從Paxos到Raft，分佈式一致性算法解析

{"type":"doc","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"一、CAP理論和BASE理論"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"理論是指導業界實現的綱領，也是提煉了多年研究的精華，在分佈式一致性領域，最主要的指導理論是CAP和BASE兩個。"}]},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"1. CAP理論"}]},{"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":"CAP理論是Eric Brewer教授在2000年提出 的，是描述分佈式一致性的三個維度，分別是指："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（1）一致性（Consistency）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"每次讀操作都能保證返回的是最新數據；在分佈式系統中，如果能針對一個數據項的更新執行成功後，所有的請求都可以讀到其最新的值，這樣的系統就被認爲具有嚴格的一致性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（2）可用性（Availablity）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"任何一個沒有發生故障的節點，會在合理的時間內返回一個正常的結果，也就是對於每一個請求總能夠在有限時間內返回結果。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（3）分區容忍性（Partition-torlerance）"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當節點間出現網絡分區，照樣可以提供滿足一致性和可用性的服務，除非整個網絡環境都發生了故障。那麼，什麼是網絡分區呢？分區是系統中可能發生的故障之一，可能有節點暫時無法提供服務：發生了像 長時間GC、CPU死循環、鏈接池耗盡或者是網絡通信故障等問題。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/16\/16c423c830343d5e237403e10b63eeec.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"CAP指出，"},{"type":"text","marks":[{"type":"strong"}],"text":"一個分佈式系統只能滿足三項中的兩項而不可能滿足全部三項"},{"type":"text","text":"。舉例來看：如下圖所示，假設有五個節點：n1~n5 ，因網絡分區故障被分成兩組："}]},{"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":"[n1、n2]和[n3~n5]，那麼當n1處理客戶端請求時（爲了支持\"容忍網絡分區\"，即支持 P）："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"如果要保證C(一致性)，那麼它需要把消息複製到所有節點，但是網絡分區導致無法成功複製到n3~n5，所以它只能返回\"處理失敗\"的結果給客戶端。（這時系統就處於不可用狀態，即喪失了A） "}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果要保證可用性A，那麼n1就只能把消息複製到n2，而不用複製到n3~n5(或者無視複製失敗\/超時)，但n3同時也可能在處理客戶端請求，n3也爲了保證A而做了同樣的處理。那麼 [n1、n2]和[n3~n5]的狀態就不一致了，於是就喪失了C(一致性)。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null},"content":[{"type":"text","text":"如果不容忍網絡分區，則P（分區容忍性）不能滿足。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":4,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/06\/0603f4296dde3ac03a918f0ed1c91d4e.webp","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"既然CAP理論無法全部滿足，爲了指導工業實現，就需要降低標準，因此出現了BASE理論。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"2. BASE理論"}]},{"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":"BASE理論是eBay的架構師Dan Pritchett提出的，它的思想是：“即使無法做到強一致性，但每個應用都可以根據自身的業務特點，採用適當的方式來使系統達到最終一致性” 。"}]},{"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":"BASE理論有三項指標："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"基本可用（Basically Available）"},{"type":"text","text":"：是指分佈式系統在出現不可預知故障的時候，允許損失部分可用性：比如響應時間、功能降級等；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"軟狀態（ Soft State）"},{"type":"text","text":"：也稱爲弱狀態，是指允許系統中的數據存在中間狀態，並認爲該中間狀態的存在不會影響系統的整體可用性，即允許系統在不同節點之間進行數據同步的過程存在延時；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"最終一致性（ Eventual Consistency）"},{"type":"text","text":"：強調的是系統中所有的數據副本，在經過一段時間的同步後，最終能夠達成一致的狀態。"}]}]}]},{"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","marks":[{"type":"strong"}],"text":"BASE理論面向的是大型高可用、可擴展的分佈式系統。BASE通過犧牲強一致性來獲得可用性，並允許數據短時間內的不一致，但是最終達到一致狀態。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/07\/07d78e24b0e4a8f1e7d63d6fe66dd687.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"接下來，我們一起學習經典的分佈式算法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"二、經典分佈式算法"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"1. PAXOS算法"}]},{"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":"Paxos算法是Leslie Lamport 在1990年提出的，經典且完備的分佈式一致性算法。