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分布式一致性协议Raft原理与实例
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來源:http://m635674608.iteye.com/blog/2283621
1.Raft協(xié)議
1.1 Raft簡介
Raft是由Stanford提出的一種更易理解的一致性算法，意在取代目前廣為使用的Paxos算法。目前，在各種主流語言中都有了一些開源實現(xiàn)，比如本文中將使用的基于JGroups的Raft協(xié)議實現(xiàn)。關(guān)于Raft的原理，強烈推薦動畫版Raft講解。
1.2 Raft原理
在Raft中，每個結(jié)點會處于下面三種狀態(tài)中的一種：
- follower：所有結(jié)點都以follower的狀態(tài)開始。如果沒收到leader消息則會變成candidate狀態(tài)
- candidate：會向其他結(jié)點“拉選票”，如果得到大部分的票則成為leader。這個過程就叫做Leader選舉(Leader Election)
- leader：所有對系統(tǒng)的修改都會先經(jīng)過leader。每個修改都會寫一條日志(log entry)。leader收到修改請求后的過程如下，這個過程叫做日志復(fù)制(Log Replication)：?
- 復(fù)制日志到所有follower結(jié)點(replicate entry)
- 大部分結(jié)點響應(yīng)時才提交日志
- 通知所有follower結(jié)點日志已提交
- 所有follower也提交日志
- 現(xiàn)在整個系統(tǒng)處于一致的狀態(tài)
1.2.1 Leader Election
當(dāng)follower在選舉超時時間(election timeout)內(nèi)未收到leader的心跳消息(append entries)，則變成candidate狀態(tài)。為了避免選舉沖突，這個超時時間是一個150~300ms之間的隨機數(shù)。
成為candidate的結(jié)點發(fā)起新的選舉期(election term)去“拉選票”：
重置自己的計時器投自己一票發(fā)送?Request Vote消息
如果接收結(jié)點在新term內(nèi)沒有投過票那它就會投給此candidate，并重置它自己的選舉超時時間。candidate拉到大部分選票就會成為leader，并定時發(fā)送心跳——Append Entries消息，去重置各個follower的計時器。當(dāng)前Term會繼續(xù)直到某個follower接收不到心跳并成為candidate。
如果不巧兩個結(jié)點同時成為candidate都去“拉票”怎么辦？這時會發(fā)生Splite Vote情況。兩個結(jié)點可能都拉到了同樣多的選票，難分勝負(fù)，選舉失敗，本term沒有l(wèi)eader。之后又有計時器超時的follower會變成candidate，將term加一并開始新一輪的投票。
1.2.2 Log Replication
當(dāng)發(fā)生改變時，leader會復(fù)制日志給follower結(jié)點，這也是通過Append Entries心跳消息完成的。前面已經(jīng)列舉了Log Replication的過程，這里就不重復(fù)了。
Raft能夠正確地處理網(wǎng)絡(luò)分區(qū)（“腦裂”）問題。假設(shè)A~E五個結(jié)點，B是leader。如果發(fā)生“腦裂”，A、B成為一個子分區(qū)，C、D、E成 為一個子分區(qū)。此時C、D、E會發(fā)生選舉，選出C作為新term的leader。這樣我們在兩個子分區(qū)內(nèi)就有了不同term的兩個leader。這時如果 有客戶端寫A時，因為B無法復(fù)制日志到大部分follower所以日志處于uncommitted未提交狀態(tài)。而同時另一個客戶端對C的寫操作卻能夠正確 完成，因為C是新的leader，它只知道D和E。
當(dāng)網(wǎng)絡(luò)通信恢復(fù)，B能夠發(fā)送心跳給C、D、E了，卻發(fā)現(xiàn)“改朝換代”了，因為C的term值更大，所以B自動降格為follower。然后A和B都回滾未提交的日志，并從新leader那里復(fù)制最新的日志。但這樣是不是就會丟失更新？
2.JGroups-raft介紹
2.1 JGroups中的Raft
JGroups是Java里比較流行的網(wǎng)絡(luò)通信框架，近期順應(yīng)潮流，它也推出了Raft基于JGroups的實現(xiàn)。簡單試用了一下，還比較容易上 手，底層Raft的內(nèi)部機制都被API屏蔽掉了。下面就通過一個分布式計數(shù)器的實例來學(xué)習(xí)一下Raft協(xié)議在JGroups中的實際用法。
Maven依賴如下：
Prettyprint代碼??
<code?class="language-xml?hljs??has-numbering">????<span?class="hljs-tag"><<span?class="hljs-title">dependency</span>></span>?? ????????<span?class="hljs-tag"><<span?class="hljs-title">groupId</span>></span>org.jgroups<span?class="hljs-tag"></<span?class="hljs-title">groupId</span>></span>?? ????????<span?class="hljs-tag"><<span?class="hljs-title">artifactId</span>></span>jgroups-raft<span?class="hljs-tag"></<span?class="hljs-title">artifactId</span>></span>?? ????????<span?class="hljs-tag"><<span?class="hljs-title">version</span>></span>0.2<span?class="hljs-tag"></<span?class="hljs-title">version</span>></span>?? ????<span?class="hljs-tag"></<span?class="hljs-title">dependency</span>></span></code>??
