Geode provides APIs such that a distributed system can capture events, invoking callbacks to process those events either synchronously or asynchronously.

This document covers best practices for the CacheWriter and the CacheListener.

Event Model

Cache Writers

A CacheWriter is an event handler invoked synchronously prior to an event. A cache writer is often used to validate data prior to an update of that data. It may also do a synchronization with external data sources. This provides a write-through capability for regions handling events that can be local, within the same JVM, or remote, in the case of replicated or partitioned region.

Basic rules:

• There can be only one CacheWriter per region.
• For partitioned regions, the node that hosts the primary bucket of the data will be the one that invokes the cache writer.
• For replicated regions, only the first node to successfully execute the writer will process the event.
• For local regions, only the local cache writer (if defined) will process the event.
• CacheWriter can abort operations (fail-fast), and a CacheWriterException will propagate back to the caller.
• Being a synchronous callback, it blocks the application's execution until the handler completes.

CacheWriter events and callbacks:

beforeCreate(EntryEvent event)- Invoked before an entry is created
beforeUpdate(EntryEvent event) - Invoked before an entry is updated
beforeDestroy(EntryEvent event) - Invoked before an entry is destroyed
beforeRegionClear(RegionEvent event) - Invoked before a region is cleared
beforeRegionDestroy(RegionEvent event) - Invoked before a region is destroyed

Because CacheWriter handlers are called synchronously, the application does not continue until the handler returns. Therefore, do not do long-running operations inside the handler. If a long-running operation is needed, consider processing the operation asynchronously through an AsyncEventListener. Using an ExecutorService to delegate the execution to a different thread is possible, but it is an anti-pattern, as it no longer implements the fail-fast property, and the handling of the event is no longer synchronous, so its timing would not be guaranteed relative to the application's completion of the event.

Cache Listeners

A CacheListener is an event handler invoked synchronously after modifications to a region occur. The main use cases for a CacheListener are synchronous write-behind and notifications. The CacheListener can handle cache events related to entries (EntryEvent) and regions (RegionEvent), but events can be processed in a different order than the order in which they’ are applied to the region.

Basic rules:

• You can install multiple CacheListener handlers in the same region.
• When multiple listeners are installed, the handlers are invoked serially.  The invocation ordering is the same as the in which the listeners were registered.
• For partitioned regions, the node that hosts the primary bucket of the data will be the one that invokes the cache listeners.
• For replicated regions, all nodes with the listener installed will process the event.
• For local regions, only local listeners (if defined) will process the event.
• For long running or batch processing, consider using an AsynchronousEventListener.
• Being a synchronous callback, the execution of each handler blocks the application's execution until the handler completes.

CacheListener events and callbacks:

afterCreate(EntryEvent<K,V> event) - Invoked after a new key is added to a region
afterDestroy(EntryEvent<K,V> event) - Invoked after an entry is destroyed
afterInvalidate(EntryEvent<K,V> event) - Invoked after an entry's value is invalidated
afterRegionClear(RegionEvent<K,V> event) - Invoked after a region is cleared
afterRegionCreate(RegionEvent<K,V> event) - Invoked after a region is created
afterRegionDestroy(RegionEvent<K,V> event) - Invoked after a region is destroyed
afterRegionInvalidate(RegionEvent<K,V> event) - Invoked after a region is invalidated
afterRegionLive(RegionEvent<K,V> event) - Invoked after a region becomes live after receiving the marker from the server
afterUpdate(EntryEvent<K,V> event) - Invoked after an entry's value is modified

General recommendations

When dealing with Geode callbacks, there are some operations that should be avoided or used with extra attention. Some general recommendations are:

• Do not perform distributed operations, such as using the Distributed Lock service.
• Avoid calling Region methods, particularly on non-colocated, partitioned regions.
• Avoid calling functions through FunctionService, since the function's execution can cause distributed deadlock.
• Do not use any Geode APIs inside a CacheListener if you have conserve-sockets set to true.
• Do not modify region attributes, since those messages will have priority and can cause blocks.
• Avoid configurations in which listeners or writers are deployed in a few nodes of the distributed system. Prefer a cluster-wide installation where every node can process the callback.
• Any exceptions thrown are caught and logged, so users can troubleshoot using Geode logs.
• EntryEvent.getNewValue() or EntryEvent.getOldValue() can result in deserializations, unless PDX and read-serialized=true are used.
• Operations inside a CacheListener or a CacheWriter are thread-safe, and entries are locked for the current thread.

When using transactions:

• A CacheWriter should not start transactions.
• Both CacheWriter and any CacheListener will receive all individual operations as part of a transaction, unlike their transactional counterparts TransactionWriter and TransactionListener.
• When a rollback or commit happens, a CacheWriter can only be notified by a TransactionWriter, and should handle rollback or failures properly.
• CacheWriterException is still propagated to the application, and it should handle the failures in the context of the transaction by continuing or aborting; JTA is the recommended alternative.
• In most cases when dealing with transactions, consider using a TransactionWriter, instead of a CacheWriter.
• With global transactions, EntryEvent.getTransactionId() will return the current internal transaction ID.
• Use the same transactional data source and make sure it is JTA-enabled, so database operations inside a CacheWriter can be rolled back and participate in the same global transaction.

When dealing with transactions always consider using TransactionListener or TransactionWriter for handling transaction events, but do notice that they are cache-wide handlers.