esx_core-esx-framework/server/main.lua

WhiteList = {}

function loadWhiteList(cb)
	Whitelist = {}

	MySQL.Async.fetchAll('SELECT identifier FROM whitelist', {}, function(result)
		for k,v in ipairs(result) do
			WhiteList[v.identifier] = true
		end

		if cb then
			cb()
		end
	end)
end

MySQL.ready(function()
	loadWhiteList()
end)

AddEventHandler('playerConnecting', function(name, setCallback, deferrals)
	-- Mark this connection as deferred, this is to prevent problems while checking player identifiers.
	deferrals.defer()

	local playerId, kickReason, identifier = source
	
	-- Letting the user know what's going on.
	deferrals.update(_U('whitelist_check'))
	
	-- Needed, not sure why.
	Citizen.Wait(100)

	for k,v in ipairs(GetPlayerIdentifiers(playerId)) do
		if string.match(v, 'license:') then
			identifier = string.sub(v, 9)
			break
		end
	end

	if ESX.Table.SizeOf(WhiteList) == 0 then
		kickReason = _U('whitelist_empty')
	elseif not identifier then
		kickReason = _U('license_missing')
	elseif not WhiteList[identifier] then
		kickReason = _U('not_whitelisted')
	end

	if kickReason then
		deferrals.done(kickReason)
	else
		deferrals.done()
	end
end)