Authoritative Server

Authoritative Server — Deep Engineering Guide

The server is the single source of truth. Clients only intend; the server decides. This skill covers the server tick loop, world-state serialization, input authority, reconciliation/rollback, lag compensation and session/connection management that make online games correct, cheat-resistant and smooth under real network loss.

1. Why Authoritative

• Clients are untrusted: a modified client shouldn't teleport, duplicate items, or confirm kills it didn't earn.
• Server-authority = the only place where state can be validated + validated state is shared.

client sends INPUT (intent)        → net
server runs sim (authoritative)    → sim decides
server sends SNAPSHOT (state)      → net
client renders (interpolated)      → visual

1.1 The Trust Boundary

Pick: full authoritative (server runs everything; client is dumb viewer) vs client-predicted (client predicts its own move; server reconciles) vs client-side prediction + server rollback (fast action games). All start from "server is truth".

2. Server-Side Simulation Can't Afford a Game-Loop Problem

2.1 Fixed Timestep

• Server tick: fixed 30–64 Hz (30 for strategy, 60 for aimful shooters; 128 for esports aimers).
• Do NOT sleep(frame): use deltaTime accumulation (see game-loop-and-timestep). Server ticks at exact fixed dt regardless of wall clock.

// server main loop:
tick = 0;
while (running) {
    dtFromWallClock = now - last;
    accumulator += dtFromWallClock;
    while (accumulator >= TICK_MS) {
        SimulateTick(tick);         // deterministic, fixed dt
        accumulator -= TICK_MS;
        tick++;
    }
    netOutputs[ticksToSend].Flush();
}

2.2 Deterministic Sim

The server sim must be deterministic given a seed: lockstep (see game-engine/multiplayer-netcode determinism refs) so replay, spec or tests reproduce identically. No fast-math, no wall-clock, no rand() global.

• RNG: per-entity/instance seeded, tick-advanced — reproducible.
• Physics stepping: fixed dt = stable.

3. Input Authority & The Input Pipeline

client: sample input → serialized command {tick, seq, buttons, aim} → UDP
server: collect per-tick input, run queued commands, apply at tick boundary, optionally re-sim (rollback)

Key properties:

• Attack later: authoritative server processes input at the tick it was generated (with a commandTick), not when it arrives.
• Input history buffer on server: replay recent inputs if a rolled-back tick needs them.
• Client command throttling: cap commands/second; server drops excess (anti-abuse).

3.1 Client Prediction ↔ Server Reconciliation

• Client predicts its own motion (renders its projection).
• Server reconciles the actual authoritative value each tick; client "rubber-bands" only if mismatch > tolerance (e.g., > 100 ms of error).
• Tolerance + extrapolation clipping prevent wild rubber-banding (see rollback-and-reconciliation.md).

4. World-State Snapshot Serialization

Server serializes deltas, not full state:

Snapshot minimal components:

• matchId, tick, seq, per-entity: {entityId, flags, bits...}.
• Compressed position: quantized int16 (glm-style fixed Q for flakeless determinism), deltas rather than absolute.
• Entities unchanged since last snapshot: omitted (per-entity dirty flags).

Standard formats: UE replicas, OSSNet Snapshot = "columns of bytes" per entity with networked mask; netcode in game-engine/multiplayer skill.

5. Reconciliation, Rollback & Lag Compensation

5.1 Rollback (Fighters, Fast Games)

Client predicts; server re-sims when input ran in the wrong order (delayed arrival). Server stores the last N-tick state frames; on late arrival re-runs tick with the corrected input:

server keeps history[40 ticks]
incoming input says "my tick 1000 shot"
if history[1000] exists → re-run 1000..now with that input
→ send diff to client (rollback) ≤ threshold

5.2 Lag Compensation (Hit Scan)

Server can't "aim at where client renders" — it rewinds to the client's requested timestamp to check the shot hit:

• Store per-tick entity transforms for the last ~100 ms.
• On shot incoming, rewind to (arrivalTime - ping/2), run raycast vs stored poses.
• Crosshair hits what the shooter saw = fair for the shooter, deterministic to server.

5.3 The "Client is Right?" Test

The criterion: a valid shot = the server re-sim with client's inputs would have produced a hit. This is authoritative but player-fair. Cheat-resistant because "position" is server-side truth, not client-reported.

6. Connection & Session Handling

6.1 The Connection State Machine

UNRESPONSIVE → HANDSHAKE (challenge/response) → ESTABLISHED
   → RESYNC (after rejoin) → (periodic heartbeat) → TIMEDOUT (kicked)

• Handshake: server->client challenge (nonce) — client signs → avoids spoofed connects.
• Heartbeats: client ping every ~1 s; server drops idle > 5–10 s (configurable).
• Backfill: server assigns sessionToken; a client reconnecting with an old token resumes state (or is denied on match close).

6.2 The "Replay Value" Problem

If a client drops in a rank/match, on return server either:

1. Fast-recovery: restore to last ack'd snapshot + client resyncs from ticklet (simple, standard).
2. Late-join backfill: watch a spectator cam until slot opens (complex; for dedicated shows).

Pick 1 for most titles; 2 only for spectator/require.

7. Exploit & Cheat Resistance (Server Facts)

Combined with validation hooks in anti-cheat-server.

8. Scalability & Cost (Server-Side Budgets)

Under load: more than 60 players/tick ● split world (zones/chunks) or shard (interest-management). If a tick > budget, drop non-crucial systems (network simulation throttle — not gameplay).

9. Testing the Server

• Determinism replay: run a match twice with the same input stream → byte-identical state hash.
• Network fault injection: drop%, reorder, duplicate, delay 50–500 ms → assert no desync, no crash, bounded resync.
• Load test: N bots with synthetic input; assert tick under budget + no queue backlog.
• Chaos: kill a tick mid-sim, restart, restore last ack.

10. References

• references/tick-loop.md — fixed timestep, accumulator, tick queue, determinism under load
• references/world-state-snapshot.md — entity serialization, deltas, dirty flags, quantization, ack/gap detection
• references/input-authority.md — input command flow, server queues, anti-abuse caps
• references/rollback-and-reconciliation.md — client prediction, server reconcile, rubber-banding, rollback replay
• references/lag-compensation.md — hit-scan rewinding, stored poses, fair-shot verification
• references/connection-and-session.md — handshake, heartbeat, timeouts, resume, backfill