turns.
  • Common Approach: Client-Server often used for matchmaking and persistence, but the core turn logic is less demanding on real-time sync. Simple request/response or event broadcasting often suffices.

MMOs/Persistent Worlds: Scalability 🏰

  • Examples: World of Warcraft, EVE Online, Runescape
  • Requirements: Scalability to handle hundreds or thousands of players concurrently in a shared world, robust persistence, effective interest management.
  • Common Approach: Complex Client-Server architecture, often involving multiple servers (sharding), heavy use of interest management, and database integration for persistence.

Browser-Based Strategy: Tick-based ⏳

  • Examples: Travian Kingdoms, OGame, Ikariam
  • Requirements: Often less real-time; actions resolve over minutes or hours. Needs reliable state persistence and scheduled event processing.
  • Common Approach: Client-Server model. Clients send commands ("Build X", "Attack Y"), server schedules them, and updates happen periodically (ticks). WebSockets or even simple HTTP requests might be used.

Popular Engines & Libraries ⚙️

While you can build networking from scratch (like in our From Zero Tutorial!), these tools can significantly speed up development:

  • Socket.IO: (JavaScript) Popular library for real-time, bidirectional event-based communication between web clients and servers. Great for web games, chat applications.
  • WebRTC: (JavaScript/Web APIs) Enables direct peer-to-peer connections for audio, video, and data transfer within web browsers. Often used for P2P games or voice chat integration.
  • Nakama: (Go) Open-source, scalable game server that handles common features like authentication, matchmaking, storage, leaderboards, and real-time multiplayer.
  • Photon Engine: (Various Languages) Cross-platform multiplayer service offering different products like PUN (for Unity), Realtime, and Bolt for various needs and architectures.
  • Mirror Networking: (Unity/C#) High-level networking library for Unity, successor to the deprecated UNet.
  • Colyseus: (Node.js/TypeScript) Authoritative multiplayer game server framework for Node.js.

Choosing the Right Approach 🤔

Consider these factors:

  • Game Genre: Real-time action vs. turn-based?
  • Platform: Web browser, desktop, mobile?
  • Scale: How many concurrent players per match/world?
  • Team Size/Expertise: Do you have experience with networking? Can you afford dedicated server hosting?
  • Security Needs: How critical is cheat prevention?
  • Budget: Are you using free open-source tools or paid services?

For many web-based games on ARCD, starting with Socket.IO for Client-Server communication or exploring WebRTC for small P2P sessions are good starting points. Frameworks like Nakama or Colyseus offer more features out-of-the-box if you need things like matchmaking and persistence.

Conclusion: Building Connected Worlds ✨

Multiplayer development adds complexity but unlocks immense potential for engaging gameplay. Understanding the trade-offs between Client-Server and P2P, mastering state synchronization, and implementing techniques to handle latency are key steps.

Start simple, choose the right tools for your specific needs, and leverage the power of libraries and frameworks to avoid reinventing the wheel. Good luck building your connected experiences on ARCD!