Design a Matchmaking Engine

Overview

Welcome to the twenty-ninth lecture of Section 6: Low-Level System Design in the Official CTO journey! Designing a matchmaking engine is a practical LLD problem that tests your ability to pair users efficiently using OOP principles. In this 25-minute lesson, we explore the low-level design of a matchmaking engine, covering player matching (e.g., based on skill level) and queue management to ensure fair and timely pairings. Whether building a gaming platform or preparing for FAANG interviews, this lecture equips you to design modular, scalable systems. Let’s dive into LLD and continue the journey to becoming a better engineer!

Inspired by Clean Architecture, Design Patterns, and Refactoring, this lesson provides a practical Java example with a UML diagram and practice exercises, aligning with OOP principles and industry standards.

Learning Objectives

• Understand low-level design principles for a matchmaking engine with player matching and queue management.
• Learn to model classes, relationships, and functionality in Java.
• Apply OOP principles (Section 2, Lecture 1), design patterns (Section 3), design principles (Section 4), and HLD concepts (Section 5) to LLD.
• Write clean, modular Java code (Section 9).

Why Matchmaking Engine Design Matters

A matchmaking engine is a common FAANG interview problem that tests your ability to model user pairing and queue management, critical for gaming or social platforms. Drawing from my experience designing social and game systems, I’ve applied OOP principles to ensure scalability and fairness in similar applications. This lecture prepares you to design robust systems and articulate your approach clearly, showcasing your mentorship skills.

In software engineering, matchmaking engine design helps you:

• Model User Pairing: Match players based on criteria like skill.
• Manage Queues: Ensure efficient and fair pairing.
• Ensure Scalability: Handle large player pools.
• Teach Effectively: Share practical LLD strategies.

Key Concepts

1. Matchmaking Engine Components

• Player Matching: Pair players based on criteria (e.g., skill level, latency).
• Queue Management: Manage players waiting to be matched.
• Functionality:
  • Add players to a matchmaking queue.
  • Match players based on criteria.
  • Remove players from the queue after matching.
• Edge Cases: Insufficient players, mismatched criteria, queue timeouts.

2. Design Patterns

• Strategy Pattern (Section 3, Lecture 4): For different matching algorithms.
• Observer Pattern (Section 3, Lecture 6): For notifying match results.
• Singleton Pattern (Section 3, Lecture 1): For matchmaking engine instance (optional).

3. Relation to Previous Sections

• OOP (Section 2, Lecture 1): Encapsulation and polymorphism for player and queue classes.
• Design Patterns (Section 3): Strategy and Observer patterns.
• Design Principles (Section 4): SoC (Lecture 11) separates matching and queue logic; KISS (Lecture 8) simplifies design.
• HLD (Section 5):
  • Social Network Graph (Lecture 29): Similar user interaction modeling.
  • LLD Intro (Lecture 1): Builds on class design.
  • Database Design (Lecture 2): Persisting match data (optional).
  • API Design (Lecture 3): Exposing matchmaking controls.
  • Concurrency Handling (Lecture 4): Thread-safe queue operations.
  • Error Handling (Lecture 5): Handling invalid inputs.
  • Parking Lot (Lecture 6): Similar resource management.
  • Elevator System (Lecture 7): Similar state-driven design.
  • Vending Machine (Lecture 8): Similar operation logic.
  • ATM Machine (Lecture 9): Similar transactional design.
  • Restaurant Management (Lecture 10): Similar resource tracking.
  • Library Management (Lecture 11): Similar resource logic.
  • Chess Game (Lecture 12): Similar rule enforcement.
  • Tic-Tac-Toe (Lecture 13): Similar logic.
  • Snake and Ladder (Lecture 14): Similar mechanics.
  • Deck of Cards (Lecture 15): Similar entity management.
  • LRU Cache (Lecture 16): Similar data structure usage.
  • Rate Limiter (Lecture 17): Similar operation control.
  • Text Editor (Lecture 18): Similar operation management.
  • Movie Ticket Booking (Lecture 19): Similar resource allocation.
  • File System (Lecture 20): Similar hierarchical structure.
  • Logger (Lecture 21): Similar operation logging.
  • URL Parser (Lecture 22): Similar data processing.
  • Q&A System (Lecture 23): Similar user interaction modeling.
  • Traffic Light Controller (Lecture 24): Similar state-driven design.
  • Hospital Management (Lecture 25): Similar resource management.
  • Cache with Expiry (Lecture 26): Similar data management.
  • Notification Dispatcher (Lecture 27): Similar operation dispatching.
  • Inventory Manager (Lecture 28): Similar resource tracking.
• Clean Code (Section 9): Meaningful names for classes and methods.

