快捷導(dǎo)航

Redis解決緩存一致性問(wèn)題

更新時(shí)間：2023年10月26日 09:02:40 作者：紫電清霜

本文主要介紹了Redis?解決緩存一致性問(wèn)題，文中通過(guò)示例代碼介紹的非常詳細(xì)，對(duì)大家的學(xué)習(xí)或者工作具有一定的參考學(xué)習(xí)價(jià)值，需要的朋友們下面隨著小編來(lái)一起學(xué)習(xí)學(xué)習(xí)吧

背景

這是我校招剛?cè)肼?Shopee 時(shí)遇到的一個(gè)問(wèn)題。Shopee 私有云上 WAF 給內(nèi)部用戶提供了設(shè)置 IP 黑白名單規(guī)則的能力，所有規(guī)則存儲(chǔ)在 MySQL 中。我校招剛?cè)肼殨r(shí)從已離職前輩的手中接過(guò)了這套系統(tǒng)。但很快發(fā)現(xiàn)每次修改規(guī)則后的 5min 內(nèi)讀到的數(shù)據(jù)不穩(wěn)定——新規(guī)則時(shí)而查得到，時(shí)而查不到，也經(jīng)常有用戶反饋這個(gè)問(wèn)題。排查發(fā)現(xiàn)原因是服務(wù)代碼中使用了內(nèi)存緩存，而這個(gè)服務(wù)部署了兩個(gè)實(shí)例，實(shí)例之間沒(méi)有同步寫(xiě)請(qǐng)求。如果寫(xiě)后讀的讀寫(xiě)請(qǐng)求被路由到不同的實(shí)例上，就無(wú)法讀到最新數(shù)據(jù)。而內(nèi)存緩存的過(guò)期時(shí)間被設(shè)置為 5min。

查了下這個(gè)服務(wù)的運(yùn)維記錄，在我入職之前做過(guò)一次擴(kuò)容，從單實(shí)例擴(kuò)容到雙實(shí)例。之前的研發(fā)同事維護(hù) WAF 時(shí)一直是單實(shí)例運(yùn)行，所以沒(méi)出過(guò)問(wèn)題。后來(lái)他離職了，別的同事擴(kuò)容時(shí)可能也沒(méi)意識(shí)到會(huì)造成不一致的問(wèn)題。于是問(wèn)題就到了我這兒。

引入 Redis

我首先想到的解決辦法是把內(nèi)存緩存換成了 Redis，但上線灰度階段 Redis 帶寬被打滿，排查發(fā)現(xiàn)是因?yàn)橛行┮?guī)則的封禁 IP 列表很長(zhǎng)，導(dǎo)致傳輸數(shù)據(jù)量非常大。

最終方案

由于 WAF 規(guī)則讀多寫(xiě)少，絕大多數(shù)時(shí)候從 Redis 讀到的數(shù)據(jù)不會(huì)有變化。有經(jīng)驗(yàn)的老同事建議用 Redis 維護(hù)版本號(hào)，規(guī)則數(shù)據(jù)仍然存在內(nèi)存緩存中。經(jīng)過(guò)反復(fù)推敲，最終的設(shè)計(jì)的架構(gòu)如下。

讀寫(xiě)邏輯：

寫(xiě)操作比較簡(jiǎn)單，使用當(dāng)前微秒時(shí)間戳作為新的版本號(hào)，做如下四件事：寫(xiě) DB，更新 redis 版本號(hào)，更新本地內(nèi)存緩存中的數(shù)據(jù)和版本號(hào)，四件事的順序可交換
讀操作稍微復(fù)雜一點(diǎn)：先讀 redis 中的版本號(hào)，如果本地版本號(hào)沒(méi)有過(guò)期（絕大多數(shù)情況）就直接從本地內(nèi)存緩存中讀數(shù)據(jù)。對(duì)于 redis 與內(nèi)存中版本號(hào)不一致和 redis 沒(méi)讀到（expired）的情況要單獨(dú)處理，處理邏輯如偽代碼所示
如果一微秒內(nèi)有多個(gè)寫(xiě)請(qǐng)求，仍然可能出現(xiàn)不一致。不過(guò) Shopee WAF 的實(shí)際使用場(chǎng)景不太會(huì)有如此頻繁的更新，所以我就沒(méi)做處理了。不過(guò)時(shí)間戳在這里只用來(lái)判等，不會(huì)比較大小，因此可以用任何一種分布式唯一 ID 解決方案替換時(shí)間戳
版本號(hào)不對(duì)用戶暴露，事實(shí)上同一版本號(hào)可能會(huì)讀到不同的規(guī)則數(shù)據(jù)，但這并不會(huì)破壞最終一致性

func Set(key, data) {
    newVer := time()
    localCacheVer.Set(newVer)
    localCacheData.Set(data)

    WriteMySQL(key, data)
    redis.Set(key, newVer, exprire=5min)
}

func Read(key) Data {
    ver := redis.Get(key)
    if ver != nil {
        if localCacheVer.Load() == ver {
            // Local cache is up-to-date, just use it
            return localCacheData.Load()
        }
    } else {    // This version has expired
        ver := time()
        res := redis.SetNX(key, ver, expire=5min)
        if res == false {
            // Another instance has proceded, use that version
            ver = redis.Get(key)
        }
    }
    
    data := ReadFromMySQL(key)
    localCacheVer.Store(ver)
    localCacheData.Store(data)
    return data
}

