narendly commented on a change in pull request #388: Add ChangeDetector 
interface and ResourceChangeDetector implementation
URL: https://github.com/apache/helix/pull/388#discussion_r314494766
 
 

 ##########
 File path: 
helix-core/src/main/java/org/apache/helix/controller/changedetector/ResourceChangeDetector.java
 ##########
 @@ -0,0 +1,167 @@
+package org.apache.helix.controller.changedetector;
+
+/*
+ * 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 com.google.common.collect.Sets;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import org.apache.helix.HelixConstants;
+import org.apache.helix.HelixException;
+import org.apache.helix.HelixProperty;
+import 
org.apache.helix.controller.dataproviders.ResourceControllerDataProvider;
+
+/**
+ * ResourceChangeDetector implements ChangeDetector. It caches 
resource-related metadata from
+ * Helix's main resource pipeline cache (DataProvider) and the computation 
results of change
+ * detection.
+ * WARNING: the methods of this class are not thread-safe.
+ */
+public class ResourceChangeDetector implements ChangeDetector {
+
+  private ResourceChangeSnapshot _oldSnapshot; // snapshot for previous 
pipeline run
+  private ResourceChangeSnapshot _newSnapshot; // snapshot for this pipeline 
run
+
+  // The following caches the computation results
+  private Map<HelixConstants.ChangeType, Collection<String>> _changedItems = 
new HashMap<>();
+  private Map<HelixConstants.ChangeType, Collection<String>> _addedItems = new 
HashMap<>();
+  private Map<HelixConstants.ChangeType, Collection<String>> _removedItems = 
new HashMap<>();
+
+  /**
+   * Compare the underlying HelixProperty objects and produce a collection of 
names of changed
+   * properties.
+   * @return
+   */
+  private Collection<String> getChangedItems(Map<String, ? extends 
HelixProperty> oldPropertyMap,
+      Map<String, ? extends HelixProperty> newPropertyMap) {
+    Collection<String> changedItems = new HashSet<>();
+    oldPropertyMap.forEach((name, property) -> {
+      if (newPropertyMap.containsKey(name)
+          && 
!property.getRecord().equals(newPropertyMap.get(name).getRecord())) {
+        changedItems.add(name);
+      }
+    });
+    return changedItems;
+  }
+
+  /**
+   * Return a collection of names that are newly added.
+   * @return
+   */
+  private Collection<String> getAddedItems(Map<String, ? extends 
HelixProperty> oldPropertyMap,
+      Map<String, ? extends HelixProperty> newPropertyMap) {
+    return Sets.difference(newPropertyMap.keySet(), oldPropertyMap.keySet());
+  }
+
+  /**
+   * Return a collection of names that were removed.
+   * @return
+   */
+  private Collection<String> getRemovedItems(Map<String, ? extends 
HelixProperty> oldPropertyMap,
+      Map<String, ? extends HelixProperty> newPropertyMap) {
+    return Sets.difference(oldPropertyMap.keySet(), newPropertyMap.keySet());
+  }
+
+  private void clearCachedComputation() {
+    _changedItems.clear();
+    _addedItems.clear();
+    _removedItems.clear();
+  }
+
+  /**
+   * Initializes old and new snapshots when ResourceChangeDetector gets its 
first update.
+   */
+  private void initializeSnapshots() {
+    _oldSnapshot = new ResourceChangeSnapshot();
+    _newSnapshot = new ResourceChangeSnapshot();
+  }
+
+  /**
+   * Based on the change type given and propertyMap type, call the right 
getters for propertyMap.
+   * @param changeType
+   * @param snapshot
+   * @return
+   */
+  private Map<String, ? extends HelixProperty> determinePropertyMapByType(
+      HelixConstants.ChangeType changeType, ResourceChangeSnapshot snapshot) {
+    switch (changeType) {
+    case INSTANCE_CONFIG:
+      return snapshot.getInstanceConfigMap();
+    case IDEAL_STATE:
+      return snapshot.getIdealStateMap();
+    case RESOURCE_CONFIG:
+      return snapshot.getResourceConfigMap();
+    case LIVE_INSTANCE:
+      return snapshot.getLiveInstances();
+    default:
+      throw new HelixException(String.format(
+          "ResourceChangeDetector cannot compute the names of changes for the 
given ChangeType: %s",
+          changeType));
+    }
+  }
+
+  /**
+   * Makes the current newSnapshot the oldSnapshot and reads in the up-to-date 
snapshot for change
+   * computation. To be called in the controller pipeline.
+   * @param dataProvider newly refreshed DataProvider (cache)
+   */
+  public synchronized void updateSnapshots(ResourceControllerDataProvider 
dataProvider) {
+    // If snapshots are null, initialize them
+    if (_oldSnapshot == null || _newSnapshot == null) {
+      initializeSnapshots();
+    }
+
+    // If there are changes, update internal states
+    _oldSnapshot = new ResourceChangeSnapshot(_newSnapshot);
+    _newSnapshot = new ResourceChangeSnapshot(dataProvider);
+    dataProvider.clearRefreshedChangeTypes();
 
 Review comment:
   I get your comment now @jiajunwang. Thanks for clarifying. Yes, there was a 
duplicate call. I removed it in the constructor :)

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