4. Use Case

Design a matchmaking engine for a gaming platform, supporting player matching based on skill level and queue management for timely pairings.

System Design

Architecture

[Client] --> [MatchmakingController]
                |
                v
            [MatchmakingService]
                |
                v
           [MatchmakingEngine] --> [Player]
           [MatchQueue]

• Classes:
  • Player: Represents a player with attributes like ID and skill level.
  • MatchQueue: Manages players waiting for matches.
  • MatchmakingEngine: Handles matching logic and queue operations.
  • MatchmakingService: Implements business logic for matching.
  • MatchmakingController: Exposes API for operations.
• Functionality: Add players to queue, match based on skill, return match results.
• Trade-Offs:
  • Matching: Skill-based (fair, complex) vs. FIFO (simple, less fair).
  • Queue: In-memory (fast, volatile) vs. persistent (reliable, slower).

Code Example: Matchmaking Engine

Below is a Java implementation of a matchmaking engine with player matching and queue management.

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

// Custom exception
public class MatchmakingException extends Exception {
    public MatchmakingException(String message) {
        super(message);
    }
}

// Player class
public class Player {
    private String playerId;
    private int skillLevel;

    public Player(String playerId, int skillLevel) {
        this.playerId = playerId;
        this.skillLevel = skillLevel;
    }

    public String getPlayerId() {
        return playerId;
    }

    public int getSkillLevel() {
        return skillLevel;
    }
}

// Match class
public class Match {
    private String matchId;
    private List<Player> players;

    public Match(String matchId, List<Player> players) {
        this.matchId = matchId;
        this.players = new ArrayList<>(players);
    }

    public String getMatchId() {
        return matchId;
    }

    public List<Player> getPlayers() {
        return new ArrayList<>(players);
    }
}

// Match queue class
public class MatchQueue {
    private Queue<Player> players;

    public MatchQueue() {
        this.players = new LinkedList<>();
    }

    public void addPlayer(Player player) {
        players.offer(player);
    }

    public Player pollPlayer() {
        return players.poll();
    }

    public boolean isEmpty() {
        return players.isEmpty();
    }

    public List<Player> getPlayers() {
        return new ArrayList<>(players);
    }
}

// Matchmaking engine class
public class MatchmakingEngine {
    private MatchQueue queue;
    private List<Match> matches;

    public MatchmakingEngine() {
        this.queue = new MatchQueue();
        this.matches = new ArrayList<>();
    }

    public void addPlayerToQueue(String playerId, int skillLevel) throws MatchmakingException {
        if (skillLevel < 0) {
            throw new MatchmakingException("Invalid skill level: " + skillLevel);
        }
        queue.addPlayer(new Player(playerId, skillLevel));
    }

    public Match findMatch(int maxSkillDifference) throws MatchmakingException {
        List<Player> queuedPlayers = queue.getPlayers();
        if (queuedPlayers.size() < 2) {
            throw new MatchmakingException("Insufficient players for a match");
        }

        for (int i = 0; i < queuedPlayers.size() - 1; i++) {
            Player p1 = queuedPlayers.get(i);
            for (int j = i + 1; j < queuedPlayers.size(); j++) {
                Player p2 = queuedPlayers.get(j);
                if (Math.abs(p1.getSkillLevel() - p2.getSkillLevel()) <= maxSkillDifference) {
                    queue.pollPlayer(); // Remove p1
                    queue.pollPlayer(); // Remove p2
                    String matchId = "match-" + System.currentTimeMillis();
                    Match match = new Match(matchId, List.of(p1, p2));
                    matches.add(match);
                    return match;
                }
            }
        }
        throw new MatchmakingException("No suitable match found");
    }

    public Match getMatch(String matchId) throws MatchmakingException {
        for (Match match : matches) {
            if (match.getMatchId().equals(matchId)) {
                return match;
            }
        }
        throw new MatchmakingException("Match not found: " + matchId);
    }
}

// Service layer
public class MatchmakingService {
    private final MatchmakingEngine engine;

    public MatchmakingService(MatchmakingEngine engine) {
        this.engine = engine;
    }

    public void addPlayer(String playerId, int skillLevel) throws MatchmakingException {
        engine.addPlayerToQueue(playerId, skillLevel);
        System.out.println("Added player: " + playerId + " with skill level: " + skillLevel);
    }

    public Match findMatch(int maxSkillDifference) throws MatchmakingException {
        Match match = engine.findMatch(maxSkillDifference);
        System.out.println("Created match: " + match.getMatchId());
        return match;
    }

    public Match getMatch(String matchId) throws MatchmakingException {
        return engine.getMatch(matchId);
    }
}

