zookeeper 实现分布式队列

zookeeper是一种高性能，可扩展的服务。其本身提供了一致性保证，特点如下：

（1）顺序一致性：客户端的更新顺序与它们被发送的顺序一致。

（2）原子性：更新操作要么成功，要么失败，没有第三种结果。

（3）单系统镜像：无论客户端连接到哪一个服务器，他都将看到相同的zookeeper视图。

（4）可靠性：一旦一个更新操作被应用，那么在客户端再次更新之前，其值不会再改变。

zookeeper实现分布式队列

基于zookeeper 提供的一致性保证，我们可以实现一个分布式队列DistributedQueue。

主要的思路如下：

（1）首先DistributedQueue 的构造器中要传入

ZooKeeper：连接服务端的核心类；

dir：表示znode组的结点

acl：每个结点的访问控制列表

（2）所谓的入队，也就是在dir组下新建一个znode结点，不过结点类型是CreateMode.PERSISTENT_SEQUENTIAL，这就表明了新建结点的值根据序号是有一定顺序性的。其核心实现就是：

zookeeper.create(dir+"/"+prefix, data, acl, CreateMode.PERSISTENT_SEQUENTIAL);

（3）所谓的出队，其实就是取出所有的子结点，然后用treeMap维护有序性，然后将最小的结点出队(也就是在服务端删除结点最小的结点)

（4）该分布式队列的实现严重依赖于zookeeper的一致性保证。

具体的实现代码如下：

package com.zookeeper.lyt.queue;

import org.apache.zookeeper.*;
import org.apache.zookeeper.data.ACL;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.List;
import java.util.NoSuchElementException;
import java.util.TreeMap;
import java.util.concurrent.CountDownLatch;

/**
*
* A <a href="package.html">protocol to implement a distributed queue</a>.
* 利用 zookeeper 构建分布式 队列
*
* 其依据的就是 zookeeper的一致性:顺序一致性,原子性,单系统镜像,可靠性.
*
*/
public class DistributedQueue {
private static final Logger LOG = LoggerFactory.getLogger(DistributedQueue.class);

private final String dir;

private ZooKeeper zookeeper;
private List<ACL> acl = ZooDefs.Ids.OPEN_ACL_UNSAFE;

private final String prefix = "qn-";

public DistributedQueue(ZooKeeper zookeeper, String dir, List<ACL> acl){
this.dir = dir;

if(acl != null){
this.acl = acl;
}
this.zookeeper = zookeeper;

}

/**
* Returns a Map of the children, ordered by id.
*
* 获取children 的Map集合, 根据ID来排序
*
* @param watcher optional watcher on getChildren() operation.
* @return map from id to child name for all children
*/
private TreeMap<Long,String> orderedChildren(Watcher watcher) throws KeeperException, InterruptedException {
TreeMap<Long,String> orderedChildren = new TreeMap<Long,String>();

List<String> childNames = null;
try{
childNames = zookeeper.getChildren(dir, watcher);
}catch (KeeperException.NoNodeException e){
throw e;
}

for(String childName : childNames){
try{
//Check format
if(!childName.regionMatches(0, prefix, 0, prefix.length())){
LOG.warn("Found child node with improper name: " + childName);
continue;
}
String suffix = childName.substring(prefix.length());
Long childId = new Long(suffix);
orderedChildren.put(childId,childName);
}catch(NumberFormatException e){
LOG.warn("Found child node with improper format : " + childName + " " + e,e);
}
}

return orderedChildren;
}

/**
* Find the smallest child node.
*
* 找到最小的结点
* @return The name of the smallest child node.
*/
private String smallestChildName() throws KeeperException, InterruptedException {
long minId = Long.MAX_VALUE;
String minName = "";

List<String> childNames = null;

try{
childNames = zookeeper.getChildren(dir, false);
}catch(KeeperException.NoNodeException e){
LOG.warn("Caught: " +e,e);
return null;
}

for(String childName : childNames){
try{
//Check format
if(!childName.regionMatches(0, prefix, 0, prefix.length())){
LOG.warn("Found child node with improper name: " + childName);
continue;
}
String suffix = childName.substring(prefix.length());
long childId = Long.parseLong(suffix);
if(childId < minId){
minId = childId;
minName = childName;
}
}catch(NumberFormatException e){
LOG.warn("Found child node with improper format : " + childName + " " + e,e);
}
}

if(minId < Long.MAX_VALUE){
return minName;
}else{
return null;
}
}

/**
* Return the head of the queue without modifying the queue.
* 返回队定的元素
* @return the data at the head of the queue.
* @throws NoSuchElementException
* @throws KeeperException
* @throws InterruptedException
*/
public byte[] element() throws NoSuchElementException, KeeperException, InterruptedException {
TreeMap<Long,String> orderedChildren;

