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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.contrib.cassandra;
  
  import java.util.Collection;
  
  import javax.annotation.Nonnull;
  
  import org.apache.apex.malhar.lib.db.AbstractBatchTransactionableStoreOutputOperator;
  
  import com.datastax.driver.core.BatchStatement;
  import com.datastax.driver.core.PreparedStatement;
  import com.datastax.driver.core.Statement;
  import com.datastax.driver.core.exceptions.DriverException;
  import com.datatorrent.api.Context;
  import com.datatorrent.api.Context.OperatorContext;
  import com.datatorrent.api.Operator.ActivationListener;
  
  /**
   * <p>
   * Generic Cassandra output adaptor which creates a transaction at the start of window.&nbsp; Subclasses should provide implementation for getting the update statement and setting the statement parameters.  <br/>
   * </p>
   *
   * <p>
   * Executes batch of CQL updates and closes the transaction at the end of the window.
   * Each tuple corresponds to an CQL update statement. The operator groups the updates in a batch
   * and submits them with one call to the database. Batch processing improves performance considerably and also provides atomicity.<br/>
   * The size of a batch is equal to the size of the window.
   * </p>
   *
   * <p>
   * The tuples in a window are stored in check-pointed collection which is cleared in the endWindow().
   * This is needed for the recovery. The operator writes a tuple exactly once in the database, which is why
   * only when all the updates are executed, the transaction is committed in the end window call.
   * </p>
   * @displayName Abstract Cassandra Transactionable Output
   * @category Output
   * @tags cassandra, batch, transactionable
   * @param <T>type of tuple</T>
   * @since 1.0.2
   */
  public abstract class AbstractCassandraTransactionableOutputOperator<T> extends AbstractBatchTransactionableStoreOutputOperator<T, CassandraTransactionalStore> implements ActivationListener<Context.OperatorContext>
  {
    private transient PreparedStatement updateCommand;
  
    @Override
    public void activate(OperatorContext context)
    {
      updateCommand = getUpdateCommand();
    }
  
    /**
     * Gets the statement which insert/update the table in the database.
     *
     * @return the cql statement to update a tuple in the database.
     */
    @Nonnull
    protected abstract PreparedStatement getUpdateCommand();
  
    /**
     * Sets the parameter of the insert/update statement with values from the tuple.
     *
     * @param tuple     tuple
     * @return statement The statement to execute
     * @throws DriverException
     */
    protected abstract Statement setStatementParameters(PreparedStatement updateCommand, T tuple) throws DriverException;
  
  
    @Override
    public void processBatch(Collection<T> tuples)
    {
      BatchStatement batchCommand = store.getBatchCommand();
      for (T tuple: tuples) {
        batchCommand.add(setStatementParameters(updateCommand, tuple));
      }
    }
  
    @Override
    public void deactivate()
    {
    }
  }
  