Beach Slap Technical Postmortem: Fast Online Multiplayer the Hard-Won Way

Making a "fast" online-multiplayer game is one of the hardest things a small studio can attempt. Beach Slap, Team KwaKwa's party brawler, does it in Unity, with plain MonoBehaviours and the Burst compiler (no DOTS), and still runs on an iPad Air 2. This is the honest engineering postmortem.

We start with the constraints that shaped everything: no dynamic lighting, a fixed performance budget on old mobile hardware, and the decision to write one gameplay codebase that serves both local and online multiplayer. That choice forced an MVC-like architecture, with a deterministic GameSystem (the simulation) cleanly separated from a GameView that only reacts to it. Get this separation wrong and online play is impossible. Get it right and rollback becomes tractable.

From there we go into the netcode itself: why network latency means you are always working with inputs from the past, why deterministic simulation lets you send almost nothing but inputs, and why lock-step is not enough for a fast game. We cover the rollback implementation in practical terms, including rolling back only the model while the view follows along, the network topology (peer-to-peer with a relay, server-authoritative confirmation), session synchronization and ping, a 3-frame input delay at 50 Hz, and input compression to keep packets small.

Finally, the hard part: detecting and debugging desyncs with Adler-32 confirmation checksums, including per-system checksums to localize which system drifted, plus concrete examples from chasing those bugs in a shipped game.

Audience: intermediate gameplay and engine programmers. Familiarity with Unity and game loops helps. No prior netcode experience is required, since the core concepts are built up from first principles with concrete examples.

You will leave knowing how to architect a game so rollback is even possible, what determinism actually demands of your code, and a realistic picture of the effort fast online multiplayer takes, mistakes included.