Zookeeper筆記之使用zk實現集羣選主
1、需求
在主從結構的集羣中，咱們假設硬件機器是很脆弱的，隨時可能會宕機，當master掛掉以後須要從slave中選出一個節點做爲新的master，使用zookeeper能夠很簡單的實現集羣選主功能。html
2、分析
下面爲了方便敘述，將使用更通用的技術術語，即便用leader表示master，使用follower表示slave。java
集羣選主涉及到兩個問題：node
1. 誰來作leaderapache
2. leader掛掉了怎麼被follower感知到網絡
首先是第一個問題，誰來作leader，其實能夠將這個問題看作是多線程中的互斥鎖搶佔，鎖只有一把，而且只能被一我的搶到，這裏就把一個zookeeper上的一個節點/leader-info看作是鎖，集羣中的每臺機器都嘗試去建立這個節點（搶佔鎖），由於zookeeper建立節點是原子性操做，因此只有一臺機器可以建立成功其它都會失敗，建立成功的那臺機器就做爲leader，其它機器作follower，通常leader搶佔成功了以後會在/leader-info節點上存儲一些與本身相關的信息，好比hostname、id之類的，以讓follower知道誰搶佔成功成爲了leader，而後去鏈接leader進行一些數據交換或指令控制之類的，那就是選主以後的事了不在此篇文章的討論範圍以內。session
第二個問題是leader掛掉了怎麼通知其它的follower，zookeeper中的節點按照有效時間分爲持久節點和臨時節點，臨時節點跟session綁定，而一個session表示一個客戶端，當客戶端下線的時候session失效，當session失效的時候跟它綁定的臨時節點就會被刪除，利用這個特性能夠檢測節點是否還在存活狀態，實現follower對leader下線的感知，只是須要注意在建立/leader-info節點的時候要將其建立爲臨時節點，而後衆多follower都在這個節點上添加一個watcher監聽其刪除事件，這樣當leader掛掉的時候session失效，而後與此session綁定的臨時節點會被刪除，即/leader-info節點將被刪除，同時給全部的follower發送事件通知，follower一看leader掛了就燥起來了，將本身的狀態置爲looking，開始新一輪的選舉。多線程
總結一下選主的流程：dom
1. 集羣中的全部機器將本身置爲looking狀態，準備開始選舉。分佈式
2. 全部looking狀態的機器嘗試去建立/leader-info節點。優化
3. 建立成功的將本身的狀態修改成leader，同時將本身的一些信息寫入到/leader-info這個節點上。
4. 建立失敗的將本身的狀態置爲follower，同時嘗試從/leader-info獲取leader信息進行一些leader改變的邏輯（在這裏這個不是重點，打印一下便可），follower在獲取/leader-info節點數據的同時要設置一個watcher，監聽此節點的刪除事件，當節點被刪除事件觸發時啓動新一輪的選舉，由於獲取數據設置watcher這個操做是原子性的，因此要麼這個節點存在獲取數據成功，而且設置watcher也成功，要麼節點不存在拋出KeeperException.NoNodeException異常。
5. 爲何在follower設置watcher的時候還有可能會拋異常呢，leader不是已經建立了這個節點了嗎？由於follower從嘗試建立/leader-info節點失敗到去獲取此節點的數據同時設置watcher這一段操做不是原子性的，在這中間可能會發生一些變故，leader可能剛成爲leader就掛掉了（或者由於一些網絡抖動緣由，總之是session失效了），leader掛掉以後它建立的臨時節點就被zookeeper刪除了，因此當follower在設置watcher的時候若是檢測到KeeperException.NoNodeException，說明以前的leader掛掉了，此時集羣中已經沒有了leader，follower又燥起來了，它將本身的狀態置爲looking開始新一輪的選舉。
3、實現
Node.java：
package cc11001100.zookeeper.leaderElection;
import cc11001100.zookeeper.utils.ZooKeeperUtil;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import java.io.IOException;
import java.io.UnsupportedEncodingException;
/**
* 表示集羣中的一個節點
*
* @author CC11001100
*/
public class Node {
private Status status;
private String nodeForLeaderInfo;
private ZooKeeper zooKeeper;
public Node(String listenerNodeForLeader) throws IOException {
this.nodeForLeaderInfo = listenerNodeForLeader;
this.zooKeeper = ZooKeeperUtil.getZooKeeper();
lookingForLeader();
}
public void lookingForLeader() {
status = Status.LOOKING;
try {
String leaderInfo = Thread.currentThread().getName();
// 須要注意這裏建立的是臨時節點
zooKeeper.create(nodeForLeaderInfo, leaderInfo.getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.EPHEMERAL);
// 若是上一步沒有拋異常，說明本身已是leader了
status = Status.LEADER;
String logMsg = Thread.currentThread().getName() + " is leader";
System.out.println(logMsg);
} catch (KeeperException.NodeExistsException e) {
// 節點已經存在，說明leader已經被別人註冊成功了，本身是follower
status = Status.FOLLOWER;
try {
byte[] leaderInfoBytes = zooKeeper.getData(nodeForLeaderInfo, event -> {
if (event.getType() == Watcher.Event.EventType.NodeDeleted) {
lookingForLeader();
}
}, null);
String logMsg = Thread.currentThread().getName() + " is follower, master is " + new String(leaderInfoBytes, "UTF-8");
System.out.println(logMsg);
} catch (KeeperException.NoNodeException e1) {
// 若是在獲取leader信息的時候報了節點不存在，說明這個leader比較短命，剛搶到leader就又掛掉了
lookingForLeader();
} catch (KeeperException | InterruptedException | UnsupportedEncodingException e1) {
e1.printStackTrace();
}
} catch (KeeperException | InterruptedException e) {
e.printStackTrace();
}
}
public void shutdown() {
try {
if (zooKeeper != null) {
zooKeeper.close();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public Status getStatus() {
