geode-core/src/main/java/org/apache/geode/Delta.java

/*
 * 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;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;


/**
 * This interface defines a contract between the application and GemFire that allows GemFire to
 * determine whether an application object contains a delta, allows GemFire to extract the delta
 * from an application object, and generate a new application object by applying a delta to an
 * existing application object. The difference in object state is contained in the {@link
 * DataOutput} and {@link DataInput} parameters.
 *
 * @since GemFire 6.1
 */
public interface Delta {

  /**
   * Returns true if this object has pending changes it can write out as a delta. Returns false if
   * this object must be transmitted in its entirety.
   *
   * @return whether this object has pending changes it can write out as a delta
   */
  boolean hasDelta();

  /**
   * This method is invoked on an application object at the delta sender, if GemFire determines the
   * presence of a delta by calling {@link Delta#hasDelta()} on the object. The delta is written to
   * the {@link DataOutput} object provided by GemFire.
   *
   * <p>
   * Any delta state should be reset in this method.
   *
   * @param out the {@link DataOutput} to write to
   * @throws IOException if a problem occurs while writing to <code>out</code>
   */
  void toDelta(DataOutput out) throws IOException;


  /**
   * This method is invoked on an existing application object when an update is received as a delta.
   * This method throws an {@link InvalidDeltaException} when the delta in the {@link DataInput}
   * cannot be applied to the object. GemFire automatically handles an {@link InvalidDeltaException}
   * by reattempting the update by sending the full application object.
   *
   * @param in the {@link DataInput} to read from
   * @throws IOException if a problem occurs while reading from <code>in</code>
   */
  void fromDelta(DataInput in) throws IOException, InvalidDeltaException;

  /**
   * By default, entry sizes are not recalculated when deltas are applied. This optimizes for the
   * case where the size of an entry does not change. However, if an entry size does increase or
   * decrease, this default behavior can result in the memory usage statistics becoming inaccurate.
   * These are used to monitor the health of Geode instances, and for balancing memory usage across
   * partitioned regions.
   *
   * <p>
   * There is a system property, gemfire.DELTAS_RECALCULATE_SIZE, which can be used to cause all
   * deltas to trigger entry size recalculation when deltas are applied. By default, this is set
   * to 'false' because of potential performance impacts when every delta triggers a recalculation.
   *
   * <p>
   * To allow entry size recalculation on a per-delta basis, classes that extend the Delta interface
   * should override this method to return 'true'. This may impact performance of specific delta
   * types, but will not globally affect the performance of other Geode delta operations.
   *
   * @return whether entry sizes should be recalculated when deltas are applied
   * @since 1.14
   */
  default boolean getForceRecalculateSize() {
    return false;
  }
}