其實JGroups-raft的Jar包中已經(jīng)自帶了一個Counter的Demo，但仔細(xì)看了一下，有的地方寫的有些麻煩，不太容易把握住Raft這根主線。所以這里就參照官方的例子，進行了簡寫，突出Raft協(xié)議的基本使用方法。JGroups-raft目前資料不多，InfoQ上的這篇文章很不錯，還有官方文檔。
2.2 核心API
使用JGroups-raft時，我們一般會實現(xiàn)兩個接口：RAFT.RoleChange和StateMachine：
- 實現(xiàn)RAFT.RoleChange接口的方法能通知我們當(dāng)前哪個結(jié)點是leader
- 實現(xiàn)StateMachine執(zhí)行要實現(xiàn)一致性的操作
典型單點服務(wù)實現(xiàn)方式就是：
Prettyprint代碼??
<code?class="language-java?hljs??has-numbering">JChannel?ch?=?<span?class="hljs-keyword">null</span>;?? RaftHandle?handle?=?<span?class="hljs-keyword">new</span>?RaftHandle(ch,?<span?class="hljs-keyword">this</span>);?? handle.addRoleListener(role?->?{?? ????<span?class="hljs-keyword">if</span>(role?==?Role.Leader)?? ????????<span?class="hljs-comment">//?start?singleton?services</span>?? ????<span?class="hljs-keyword">else</span>?? ????????<span?class="hljs-comment">//?stop?singleton?services</span>?? });</code>??
2.3 默認(rèn)配置
jgroups-raft.jar中已經(jīng)帶了一個raft.xml配置文件，作為實例程序我們可以直接使用它。
簡要解釋一下最核心的幾個配置項，參照GitHub上的文檔：
- UDP：IP多播配置
- raft.NO_DUPES：是否檢測新加入結(jié)點的ID與老結(jié)點有重復(fù)
- raft.ELECTION：選舉超時時間的隨機化范圍
- raft.RAFT：所有Raft集群的成員必須在這里聲明，也可以在運行時通過addServer/removeServer動態(tài)修改
- raft.REDIRECT：是否轉(zhuǎn)發(fā)請求給leader
- raft.CLIENT：在哪個IP和端口上接收客戶端請求
Prettyprint代碼??
<code?class="language-xml?hljs??has-numbering"><span?class="hljs-comment"><!--?? ??Default?stack?using?IP?multicasting.?It?is?similar?to?the?"udp"?? ??stack?in?stacks.xml,?but?doesn't?use?streaming?state?transfer?and?flushing?? ??author:?Bela?Ban?? --></span>?? ?? <span?class="hljs-tag"><<span?class="hljs-title">config</span>?<span?class="hljs-attribute">xmlns</span>=<span?class="hljs-value">"urn:org:jgroups"</span>?? ????????<span?class="hljs-attribute">xmlns:xsi</span>=<span?class="hljs-value">"http://www.w3.org/2001/XMLSchema-instance"</span>?? ????????<span?class="hljs-attribute">xsi:schemaLocation</span>=<span?class="hljs-value">"urn:org:jgroups?http://www.jgroups.org/schema/jgroups.xsd"</span>></span>?? ????<span?class="hljs-tag"><<span?class="hljs-title">UDP?? </span>?????????<span?class="hljs-attribute">mcast_addr</span>=<span?class="hljs-value">"228.5.5.5"</span>?? ?????????<span?class="hljs-attribute">mcast_port</span>=<span?class="hljs-value">"${jgroups.udp.mcast_port:45588}"</span>?? ?????????<span?class="hljs-attribute">...</span>?/></span>?? ????...?? ????<span?class="hljs-tag"><<span?class="hljs-title">raft.NO_DUPES</span>/></span>?? ????<span?class="hljs-tag"><<span?class="hljs-title">raft.ELECTION</span>?<span?class="hljs-attribute">election_min_interval</span>=<span?class="hljs-value">"100"</span>?<span?class="hljs-attribute">election_max_interval</span>=<span?class="hljs-value">"500"</span>/></span>?? ????<span?class="hljs-tag"><<span?class="hljs-title">raft.RAFT</span>?<span?class="hljs-attribute">members</span>=<span?class="hljs-value">"A,B,C"</span>?<span?class="hljs-attribute">raft_id</span>=<span?class="hljs-value">"${raft_id:undefined}"</span>/></span>?? ????<span?class="hljs-tag"><<span?class="hljs-title">raft.REDIRECT</span>/></span>?? ????<span?class="hljs-tag"><<span?class="hljs-title">raft.CLIENT</span>?<span?class="hljs-attribute">bind_addr</span>=<span?class="hljs-value">"0.0.0.0"</span>?/></span>?? <span?class="hljs-tag"></<span?class="hljs-title">config</span>></span></code>??
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3.JGroups-raft實例
實例很簡單，只有JGroupsRaftTest和CounterService兩個類組成。JGroupsRaftTest是測試啟動類，而CounterService就是利用Raft協(xié)議實現(xiàn)的分布式計數(shù)服務(wù)類。
3.1 JGroupsRaftTest
JGroupsRaftTest的職責(zé)主要有三個：
- 創(chuàng)建Raft協(xié)議的JChannel
- 創(chuàng)建CounterService
- 循環(huán)讀取用戶輸入
目前簡單實現(xiàn)了幾種操作包括：初始化計數(shù)器、加一、減一、讀取計數(shù)器、查看Raft日志、做Raft快照（用于壓縮日志文件）等。其中對計數(shù)器的操作，因為要與其他Raft成員進行分布式通信，所以當(dāng)前集群必須要多于一個結(jié)點時才能進行操作。如果要支持單結(jié)點時的操作，需要做特殊處理。
Prettyprint代碼??
