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Apache Geode provides a database-like consistency model, reliable transaction processing and a shared-nothing architecture to maintain very low latency performance with high concurrency processing
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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.geode.cache.execute;

import java.util.concurrent.TimeUnit;

import org.apache.geode.distributed.DistributedMember;

/**
 * 
 * Defines the interface for a container that gathers results from function execution.

 * GemFire provides a default implementation for ResultCollector. Applications can choose to
 * implement their own custom ResultCollector. A custom ResultCollector facilitates result sorting
 * or aggregation. Aggregation functions like sum, minimum, maximum and average can also be applied
 * to the result using a custom ResultCollector. Results arrive as they are sent using the
 * {@link ResultSender#sendResult(Object)} and can be used as they arrive. To indicate that all
 * results have been received {@link #endResults()} is called.
 * 
 * 
 * Example: 

 * 
 *  *  Region region ; 
 *  Set keySet = Collections.singleton("myKey");
 *  Function multiGetFunction ;
 *  Object args ;
 *  ResultCollector rc = FunctionService.onRegion(region)
 *                                      .withArgs(args)
 *                                      .withFilter(keySet)
 *                                      .withCollector(new MyCustomResultCollector())
 *                                      .execute(multiGetFunction.getId());
 *  Application can do something else here before retrieving the result
 *  Or it can have some logic in {{@link #addResult(DistributedMember, Object)} to use the partial results.
 *  If it wants to see all the results it can use 
 *  Object functionResult = rc.getResult();
 *  or
 *  Object functionResult = rc.getResult(timeout,TimeUnit);
 *  depending on if it wants to wait for all the results or wait for a timeout period.
 * 
 * 
 * GemFire provides default implementation of ResultCollector which collects results in Arraylist.
 * There is no need to provide a synchronization mechanism in the user implementations of
 * ResultCollector
 * 
 * @since GemFire 6.0
 * 
 */
public interface ResultCollector {

  /**
   * Method used to pull results from the ResultCollector. It returns the result of function
   * execution, potentially blocking until {@link #endResults() all the results are available} has
   * been called.
   * 
   * @return the result
   * @throws FunctionException if result retrieval fails
   * @since GemFire 6.0
   */
  public S getResult() throws FunctionException;

  /**
   * Method used to pull results from the ResultCollector. It returns the result of function
   * execution, blocking for the timeout period until {@link #endResults() all the results are
   * available}. If all the results are not received in provided time a FunctionException is thrown.
   * 
   * @param timeout the maximum time to wait
   * @param unit the time unit of the timeout argument
   * @return the result
   * @throws FunctionException if result retrieval fails within timeout provided
   * @throws InterruptedException if the current thread was interrupted while waiting
   * @since GemFire 6.0
   * 
   */
  public S getResult(long timeout, TimeUnit unit) throws FunctionException, InterruptedException;

  /**
   * Method used to feed result to the ResultCollector. It adds a single function execution result
   * to the ResultCollector It is invoked every time a result is sent using ResultSender.
   * 
   * @param resultOfSingleExecution
   * @since GemFire 6.0
   * @param memberID DistributedMember ID to which result belongs
   */
  public void addResult(DistributedMember memberID, T resultOfSingleExecution);

  /**
   * GemFire will invoke this method when function execution has completed and all results for the
   * execution have been obtained and {@link #addResult(DistributedMember, Object) added to the
   * ResultCollector} Unless the {@link ResultCollector} has received
   * {@link ResultSender#lastResult(Object) last result} from all the executing nodes, it keeps
   * waiting for more results to come.
   * 
   * @since GemFire 6.0
   * 
   * @see ResultSender#lastResult(Object)
   */
  public void endResults();

  /**
   * GemFire will invoke this method before re-executing function (in case of Function Execution
   * HA). This is to clear the previous execution results from the result collector
   * 
   * @since GemFire 6.5
   */
  public void clearResults();

}