Leslie大佬也在這個算法中展示了他不同常人的智商。"}]},{"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":"該算法的目標是：在不同的節點間，對一個key的取值達成共識（同一個值）。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（1）角色"}]},{"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":"協議將系統中的節點分爲三種角色：Proposer（提案者）、Acceptor（決議者）、Leaner（學習者），他們的具體職責爲："}]},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"提案者：對key的值提出自己的值；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"決議者：對提案者的提議進行投票，選定一個提案，形成最終決策；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"學習者：學習決議者達成共識的決策。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/60\/602396b5472822d57cc394a094ced232.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（2）約束條件推導"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本小節將通過推導來引出Paxos算法的約束條件，故事靈感來自於分佈式理論：深入淺出Paxos算法【1】，在原文基礎上優化了可讀性。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"italic"}],"text":"https:\/\/my.oschina.net\/u\/150175\/blog\/2992187"}]}]},{"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":"在介紹paxos算法協議前，我們先來看一個故事，故事名叫《小明姓什麼》。在孤兒院長大的小明馬上就要步入社會了，但是孤兒院的老師還沒有找到小明的姓氏，因此決定開會解決這個問題。"}]},{"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":"在會議上，由提案者P提出姓氏，然後決議者A來決定選擇哪一個。"}]},{"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","marks":[{"type":"strong"}],"text":"場景1"},{"type":"text","text":": Pa向 A1提出“趙”， Pb向A2，A3提出“錢”。現在有超過半數的決策者接受了“錢”，所以最終的決策結果就是“錢”。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"這裏引出了一個基礎的約束條件："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P0. 當集羣中，超過半數的Acceptor接受了一個議案，那我們就可以說這個議案被選定了（Chosen）"}]}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b0\/b08b0ba2327771e7e72ed509719ffad8.webp","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"P0 是完備的條件，但是不實用。"}]},{"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","marks":[{"type":"strong"}],"text":"場景2 "},{"type":"text","text":"：Pa向 A1提出“趙”， Pb向A2提出“錢”, Pc向A3提出“孫”，這樣就出現了三個決議者各執一票，無法形成多數派決議的局面。"}]},{"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":"因此，必須允許一個決策者能夠接受多個議案，後接受的議案可以覆蓋之前的議案。這樣，Pc可以向所有決策者提起議案“孫”，形成一個多數派決議。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/0f\/0fdb3c0ea2a40c28a5f8163a991fb289.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"但是，現在Pa Pb提案者也可以利用重新提案的能力，來重新提出自己的提案，導致形成的共識重新被推翻。會議亂成了一鍋粥。因此還需要新的約束條件來保證會議正常進行。"}]},{"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":"我們提煉一下問題：在不同版本的提案中，選擇一個固定的值作爲全局決議。"}]},{"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":"Paxos算法就是爲解決這種不一致問題而提出的，算法目標有兩個："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"T1：一次選舉必須要選定一個議案（不能出現所有議案都被拒絕的情況）；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"T2：一次選舉必須只選定一個議案（不能出現兩個議案有不同的值，卻都被選定的情況）。"}]}]}]},{"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":"首先介紹一個概念 “"},{"type":"text","marks":[{"type":"strong"}],"text":"編號"},{"type":"text","text":"”：編號在算法中是唯一自增的鍵值，假設第一個提出的議案版本是n1， 接下來提出的第二個議案版本 n2 = n1+1 。有了編號之後，一條提案也變成了（Num， Value）二元組。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/16\/16fb5f9c2666c6b03eaab1266a531d34.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"爲了達成上述目標，算法提出了一組約束條件："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P1：一個Acceptor必須接受它收到的第一個議案。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P2：如果一個值爲v的議案被選定了，那麼被選定的更大編號的議案，它的值必須也是v。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"a：如果一個值爲v的議案被選定了，那麼Acceptor接受的更大編號的議案，它的值必須也是v。