Design Patterns for Game Servers

Apply proven design patterns for scalable, maintainable game server architecture.

Pattern Selection Guide

Pattern	Purpose	Use Case
ECS	Data-oriented design	Entity management
Command	Action encapsulation	Input replay, undo
Observer	Event notification	State changes
State Machine	State transitions	Player states
Object Pool	Memory efficiency	Bullets, particles
Event Sourcing	Audit trail	Match replay

Entity Component System (ECS)

// Components - pure data, no logic
struct Position { float x, y, z; };
struct Velocity { float dx, dy, dz; };
struct Health { int current, max; };
struct NetworkSync { uint32_t last_sync_tick; };

// Entity is just an ID
using Entity = uint32_t;

// Component storage
template<typename T>
class ComponentArray {
    std::unordered_map<Entity, T> components;

public:
    void add(Entity e, T component) {
        components[e] = component;
    }

    T* get(Entity e) {
        auto it = components.find(e);
        return it != components.end() ? &it->second : nullptr;
    }

    void remove(Entity e) {
        components.erase(e);
    }
};

// System - logic that operates on components
class MovementSystem {
public:
    void update(float dt, World& world) {
        for (auto& [entity, pos] : world.query<Position, Velocity>()) {
            auto& vel = world.get<Velocity>(entity);
            pos.x += vel.dx * dt;
            pos.y += vel.dy * dt;
            pos.z += vel.dz * dt;
        }
    }
};

class DamageSystem {
public:
    void applyDamage(Entity target, int amount, World& world) {
        if (auto* health = world.get<Health>(target)) {
            health->current -= amount;
            if (health->current <= 0) {
                world.emit<EntityDied>(target);
            }
        }
    }
};

Command Pattern

struct GameCommand {
    uint64_t tick;
    uint32_t playerId;

    virtual ~GameCommand() = default;
    virtual void execute(GameState& state) = 0;
    virtual void undo(GameState& state) = 0;
    virtual std::unique_ptr<GameCommand> clone() const = 0;
};

struct MoveCommand : GameCommand {
    Vector3 direction;
    Vector3 previousPosition;  // For undo

    void execute(GameState& state) override {
        auto& player = state.players[playerId];
        previousPosition = player.position;
        player.velocity = direction * player.speed;
    }

    void undo(GameState& state) override {
        state.players[playerId].position = previousPosition;
    }

    std::unique_ptr<GameCommand> clone() const override {
        return std::make_unique<MoveCommand>(*this);
    }
};

// Command history for replay
class CommandHistory {
    std::vector<std::unique_ptr<GameCommand>> history;

public:
    void record(std::unique_ptr<GameCommand> cmd) {
        history.push_back(std::move(cmd));
    }

    GameState replay(const GameState& initial) {
        GameState state = initial;
        for (const auto& cmd : history) {
            cmd->execute(state);
        }
        return state;
    }

    void undoLast(GameState& state) {
        if (!history.empty()) {
            history.back()->undo(state);
            history.pop_back();
        }
    }
};

Event Sourcing

// Events are immutable facts
struct GameEvent {
    uint64_t timestamp;
    uint64_t sequence;

    virtual ~GameEvent() = default;
    virtual void apply(GameState& state) const = 0;
    virtual std::string serialize() const = 0;
};

struct PlayerDamaged : GameEvent {
    uint32_t playerId;
    uint32_t sourceId;
    int damage;
    DamageType type;

    void apply(GameState& state) const override {
        auto& player = state.players[playerId];
        player.health -= damage;
        player.lastDamageSource = sourceId;
    }

    std::string serialize() const override {
        return fmt::format("{{\"type\":\"damage\",\"player\":{},\"damage\":{}}}",
                          playerId, damage);
    }
};

class EventStore {
    std::vector<std::unique_ptr<GameEvent>> events;

public:
    void append(std::unique_ptr<GameEvent> event) {
        event->sequence = events.size();
        events.push_back(std::move(event));
    }

    GameState rebuildState() const {
        GameState state;
        for (const auto& event : events) {
            event->apply(state);
        }
        return state;
    }

    GameState rebuildAtTime(uint64_t timestamp) const {
        GameState state;
        for (const auto& event : events) {
            if (event->timestamp > timestamp) break;
            event->apply(state);
        }
        return state;
    }
};

Object Pool

template<typename T, size_t N>
class ObjectPool {
    struct PoolEntry {
        T object;
        bool in_use = false;
    };

    std::array<PoolEntry, N> pool;
    std::stack<size_t> free_indices;
    std::mutex mutex;

public:
    ObjectPool() {
        for (size_t i = 0; i < N; ++i) {
            free_indices.push(i);
        }
    }

    T* acquire() {
        std::lock_guard<std::mutex> lock(mutex);
        if (free_indices.empty()) return nullptr;

        size_t idx = free_indices.top();
        free_indices.pop();
        pool[idx].in_use = true;
        return &pool[idx].object;
    }

    void release(T* obj) {
        std::lock_guard<std::mutex> lock(mutex);
        size_t idx = obj - &pool[0].object;
        pool[idx].object.reset();
        pool[idx].in_use = false;
        free_indices.push(idx);
    }

    size_t available() const {
        return free_indices.size();
    }
};

