narendly commented on a change in pull request #727: Add MetadataStoreRoutingDataWriter with DistributedLeaderElection URL: https://github.com/apache/helix/pull/727#discussion_r377378006
########## File path: helix-rest/src/main/java/org/apache/helix/rest/metadatastore/concurrency/ZkDistributedLeaderElection.java ########## @@ -0,0 +1,142 @@ +package org.apache.helix.rest.metadatastore.concurrency; + +/* + * 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. + */ + +import java.util.Collections; +import java.util.List; + +import org.I0Itec.zkclient.IZkDataListener; +import org.I0Itec.zkclient.exception.ZkNodeExistsException; +import org.apache.helix.ZNRecord; +import org.apache.helix.manager.zk.ZNRecordSerializer; +import org.apache.helix.manager.zk.client.HelixZkClient; +import org.apache.helix.manager.zk.zookeeper.IZkStateListener; +import org.apache.zookeeper.CreateMode; +import org.apache.zookeeper.Watcher; +import org.slf4j.Logger; +import org.slf4j.LoggerFactory; + + +public class ZkDistributedLeaderElection implements IZkDataListener, IZkStateListener { + private static final Logger LOG = LoggerFactory.getLogger(ZkDistributedLeaderElection.class); + private static final String PREFIX = "MSDS_SERVER_"; + + private final HelixZkClient _zkClient; + private final String _basePath; + private final ZNRecord _participantInfo; + private ZNRecord _currentLeaderInfo; + + private String _myEphemeralSequentialPath; + private volatile boolean _isLeader; + + public ZkDistributedLeaderElection(HelixZkClient zkClient, String basePath, + ZNRecord participantInfo) { + synchronized (this) { + if (zkClient == null || zkClient.isClosed()) { + throw new IllegalArgumentException("ZkClient cannot be null or closed!"); + } + _zkClient = zkClient; + _zkClient.setZkSerializer(new ZNRecordSerializer()); + if (basePath == null || basePath.isEmpty()) { + throw new IllegalArgumentException("lockBasePath cannot be null or empty!"); + } + _basePath = basePath; + _participantInfo = participantInfo; + _isLeader = false; + } + init(); + } + + /** + * Create the base path if it doesn't exist and create an ephemeral sequential ZNode. + */ + private void init() { + try { + _zkClient.createPersistent(_basePath, true); + } catch (ZkNodeExistsException e) { + // Okay if it exists already + } + + // Create my ephemeral sequential node with my information + _myEphemeralSequentialPath = _zkClient + .create(_basePath + "/" + PREFIX, _participantInfo, CreateMode.EPHEMERAL_SEQUENTIAL); + if (_myEphemeralSequentialPath == null) { + throw new IllegalStateException( + "Unable to create ephemeral sequential node at path: " + _basePath); + } + tryAcquiringLeadership(); + } + + private void tryAcquiringLeadership() { + List<String> children = _zkClient.getChildren(_basePath); + Collections.sort(children); + String leaderName = children.get(0); + ZNRecord leaderInfo = _zkClient.readData(_basePath + "/" + leaderName, true); + + String[] myNameArray = _myEphemeralSequentialPath.split("/"); + String myName = myNameArray[myNameArray.length - 1]; + + if (leaderName.equals(myName)) { + // My turn for leadership + _isLeader = true; + _currentLeaderInfo = leaderInfo; + LOG.info("{} acquired leadership! Info: {}", myName, leaderInfo); + } else { + // Watch the ephemeral ZNode before me for a deletion event + String beforeMe = children.get(children.indexOf(myName) - 1); + _zkClient.subscribeDataChanges(_basePath + "/" + beforeMe, this); + } + } + + public synchronized boolean isLeader() { + return _isLeader; + } + + public synchronized ZNRecord getCurrentLeaderInfo() { + return _currentLeaderInfo; + } + + @Override + public synchronized void handleStateChanged(Watcher.Event.KeeperState state) { + if (state == Watcher.Event.KeeperState.SyncConnected) { + init(); + } + } + + @Override + public void handleNewSession(String sessionId) { + return; + } + + @Override + public void handleSessionEstablishmentError(Throwable error) { + return; + } + + @Override + public void handleDataChange(String s, Object o) { + return; + } + + @Override + public void handleDataDeleted(String s) { Review comment: I'm not sure what you're asking about exactly... > private void fireDataChangedEvents(final String path, Set<IZkDataListenerEntry> listeners, > final OptionalLong notificationTime) { > try { > final ZkPathStatRecord pathStatRecord = new ZkPathStatRecord(path); > // Trigger listener callbacks > for (final IZkDataListenerEntry listener : listeners) { > _eventThread.send(new ZkEvent("Data of " + path + " changed sent to " > + listener.getDataListener() + " prefetch data: " + listener.isPrefetchData()) { > @Override > public void run() throws Exception { > // Reinstall watch before listener callbacks to check the znode status > if (!pathStatRecord.pathChecked()) { > pathStatRecord.recordPathStat(getStat(path, true), notificationTime); > } > if (!pathStatRecord.pathExists()) { > // no znode found at the path, trigger data deleted handler. > listener.getDataListener().handleDataDeleted(path); > } else { > Object data = null; > if (listener.isPrefetchData()) { > if (LOG.isDebugEnabled()) { > LOG.debug("Prefetch data for path: {}", path); > } > try { > data = readData(path, null, true); > } catch (ZkNoNodeException e) { > LOG.warn("Prefetch data for path: {} failed.", path, e); > listener.getDataListener().handleDataDeleted(path); > return; > } > } > listener.getDataListener().handleDataChange(path, data); > } > } > }); > } Does this help? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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