Synvael/docs/adr/0004-server-authoritative-simulation.md

0004. Server-authoritative simulation

• Status: Accepted
• Date: 2026-06-27

Context

The game supports single-player and multiplayer through one dedicated server crate; single-player runs that same server logic rather than a separate offline path. A networked simulation must decide where authority lives and how much determinism the simulation is held to.

One option is full simulation determinism (lockstep, rollback, or replay-from-inputs), which permits clients to advance the simulation in agreement and exchange only inputs. It is powerful but imposes a heavy, ongoing cost: every float, every hash-map iteration, and all platform-specific math must be made cross-platform reproducible.

Decision

The simulation is server-authoritative. The server computes the truth; clients send inputs and receive state snapshots, predicting locally for responsiveness and reconciling on disagreement. Combat, physics, mob AI, and item drops are computed once, on the server.

Full simulation determinism (lockstep / rollback / replay-from-inputs) is an explicit non-goal. Outside of worldgen (see ADR-0003), floats, hash-map iteration order, and platform-specific math are all permitted.

Consequences

• The engine does not pay the cost of cross-platform float reproducibility for the simulation — only worldgen carries that burden.
• Clients require prediction and reconciliation logic to stay responsive against an authoritative server.
• Cheat resistance follows from authority residing on the server.
• Features that would require deterministic replay of the full simulation are out of scope by this decision; reopening them would mean revisiting the determinism cost deliberately avoided here.