SetNet.Lockstep 1.1.0

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SetNet.Lockstep

Deterministic lockstep engine for SetNet.

Two ways to network a game: replicate state (server streams positions — see SetNet.StateSync), or replicate inputs (every client runs the same simulation from the same inputs). The second is lockstep, and it's how most RTS games work: sending "player 2 ordered these 40 units to move" is tiny compared to streaming 40 unit positions every tick.

Here, the server collects each participant's input for a turn and — once all inputs are in (or the turn times out) — broadcasts the complete input set. Every client then advances its simulation by one turn, identically.

Install

dotnet add package SetNet
dotnet add package SetNet.Lockstep

Setup

LockstepRuntime.Enable();     // once at startup, both ends, before creating client/server

Server

server.UseLockstep(new LockstepOptions { TurnTimeoutMs = 200 });

The server auto-enrolls every connected peer as a participant, runs the turn clock, and relays inputs opaquely (it never deserializes them, so it stays serializer-agnostic).

Client (typed)

// choose your per-turn input type — (de)serialized via the registered serializer:
var ls = client.UseLockstep<PlayerCommand>();

ls.TurnReady += (turn, inputs) =>       // inputs: playerId -> PlayerCommand
{
    foreach (var (playerId, cmd) in inputs)
        Simulation.Apply(playerId, cmd);
    Simulation.Step();                  // advance exactly one deterministic tick
};

// each turn, submit your command:
ls.SubmitInput(new PlayerCommand { Move = dir, Fire = firing });

TurnReady hands you a IReadOnlyDictionary<string, PlayerCommand> — fully typed, no raw bytes. Use client.UseLockstep<byte[]>() if you want to manage serialization yourself.

Determinism is on you

Lockstep only guarantees every client receives the same inputs in the same order. Whether they compute the same result depends on your simulation:

• Prefer fixed-point or integer math; be careful with float (order-of-operations and platform differences can diverge).
• Don't branch on non-replicated state (wall-clock time, Random without a shared seed, iteration order of unordered collections).
• Step the simulation only in TurnReady, never in a render/Update loop.

A divergence (desync) means one client computed a different world — usually fatal for lockstep, so test with a periodic state checksum.

Options

How it works

• The server holds a turn number T. Clients submit an input tagged for their current turn.
• A turn finalizes when all participants have submitted for it (fast path) or after TurnTimeoutMs (safety net). The finalized input set is broadcast; T advances.
• Clients run a turn or two behind real time (the input delay), which is the standard lockstep trade-off for perfect consistency.
• Reserved wire types 65514 / 65515.

Notes

• Best for turn-based and RTS-style games; for fast action shooters prefer state replication + prediction.
• A dropped/slow player is dropped from a turn after the timeout — handle "missing input" gracefully (e.g. repeat last command).

Documentation & source

• 🐙 https://github.com/Povstalez/SetNet — full module catalog in docs/MODULES.md

License

MIT