TLA+ 形式化驗(yàn)證

恰好當(dāng)時(shí)自學(xué)了 TLA+，順手寫(xiě)了下這個(gè)設(shè)計(jì)對(duì)應(yīng)的 TLA+ 公式，果然成功通過(guò)了最終一致性的驗(yàn)證。寫(xiě)這篇總結(jié)的時(shí)候感覺(jué)應(yīng)該是線性一致的，但沒(méi)有驗(yàn)證。
最開(kāi)始的持續(xù) 5min 的接口返回?cái)?shù)據(jù)不一致問(wèn)題成功得到了解決。

// ================ tla file ================

---- MODULE waf ----
EXTENDS Integers, TLC

VARIABLE redisVer, localVer, pc, threadVer, DBData, localData, threadData
CONSTANTS DataDomain, ProcSet, r1, r2, r3, t1, t2, t3

vars == << redisVer, localVer, pc, threadVer, localData, threadData, DBData>>

Init == /\ redisVer = -1 /\ localVer = -1 /\ localData = "" /\ DBData = ""
        /\ threadVer = [self \in ProcSet |-> -1]
        /\ pc = [self \in ProcSet |-> "A"]
        /\ threadData = [self \in ProcSet |-> ""]

RedisExpire == /\ threadData = [self \in ProcSet |-> DBData]
               /\ redisVer' = -1
               /\ DBData' \in DataDomain
               /\ UNCHANGED <<localVer, threadVer, localData, threadData, pc>>

ReadRedis(self) == /\ pc[self] = "A"
                   /\ threadVer' = [threadVer EXCEPT ![self] = redisVer]
                   /\ / /\ redisVer = -1
                         /\ pc' = [pc EXCEPT ![self] = "C"]
                      / /\ redisVer # -1
                         /\ pc' = [pc EXCEPT ![self] = "F"]
                   /\ UNCHANGED <<localVer, redisVer, localData, threadData, DBData>>

SetRedis(self) == /\ pc[self] = "C"
                  /\ / /\ redisVer # -1    * SetNX failed => use existing redis
                        /\ redisVer' = redisVer
                        /\ threadVer' = [threadVer EXCEPT ![self] = redisVer] * Not strictly the same!
                     / /\ redisVer = -1    * SetNX ok => change redis
                        /\ redisVer' \in 1600012345..1600012350
                        /\ threadVer' = [threadVer EXCEPT ![self] = redisVer']
                  /\ pc' = [pc EXCEPT ![self] = "I"]
                  /\ UNCHANGED <<localVer, localData, threadData, DBData>>

CheckLocal(self) == /\ pc[self] = "F"
                    /\ / /\ localVer = threadVer[self]    * Normal case
                          /\ threadData' = [threadData EXCEPT ![self] = localData]
                          /\ pc' = [pc EXCEPT ![self] = "H"]
                       / /\ localVer # threadVer[self]
                          /\ pc' = [pc EXCEPT ![self] = "I"]
                          /\ threadData' = threadData
                    /\ UNCHANGED <<redisVer, localVer, localData, threadVer, DBData>>

SetLocal(self) == /\ pc[self] = "I"
                  /\ localVer' = threadVer[self]
                  /\ localData' = DBData
                  /\ threadData' = [threadData EXCEPT ![self] = DBData]
                  /\ pc' = [pc EXCEPT ![self] = "H"]
                  /\ UNCHANGED <<redisVer, threadVer, DBData>>

ReturnResult(self) == /\ pc[self] = "H"
                      /\ pc' = [pc EXCEPT ![self] = "Done"]
                      /\ UNCHANGED <<redisVer, localVer, threadVer, localData, threadData, DBData>>

Again(self) == /\ pc[self] = "Done"
               /\ pc' = [pc EXCEPT ![self] = "A"]
               /\ UNCHANGED <<redisVer, localVer, threadVer, localData, threadData, DBData>>

Terminating == /\ \A self \in ProcSet: pc[self] = "Done"
               /\ UNCHANGED vars

Proceed(t) == ReadRedis(t) / SetRedis(t) / CheckLocal(t) / SetLocal(t) / ReturnResult(t) / Again(t)

Next == / RedisExpire
        / \E t \in ProcSet: Proceed(t)

FairForEveryone == \A t \in ProcSet: SF_vars(Proceed(t))

Spec == /\ Init /\ [][Next]_vars /\ FairForEveryone

symm == Permutations({r1, r2, r3}) \union Permutations({t1, t2, t3})

EventualCons == \A v \in DataDomain: DBData = v ~> threadData = [t \in ProcSet |-> v]
ECSpec == Spec /\ EventualCons


// ======= cfg file ========
SPECIFICATION Spec

CONSTANTS
    DataDomain = {r1, r2}
    r1 = r1
    r2 = r2
    r3 = r3
    ProcSet = {t1, t2, t3}
    t1 = t1
    t2 = t2
    t3 = t3

SYMMETRY symm
PROPERTIES EventualCons

到此這篇關(guān)于Redis 解決緩存一致性問(wèn)題的文章就介紹到這了,更多相關(guān)Redis 緩存一致性內(nèi)容請(qǐng)搜索腳本之家以前的文章或繼續(xù)瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！

您可能感興趣的文章: