contrib/src/main/java/com/datatorrent/contrib/cassandra/AbstractCassandraInputOperator.java

/*
 * Copyright (c) 2014 DataTorrent, Inc. ALL Rights Reserved.
 *
 * Licensed 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 com.datatorrent.contrib.cassandra;


import com.datastax.driver.core.ResultSet;
import com.datastax.driver.core.Row;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.datatorrent.lib.db.AbstractStoreInputOperator;

import com.datatorrent.api.DefaultOutputPort;

import com.datatorrent.common.util.DTThrowable;

/**
 * Base input adapter which reads data from persistence database through DATASTAX API and writes into output port(s). 
 * Subclasses should provide implementation to get tuples and querying to retrieve data.
 * <p>
 * This is an abstract class. Sub-classes need to implement {@link #queryToRetrieveData()} and {@link #getTuple(Row)}.
 * </p>
 * @param <T>
 * @displayName Abstract Cassandra Input
 * @category Store
 * @tags input operator
 * @since 1.0.2
 */
public abstract class AbstractCassandraInputOperator<T> extends AbstractStoreInputOperator<T, CassandraStore> {

  private static final Logger logger = LoggerFactory.getLogger(AbstractCassandraInputOperator.class);

  int waitForDataTimeout = 100;

  /**
   * Get the amount of time to wait for data in milliseconds.
   * When there is no data available this timeout is used to throttle requests to the store so as to not continuously
   * slam it with requests. This is specified in milliseconds.
   *
   * @return The wait timeout in milliseconds
   */
  public int getWaitForDataTimeout()
  {
    return waitForDataTimeout;
  }

  /**
   * Set the amount of time to wait for data in milliseconds.
   * When there is no data available this timeout is used to throttle requests to the store so as to not continuously
   * slam it with requests. This is specified in milliseconds.
   *
   * @param waitForDataTimeout The wait timeout in milliseconds
   */
  public void setWaitForDataTimeout(int waitForDataTimeout)
  {
    this.waitForDataTimeout = waitForDataTimeout;
  }

  /**
   * Any concrete class has to override this method to convert a Database row into Tuple.
   *
   * @param row a single row that has been read from database.
   * @return Tuple a tuples created from row which can be any Java object.
   */
  public abstract T getTuple(Row row);

  /**
   * Any concrete class has to override this method to return the query string which will be used to
   * retrieve data from database.
   *
   * @return Query string
   */
  public abstract String queryToRetrieveData();

  /**
   * The output port that will emit tuple into DAG.
   */
  public final transient DefaultOutputPort<T> outputPort = new DefaultOutputPort<T>();

  /**
   * This executes the query to retrieve result from database.
   * It then converts each row into tuple and emit that into output port.
   */
  @Override
  public void emitTuples()
  {
    String query = queryToRetrieveData();
    logger.debug(String.format("select statement: %s", query));

    try {
      ResultSet result = store.getSession().execute(query);
      if (!result.isExhausted()) {
        for (Row row : result) {
          T tuple = getTuple(row);
          outputPort.emit(tuple);
        }
      } else {
        // No rows available wait for some time before retrying so as to not continuously slam the database
        Thread.sleep(waitForDataTimeout);
      }
    }
    catch (Exception ex) {
      store.disconnect();
      DTThrowable.rethrow(ex);
    }
  }
}