return status;
}
// 當前節點的狀態，節點的狀態必須在這三個中的一個
public enum Status {
LOOKING, // 選舉中
LEADER, // 選舉完畢，當前節點爲leader
FOLLOWER; // 選舉完畢，當前節點爲follower
}
}
LeaderElectionTest.java：
package cc11001100.zookeeper.leaderElection;
import java.io.IOException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
/**
* @author CC11001100
*/
public class LeaderElectionTest {
private static void sleep(long mils) {
try {
TimeUnit.MILLISECONDS.sleep(mils);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) throws IOException {
final String LEADER_INFO_NODE = "/leader-info";
int nodeNum = 10;
AtomicLong idGenerator = new AtomicLong();
AtomicInteger activeNodeCount = new AtomicInteger();
while (true) {
if (activeNodeCount.get() >= nodeNum) {
sleep(10);
continue;
}
// 線程啓動須要必定時間，將線程啓動看作開機過程，在開機以前就算一臺新的機器加入了
activeNodeCount.incrementAndGet();
new Thread(() -> {
try {
Node node = new Node(LEADER_INFO_NODE);
while (true) {
sleep(1000);
// 這裏爲了試驗就讓leader有輕微自殺傾向...
if (node.getStatus() == Node.Status.LEADER && Math.random() < 0.3) {
String logMsg = "----------------------------- " + Thread.currentThread().getName() + " shutdown -----------------------------";
System.out.println(logMsg);
node.shutdown();
break;
}
}
} catch (IOException e) {
e.printStackTrace();
} finally {
activeNodeCount.decrementAndGet();
}
}, "node-" + idGenerator.getAndIncrement()).start();
}
}
}
控制檯輸出：
... node-4 is leader node-3 is follower, master is node-4 node-0 is follower, master is node-4 node-9 is follower, master is node-4 node-7 is follower, master is node-4 node-5 is follower, master is node-4 node-1 is follower, master is node-4 node-6 is follower, master is node-4 node-8 is follower, master is node-4 node-2 is follower, master is node-4 ----------------------------- node-4 shutdown ----------------------------- node-0-EventThread is leader node-6-EventThread is follower, master is node-0-EventThread node-3-EventThread is follower, master is node-0-EventThread node-7-EventThread is follower, master is node-0-EventThread node-1-EventThread is follower, master is node-0-EventThread node-5-EventThread is follower, master is node-0-EventThread node-9-EventThread is follower, master is node-0-EventThread node-2-EventThread is follower, master is node-0-EventThread node-8-EventThread is follower, master is node-0-EventThread node-10 is follower, master is node-0-EventThread ----------------------------- node-0 shutdown ----------------------------- node-6-EventThread is leader node-7-EventThread is follower, master is node-6-EventThread node-1-EventThread is follower, master is node-6-EventThread node-3-EventThread is follower, master is node-6-EventThread node-10-EventThread is follower, master is node-6-EventThread node-9-EventThread is follower, master is node-6-EventThread node-5-EventThread is follower, master is node-6-EventThread node-2-EventThread is follower, master is node-6-EventThread node-8-EventThread is follower, master is node-6-EventThread node-11 is follower, master is node-6-EventThread ----------------------------- node-6 shutdown ----------------------------- node-1-EventThread is leader node-10-EventThread is follower, master is node-1-EventThread node-7-EventThread is follower, master is node-1-EventThread node-11-EventThread is follower, master is node-1-EventThread node-8-EventThread is follower, master is node-1-EventThread node-5-EventThread is follower, master is node-1-EventThread node-9-EventThread is follower, master is node-1-EventThread node-3-EventThread is follower, master is node-1-EventThread