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這篇文章主要介紹“hadoop 2.4 namenode源碼分析”,在日常操作中,相信很多人在hadoop 2.4 namenode源碼分析問題上存在疑惑,小編查閱了各式資料,整理出簡單好用的操作方法,希望對大家解答”hadoop 2.4 namenode源碼分析”的疑惑有所幫助!接下來,請跟著小編一起來學(xué)習(xí)吧!
在hadoop nn的HA,對于主備節(jié)點(diǎn)的選舉,是通過ActiveStandbyElector來實(shí)現(xiàn)的。源碼上有針對該類的解釋。
小弟英文不才,翻譯一下。該類主要使用了zookeeper實(shí)現(xiàn)了主節(jié)點(diǎn)的選舉,對于成功選舉的主節(jié)點(diǎn),會在zookeeper上創(chuàng)建零時節(jié)點(diǎn)。如果創(chuàng)建成功,NN會變成active,而其余nn節(jié)點(diǎn)會成備用節(jié)點(diǎn)。
下面還是來具體分析一下ActiveStandbyElector類的作用,ActiveStandbyElector主要實(shí)現(xiàn)了選舉,選舉流程主要是通過創(chuàng)建零時節(jié)點(diǎn)的方式實(shí)現(xiàn),如果創(chuàng)建成功??梢哉J(rèn)為是獲取到對應(yīng)的LOCK,該節(jié)點(diǎn)可以成為active。如果沒有成功創(chuàng)建該節(jié)點(diǎn),可以認(rèn)為為standby節(jié)點(diǎn),對于standby節(jié)點(diǎn),需要一直監(jiān)聽該LOCK節(jié)點(diǎn)的狀態(tài)。如果發(fā)生節(jié)點(diǎn)的事件,就去嘗試選舉。基本流程就是這樣。
下面,來看一下ActiveStandbyElector類的主要方法和流程。對于熟悉zookeeper的同學(xué)來說,zookeeper的必須要實(shí)現(xiàn)watcher接口,其中可以實(shí)現(xiàn)自己的各種事件的處理邏輯。
在ActiveStandbyElector中,采用了
內(nèi)部類來實(shí)現(xiàn)Watcher接口,其process方法,調(diào)用了processWatchEvent來實(shí)現(xiàn)具體的業(yè)務(wù)處理。
下面來分析該processWatchEvent的具體邏輯:
//處理zk的事件
synchronized void processWatchEvent(ZooKeeper zk, WatchedEvent event) { Event.EventType eventType = event.getType(); if (isStaleClient(zk)) return; LOG.debug("Watcher event type: " + eventType + " with state:" + event.getState() + " for path:" + event.getPath() + " connectionState: " + zkConnectionState + " for " + this); if (eventType == Event.EventType.None) { //會話本身的時間,如連接。失去連接。 // the connection state has changed switch (event.getState()) { case SyncConnected: LOG.info("Session connected."); // if the listener was asked to move to safe state then it needs to // be undone ConnectionState prevConnectionState = zkConnectionState; zkConnectionState = ConnectionState.CONNECTED; if (prevConnectionState == ConnectionState.DISCONNECTED && wantToBeInElection) { monitorActiveStatus();//監(jiān)控節(jié)點(diǎn) } break; case Disconnected: LOG.info("Session disconnected. Entering neutral mode..."); // ask the app to move to safe state because zookeeper connection // is not active and we dont know our state zkConnectionState = ConnectionState.DISCONNECTED; enterNeutralMode(); break; case Expired: // the connection got terminated because of session timeout // call listener to reconnect LOG.info("Session expired. Entering neutral mode and rejoining..."); enterNeutralMode(); reJoinElection(0);//參與選舉 break; case SaslAuthenticated: LOG.info("Successfully authenticated to ZooKeeper using SASL."); break; default: fatalError("Unexpected Zookeeper watch event state: " + event.getState()); break; } return; } // a watch on lock path in zookeeper has fired. so something has changed on // the lock. ideally we should check that the path is the same as the lock // path but trusting zookeeper for now //節(jié)點(diǎn)事件 String path = event.getPath(); if (path != null) { switch (eventType) { case NodeDeleted: if (state == State.ACTIVE) { enterNeutralMode();//該方法目前未實(shí)現(xiàn) } joinElectionInternal();//開始選舉 break; case NodeDataChanged: monitorActiveStatus();//繼續(xù)監(jiān)控該節(jié)點(diǎn),嘗試成為active break; default: LOG.debug("Unexpected node event: " + eventType + " for path: " + path); monitorActiveStatus(); } return; } // some unexpected error has occurred fatalError("Unexpected watch error from Zookeeper"); }
而joinElectionInternal,選舉的核心方法就是,
選舉就是通過對zkLokFilePath節(jié)點(diǎn)的創(chuàng)建,來完成。這個采用了zk的異步回調(diào)。
從該類的定義,可以看出,本身就是實(shí)現(xiàn)了zk的兩個接口。
StatCallback需要實(shí)現(xiàn)的方法,如下:
對于兩個方法的實(shí)現(xiàn),ActiveStandbyElector內(nèi)部實(shí)現(xiàn)幾乎是一樣的。這里不再貼上源碼,有興趣的可以自己去看源碼。
貼上實(shí)現(xiàn)方法,有注釋。呵呵
public synchronized void processResult(int rc, String path, Object ctx, String name) { if (isStaleClient(ctx)) return; LOG.debug("CreateNode result: " + rc + " for path: " + path + " connectionState: " + zkConnectionState + " for " + this); Code code = Code.get(rc);//為了方便使用,這里自定義了一組狀態(tài) if (isSuccess(code)) {//成功返回,成功創(chuàng)建zklocakpath節(jié)點(diǎn) // we successfully created the znode. we are the leader. start monitoring if (becomeActive()) {//要將本節(jié)點(diǎn)上的NN變成active monitorActiveStatus();//繼續(xù)監(jiān)控節(jié)點(diǎn)狀態(tài) } else { reJoinElectionAfterFailureToBecomeActive();//失敗,繼續(xù)選舉嘗試 } return; } if (isNodeExists(code)) {//節(jié)點(diǎn)存在,說明已經(jīng)有active,wait即可 if (createRetryCount == 0) { // znode exists and we did not retry the operation. so a different // instance has created it. become standby and monitor lock. becomeStandby(); } // if we had retried then the znode could have been created by our first // attempt to the server (that we lost) and this node exists response is // for the second attempt. verify this case via ephemeral node owner. this // will happen on the callback for monitoring the lock. monitorActiveStatus();//不過努力成為active的動作不能停 return; } String errorMessage = "Received create error from Zookeeper. code:" + code.toString() + " for path " + path; LOG.debug(errorMessage); if (shouldRetry(code)) { if (createRetryCount < maxRetryNum) { LOG.debug("Retrying createNode createRetryCount: " + createRetryCount); ++createRetryCount; createLockNodeAsync(); return; } errorMessage = errorMessage + ". Not retrying further znode create connection errors."; } else if (isSessionExpired(code)) { // This isn't fatal - the client Watcher will re-join the election LOG.warn("Lock acquisition failed because session was lost"); return; } fatalError(errorMessage); }
對于becomeStandby,becomeActive這些狀態(tài)的改變,有ZKFailoverController來實(shí)現(xiàn)。
到此,關(guān)于“hadoop 2.4 namenode源碼分析”的學(xué)習(xí)就結(jié)束了,希望能夠解決大家的疑惑。理論與實(shí)踐的搭配能更好的幫助大家學(xué)習(xí),快去試試吧!若想繼續(xù)學(xué)習(xí)更多相關(guān)知識,請繼續(xù)關(guān)注億速云網(wǎng)站,小編會繼續(xù)努力為大家?guī)砀鄬?shí)用的文章!
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