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"b：如果一個值爲v的議案被選定了，那麼Proposer提出的更大編號的議案，它的值必須也是v。"}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/4a\/4a080d2d322537ae8daf6ee185e737bd.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"P2 和 P2a、P2b 理解起來比較費勁。P2這個約束是爲了解決上述場景2中，形成決議後又被推翻覆蓋的問題。有了約束2之後，當形成了決議“孫”，則後續提出的議案的值必須爲“孫”。"}]},{"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":"那麼，如何才能滿足P2呢，P2a約束是在場景2的問題決策域提出的。爲了能夠滿足P2a，自然可以在問題的產生域也提出約束， 既P2b。如果算法能夠滿足P2b，也就是將解決“產生一致性問題”的時機提前。所以，P2b是P2a的充分條件，只要能滿足P2b，那P2a就自動滿足。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/82\/82f6a1ddfdaddd97de5129a74134005d.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"那麼，作爲提案者，如何提前知道一個目前的決議（多數派）議案呢？接下來，我們就提出了新的約束P2c："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P2c: 在所有Acceptor中，任意選取半數以上的集合，稱這個集合爲S。Proposal新的提案Pnew （Nnew , Vnew ）必須符合下面兩個條件之一：  1）如果S中所有Acceptor都沒有接受過議案，那麼Pnew的編號保證唯一性和遞增，Pnew的值可以是任意值。  2）如果S中有Acceptor曾經接受過議案，找出編號最大的議案PN (N,V) 。那麼Pnew的編號必須大於N，Pnew的值必須等於V。"}]}]},{"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":"在P2c中，引入了多數派集合S。根據集合S中是否有被接受的議案，來推斷全局節點是否已經形成決議。根據約束P0，如果一個議案（Nchosen, Vchosen）是被選中的，那麼它必然是被多於半數節點選中的。那麼，在任意一個多數派集合S中，一定會存在至少一個節點A，它的數據中已經決議的議案是（Nchosen, Vchosen）。"}]},{"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":"反之，如果多數派集合中沒有被決議的議案，表示此時系統中還沒有一個被選定的決議。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/87\/87462d69cef2314c6439b0bcbdb9fe00.webp","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"看這個例子，假設目前選定了（3， Va）狀態；下次發起提案時，proposer選定A3、4、5作爲集合S進行查詢；提出了編號爲4的提案，在P2c約束條件下，提案必定爲（4，Va）。"}]},{"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":"因此，滿足P2c的算法也滿足P2b算法。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"但是也可能存在這種情況，當proposer發查詢請求時，還沒有一個被確認的提案，所以發出了(4, Vb)提案，這樣還是會破壞P2b約束。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b3\/b3cd2330b6654dc94e2fb6961b49e4c7.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"因此，當一個議案在提出時（即使已經在發送的半路上了），它必須能夠知道當前已經提出的議案的最大編號。這聽起來很魔幻，畢竟跑在網線上的請求很難被召回。因此，我們提出了新的約束條件；"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P3：議案（n，v）在提出前，必須將自己的編號通知給半數以上的Acceptor。收到通知的Acceptor將n跟自己之前收到的通知進行比較，如果n更大，就將n確認爲最大編號。當半數以上Acceptor確認n是最大編號時，議案（n，v）才能正式提交。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"P4：Acceptor收到一個新的編號n，在確認n比自己之前收到的編號大時，必須承諾不再接受比n小的議案。"}]}]},{"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":"有了P3的約束，則能夠保證兩個不同編號的議案，不可能同時被認爲是最大編號；同時再加上P4約束，保證了一個決議者不會對自己接受的議案版本進行回退。"}]},{"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":"至此，Paxos算法的約束條件就介紹完了。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/bd\/bda24a806e32f9d6ec8d65bc6d50759d.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"href":null,"fromPaste":true,"pastePass":true}},{"type":"heading","attrs":{"align":null,"level":3}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（3）協議流程"}]},{"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":"在Paxos中，一個決策過程（Round、Phase）分爲兩個階段："}]},{"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","marks":[{"type":"strong"}],"text":"phase1（準備階段）："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"Proposer向超過半數（n\/2+1）Acceptor發起prepare消息(發送編號)；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"如果Acceptor 收到一個編號爲 N 的 Prepare 請求，且 N 大於該 Acceptor 已經響應過的所有 Prepare 請求的編號，那麼它就會將它已經接受過的編號最大的提案（如果有的話）作爲響應反饋給 Proposer，同時該Acceptor 承諾不再接受任何編號小於 N 的提案。否則拒絕返回。