What's the best way to handle timer-based events in a multiplayer game where timing needs to be synchronized?

Question

Ryan McCombe · Accepted Answer

Synchronizing timers in multiplayer games requires careful consideration of network latency and state management. Here's a pattern that helps maintain consistency across clients while being resistant to network issues.

Basic Approach

Instead of letting each client manage its own timers independently, we synchronize based on a shared start time:

1#include <SDL.h>
2
3#include <iostream>
4
5class NetworkSpawner {
6public:
7  struct SpawnConfig {
8    Uint32 StartTime;
9    Uint32 Interval;
10    int WaveSize;
11  };
12
13  void StartSpawning(
14    const SpawnConfig& Config) {
15    MyConfig = Config;
16    IsSpawning = true;
17    LastSpawnTime = Config.StartTime;
18
19    std::cout << "Starting wave at time "
20      << Config.StartTime << "ms\n";
21  }
22
23  void Update() {
24    if (!IsSpawning) { return; }
25
26    Uint32 CurrentTime{SDL_GetTicks()};
27    Uint32 TimeSinceStart{
28      CurrentTime - MyConfig.StartTime};
29    Uint32 ExpectedSpawns{
30      TimeSinceStart / MyConfig.Interval};
31
32    // Spawn any missing enemies to catch up
33    while (SpawnCount < ExpectedSpawns &&
34      SpawnCount < MyConfig.WaveSize) {
35      SpawnEnemy(CurrentTime);
36      SpawnCount++;
37    }
38
39    // Check if wave is complete
40    if (SpawnCount >= MyConfig.WaveSize) {
41      IsSpawning = false;
42    }
43  }
44
45private:
46  void SpawnEnemy(Uint32 Time) {
47    std::cout << "Spawning enemy at time "
48      << Time << "ms\n";
49    LastSpawnTime = Time;
50  }
51
52  SpawnConfig MyConfig;
53  bool IsSpawning{false};
54  Uint32 LastSpawnTime{0};
55  Uint32 SpawnCount{0};
56};
57
58// Simulate network message received from server
59void HandleServerMessage(
60  NetworkSpawner& Spawner) {
61  NetworkSpawner::SpawnConfig Config;
62  Config.StartTime = SDL_GetTicks();
63  Config.Interval = 1000;
64  Config.WaveSize = 3;
65  Spawner.StartSpawning(Config);
66}
67
68int main(int argc, char** argv) {
69  SDL_Init(SDL_INIT_TIMER);
70
71  NetworkSpawner Spawner;
72  HandleServerMessage(Spawner);
73
74  // Simulate game loop
75  for (int i = 0; i < 4; ++i) {
76    Spawner.Update();
77    SDL_Delay(1000);
78  }
79
80  SDL_Quit();
81  return 0;
82}

1Starting wave at time 0ms
2Spawning enemy at time 1008ms
3Spawning enemy at time 2008ms
4Spawning enemy at time 3009ms

Key Concepts

Server Authority: The server sends:

Wave start times
Spawn intervals
Number of enemies to spawn

Time Synchronization: Clients convert server time to local time:

1Uint32 ServerToLocalTime(Uint32 ServerTime) {
2  return ServerTime + EstimatedTimeOffset;
3}

Catch-up Mechanism: If a client falls behind (lag spike, etc), it can catch up by spawning multiple enemies in one frame, maintaining consistency with other clients.

State Verification: Periodically verify game state with the server:

1void VerifyState(const ServerState& State) {
2  if (SpawnCount != State.ExpectedSpawns) {
3    // Reconcile difference
4    SpawnCount = State.ExpectedSpawns;
5  }
6}

Remember to:

Account for network latency in timing calculations
Handle disconnections gracefully
Consider interpolation for smooth visual presentation
Implement proper error handling for desyncs

This approach provides a good balance between responsiveness and consistency while being resistant to network issues.

SDL2 Timers and Callbacks

Syncing Timers in Multiplayer

Basic Approach

Key Concepts

SDL2 Timers and Callbacks

Game Development with SDL2

Questions & Answers