node-2-EventThread is follower, master is node-1-EventThread node-12 is follower, master is node-1-EventThread ----------------------------- node-1 shutdown ----------------------------- node-3-EventThread is leader node-12-EventThread is follower, master is node-3-EventThread node-11-EventThread is follower, master is node-3-EventThread node-5-EventThread is follower, master is node-3-EventThread node-7-EventThread is follower, master is node-3-EventThread node-9-EventThread is follower, master is node-3-EventThread node-2-EventThread is follower, master is node-3-EventThread node-10-EventThread is follower, master is node-3-EventThread node-8-EventThread is follower, master is node-3-EventThread node-13 is follower, master is node-3-EventThread ----------------------------- node-3 shutdown ----------------------------- node-5-EventThread is leader node-13-EventThread is follower, master is node-5-EventThread node-12-EventThread is follower, master is node-5-EventThread node-7-EventThread is follower, master is node-5-EventThread node-11-EventThread is follower, master is node-5-EventThread node-10-EventThread is follower, master is node-5-EventThread node-9-EventThread is follower, master is node-5-EventThread node-2-EventThread is follower, master is node-5-EventThread node-8-EventThread is follower, master is node-5-EventThread node-14 is follower, master is node-5-EventThread ----------------------------- node-5 shutdown ----------------------------- node-7-EventThread is leader node-13-EventThread is follower, master is node-7-EventThread node-12-EventThread is follower, master is node-7-EventThread node-9-EventThread is follower, master is node-7-EventThread node-11-EventThread is follower, master is node-7-EventThread node-14-EventThread is follower, master is node-7-EventThread node-10-EventThread is follower, master is node-7-EventThread node-8-EventThread is follower, master is node-7-EventThread node-2-EventThread is follower, master is node-7-EventThread node-15 is follower, master is node-7-EventThread ----------------------------- node-7 shutdown ----------------------------- node-14-EventThread is leader node-13-EventThread is follower, master is node-14-EventThread node-11-EventThread is follower, master is node-14-EventThread node-2-EventThread is follower, master is node-14-EventThread node-12-EventThread is follower, master is node-14-EventThread node-15-EventThread is follower, master is node-14-EventThread node-10-EventThread is follower, master is node-14-EventThread node-9-EventThread is follower, master is node-14-EventThread node-8-EventThread is follower, master is node-14-EventThread node-16 is follower, master is node-14-EventThread ----------------------------- node-14 shutdown ----------------------------- node-13-EventThread is leader node-12-EventThread is follower, master is node-13-EventThread node-15-EventThread is follower, master is node-13-EventThread node-9-EventThread is follower, master is node-13-EventThread node-10-EventThread is follower, master is node-13-EventThread node-2-EventThread is follower, master is node-13-EventThread node-8-EventThread is follower, master is node-13-EventThread node-11-EventThread is follower, master is node-13-EventThread node-16-EventThread is follower, master is node-13-EventThread node-17 is follower, master is node-13-EventThread ...
最後有個須要注意的地方就是是否須要將leader節點設置爲順序臨時節點呢？相似於分佈式鎖那樣，這樣的話每次喚醒一個節點就能夠了，這看上去像是一個能夠優化的點。
其實並非，當leader掛掉的時候必須全部follower都可以感知到leader的變動，即便他們不參與搶主也必須醒來執行leader變動的邏輯。
.
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