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"phase2（決議階段或投票階段）："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"numberedlist","attrs":{"start":null,"normalizeStart":1},"content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":1,"align":null,"origin":null},"content":[{"type":"text","text":"如果超過半數Acceptor回覆promise，Proposer向Acceptor發送accept消息。注意此時的V：V 就是收到的響應中編號最大的提案的，如果響應中不包含任何提案，那麼V 就由 Proposer 自己決定；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":2,"align":null,"origin":null},"content":[{"type":"text","text":"Acceptor檢查accept消息是否符合規則，只要該 Acceptor 沒有對編號大於 N 的 Prepare 請求做出過響應，它就接受該提案。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":3,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/0c\/0c57cc9442780d53b03a529ea5647551.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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","marks":[{"type":"strong"}],"text":"在實際發展中，Paxos算法也演變出一系列變種"},{"type":"text","text":"："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"PBFT（實用拜占庭容錯）算法：是一種共識算法，較高效地解決了拜占庭將軍問題；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"Multi-Paxos算法：優化了prepare階段的效率，同時允許多個Leader併發提議；以及FastPaxos、EPaxos等，這些演變是針對某些問題進行的優化，內核思想還是依託於Paxos。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":3},"content":[{"type":"text","text":"2. Raft算法"}]},{"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":"Paxos協議從被提出，一直是分佈式一致性算法的標準協議，但是它不易理解，對於工程師來說實現起來很複雜，以至於算法提出近30年都沒有一個完全版的實現方案。業界很多實現都是Paxos-like。"}]},{"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":"Raft算法是斯坦福大學2017年提出，論文名稱《In Search of an Understandable Consensus Algorithm》，望文生義，該算法的目的是易於理解。Raft這一名字來源於\"Reliable, Replicated, Redundant, And Fault-Tolerant\"（“可靠、可複製、可冗餘、可容錯”）的首字母縮寫。"}]},{"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":"Raft使用了分治思想把算法流程分爲三個子問題：選舉（Leader election）、日誌複製（Log replication）、安全性（Safety）三個子問題。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Leader選舉"},{"type":"text","text":"：當前leader跪了或集羣初始化的情況下，新leader被選舉出來。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"日誌複製"},{"type":"text","text":"：leader必須能夠從客戶端接收請求，然後將它們複製給其他機器，強制它們與自己的數據一致。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"安全性"},{"type":"text","text":"：如何保證上述選舉和日誌複製的安全，使系統滿足最終一致性。"}]}]}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（1）概念介紹"}]},{"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":"在Raft中，節點被分爲Leader Follower Cabdidate三種角色："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Leader"},{"type":"text","text":"：處理與客戶端的交互和與follower的日誌複製等，一般只有一個Leader；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Follower"},{"type":"text","text":"：被動學習Leader的日誌同步，同時也會在leader超時後轉變爲Candidate參與競選；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","marks":[{"type":"strong"}],"text":"Candidate"},{"type":"text","text":"：在競選期間參與競選；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}}]}]},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/b1\/b1a40bab271b924a959abbaac49f8182.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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","marks":[{"type":"strong"}],"text":"Term"},{"type":"text","text":"：是有連續單調遞增的編號，每個term開始於選舉階段，一般由選舉階段和領導階段組成。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/ec\/ec2cdafb4206d759b8bafd342c107737.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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","marks":[{"type":"strong"}],"text":"隨機超時時間"},{"type":"text","text":"：Follower節點每次收到Leader的心跳請求後，會設置一個隨機的，區間位於[150ms, 300ms)的超時時間。如果超過超時時間，還沒有收到Leader的下一條請求，則認爲Leader過期\/故障了。"