【高级技巧】缓存一致性：多级缓存的失效同步设计

local rankCache = { mem = nil, memTs = 0 }
local TTL = 60
local function rankGet()
    local now = os.time()
    if rankCache.mem and now - rankCache.memTs < TTL then
        return rankCache.mem, "mem"
    end
    local snap = getsysvar("RankSnapshot")
    if snap ~= "" and snap ~= nil then
        rankCache.mem = snap
        rankCache.memTs = now
        return snap, "global"
    end
    local fresh = rebuildRank()
    rankInvalidate(fresh)
    return fresh, "source"
end
local function rankInvalidate(freshValue)
    if freshValue then
        rankCache.mem = freshValue
        rankCache.memTs = os.time()
        setsysvarex("RankSnapshot", freshValue, 1)
    else
        rankCache.mem = nil
        rankCache.memTs = 0
        setsysvarex("RankSnapshot", "", 1)
    end
end

local function onScoreChange(actorName, delta)
    applyScore(actorName, delta)
    rankInvalidate()
end