Overview

Ledger keeps player data as a log of changes instead of a document you overwrite. You never write state. You write down the change you want, a function you own decides if it's allowed, and state is what falls out of replaying those changes.

local Store = Ledger.New({
	Name = "PlayerData",
	Default = { Gold = 100 },
	Reducer = function(State, Op)
		if Op.Kind == "Earn" then
			if type(Op.Amount) ~= "number" or Op.Amount <= 0 then
				return nil
			end
			return { Gold = State.Gold + Op.Amount }
		end

		if Op.Kind == "SpendGold" then
			if type(Op.Amount) ~= "number" or Op.Amount > State.Gold then
				return nil -- refused, on every server, forever
			end
			return { Gold = State.Gold - Op.Amount }
		end

		return nil -- an op this build has never heard of
	end,
})

Store:Load(Player)
Store:Expect(Player):Apply("SpendGold", { Amount = 25 })

Two servers spend the same 100 gold, both writes go through, the fold takes one and refuses the other. Every server agrees, every time.

Why there's no session lock

A session lock stops the second writer. You pay for that in three ways. A dead server leaves a lease someone has to wait out. Whoever is locked waits at the join screen. And nothing can reach a player who is offline or on another server.

A fold that validates makes the bad state unreachable, so nothing needs unlocking when a server crashes. It costs discipline instead. Changes have to be ops with names, and you have to write a reducer.

What's in it

What it isn't

Ledger is server side only. It doesn't replicate to clients, it doesn't do leaderboards or ordered stores, and it won't hide a reducer that's wrong. If your reducer reads os.time() or math.random(), two servers will fold the same log into different state. Ledger warns about that in Studio rather than hiding it.