// Usage
ObjectPool<Bullet, 1000> bulletPool;

void fireBullet(Player& player) {
    Bullet* bullet = bulletPool.acquire();
    if (bullet) {
        bullet->init(player.position, player.aimDirection);
        activeBullets.push_back(bullet);
    }
}

void onBulletHit(Bullet* bullet) {
    activeBullets.remove(bullet);
    bulletPool.release(bullet);
}

State Machine

enum class PlayerState {
    Idle, Running, Jumping, Attacking, Dead
};

class PlayerStateMachine {
    PlayerState current = PlayerState::Idle;
    std::unordered_map<PlayerState,
        std::unordered_set<PlayerState>> transitions;

public:
    PlayerStateMachine() {
        // Define valid transitions
        transitions[PlayerState::Idle] = {
            PlayerState::Running,
            PlayerState::Jumping,
            PlayerState::Attacking,
            PlayerState::Dead
        };
        transitions[PlayerState::Running] = {
            PlayerState::Idle,
            PlayerState::Jumping,
            PlayerState::Dead
        };
        transitions[PlayerState::Jumping] = {
            PlayerState::Idle,
            PlayerState::Dead
        };
        transitions[PlayerState::Attacking] = {
            PlayerState::Idle,
            PlayerState::Dead
        };
        // Dead is terminal - no transitions out
        transitions[PlayerState::Dead] = {};
    }

    bool canTransition(PlayerState newState) const {
        auto it = transitions.find(current);
        return it != transitions.end() &&
               it->second.count(newState) > 0;
    }

    bool transition(PlayerState newState) {
        if (!canTransition(newState)) return false;

        onExit(current);
        current = newState;
        onEnter(current);
        return true;
    }

private:
    void onEnter(PlayerState state) {
        switch (state) {
            case PlayerState::Jumping:
                player->velocity.y = JUMP_FORCE;
                break;
            case PlayerState::Dead:
                player->onDeath();
                break;
        }
    }

    void onExit(PlayerState state) {
        // Cleanup for previous state
    }
};

Troubleshooting

Common Failure Modes

Pattern	Problem	Root Cause	Solution
ECS	Component fragmentation	Sparse storage	Archetype-based storage
Command	Memory bloat	Unbounded history	Limit history size
Event Sourcing	Slow replay	Too many events	Periodic snapshots
Object Pool	Exhaustion	Leaks	Track allocations
State Machine	Invalid state	Missing transition	Validate all paths

Debug Checklist

// ECS diagnostics
void debugECS(World& world) {
    std::cout << "Entities: " << world.entityCount() << std::endl;
    std::cout << "Components per type:\n";
    world.forEachArchetype([](const Archetype& a) {
        std::cout << "  " << a.signature() << ": " << a.size() << "\n";
    });
}

// Object pool health
void debugPool(ObjectPool& pool) {
    std::cout << "Pool utilization: "
              << (pool.capacity() - pool.available())
              << "/" << pool.capacity() << std::endl;
}

// Event store stats
void debugEventStore(EventStore& store) {
    std::cout << "Events: " << store.size() << std::endl;
    std::cout << "Rebuild time: " << measureRebuildTime(store) << "ms\n";
}

Unit Test Template

#include <gtest/gtest.h>

TEST(ECS, CreatesAndQueriesEntities) {
    World world;

    Entity player = world.createEntity();
    world.add<Position>(player, {0, 0, 0});
    world.add<Velocity>(player, {1, 0, 0});

    auto entities = world.query<Position, Velocity>();
    EXPECT_EQ(entities.size(), 1);
}

TEST(Command, UndoRestoresState) {
    GameState state;
    state.players[1].position = {0, 0, 0};

    auto cmd = std::make_unique<MoveCommand>();
    cmd->playerId = 1;
    cmd->direction = {1, 0, 0};

    cmd->execute(state);
    EXPECT_NE(state.players[1].velocity.x, 0);

    cmd->undo(state);
    EXPECT_EQ(state.players[1].position.x, 0);
}

TEST(EventSourcing, RebuildMatchesLive) {
    EventStore store;
    GameState liveState;

    for (int i = 0; i < 100; ++i) {
        auto event = std::make_unique<PlayerDamaged>();
        event->playerId = 1;
        event->damage = 1;
        event->apply(liveState);
        store.append(std::move(event));
    }

    auto rebuiltState = store.rebuildState();
    EXPECT_EQ(rebuiltState.players[1].health, liveState.players[1].health);
}

TEST(ObjectPool, AcquireAndRelease) {
    ObjectPool<Bullet, 10> pool;

    std::vector<Bullet*> acquired;
    for (int i = 0; i < 10; ++i) {
        acquired.push_back(pool.acquire());
        EXPECT_NE(acquired.back(), nullptr);
    }

    // Pool exhausted
    EXPECT_EQ(pool.acquire(), nullptr);

    // Release one
    pool.release(acquired.back());
    acquired.pop_back();

    // Can acquire again
    EXPECT_NE(pool.acquire(), nullptr);
}

Resources

• assets/ - Pattern implementations
• references/ - Design pattern catalogs