<code?class="language-java?hljs??has-numbering"><span?class="hljs-keyword">import</span>?org.jgroups.JChannel;?? <span?class="hljs-keyword">import</span>?org.jgroups.protocols.raft.RAFT;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.Util;?? ?? <span?class="hljs-javadoc">/**?? ?*?Test?jgroups?raft?algorithm?implementation.?? ?*/</span>?? <span?class="hljs-keyword">public</span>?<span?class="hljs-class"><span?class="hljs-keyword">class</span>?<span?class="hljs-title">JGroupsRaftTest</span>?{</span>?? ?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">final</span>?String?CLUSTER_NAME?=?<span?class="hljs-string">"ctr-cluster"</span>;?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">final</span>?String?COUNTER_NAME?=?<span?class="hljs-string">"counter"</span>;?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">final</span>?String?RAFT_XML?=?<span?class="hljs-string">"raft.xml"</span>;?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">main</span>(String[]?args)?<span?class="hljs-keyword">throws</span>?Exception?{?? ????????JChannel?ch?=?<span?class="hljs-keyword">new</span>?JChannel(RAFT_XML).name(args[<span?class="hljs-number">0</span>]);?? ????????CounterService?counter?=?<span?class="hljs-keyword">new</span>?CounterService(ch);?? ?? ????????<span?class="hljs-keyword">try</span>?{?? ????????????doConnect(ch,?CLUSTER_NAME);?? ????????????doLoop(ch,?counter);?? ????????}?<span?class="hljs-keyword">finally</span>?{?? ????????????Util.close(ch);?? ????????}?? ????}?? ?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">doConnect</span>(JChannel?ch,?String?clusterName)?<span?class="hljs-keyword">throws</span>?Exception?{?? ????????ch.connect(clusterName);?? ????}?? ?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">static</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">doLoop</span>(JChannel?ch,?CounterService?counter)?{?? ????????<span?class="hljs-keyword">boolean</span>?looping?=?<span?class="hljs-keyword">true</span>;?? ????????<span?class="hljs-keyword">while</span>?(looping)?{?? ????????????<span?class="hljs-keyword">int</span>?key?=?Util.keyPress(<span?class="hljs-string">"\n[0]?Create?[1]?Increment?[2]?Decrement?[3]?Dump?log?[4]?Snapshot?[x]?Exit\n"</span>?+?? ????????????????????<span?class="hljs-string">"first-applied="</span>?+?((RAFT)?ch.getProtocolStack().findProtocol(RAFT.class)).log().firstApplied()?+?? ????????????????????<span?class="hljs-string">",?last-applied="</span>?+?counter.lastApplied()?+?? ????????????????????<span?class="hljs-string">",?commit-index="</span>?+?counter.commitIndex()?+?? ????????????????????<span?class="hljs-string">",?log?size="</span>?+?Util.printBytes(counter.logSize())?+?<span?class="hljs-string">":?"</span>);?? ?? ????????????<span?class="hljs-keyword">if</span>?((key?==?<span?class="hljs-string">'0'</span>?||?key?==?<span?class="hljs-string">'1'</span>?||?key?==?<span?class="hljs-string">'2'</span>)?&&?!counter.isLeaderExist())?{?? ????????????????System.out.println(<span?class="hljs-string">"Cannot?perform?cause?there?is?no?leader?by?now"</span>);?? ????????????????<span?class="hljs-keyword">continue</span>;?? ????????????}?? ?? ????????????<span?class="hljs-keyword">long</span>?val;?? ????????????<span?class="hljs-keyword">switch</span>?(key)?{?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'0'</span>:?? ????????????????????counter.getOrCreateCounter(COUNTER_NAME,?<span?class="hljs-number">1</span>L);?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'1'</span>:?? ????????????????????val?=?counter.incrementAndGet(COUNTER_NAME);?? ????????????????????System.out.printf(<span?class="hljs-string">"%s:?%s\n"</span>,?COUNTER_NAME,?val);?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'2'</span>:?? ????????????????????val?=?counter.decrementAndGet(COUNTER_NAME);?? ????????????????????System.out.printf(<span?class="hljs-string">"%s:?%s\n"</span>,?COUNTER_NAME,?val);?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'3'</span>:?? ????????????????????counter.dumpLog();?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'4'</span>:?? ????????????????????counter.snapshot();?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'x'</span>:?? ????????????????????looping?=?<span?class="hljs-keyword">false</span>;?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????<span?class="hljs-keyword">case</span>?<span?class="hljs-string">'\n'</span>:?? ????????????????????System.out.println(COUNTER_NAME?+?<span?class="hljs-string">":?"</span>?+?counter.get(COUNTER_NAME)?+?<span?class="hljs-string">"\n"</span>);?? ????????????????????<span?class="hljs-keyword">break</span>;?? ????????????}?? ????????}?? ????}?? ?? }</code>??
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3.2 CounterService
CounterService是我們的核心類，利用Raft實現(xiàn)了分布式的計數(shù)器操作，它的API主要由四部分組成：
- Raft Local API：操作本地Raft的狀態(tài)，像日志大小、做快照等
- Raft API：實現(xiàn)Raft的監(jiān)聽器和狀態(tài)機的方法?
