《我想進大廠》之Redis奪命連環11問

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這是面試題系列第三篇--redis專題。"}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"說說Redis基本數據類型有哪些吧"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"字符串:redis沒有直接使用C語言傳統的字符串表示,而是自己實現的叫做簡單動態字符串SDS的抽象類型。C語言的字符串不記錄自身的長度信息,而SDS則保存了長度信息,這樣將獲取字符串長度的時間由O(N)降低到了O(1),同時可以避免緩衝區溢出和減少修改字符串長度時所需的內存重分配次數。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"鏈表linkedlist:redis鏈表是一個雙向無環鏈表結構,很多發佈訂閱、慢查詢、監視器功能都是使用到了鏈表來實現,每個鏈表的節點由一個listNode結構來表示,每個節點都有指向前置節點和後置節點的指針,同時表頭節點的前置和後置節點都指向NULL。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"字典hashtable:用於保存鍵值對的抽象數據結構。redis使用hash表作爲底層實現,每個字典帶有兩個hash表,供平時使用和rehash時使用,hash表使用鏈地址法來解決鍵衝突,被分配到同一個索引位置的多個鍵值對會形成一個單向鏈表,在對hash表進行擴容或者縮容的時候,爲了服務的可用性,rehash的過程不是一次性完成的,而是漸進式的。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"跳躍表skiplist:跳躍表是有序集合的底層實現之一,redis中在實現有序集合鍵和集羣節點的內部結構中都是用到了跳躍表。redis跳躍表由zskiplist和zskiplistNode組成,zskiplist用於保存跳躍表信息(表頭、表尾節點、長度等),zskiplistNode用於表示表跳躍節點,每個跳躍表的層高都是1-32的隨機數,在同一個跳躍表中,多個節點可以包含相同的分值,但是每個節點的成員對象必須是唯一的,節點按照分值大小排序,如果分值相同,則按照成員對象的大小排序。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"整數集合intset:用於保存整數值的集合抽象數據結構,不會出現重複元素,底層實現爲數組。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":6,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"壓縮列表ziplist:壓縮列表是爲節約內存而開發的順序性數據結構,他可以包含多個節點,每個節點可以保存一個字節數組或者整數值。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"基於這些基礎的數據結構,redis封裝了自己的對象系統,包含字符串對象string、列表對象list、哈希對象hash、集合對象set、有序集合對象zset,每種對象都用到了至少一種基礎的數據結構。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"redis通過encoding屬性設置對象的編碼形式來提升靈活性和效率,基於不同的場景redis會自動做出優化。不同對象的編碼如下:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"字符串對象string:int整數、embstr編碼的簡單動態字符串、raw簡單動態字符串"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"列表對象list:ziplist、linkedlist"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"哈希對象hash:ziplist、hashtable"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"集合對象set:intset、hashtable"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"有序集合對象zset:ziplist、skiplist"}]}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"Redis爲什麼快呢?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"redis的速度非常的快,單機的redis就可以支撐每秒10幾萬的併發,相對於mysql來說,性能是mysql的幾十倍。速度快的原因主要有幾點:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"完全基於內存操作"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"C語言實現,優化過的數據結構,基於幾種基礎的數據結構,redis做了大量的優化,性能極高"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"使用單線程,無上下文的切換成本"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"基於非阻塞的IO多路複用機制"}]}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"那爲什麼Redis6.0之後又改用多線程呢?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"redis使用多線程並非是完全摒棄單線程,redis還是使用單線程模型來處理客戶端的請求,只是使用多線程來處理數據的讀寫和協議解析,執行命令還是使用單線程。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣做的目的是因爲redis的性能瓶頸在於網絡IO而非CPU,使用多線程能提升IO讀寫的效率,從而整體提高redis的性能。"}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"知道什麼是熱key嗎?熱key問題怎麼解決?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"所謂熱key問題就是,突然有幾十萬的請求去訪問redis上的某個特定key,那麼這樣會造成流量過於集中,達到物理網卡上限,從而導致這臺redis的服務器宕機引發雪崩。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/db/dbf60ad2707e0f0f8513c13780c67185.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"針對熱key的解決方案:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"提前把熱key打散到不同的服務器,降低壓力"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"加入二級緩存,提前加載熱key數據到內存中,如果redis宕機,走內存查詢"}]}]}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"什麼是緩存擊穿、緩存穿透、緩存雪崩?"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"緩存擊穿"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"緩存擊穿的概念就是單個key併發訪問過高,過期時導致所有請求直接打到db上,這個和熱key的問題比較類似,只是說的點在於過期導致請求全部打到DB上而已。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"解決方案:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"加鎖更新,比如請求查詢A,發現緩存中沒有,對A這個key加鎖,同時去數據庫查詢數據,寫入緩存,再返回給用戶,這樣後面的請求就可以從緩存中拿到數據了。"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"將過期時間組合寫在value中,通過異步的方式不斷的刷新過期時間,防止此類現象。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/2c/2c91b29fb027385f045ec915e5f7f056.