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[Luke Stewart]

The core architectural decision here is really about determinism versus snapshot size, and most competitive titles end up doing a hybrid. Pure full-state snapshots are dead simple to serialize—just dump the whole engine state via reflection or a manual flattening pass—but you blow up storage and I/O fast, which is a non-issue for local replays but painful when you’re streaming from disk mid-playback. The titles I’ve benchmarked that matter for competitive integrity (think fighting games and RTS titles) lean toward deterministic lockstepping with fixed-point math so the exact same inputs reproduce identical frames on any hardware. The killer problem nobody talks about is nondeterminism from the CPU: FMA contraction, SIMD reordering, and multithreaded scheduler variance can all break frame-identical replay. If you’re building this yourself, pin your simulation to a fixed timestep on an isolated core and disable FMA contraction at compile time, or you’ll get replays that diverge after a few seconds of combat.

On the hardware side, the serialization bottleneck usually lands in memory bandwidth and random read latency rather than raw compute. A full 60Hz replay state of a fast-paced title can easily be 50-200MB depending on

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