CacheWriter (Apache Geode 1.11.0)
Date: 2019-12-20

org.apache.geode.cache

## Interface CacheWriter<K,V>

- All Superinterfaces:
    CacheCallback, Declarable
  All Known Implementing Classes:
    CacheWriterAdapter, JdbcWriter, LocalSessionCacheWriter

      public interface CacheWriter<K,V>
      extends CacheCallback

  A user-defined object defined in the `RegionAttributes` that is called synchronously before a region or entry in the cache is modified. The typical use for a `CacheWriter` is to update a database. Application writers should implement these methods to execute application-specific behavior before the cache is modified.

  Before the region is updated via a put, create, or destroy operation, GemFire will call a `CacheWriter` that is installed anywhere in any participating cache for that region, preferring a local `CacheWriter` if there is one. Usually there will be only one `CacheWriter` in the distributed system. If there are multiple `CacheWriter`s available in the distributed system, the GemFire implementation always prefers one that is stored locally, or else picks one arbitrarily; in any case only one `CacheWriter` will be invoked.

  The `CacheWriter` is capable of aborting the update to the cache by throwing a `CacheWriterException`. This exception or any runtime exception thrown by the `CacheWriter` will abort the operation and the exception will be propagated to the initiator of the operation, regardless of whether the initiator is in the same VM as the `CacheWriter`.

  WARNING: To avoid risk of deadlock, do not invoke CacheFactory.getAnyInstance() from within any callback methods. Instead use EntryEvent.getRegion().getCache() or RegionEvent.getRegion().getCache().

  Since:
    GemFire 3.0
  See Also:
    `AttributesFactory.setCacheWriter(org.apache.geode.cache.CacheWriter<K, V>)`, `RegionAttributes.getCacheWriter()`, `AttributesMutator.setCacheWriter(org.apache.geode.cache.CacheWriter<K, V>)`

- - ### Method Summary

    All Methods Instance Methods Abstract Methods
    Modifier and Type | Method and Description
    --- | ---
    `void` | `beforeCreate(EntryEvent<K,V> event)` Called before an entry is created.
    `void` | `beforeDestroy(EntryEvent<K,V> event)` Called before an entry is destroyed.
    `void` | `beforeRegionClear(RegionEvent<K,V> event)` Called before a region is cleared.
    `void` | `beforeRegionDestroy(RegionEvent<K,V> event)` Called before a region is destroyed.
    `void` | `beforeUpdate(EntryEvent<K,V> event)` Called before an entry is updated.

    - ### Methods inherited from interface org.apache.geode.cache.CacheCallback

      `close`

    - ### Methods inherited from interface org.apache.geode.cache.Declarable

      `init, initialize`

- - ### Method Detail

    - #### beforeUpdate

          void beforeUpdate(EntryEvent<K,V> event)
                     throws CacheWriterException

      Called before an entry is updated. The entry update is initiated by a `put` or a `get` that causes the loader to update an existing entry. The entry previously existed in the cache where the operation was initiated, although the old value may have been null. The entry being updated may or may not exist in the local cache where the CacheWriter is installed.
      Parameters:
        `event` - an EntryEvent that provides information about the operation in progress
      Throws:
        `CacheWriterException` - if thrown will abort the operation in progress, and the exception will be propagated back to caller that initiated the operation
      See Also:
        `Region.put(Object, Object)`, `Region.get(Object)`

    - #### beforeCreate

          void beforeCreate(EntryEvent<K,V> event)
                     throws CacheWriterException

      Called before an entry is created. Entry creation is initiated by a `create`, a `put`, or a `get`. The `CacheWriter` can determine whether this value comes from a `get` or not by evaluating the `Operation`'s `Operation.isLoad()` method. The entry being created may already exist in the local cache where this `CacheWriter` is installed, but it does not yet exist in the cache where the operation was initiated.
      Parameters:
        `event` - an EntryEvent that provides information about the operation in progress
      Throws:
        `CacheWriterException` - if thrown will abort the operation in progress, and the exception will be propagated back to caller that initiated the operation
      See Also:
        `Region.create(Object, Object)`, `Region.put(Object, Object)`, `Region.get(Object)`

    - #### beforeDestroy

          void beforeDestroy(EntryEvent<K,V> event)
                      throws CacheWriterException

      Called before an entry is destroyed. The entry being destroyed may or may not exist in the local cache where the CacheWriter is installed. This method is not called as a result of expiration or `Region.localDestroy(Object)`.
      Parameters:
        `event` - an EntryEvent that provides information about the operation in progress
      Throws:
        `CacheWriterException` - if thrown will abort the operation in progress, and the exception will be propagated back to caller that initiated the operation
      See Also:
        `Region.destroy(Object)`

    - #### beforeRegionDestroy

          void beforeRegionDestroy(RegionEvent<K,V> event)
                            throws CacheWriterException

      Called before a region is destroyed. The `CacheWriter` will not additionally be called for each entry that is destroyed in the region as a result of a region destroy. If the region's subregions have `CacheWriter`s installed, then they will be called for the cascading subregion destroys. This method is not called as a result of `Region.close()`, `Cache.close(boolean)`, or `Region.localDestroyRegion()`. However, the `Region.close()` method is invoked regardless of whether a region is destroyed locally. A non-local region destroy results in an invocation of `beforeRegionDestroy(org.apache.geode.cache.RegionEvent<K, V>)` followed by an invocation of `Region.close()`.

      WARNING: This method should not destroy or create any regions itself or a deadlock will occur.

      Parameters:
        `event` - a RegionEvent that provides information about the operation
      Throws:
        `CacheWriterException` - if thrown, will abort the operation in progress, and the exception will be propagated back to the caller that initiated the operation
      See Also:
        `Region.destroyRegion()`

    - #### beforeRegionClear

          void beforeRegionClear(RegionEvent<K,V> event)
                          throws CacheWriterException

      Called before a region is cleared. The `CacheWriter` will not additionally be called for each entry that is cleared in the region as a result of a region clear.

      WARNING: This method should not clear/destroy any regions

      Parameters:
        `event` - a RegionEvent that provides information about the operation
      Throws:
        `CacheWriterException` - if thrown, will abort the operation in progress, and the exception will be propagated back to the caller that initiated the operation
      See Also:
        `Region.clear()`