- roleChanged：本地Raft的角色發(fā)生變化
- apply：分布式通信消息
- readContentFrom/writeContentTo：讀寫快照
- Counter API：計數(shù)器的分布式API
- Counter Native API：計數(shù)器的本地API。直接使用的話相當(dāng)于臟讀
Prettyprint代碼??
<code?class="language-java?hljs??has-numbering"><span?class="hljs-keyword">import</span>?org.jgroups.Channel;?? <span?class="hljs-keyword">import</span>?org.jgroups.protocols.raft.RAFT;?? <span?class="hljs-keyword">import</span>?org.jgroups.protocols.raft.Role;?? <span?class="hljs-keyword">import</span>?org.jgroups.protocols.raft.StateMachine;?? <span?class="hljs-keyword">import</span>?org.jgroups.raft.RaftHandle;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.AsciiString;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.Bits;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.ByteArrayDataInputStream;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.ByteArrayDataOutputStream;?? <span?class="hljs-keyword">import</span>?org.jgroups.util.Util;?? ?? <span?class="hljs-keyword">import</span>?java.io.DataInput;?? <span?class="hljs-keyword">import</span>?java.io.DataOutput;?? <span?class="hljs-keyword">import</span>?java.io.IOException;?? <span?class="hljs-keyword">import</span>?java.text.SimpleDateFormat;?? <span?class="hljs-keyword">import</span>?java.util.Date;?? <span?class="hljs-keyword">import</span>?java.util.HashMap;?? <span?class="hljs-keyword">import</span>?java.util.Map;?? ?? <span?class="hljs-javadoc">/**?? ?*?Distribute?counter?service?based?on?Raft?consensus?algorithm.?? ?*/</span>?? class?CounterService?implements?StateMachine,?RAFT.RoleChange?{?? ?? ????<span?class="hljs-keyword">private</span>?RaftHandle?raft;?? ?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">final</span>?Map<String,?Long>?counters;?? ?? ????<span?class="hljs-keyword">private</span>?<span?class="hljs-keyword">enum</span>?Command?{?? ????????CREATE,?INCREMENT_AND_GET,?DECREMENT_AND_GET,?GET,?SET?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-title">CounterService</span>(Channel?ch)?{?? ????????<span?class="hljs-keyword">this</span>.raft?=?<span?class="hljs-keyword">new</span>?RaftHandle(ch,?<span?class="hljs-keyword">this</span>);?? ????????<span?class="hljs-keyword">this</span>.counters?=?<span?class="hljs-keyword">new</span>?HashMap<>();?? ?? ????????raft.raftId(ch.getName())?? ????????????.addRoleListener(<span?class="hljs-keyword">this</span>);?? ????}?? ?? ????<span?class="hljs-comment">//?===========================================</span>?? ????<span?class="hljs-comment">//??????????????Raft?Status?API</span>?? ????<span?class="hljs-comment">//?===========================================</span>?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">int</span>?<span?class="hljs-title">lastApplied</span>()?{?? ????????<span?class="hljs-keyword">return</span>?raft.lastApplied();?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">int</span>?<span?class="hljs-title">commitIndex</span>()?{?? ????????<span?class="hljs-keyword">return</span>?raft.commitIndex();?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">int</span>?<span?class="hljs-title">logSize</span>()?{?? ????????<span?class="hljs-keyword">return</span>?raft.logSize();?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">dumpLog</span>()?{?? ????????System.out.println(<span?class="hljs-string">"\nindex?(term):?command\n---------------------"</span>);?? ????????raft.logEntries((entry,?index)?->?{?? ????????????StringBuilder?log?=?<span?class="hljs-keyword">new</span>?StringBuilder()?? ????????????????????.append(index)?? ????????????????????.append(<span?class="hljs-string">"?("</span>).append(entry.term()).append(<span?class="hljs-string">"):?"</span>);?? ?? ????????????<span?class="hljs-keyword">if</span>?(entry.command()?==?<span?class="hljs-keyword">null</span>?)?{?? ????????????????System.out.println(log.append(<span?class="hljs-string">"<marker?record>"</span>));?? ????????????????<span?class="hljs-keyword">return</span>;?? ????????????}?<span?class="hljs-keyword">else</span>?<span?class="hljs-keyword">if</span>?(entry.internal())?{?? ????????????????System.out.println(log.append(<span?class="hljs-string">"<internal?command>"</span>));?? ????????????????<span?class="hljs-keyword">return</span>;?? ????????????}?? ?? ????????????ByteArrayDataInputStream?in?=?<span?class="hljs-keyword">new</span>?ByteArrayDataInputStream(?? ????????????????????entry.command(),?entry.offset(),?entry.length()?? ????????????);?? ????????????<span?class="hljs-keyword">try</span>?{?? ????????????????Command?cmd?=?Command.values()[in.readByte()];?? ????????????????String?name?=?Bits.readAsciiString(in).toString();?? ????????????????<span?class="hljs-keyword">switch</span>?(cmd)?{?? ????????????????????<span?class="hljs-keyword">case</span>?CREATE:?? ????????????????????????log.append(cmd)?? ????????????????????????????.append(<span?class="hljs-string">"("</span>).append(name).append(<span?class="hljs-string">",?"</span>)?? ????????????????????????????.append(Bits.readLong(in))?? ????????????????????????????.append(<span?class="hljs-string">")"</span>);?? ????????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????????<span?class="hljs-keyword">case</span>?GET:?? ????????????????????