rollback netcode in Godot 4 — anyone actually done this or is it just theory on reddit

yeah the snopek addon is solid as a starting point. biggest thing i learned: your _save_state() and _load_state() implementations need to be exhaustive. missed one exported var on my player node and spent a week wondering why replays were subtly wrong. there's no safety net. it just silently desyncs if you forget something. no error, no warning, just wrongness creeping in over several seconds.

also watch out for nodes that reference scene-level singletons in their state. anything non-deterministic in there will eventually ruin your day.

The RNG seeding issue is sneakier than it looks. In Godot 4 you can end up with multiple RandomNumberGenerator instances scattered across nodes, and any constructed with defaults will seed from OS time. What worked for us: wrap every RNG call behind a single global seeded deterministically at match start, then grep the whole project for direct RandomNumberGenerator.new() and bare randf() calls outside that wrapper. Found three independent instances, including one inside a particle effect that didn't touch gameplay state but was causing divergence in our debug state-comparison tool and took an embarrassingly long time to isolate.

Float accumulation is harder to fully solve. Once you're calling trig or engine physics functions you're largely at Godot's mercy on precision. We ended up accepting small drift and leaning on periodic full-state syncs to compensate rather than trying to eliminate it entirely.

The pooled entity gap is basically guaranteed unless you build something that catches it early. What worked for me was a debug-mode assertion that walks the full node tree from a known state root and checks every active node against a registry of objects that must appear in saves. Slow enough that you gate it behind a flag, but running it for a few sessions surfaces all the gaps before they turn into 2am desync mysteries.

The nastier version of this problem is when the missing node is conditionally active. It gets included in saves when active but silently skipped when inactive, so your snapshots are inconsistent depending on game state at rollback time. That one took way longer to diagnose than the straightforward missing-node case.

for a 2D fighter specifically i'd push hard toward per-tick rollback. predictive interpolation means players can briefly see frames that causally never happened, fine for a racing game, genuinely bad for a fighter where frame-perfect reads are the whole point. if someone whiff-punishes based on a ghost frame that gets corrected a moment later, that's a trust problem with your netcode. rollback keeps both clients in causally valid states at the cost of implementation complexity, and for the genre that tradeoff is almost always correct imo.

lmao the pooled entity problem is like a rite of passage. i had the exact same thing with a pooled shockwave effect that had its own hitbox as a child node — spent an afternoon convinced the desync was in my input prediction when it was just... that node never getting walked during state serialization. and the bug only showed up 6-8 frames after the missing entity would have mattered, so tracing it back was genuinely miserable. the fix was like 4 lines. the debugging was most of a day.

the 'missed one entity' issue nearly made me abandon the whole thing lol. i had a pooled bullet that wasn't being included in state saves because i forgot it lived one node level deeper than where i was iterating. rollback would correct every other entity perfectly and the bullet would just... teleport across the screen. took me almost a full day to even identify it as the bullet causing it.

switched to recursively walking the scene tree and collecting every node that implements _save_state() rather than maintaining a manual list. much harder to accidentally miss something that way.

yeah for 2D fighters it's basically non-negotiable. the whole genre runs on frame-perfect interactions, and if a player sees a frame that causally never happened, their shoryuken looks like it whiffed when it actually connected and the game feels broken. the snopek addon is implementing the same ideas as GGPO anyway so you're already on the right architecture, just gotta commit fully to determinism and exhaustive per-tick state saves. no shortcuts on that part.