}]},{"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","marks":[{"type":"strong"}],"text":"心跳續命"},{"type":"text","text":"：Leader在當選期間，會以一定時間間隔向其他節點發送心跳請求，以維護自己的Leader地位。"}]},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（2）協議流程"}]},{"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","marks":[{"type":"strong"}],"text":"a. 選舉流程"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"當某個follower節點在超時時間內未收到Leader的請求，它則認爲當前Leader宕機或者當前無Leader，將發起選舉， 既從一個Follower變成Candidate。"}]},{"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":"這個轉變過程中會發生四件事："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"增加本地節點的Current 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state."}]}]},{"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":"分佈式一致性算法的實現是基於複製狀態機的。"}]},{"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":"在Raft算法中，節點初始化後具有相同初始狀態。爲了提供相同的輸入指令集這個條件，raft將一個客戶端請求（command）封裝到一個log entry中。Leader負責將這些log entries 複製到所有的Follower節點，然後節點按照相同的順序應用commands，達到最終的一致狀態。"}]},{"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":"當Leader收到客戶端的寫請求，到將執行結果返回給客戶端的這個過程，從Leader視角來看經歷了以下步驟："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"本地追加日誌信息；"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"並行發出 AppendEntries 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RPC，直到所有Followers應用了所有日誌條目。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"image","attrs":{"src":"https:\/\/static001.geekbang.org\/infoq\/29\/295990d30c020563b7af89322f1578be.png","alt":"圖片","title":null,"style":[{"key":"width","value":"75%"},{"key":"bordertype","value":"none"}],"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":"logs由順序排列的log entry組成 ，每個log entry包含command和產生該log entry時的leader term。從上圖可以看到，五個節點的日誌並不完全一致，raft算法爲了保證高可用，並不是強一致性，而是最終一致性，leader會不斷嘗試給follower發log entries，直到所有節點的log 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split），這是非常嚴重的問題，會導致數據的覆蓋丟失。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"在raft中，通過兩點保證了這個屬性："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"bulletedlist","content":[{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"一個節點某一任期內最多隻能投一票；而節點B的term必須比A的新，A才能給B投票。"}]}]},{"type":"listitem","attrs":{"listStyle":null},"content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"只有獲得多數投票的節點纔會成爲leader。"}]}]}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":4},"content":[{"type":"text","text":"（2）日誌append 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index）如果日誌中含有不同的任期，則選最新的任期的節點；如果最新任期一致，則選最長的日誌的那個節點。"}]},{"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":"選舉安全性中包含了Leader完備性"}]},{"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","marks":[{"type":"strong"}],"text":"（5）狀態機安全性"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":"br"}},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"狀態機安全性由日誌的一致來保證。在算法中，一個日誌被複制到多數節點纔算committed， 如果一個log entry在某個任期被提交（committed），那麼這條日誌一定會出現在所有更高term的leader的日誌裏面。"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"heading","attrs":{"align":null,"level":2},"content":[{"type":"text","text":"三、總語"}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"    "}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"分佈式算法已經經歷了近40年的發展（1982- ），湧現出各種各樣的算法。正如Chubby的作者所說："}]},{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null}},{"type":"blockquote","content":[{"type":"paragraph","attrs":{"indent":0,"number":0,"align":null,"origin":null},"content":[{"type":"text","text":"“這個世界上只有一種一致性算法，那就是 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