<span?class="hljs-keyword">case</span>?INCREMENT_AND_GET:?? ????????????????????<span?class="hljs-keyword">case</span>?DECREMENT_AND_GET:?? ????????????????????????log.append(cmd)?? ????????????????????????????.append(<span?class="hljs-string">"("</span>).append(name).append(<span?class="hljs-string">")"</span>);?? ????????????????????????<span?class="hljs-keyword">break</span>;?? ????????????????????<span?class="hljs-keyword">default</span>:?? ????????????????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?IllegalArgumentException(<span?class="hljs-string">"Command?"</span>?+?cmd?+?<span?class="hljs-string">"is?unknown"</span>);?? ????????????????}?? ????????????????System.out.println(log);?? ????????????}?? ????????????<span?class="hljs-keyword">catch</span>?(IOException?e)?{?? ????????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?IllegalStateException(<span?class="hljs-string">"Error?when?dump?log"</span>,?e);?? ????????????}?? ????????});?? ????????System.out.println();?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">snapshot</span>()?{?? ????????<span?class="hljs-keyword">try</span>?{?? ????????????raft.snapshot();?? ????????}?<span?class="hljs-keyword">catch</span>?(Exception?e)?{?? ????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?IllegalStateException(<span?class="hljs-string">"Error?when?snapshot"</span>,?e);?? ????????}?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">boolean</span>?<span?class="hljs-title">isLeaderExist</span>()?{?? ????????<span?class="hljs-keyword">return</span>?raft.leader()?!=?<span?class="hljs-keyword">null</span>;?? ????}?? ?? ????<span?class="hljs-comment">//?===========================================</span>?? ????<span?class="hljs-comment">//??????????????Raft?API</span>?? ????<span?class="hljs-comment">//?===========================================</span>?? ?? ????<span?class="hljs-annotation">@Override</span>?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">roleChanged</span>(Role?role)?{?? ????????System.out.println(<span?class="hljs-string">"roleChanged?to:?"</span>?+?role);?? ????}?? ?? ????<span?class="hljs-annotation">@Override</span>?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">byte</span>[]?<span?class="hljs-title">apply</span>(<span?class="hljs-keyword">byte</span>[]?data,?<span?class="hljs-keyword">int</span>?offset,?<span?class="hljs-keyword">int</span>?length)?<span?class="hljs-keyword">throws</span>?Exception?{?? ????????ByteArrayDataInputStream?in?=?<span?class="hljs-keyword">new</span>?ByteArrayDataInputStream(data,?offset,?length);?? ????????Command?cmd?=?Command.values()[in.readByte()];?? ????????String?name?=?Bits.readAsciiString(in).toString();?? ????????System.out.println(<span?class="hljs-string">"["</span>?+?<span?class="hljs-keyword">new</span>?SimpleDateFormat(<span?class="hljs-string">"HH:mm:ss.SSS"</span>).format(<span?class="hljs-keyword">new</span>?Date())?? ????????????????+?<span?class="hljs-string">"]?Apply:?cmd=["</span>?+?cmd?+?<span?class="hljs-string">"]"</span>);?? ?? ????????<span?class="hljs-keyword">long</span>?v1,?retVal;?? ????????<span?class="hljs-keyword">switch</span>?(cmd)?{?? ????????????<span?class="hljs-keyword">case</span>?CREATE:?? ????????????????v1?=?Bits.readLong(in);?? ????????????????retVal?=?create0(name,?v1);?? ????????????????<span?class="hljs-keyword">return</span>?Util.objectToByteBuffer(retVal);?? ????????????<span?class="hljs-keyword">case</span>?GET:?? ????????????????retVal?=?get0(name);?? ????????????????<span?class="hljs-keyword">return</span>?Util.objectToByteBuffer(retVal);?? ????????????<span?class="hljs-keyword">case</span>?INCREMENT_AND_GET:?? ????????????????retVal?=?add0(name,?<span?class="hljs-number">1</span>L);?? ????????????????<span?class="hljs-keyword">return</span>?Util.objectToByteBuffer(retVal);?? ????????????<span?class="hljs-keyword">case</span>?DECREMENT_AND_GET:?? ????????????????retVal?=?add0(name,?-<span?class="hljs-number">1</span>L);?? ????????????????<span?class="hljs-keyword">return</span>?Util.objectToByteBuffer(retVal);?? ????????????<span?class="hljs-keyword">default</span>:?? ????????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?IllegalArgumentException(<span?class="hljs-string">"Command?"</span>?+?cmd?+?<span?class="hljs-string">"is?unknown"</span>);?? ????????}?? ????}?? ?? ????<span?class="hljs-annotation">@Override</span>?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">readContentFrom</span>(DataInput?in)?<span?class="hljs-keyword">throws</span>?Exception?{?? ????????<span?class="hljs-keyword">int</span>?size?=?in.readInt();?? ????????System.out.println(<span?class="hljs-string">"ReadContentFrom:?size=["</span>?+?size?+?<span?class="hljs-string">"]"</span>);?? ????????<span?class="hljs-keyword">for</span>?(<span?class="hljs-keyword">int</span>?i?=?<span?class="hljs-number">0</span>;?i?<?size;?i++)?{?? ????????????AsciiString?name?=?Bits.readAsciiString(in);?? ????????????Long?value?=?Bits.readLong(in);?? ????????????counters.put(name.toString(),?value);?? ????????}?? ????}?? ?? ????<span?class="hljs-annotation">@Override</span>?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">writeContentTo</span>(DataOutput?out)?