[GitHub] [samza] ajothomas commented on a diff in pull request #1605: SAMZA-2742: [Pipeline Drain] [WIP] Add Drain components and integrate them with SamzaContainer and JC

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

ajothomas commented on code in PR #1605:
URL: https://github.com/apache/samza/pull/1605#discussion_r876347348


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samza-core/src/main/java/org/apache/samza/drain/DrainMonitor.java:
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@@ -0,0 +1,52 @@
+/*
+ * 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.samza.drain;
+
+/**
+ * DrainMonitor implementations are intended to monitor the MetadataStore for {@link DrainNotification} and invokes
+ * the {@link Callback}.
+ * */
+public interface DrainMonitor {
+  /**
+   * Starts the DrainMonitor.
+   * */
+  void start();
+
+  /**
+   * Stops the DrainMonitor.
+   * */
+  void stop();
+
+  /**
+   * Register a callback to be executed when DrainNotification is encountered.
+   *
+   * @param callback the callback to register.
+   * @return Returns {@code true} if registration was successful and {@code false} if not.
+   * Registration can fail it the DrainMonitor is stopped or a callback is already registered.
+   * */
+  boolean registerCallback(Callback callback);

Review Comment:
   DrainMonitor is the component which checks for DrainNotification in the metadata-store. It's implementations are SinglePollDrainMonitor and PollingDrainMonitor. The former checks for drain exactly once, typically used on container startup. The latter will poll the metadata-store periodically for drain. SamzaContainer/JC will register the callback which is an action that should be invoked by the DrainMonitor whenever it encounters Drain.



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