Firehose.java

/*
* Licensed to Metamarkets Group Inc. (Metamarkets) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. Metamarkets 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 io.druid.data.input;

import java.io.Closeable;

/**
 * This is an interface that holds onto the stream of incoming data.  Realtime data ingestion is built around this
 * abstraction.  In order to add a new type of source for realtime data ingestion, all you need to do is implement
 * one of these and register it with the Main.
 *
 * This object acts a lot like an Iterator, but it doesn't extend the Iterator interface because it extends
 * Closeable and it is very important that the close() method doesn't get forgotten, which is easy to do if this
 * gets passed around as an Iterator.
 * <p>
 * The implementation of this interface only needs to be minimally thread-safe. The three methods ##hasMore(),
 * ##nextRow() and ##commit() are all called from the same thread.  ##commit(), however, returns a callback
 * which will be called on another thread, so the operations inside of that callback must be thread-safe.
 * </p>
 */
public interface Firehose extends Closeable
{
  /**
   * Returns whether there are more rows to process.  This is used to indicate that another item is immediately
   * available via ##nextRow().  Thus, if the stream is still available but there are no new messages on it, this call
   * should block until a new message is available.
   *
   * If something happens such that the stream is no longer available, this should return false.
   *
   * @return true if and when there is another row available, false if the stream has dried up
   */
  public boolean hasMore();

  /**
   * The next row available.  Should only be called if hasMore returns true.
   *
   * @return The next row
   */
  public InputRow nextRow() ;

  /**
   * Returns a runnable that will "commit" everything read up to the point at which commit() is called.  This is
   * often equivalent to everything that has been read since the last commit() call (or instantiation of the object),
   * but doesn't necessarily have to be.
   *
   * This method is called when the main processing loop starts to persist its current batch of things to process.
   * The returned runnable will be run when the current batch has been successfully persisted, there is usually
   * some time lag between when this method is called and when the runnable is run.  The Runnable is also run on
   * a separate thread so its operation should be thread-safe.
   *
   * The Runnable is essentially just a lambda/closure that is run() after data supplied by this instance has
   * been committed on the writer side of this interface protocol.
   * <p>
   * A simple implementation of this interface might do nothing when run() is called 
   * (in which case the same do-nothing instance can be returned every time), or 
   * a more complex implementation might clean up temporary resources that are no longer needed 
   * because of InputRows delivered by prior calls to ##nextRow().
   * </p>
   */
  public Runnable commit();
}