// element, take, and remove follow the same pattern.
// We want to return the child node with the smallest sequence number.
// Since other clients are remove()ing and take()ing nodes concurrently,
// the child with the smallest sequence number in orderedChildren might be gone by the time we check.
// We don't call getChildren again until we have tried the rest of the nodes in sequence order.
while(true){
try{
orderedChildren = orderedChildren(null);
}catch(KeeperException.NoNodeException e){
throw new NoSuchElementException();
}
if(orderedChildren.size() == 0 ) throw new NoSuchElementException();

for(String headNode : orderedChildren.values()){
if(headNode != null){
try{
return zookeeper.getData(dir+"/"+headNode, false, null);
}catch(KeeperException.NoNodeException e){
//Another client removed the node first, try next
}
}
}

}
}

/**
* Attempts to remove the head of the queue and return it.
* @return The former head of the queue
* @throws NoSuchElementException
* @throws KeeperException
* @throws InterruptedException
*/
public byte[] remove() throws NoSuchElementException, KeeperException, InterruptedException {
TreeMap<Long,String> orderedChildren;
// Same as for element.  Should refactor this.
while(true){
try{
orderedChildren = orderedChildren(null);
}catch(KeeperException.NoNodeException e){
throw new NoSuchElementException();
}
if(orderedChildren.size() == 0) throw new NoSuchElementException();

for(String headNode : orderedChildren.values()){
String path = dir +"/"+headNode;
try{
byte[] data = zookeeper.getData(path, false, null);
zookeeper.delete(path, -1);
return data;
}catch(KeeperException.NoNodeException e){
// Another client deleted the node first.
}
}

}
}

private class LatchChildWatcher implements Watcher {

CountDownLatch latch;

public LatchChildWatcher(){
latch = new CountDownLatch(1);
}

public void process(WatchedEvent event){
LOG.debug("Watcher fired on path: " + event.getPath() + " state: " +
event.getState() + " type " + event.getType());
latch.countDown();
}
public void await() throws InterruptedException {
latch.await();
}
}

/**
* Removes the head of the queue and returns it, blocks until it succeeds.
* @return The former head of the queue
* @throws NoSuchElementException
* @throws KeeperException
* @throws InterruptedException
*/
public byte[] take() throws KeeperException, InterruptedException {
TreeMap<Long,String> orderedChildren;
// Same as for element.  Should refactor this.
while(true){
LatchChildWatcher childWatcher = new LatchChildWatcher();
try{
orderedChildren = orderedChildren(childWatcher);
}catch(KeeperException.NoNodeException e){
zookeeper.create(dir, new byte[0], acl, CreateMode.PERSISTENT);
continue;
}
if(orderedChildren.size() == 0){
childWatcher.await();
continue;
}

for(String headNode : orderedChildren.values()){
String path = dir +"/"+headNode;
try{
byte[] data = zookeeper.getData(path, false, null);
zookeeper.delete(path, -1);
return data;
}catch(KeeperException.NoNodeException e){
// Another client deleted the node first.
}
}
}
}

/**
* Inserts data into queue.
* @param data
* @return true if data was successfully added
*/
public boolean offer(byte[] data) throws KeeperException, InterruptedException{
for(;;){
try{
zookeeper.create(dir+"/"+prefix, data, acl, CreateMode.PERSISTENT_SEQUENTIAL);
return true;
}catch(KeeperException.NoNodeException e){
zookeeper.create(dir, new byte[0], acl, CreateMode.PERSISTENT);
}
}

}

/**
* Returns the data at the first element of the queue, or null if the queue is empty.
* @return data at the first element of the queue, or null.
* @throws KeeperException
* @throws InterruptedException
*/
public byte[] peek() throws KeeperException, InterruptedException{
try{
return element();
}catch(NoSuchElementException e){
return null;
}
}

/**
* Attempts to remove the head of the queue and return it. Returns null if the queue is empty.
* @return Head of the queue or null.
* @throws KeeperException
* @throws InterruptedException
*/
public byte[] poll() throws KeeperException, InterruptedException {
try{
return remove();
}catch(NoSuchElementException e){
return null;
}
}
}