Revy

HackerNews discussion reveals Revy is an open-source proof-of-concept that records Bevy (Rust game engine) application state every frame and visualizes it in Rerun, enabling time-travel-style inspection of ECS data. The author states plainly: 'Revy is not meant to be a polished / properly maintained project — it's just a proof-of-concept,' though updates are intended when new Rerun or Bevy versions land. Technical details from the thread: Revy runs as the last system at the end of each frame with exclusive access to the World, tracking three timelines (wall-clock time, frame number, simulation time), so non-deterministic scheduling is not an issue by default; it uses snapshot deltas, syncing only components that changed. One user benchmarked the demo recording at ~145KB/s (3.58MB over a 24.6s runtime) and called it 'really decent.' Sentiment is enthusiastic — 'This is really awesome!... something that I felt was missing since day 2 of learning Bevy' — with users sharing cruder workarounds (dumping user actions to ndjson for replay). Caveats raised: at ~21k entities users will hit 'much bigger issues on the Rerun-side' since Rerun was designed for dozens-to-hundreds of massive entities; commenters also note bevy_ecs seems more mature than the rest of Bevy and question whether Revy's blanket bevy dependency pulls in more than the ECS. The thread situates Revy among time-travel debugging tools (rr, undo.io, Microsoft TTD, Green Hills Time Machine) while clarifying Revy does not replay anything — it stores state each frame and visualizes it (e.g. storing a node's post-physics transform every frame). Warning: the +96-voted comment block discusses PCA/statistics/units and is unrelated to Revy — apparent contamination from a different thread.

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