[GitHub] [helix] desaikomal commented on a change in pull request #1978: Enable HelixManager as an event listener

[GitBox <gi...@apache.org>]

desaikomal commented on a change in pull request #1978:
URL: https://github.com/apache/helix/pull/1978#discussion_r826164021



##########
File path: helix-core/src/main/java/org/apache/helix/HelixCloudProperty.java
##########
@@ -180,4 +185,20 @@ public void setCloudRequestTimeout(long cloudRequestTimeout) {
   public void setCustomizedCloudProperties(Properties customizedCloudProperties) {
     _customizedCloudProperties.putAll(customizedCloudProperties);
   }
+
+  public boolean isCloudEventCallbackEnabled() {
+    return _isCloudEventCallbackEnabled;
+  }
+
+  public void enableCloudEventCallback(boolean enabled) {
+    _isCloudEventCallbackEnabled = enabled;
+  }

Review comment:
       +1, set<> would be better naming convention. 

##########
File path: helix-core/src/main/java/org/apache/helix/cloud/event/helix/CloudEventCallbackProperty.java
##########
@@ -0,0 +1,233 @@
+package org.apache.helix.cloud.event.helix;
+
+/*
+ * 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.Arrays;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.BiConsumer;
+
+import org.apache.helix.HelixManager;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * A property for users to customize the behavior of a Helix manager as a cloud event listener
+ * Use this
+ */
+public class CloudEventCallbackProperty {
+  private static final Logger LOG =
+      LoggerFactory.getLogger(CloudEventCallbackProperty.class.getName());
+
+  // The key for user to pass in callback impl class name in the constructor
+  public static final String CALLBACK_IMPL_CLASS_NAME = "callbackImplClassName";
+  private ConcurrentHashMap<OnPauseOperations, BiConsumer<HelixManager, Object>>
+      _onPauseOperationMap;
+  private ConcurrentHashMap<OnResumeOperations, BiConsumer<HelixManager, Object>>
+      _onResumeOperationMap;
+  private String _callbackImplClassName;
+
+  /**
+   * Constructor
+   * @param userArgs A map contains information that users pass in
+   */
+  public CloudEventCallbackProperty(Map<String, String> userArgs) {
+    _callbackImplClassName = userArgs.get(CALLBACK_IMPL_CLASS_NAME);
+    _onPauseOperationMap = new ConcurrentHashMap<>();
+    _onResumeOperationMap = new ConcurrentHashMap<>();
+  }
+
+  /**
+   * A collection of types of optional Helix-supported operations
+   */
+  public enum OptionalHelixOperation {
+    MAINTENANCE_MODE
+  }
+
+  /**
+   * A collection of types and positions for user to plugin customized callback
+   */
+  public enum UserDefinedCallbackType {
+    PRE_ON_PAUSE, POST_ON_PAUSE, PRE_ON_RESUME, POST_ON_RESUME,
+  }
+
+  private enum OnPauseOperations {
+    PRE_ON_PAUSE, MAINTENANCE_MODE, DEFAULT_HELIX_OPERATION, POST_ON_PAUSE
+  }
+
+  private enum OnResumeOperations {
+    PRE_ON_RESUME, DEFAULT_HELIX_OPERATION, MAINTENANCE_MODE, POST_ON_RESUME,
+  }
+
+  /**
+   * Trigger the registered onPause callbacks one by one
+   * Order:
+   * 1. PRE onPause
+   * 2. Helix optional: Maintenance Mode
+   * 3. Helix default
+   * 4. POST onPause
+   * @param helixManager Helix manager to perform operations
+   * @param eventInfo Information of the event provided by upsteam
+   * @param callbackImplClass Implementation class for Helix default and optional operations
+   */
+  public void onPause(HelixManager helixManager, Object eventInfo,
+      AbstractCloudEventCallbackImpl callbackImplClass) {
+    Iterator<OnPauseOperations> iterator = Arrays.stream(OnPauseOperations.values()).iterator();
+    while (iterator.hasNext()) {
+      OnPauseOperations operation = iterator.next();
+      switch (operation) {
+        case DEFAULT_HELIX_OPERATION:
+          callbackImplClass.onPauseDefaultHelixOperation(helixManager, eventInfo);
+          break;
+        case MAINTENANCE_MODE:
+          callbackImplClass.onPauseMaintenanceMode(helixManager, eventInfo);
+          break;
+        default:
+          _onPauseOperationMap.getOrDefault(operation, (manager, info) -> {
+          }).accept(helixManager, eventInfo);

Review comment:
       Not code related but behavior related question: 
   - we will use the main thread for calling all the callbacks and hence if any of the callbacks block, our thread will block, is that what we want?
   - if callback fails, do we still proceed with MM (maintence mode)?
   - have we reasoned through all the failure scenarios ?




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