<span?class="hljs-keyword">throws</span>?Exception?{?? ????????<span?class="hljs-keyword">synchronized</span>?(counters)?{?? ????????????<span?class="hljs-keyword">int</span>?size?=?counters.size();?? ????????????System.out.println(<span?class="hljs-string">"WriteContentFrom:?size=["</span>?+?size?+?<span?class="hljs-string">"]"</span>);?? ????????????out.writeInt(size);?? ????????????<span?class="hljs-keyword">for</span>?(Map.Entry<String,?Long>?entry?:?counters.entrySet())?{?? ????????????????AsciiString?name?=?<span?class="hljs-keyword">new</span>?AsciiString(entry.getKey());?? ????????????????Long?value?=?entry.getValue();?? ????????????????Bits.writeAsciiString(name,?out);?? ????????????????Bits.writeLong(value,?out);?? ????????????}?? ????????}?? ????}?? ?? ????<span?class="hljs-comment">//?===========================================</span>?? ????<span?class="hljs-comment">//??????????????Counter?API</span>?? ????<span?class="hljs-comment">//?===========================================</span>?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">void</span>?<span?class="hljs-title">getOrCreateCounter</span>(String?name,?<span?class="hljs-keyword">long</span>?initVal)?{?? ????????Object?retVal?=?invoke(Command.CREATE,?name,?<span?class="hljs-keyword">false</span>,?initVal);?? ????????counters.put(name,?(Long)?retVal);?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">long</span>?<span?class="hljs-title">incrementAndGet</span>(String?name)?{?? ????????<span?class="hljs-keyword">return</span>?(<span?class="hljs-keyword">long</span>)?invoke(Command.INCREMENT_AND_GET,?name,?<span?class="hljs-keyword">false</span>);?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">long</span>?<span?class="hljs-title">decrementAndGet</span>(String?name)?{?? ????????<span?class="hljs-keyword">return</span>?(<span?class="hljs-keyword">long</span>)?invoke(Command.DECREMENT_AND_GET,?name,?<span?class="hljs-keyword">false</span>);?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">long</span>?<span?class="hljs-title">get</span>(String?name)?{?? ????????<span?class="hljs-keyword">return</span>?(<span?class="hljs-keyword">long</span>)?invoke(Command.GET,?name,?<span?class="hljs-keyword">false</span>);?? ????}?? ?? ????<span?class="hljs-keyword">private</span>?Object?<span?class="hljs-title">invoke</span>(Command?cmd,?String?name,?<span?class="hljs-keyword">boolean</span>?ignoreRetVal,?<span?class="hljs-keyword">long</span>...?values)?{?? ????????ByteArrayDataOutputStream?out?=?<span?class="hljs-keyword">new</span>?ByteArrayDataOutputStream(<span?class="hljs-number">256</span>);?? ????????<span?class="hljs-keyword">try</span>?{?? ????????????out.writeByte(cmd.ordinal());?? ????????????Bits.writeAsciiString(<span?class="hljs-keyword">new</span>?AsciiString(name),?out);?? ????????????<span?class="hljs-keyword">for</span>?(<span?class="hljs-keyword">long</span>?val?:?values)?{?? ????????????????Bits.writeLong(val,?out);?? ????????????}?? ?? ????????????<span?class="hljs-keyword">byte</span>[]?rsp?=?raft.set(out.buffer(),?<span?class="hljs-number">0</span>,?out.position());?? ????????????<span?class="hljs-keyword">return</span>?ignoreRetVal???<span?class="hljs-keyword">null</span>?:?Util.objectFromByteBuffer(rsp);?? ????????}?? ????????<span?class="hljs-keyword">catch</span>?(IOException?ex)?{?? ????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?RuntimeException(<span?class="hljs-string">"Serialization?failure?(cmd="</span>?? ????????????????????+?cmd?+?<span?class="hljs-string">",?name="</span>?+?name?+?<span?class="hljs-string">")"</span>,?ex);?? ????????}?? ????????<span?class="hljs-keyword">catch</span>?(Exception?ex)?{?? ????????????<span?class="hljs-keyword">throw</span>?<span?class="hljs-keyword">new</span>?RuntimeException(<span?class="hljs-string">"Raft?set?failure?(cmd="</span>?? ????????????????????+?cmd?+?<span?class="hljs-string">",?name="</span>?+?name?+?<span?class="hljs-string">")"</span>,?ex);?? ????????}?? ????}?? ?? ????<span?class="hljs-comment">//?===========================================</span>?? ????<span?class="hljs-comment">//??????????????Counter?Native?API</span>?? ????<span?class="hljs-comment">//?===========================================</span>?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">synchronized</span>?Long?<span?class="hljs-title">create0</span>(String?name,?<span?class="hljs-keyword">long</span>?initVal)?{?? ????????counters.putIfAbsent(name,?initVal);?? ????????<span?class="hljs-keyword">return</span>?counters.get(name);?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">synchronized</span>?Long?<span?class="hljs-title">get0</span>(String?name)?{?? ????????<span?class="hljs-keyword">return</span>?counters.getOrDefault(name,?<span?class="hljs-number">0</span>L);?? ????}?? ?? ????<span?class="hljs-keyword">public</span>?<span?class="hljs-keyword">synchronized</span>?Long?<span?class="hljs-title">add0</span>(String?name,?<span?class="hljs-keyword">long</span>?delta)?{?? ????????Long?oldVal?=?counters.getOrDefault(name,?<span?class="hljs-number">0</span>L);?? ????????<span?class="hljs-keyword">return</span>?counters.put(name,?oldVal?+?delta);?? ????}?? }</code>??
