/**
*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.zookeeper.recipes.queue;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.TreeMap;
import java.util.concurrent.CountDownLatch;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.apache.zookeeper.CreateMode;
import org.apache.zookeeper.KeeperException;
import org.apache.zookeeper.WatchedEvent;
import org.apache.zookeeper.Watcher;
import org.apache.zookeeper.ZooDefs;
import org.apache.zookeeper.ZooKeeper;
import org.apache.zookeeper.data.ACL;
import org.apache.zookeeper.data.Stat;
/**
*
* A <a href="package.html">protocol to implement a distributed queue</a>.
*
*/
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.
* @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.
* @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.
* @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;
}
}
}