Delta (Apache Geode 1.14.2)
Date: 2021-12-14

org.apache.geode

## Interface Delta

- All Known Implementing Classes:
    DeltaSession, DeltaSession7, DeltaSession8, DeltaSession9

      public interface Delta

  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 `DataOutput` and `DataInput` parameters.
  Since:
    GemFire 6.1

- - ### Method Summary

    All Methods Instance Methods Abstract Methods Default Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `fromDelta(DataInput in)` This method is invoked on an existing application object when an update is received as a delta.
    `default boolean` | `getForceRecalculateSize()` By default, entry sizes are not recalculated when deltas are applied.
    `boolean` | `hasDelta()` Returns true if this object has pending changes it can write out as a delta.
    `void` | `toDelta(DataOutput out)` This method is invoked on an application object at the delta sender, if GemFire determines the presence of a delta by calling `hasDelta()` on the object.

- - ### Method Detail

    - #### hasDelta

          boolean hasDelta()

      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.

    - #### toDelta

          void toDelta(DataOutput out)
                throws IOException

      This method is invoked on an application object at the delta sender, if GemFire determines the presence of a delta by calling `hasDelta()` on the object. The delta is written to the `DataOutput` object provided by GemFire.

      Any delta state should be reset in this method.

      Throws:
        `IOException`

    - #### fromDelta

          void fromDelta(DataInput in)
                  throws IOException,
                         InvalidDeltaException

      This method is invoked on an existing application object when an update is received as a delta. This method throws an `InvalidDeltaException` when the delta in the `DataInput` cannot be applied to the object. GemFire automatically handles an `InvalidDeltaException` by reattempting the update by sending the full application object.
      Throws:
        `IOException`
        `InvalidDeltaException`

    - #### getForceRecalculateSize

          default boolean getForceRecalculateSize()

      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.

      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.

      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.

      Since:
        1.14