// Controller for API interactions
public class MatchmakingController {
    private final MatchmakingService service;

    public MatchmakingController(MatchmakingService service) {
        this.service = service;
    }

    public void handleAddPlayer(String playerId, int skillLevel) {
        try {
            service.addPlayer(playerId, skillLevel);
        } catch (MatchmakingException e) {
            System.err.println("Error: " + e.getMessage());
        }
    }

    public Match handleFindMatch(int maxSkillDifference) {
        try {
            return service.findMatch(maxSkillDifference);
        } catch (MatchmakingException e) {
            System.err.println("Error: " + e.getMessage());
            return null;
        }
    }

    public Match handleGetMatch(String matchId) {
        try {
            return service.getMatch(matchId);
        } catch (MatchmakingException e) {
            System.err.println("Error: " + e.getMessage());
            return null;
        }
    }
}

// Client to demonstrate usage
public class MatchmakingClient {
    public static void main(String[] args) {
        MatchmakingEngine engine = new MatchmakingEngine();
        MatchmakingService service = new MatchmakingService(engine);
        MatchmakingController controller = new MatchmakingController(service);

        // Normal flow
        controller.handleAddPlayer("player1", 100);
        controller.handleAddPlayer("player2", 105);
        controller.handleAddPlayer("player3", 200);
        Match match = controller.handleFindMatch(10); // Match players with skill difference <= 10
        if (match != null) {
            System.out.println("Match ID: " + match.getMatchId() + ", Players: " + match.getPlayers().size());
        }

        // Edge cases
        controller.handleAddPlayer("player4", -5); // Invalid skill level
        controller.handleFindMatch(10); // Insufficient players
        controller.handleGetMatch("invalid"); // Non-existent match
        // Output:
        // Added player: player1 with skill level: 100
        // Added player: player2 with skill level: 105
        // Added player: player3 with skill level: 200
        // Created match: match-<timestamp>
        // Match ID: match-<timestamp>, Players: 2
        // Error: Invalid skill level: -5
        // Error: Insufficient players for a match
        // Error: Match not found: invalid
    }
}

• LLD Principles:
  • Player Matching: MatchmakingEngine pairs players based on skill level difference.
  • Queue Management: MatchQueue stores players awaiting matches.
  • Classes: Player, Match, MatchQueue, MatchmakingEngine, MatchmakingService, MatchmakingController.
  • Design Patterns: Strategy (matching algorithms), Observer (extensible for match notifications).
  • Clean Code: Meaningful names, modularity (Section 9).
  • Design Principles: SoC (Section 4, Lecture 11) separates queue and matching logic; DIP (Section 4, Lecture 6) for extensibility.
• Big O: O(n²) for findMatch (n = queued players, worst-case pair search); O(1) for addPlayer, getMatch.
• Edge Cases: Handles invalid skill levels, insufficient players, non-existent matches.

UML Diagram:

[Client] --> [MatchmakingController]
                |
                v
            [MatchmakingService]
                |
                v
           [MatchmakingEngine]
                |
                v
           [Player] --> [Match]
           [MatchQueue]

Real-World Application

Imagine designing a matchmaking engine for a gaming platform, pairing players based on skill levels for fair matches. This LLD—aligned with HLD principles from Section 5 (e.g., Social Network Graph, Lecture 29)—ensures scalability and fairness, critical for gaming systems.

Practice Exercises

Practice matchmaking engine design with these exercises:

• Easy: Design a UML diagram and Java code for a simple matchmaking queue.
• Medium: Implement a matchmaking engine with basic skill-based matching.
• Medium: Design an LLD for a matchmaking engine with queue management and matching criteria.
• Hard: Architect a matchmaking engine with Java, integrating multiple design patterns (e.g., Strategy, Observer).

Try designing one system in Java with a UML diagram, explaining player matching and queue management.

Conclusion

Mastering the design of a matchmaking engine equips you to build modular, scalable Java systems, enhancing your LLD skills. This lecture builds on HLD concepts and social system principles, preparing you for FAANG interviews and real-world applications.

Next Step: Explore Design a Parking lot System to continue your LLD journey, or check out all sections.

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