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3.3 運行測試
我們分別以A、B、C為參數(shù)，啟動三個JGroupsRaftTest服務(wù)。這樣會自動在C:\Users\cdai\AppData\Local\Temp下生成A.log、B.log、C.log三個日志文件夾。
Prettyprint代碼??
<code?class="hljs?livecodeserver?has-numbering">cdai@vm?/cygdrive/c/Users/cdai/AppData/Local/Temp?? $?tree?A.<span?class="hljs-built_in">log</span>/?B.<span?class="hljs-built_in">log</span>/?C.<span?class="hljs-built_in">log</span>/?? A.<span?class="hljs-built_in">log</span>/?? |<span?class="hljs-comment">--?000005.sst</span>?? |<span?class="hljs-comment">--?000006.log</span>?? |<span?class="hljs-comment">--?CURRENT</span>?? |<span?class="hljs-comment">--?LOCK</span>?? |<span?class="hljs-comment">--?LOG</span>?? |<span?class="hljs-comment">--?LOG.old</span>?? `<span?class="hljs-comment">--?MANIFEST-000004</span>?? B.<span?class="hljs-built_in">log</span>/?? |<span?class="hljs-comment">--?000003.log</span>?? |<span?class="hljs-comment">--?CURRENT</span>?? |<span?class="hljs-comment">--?LOCK</span>?? |<span?class="hljs-comment">--?LOG</span>?? `<span?class="hljs-comment">--?MANIFEST-000002</span>?? C.<span?class="hljs-built_in">log</span>/?? |<span?class="hljs-comment">--?000003.log</span>?? |<span?class="hljs-comment">--?CURRENT</span>?? |<span?class="hljs-comment">--?LOCK</span>?? |<span?class="hljs-comment">--?LOG</span>?? `<span?class="hljs-comment">--?MANIFEST-000002</span>?? ?? <span?class="hljs-number">0</span>?<span?class="hljs-built_in">directories</span>,?<span?class="hljs-number">17</span>?<span?class="hljs-built_in">files</span></code>??
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3.3.1 分布式一致性
首先A創(chuàng)建計數(shù)器，B“加一”，C“減一”。可以看到盡管我們是分別在A、B、C上執(zhí)行這三個操作，但三個結(jié)點都先后（leader提交日志后通知follower）通過apply()方法收到消息，并在本地的計數(shù)器Map上同步執(zhí)行操作，保證了數(shù)據(jù)的一致性。最后停掉A服務(wù)，可以看到B通過roleChanged()得到消息，提升為新的Leader，并與C一同繼續(xù)提供服務(wù)。
A的控制臺輸出：
Prettyprint代碼??
<code?class="hljs?asciidoc?has-numbering"><span?class="hljs-code">-------------------------------------------------------------------?? GMS:?address=A,?cluster=ctr-cluster,?physical?address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:50100?? -------------------------------------------------------------------</span>?? ?? [0]?Create?[1]?Increment?[2]?Decrement?[3]?Dump?log?[4]?Snapshot?[x]?Exit?? first-applied=0,?last-applied=0,?commit-index=0,?log?size=0b:??? roleChanged?to:?Candidate?? roleChanged?to:?Leader?? 0?? <span?class="hljs-attribute">[14:16:00.744]?Apply:?cmd=[CREATE]</span>?? ?? [0]?Create?[1]?Increment?[2]?Decrement?[3]?Dump?log?[4]?Snapshot?[x]?Exit?? first-applied=0,?last-applied=1,?commit-index=1,?log?size=1b:??? <span?class="hljs-attribute">[14:16:07.002]?Apply:?cmd=[INCREMENT_AND_GET]</span>?? <span?class="hljs-attribute">[14:16:14.264]?Apply:?cmd=[DECREMENT_AND_GET]</span>?? 3?? ?? <span?class="hljs-header">index?(term):?command?? ---------------------</span>?? 1?(29):?CREATE(counter,?1)?? 2?(29):?INCREMENT<span?class="hljs-emphasis">_AND_</span>GET(counter)?? 3?(29):?DECREMENT<span?class="hljs-emphasis">_AND_</span>GET(counter)</code>??
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B的控制臺輸出：
Prettyprint代碼??
