Kafka 0.9.0 output operators and unit tests.

1. Abstract Base class
2. Kafka Output operator
3. Exactly Once output operator
     Key in the Kafka message is used by the operator to track the tuples written by it.

kafka/src/main/java/org/apache/apex/malhar/kafka/AbstractKafkaOutputOperator.java

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+/**
+ * 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.apex.malhar.kafka;
+
+import java.util.Properties;
+import javax.validation.constraints.NotNull;
+
+import org.apache.kafka.clients.producer.KafkaProducer;
+import org.apache.kafka.clients.producer.Producer;
+import com.datatorrent.api.Context;
+import com.datatorrent.api.Operator;
+
+/**
+ * This is the base implementation of a Kafka output operator(0.9.0), which writes data to the Kafka message bus.
+ *
+ * @displayName Abstract Kafka Output
+ * @category Messaging
+ * @tags output operator
+ *
+ */
+@org.apache.hadoop.classification.InterfaceStability.Evolving
+public abstract class AbstractKafkaOutputOperator<K, V> implements Operator
+{
+  private transient Producer<K, V> producer;
+  @NotNull
+  private String topic;
+  private Properties properties = new Properties();
+
+  @Override
+  public void setup(Context.OperatorContext context)
+  {
+    producer = new KafkaProducer<K, V>(properties);
+  }
+
+  /**
+   * Implement Component Interface.
+   */
+  @Override
+  public void teardown()
+  {
+    producer.close();
+  }
+
+  /**
+   * Implement Operator Interface.
+   */
+  @Override
+  public void beginWindow(long windowId)
+  {
+  }
+
+  /**
+   * Implement Operator Interface.
+   */
+  @Override
+  public void endWindow()
+  {
+  }
+
+  public Properties getProperties()
+  {
+    return properties;
+  }
+
+  /**
+   * Set the Kafka producer properties.
+   *
+   * @param properties Producer properties
+   */
+  public void setProperties(Properties properties)
+  {
+    this.properties.putAll(properties);
+  }
+
+  /**
+   * Set the Kafka producer property.
+   *
+   * @param key Producer Property name
+   * @param val Producer Property value
+   */
+  public void setProperty(Object key, Object val)
+  {
+    properties.put(key,val);
+  }
+
+  public String getTopic()
+  {
+    return topic;
+  }
+
+  /**
+   * Set the Kafka topic
+   * @param  topic  Kafka topic for which the data is sent
+   */
+  public void setTopic(String topic)
+  {
+    this.topic = topic;
+  }
+
+  protected Producer<K, V> getProducer()
+  {
+    return producer;
+  }
+}
+