Defines methods that are called BEFORE entry modification. A distributed region will typically have a single cache writer. If the application is designed such that all or most updates to a region occurs on a node, it is desirable to have the cache writer for the region to be installed at that node.
Cache writer invocations are initiated by the node where the entry or region modification occurs. If there is a local cache writer on the node where the change occurs, that is invoked. If there is no local cache writer , the system knows which of the nodes that have the region defined have a cache writer defined.
Note that cache writer callbacks are synchronous callbacks and have the ability to veto the cache update. Since cache writer invocations require communications over the network, (especially if they are not colocated on the nodes where the change occurs) the use of cache writers presents a performance penalty.
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
CacheWriters 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.
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 process as the
Public Member Functions
|virtual bool||beforeCreate (const EntryEvent &event)|
|Called before an entry is created. |
|virtual bool||beforeDestroy (const EntryEvent &event)|
|Called before an entry is destroyed. |
|virtual bool||beforeUpdate (const EntryEvent &event)|
|Called before an entry is updated. |
|virtual void||close (const RegionPtr &rp)|
|Called when the region containing this callback is destroyed, when the cache is closed. |
|void||preserveSB () const|
|Atomically increment reference count. |
|void||releaseSB () const|
|Atomically decrement reference count, the SharedBase object is automatically deleted when its reference count goes to zero. |
|virtual bool gemfire::CacheWriter::beforeCreate||(||const EntryEvent &||event||)||
Called before an entry is created.
Entry creation is initiated by a
put, or a
CacheWriter can determine whether this value comes from a
get or not from EntryEvent::isLoad. 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.
|event||EntryEvent denotes the event object associated with creating the entry|
|virtual bool gemfire::CacheWriter::beforeDestroy||(||const EntryEvent &||event||)||
Called before an entry is destroyed.
|event||EntryEvent denotes the event object associated with destroying the entry|
|virtual bool gemfire::CacheWriter::beforeUpdate||(||const EntryEvent &||event||)||
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 NULLPTR. The entry being updated may or may not exist in the local cache where the CacheWriter is installed.
|event||EntryEvent denotes the event object associated with updating the entry|
|virtual void gemfire::CacheWriter::close||(||const RegionPtr &||rp||)||
Called when the region containing this callback is destroyed, when the cache is closed.
Implementations should clean up any external resources, such as database connections. Any runtime exceptions this method throws will be logged.
It is possible for this method to be called multiple times on a single callback instance, so implementations must be tolerant of this.
|void gemfire::SharedBase::preserveSB||(||)|| const
Atomically increment reference count.
|void gemfire::SharedBase::releaseSB||(||)|| const
Atomically decrement reference count, the SharedBase object is automatically deleted when its reference count goes to zero.