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"緩存穿透"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"緩存穿透是指查詢不存在緩存中的數據,每次請求都會打到DB,就像緩存不存在一樣。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/40/40ca7eb6ddc6f672e0098f43a5743567.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"針對這個問題,加一層布隆過濾器。布隆過濾器的原理是在你存入數據的時候,會通過散列函數將它映射爲一個位數組中的K個點,同時把他們置爲1。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這樣當用戶再次來查詢A,而A在布隆過濾器值爲0,直接返回,就不會產生擊穿請求打到DB了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"顯然,使用布隆過濾器之後會有一個問題就是誤判,因爲它本身是一個數組,可能會有多個值落到同一個位置,那麼理論上來說只要我們的數組長度夠長,誤判的概率就會越低,這種問題就根據實際情況來就好了。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b7/b71c876d6dea46a6b0138ccd6c4d45db.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"緩存雪崩"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當某一時刻發生大規模的緩存失效的情況,比如你的緩存服務宕機了,會有大量的請求進來直接打到DB上,這樣可能導致整個系統的崩潰,稱爲雪崩。雪崩和擊穿、熱key的問題不太一樣的是,他是指大規模的緩存都過期失效了。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/67/6727dc5a64b698dd9a1c176fa218839d.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"針對雪崩幾個解決方案:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"stron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meet命令來進行連接,節點的握手過程:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"節點A收到客戶端的cluster meet命令"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"A根據收到的IP地址和端口號,向B發送一條meet消息"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"節點B收到meet消息返回pong"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"A知道B收到了meet消息,返回一條ping消息,握手成功"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":5,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"最後,節點A將會通過gossip協議把節點B的信息傳播給集羣中的其他節點,其他節點也將和B進行握手"}]}]}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/3c/3c8e1d99a58f70abffc04c6c8bc80743.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"槽slot"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"redis通過集羣分片的形式來保存數據,整個集羣數據庫被分爲16384個slot,集羣中的每個節點可以處理0-16384個slot,當數據庫16384個slot都有節點在處理時,集羣處於上線狀態,反之只要有一個slot沒有得到處理都會處理下線狀態。通過cluster addslots命令可以將slot指派給對應節點處理。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"slot是一個位數組,數組的長度是16384/8=2048,而數組的每一位用1表示被節點處理,0表示不處理,如圖所示的話表示A節點處理0-7的slot。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/09/0902372ee0c5d5c196a82ad201fc0bfb.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當客戶端向節點發送命令,如果剛好找到slot屬於當前節點,那麼節點就執行命令,反之,則會返回一個MOVED命令到客戶端指引客戶端轉向正確的節點。(MOVED過程是自動的)"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/e5/e564d5d1199229fb67ef5b3661cddf9f.jpeg","alt":"","title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果增加或者移出節點,對於slot的重新分配也是非常方便的,redis提供了工具幫助實現slot的遷移,整個過程是完全在線的,不需要停止服務。"}]},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"故障轉移"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"如果節點A向節點B發送ping消息,節點B沒有在規定的時間內響應pong,那麼節點A會標記節點B爲pfail疑似下線狀態,同時把B的狀態通過消息的形式發送給其他節點,如果超過半數以上的節點都標記B爲pfail狀態,B就會被標記爲fail下線狀態,此時將會發生故障轉移,優先從複製數據較多的從節點選擇一個成爲主節點,並且接管下線節點的slot,整個過程和哨兵非常類似,都是基於Raft協議做選舉。"}]},{"type":"heading","attrs":{"align":null,"level":1},"content":[{"type":"text","text":"瞭解Redis事務機制嗎?"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"redis通過MULTI、EXEC、WATCH等命令來實現事務機制,事務執行過程將一系列多個命令按照順序一次性執行,並且在執行期間,事務不會被中斷,也不會去執行客戶端的其他請求,直到所有命令執行完畢。事務的執行過程如下:"}]},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"服務端收到客戶端請求,事務以MULTI開始"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"如果客戶端正處於事務狀態,則會把事務放入隊列同時返回給客戶端QUEUED,反之則直接執行這個命令"}]}]},{"type":"listitem","content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"當收到客戶端EXEC命令時,WATCH命令監視整個事務中的key是否有被修改,如果有則返回空回覆到客戶端表示失敗,否則redis會遍歷整個事務隊列,執行隊列中保存的所有命令,最後返回結果給客戶端"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"WATCH的機制本身是一個CAS的機制,被監視的key會被保存到一個鏈表中,如果某個key被修改,那麼REDIS_DIRTY_CAS標誌將會被打開,這時服務器會拒絕執行事務。"}]},{"type":"image","attrs":{"src":"https://static001.geekbang.org/infoq/b8/b87262fbfc19c33c211450107be40d5f.png","alt":null,"title":null,"style":null,"href":null,"fromPaste":true,"pastePass":true}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"- END -"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}
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