<code?class="hljs?sql?has-numbering"><span?class="hljs-comment">-------------------------------------------------------------------</span>?? GMS:?address=B,?cluster=ctr-cluster,?physical?address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:50101?? <span?class="hljs-comment">-------------------------------------------------------------------</span>?? ?? [0]?<span?class="hljs-operator"><span?class="hljs-keyword">Create</span>?[<span?class="hljs-number">1</span>]?Increment?[<span?class="hljs-number">2</span>]?Decrement?[<span?class="hljs-number">3</span>]?Dump?log?[<span?class="hljs-number">4</span>]?Snapshot?[x]?Exit?? <span?class="hljs-keyword">first</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">last</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">commit</span>-index=<span?class="hljs-number">0</span>,?log?<span?class="hljs-keyword">size</span>=<span?class="hljs-number">0</span>b:??? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">01.300</span>]?Apply:?cmd=[<span?class="hljs-keyword">CREATE</span>]?? <span?class="hljs-number">1</span>?? counter:?<span?class="hljs-number">2</span>?? ?? [<span?class="hljs-number">0</span>]?<span?class="hljs-keyword">Create</span>?[<span?class="hljs-number">1</span>]?Increment?[<span?class="hljs-number">2</span>]?Decrement?[<span?class="hljs-number">3</span>]?Dump?log?[<span?class="hljs-number">4</span>]?Snapshot?[x]?Exit?? <span?class="hljs-keyword">first</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">last</span>-applied=<span?class="hljs-number">2</span>,?<span?class="hljs-keyword">commit</span>-index=<span?class="hljs-number">1</span>,?log?<span?class="hljs-keyword">size</span>=<span?class="hljs-number">2</span>b:??? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">07.299</span>]?Apply:?cmd=[INCREMENT_AND_GET]?? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">14.304</span>]?Apply:?cmd=[DECREMENT_AND_GET]?? roleChanged?<span?class="hljs-keyword">to</span>:?Candidate?? roleChanged?<span?class="hljs-keyword">to</span>:?Leader</span></code>??
C的控制臺輸出：
Prettyprint代碼??
<code?class="hljs?sql?has-numbering"><span?class="hljs-comment">-------------------------------------------------------------------</span>?? GMS:?address=C,?cluster=ctr-cluster,?physical?address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:55800?? <span?class="hljs-comment">-------------------------------------------------------------------</span>?? ?? [0]?<span?class="hljs-operator"><span?class="hljs-keyword">Create</span>?[<span?class="hljs-number">1</span>]?Increment?[<span?class="hljs-number">2</span>]?Decrement?[<span?class="hljs-number">3</span>]?Dump?log?[<span?class="hljs-number">4</span>]?Snapshot?[x]?Exit?? <span?class="hljs-keyword">first</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">last</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">commit</span>-index=<span?class="hljs-number">0</span>,?log?<span?class="hljs-keyword">size</span>=<span?class="hljs-number">0</span>b:??? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">01.300</span>]?Apply:?cmd=[<span?class="hljs-keyword">CREATE</span>]?? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">07.299</span>]?Apply:?cmd=[INCREMENT_AND_GET]?? <span?class="hljs-number">2</span>?? counter:?<span?class="hljs-number">3</span>?? ?? [<span?class="hljs-number">0</span>]?<span?class="hljs-keyword">Create</span>?[<span?class="hljs-number">1</span>]?Increment?[<span?class="hljs-number">2</span>]?Decrement?[<span?class="hljs-number">3</span>]?Dump?log?[<span?class="hljs-number">4</span>]?Snapshot?[x]?Exit?? <span?class="hljs-keyword">first</span>-applied=<span?class="hljs-number">0</span>,?<span?class="hljs-keyword">last</span>-applied=<span?class="hljs-number">3</span>,?<span?class="hljs-keyword">commit</span>-index=<span?class="hljs-number">2</span>,?log?<span?class="hljs-keyword">size</span>=<span?class="hljs-number">3</span>b:??? [<span?class="hljs-number">14</span>:<span?class="hljs-number">16</span>:<span?class="hljs-number">14.304</span>]?Apply:?cmd=[DECREMENT_AND_GET]</span></code>??
3.3.2 服務(wù)恢復(fù)
在只有B和C的集群中，我們執(zhí)行了一次“加一”。當(dāng)我們重新啟動A服務(wù)時，它會自動執(zhí)行這條日志，保持與B和C的一致。從日志的index能夠看出，69是一個Term，也就是A為Leader時的“任期”，而70也就是B為Leader時。
A的控制臺輸出：
Prettyprint代碼??
<code?class="hljs?asciidoc?has-numbering"><span?class="hljs-code">-------------------------------------------------------------------?? GMS:?address=A,?cluster=ctr-cluster,?physical?address=2001:0:9d38:6abd:cbb:1f78:3f57:50f6:53237?? -------------------------------------------------------------------</span>?? ?? [0]?Create?[1]?Increment?[2]?Decrement?[3]?Dump?log?[4]?Snapshot?[x]?Exit?? first-applied=0,?last-applied=3,?commit-index=3,?log?size=3b:??? <span?class="hljs-attribute">[14:18:45.275]?Apply:?cmd=[INCREMENT_AND_GET]</span>?? <span?class="hljs-attribute">[14:18:45.277]?Apply:?cmd=[GET]</span>?? 3?? ?? <span?class="hljs-header">index?(term):?command?? ---------------------</span>?? 1?(69):?CREATE(counter,?1)?? 2?(69):?INCREMENT<span?class="hljs-emphasis">_AND_</span>GET(counter)?? 3?(69):?DECREMENT<span?class="hljs-emphasis">_AND_</span>GET(counter)?? 4?(70):?INCREMENT<span?class="hljs-emphasis">_AND_</span>GET(counter)?? 5?(70):?GET(counter)</code>??
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http://blog.csdn.net/dc_726/article/details/48832405
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