Setup

Install

WallynpmModel

[dependencies]
Lync = "axp3cter/lync@4.0.2"

npm install @axpecter/lync

Take Lync.rbxm from the latest release and place it in ReplicatedStorage.

One module, both sides

Put every definition in one module and require that module from the server and from every client.

ReplicatedStorage/Net.luau

local Lync = require(game.ReplicatedStorage.Lync)

return Lync.define("arena", {
    Ping = Lync.packet(Lync.empty()),
    Chat = Lync.packet(Lync.str(1, 200)),
})

On the server, attach listeners, call start, and connect flush to a fixed step.

ServerScriptService/Net.server.luau

local Lync = require(game.ReplicatedStorage.Lync)
local Net = require(game.ReplicatedStorage.Net)

-- An empty payload arrives as nil, so the player is the second argument.
Net.Ping:onServer(function(_, player)
    Net.Chat:fireClient(player, "pong")
end)

Lync.start()
RunService.PostSimulation:Connect(function()
    Lync.flush()
end)
game:BindToClose(Lync.close)

The client is wired the same way.

StarterPlayerScripts/Net.client.luau

local Lync = require(game.ReplicatedStorage.Lync)
local Net = require(game.ReplicatedStorage.Net)

Net.Chat:onClient(function(text)
    print(text)
end)

Lync.start()
RunService.PostSimulation:Connect(function()
    Lync.flush()
end)

Net.Ping:fireServer()

Rules that apply to every setup.

start runs once per side	Call it after the last definition. Adding a definition or responder after it throws.
Nothing is sent without flush	Fires, requests and set changes are buffered until the next flush. Call it from a function of your own, since a signal connected to it directly passes the frame time as a budget.
The bootstrap is the same on both sides	If you have a shared startup module, put start, the flush connection and close there.

Bounds are the compression

Every codec is bounded, and the bound decides how many bits a value takes. Pick bounds that match the ranges the game actually uses.

Codec	Bits
Lync.f32()	32	Any float.
Lync.int(0, 100)	7	101 throws on send and drops on arrival.
Lync.quant(0, 1, 0.01)	7	Rounded to the nearest 0.01.
Lync.bool()	1
Lync.bitfield({ "a", "b", "c" })	3	Three flags as one table.

All codecs are listed under Codecs.