Geode provides APIs to capture a handful of events in a distributed system providing extensive documentation on how to implement listeners and callbacks for processing those synchronously or asynchronously.
This document covers CacheWriter
and CacheListener
best practices for handlingCacheEvents
.
Event Model
Cache Writers
A CacheWriter
is an event handler that synchronously handles “happens-before” events (CacheEvent
) in an ordered fashion. It is mostly used for data validation and in some cases data synchronization with external data sources, offering 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, only the writer in the event’s primary node will process the event.
- For replicated regions, only the first node to successfully execute the writer will process the event.
- For local regions, only local writer (if defined) will process the event.
- Unlike cache listeners, you can only install one cache writer in a region for each member.
CacheWriter
can abort operations (fail-fast) and will propagate theCacheWriterException
back to the caller.- Being a synchronous callback, it will block the current execution until it completes.
Available events and callbacks:
Code Block | ||
---|---|---|
| ||
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. |
It is not recommended to perform long running operations inside a CacheWriter
within the current thread. If such a long running operation is needed consider processing it asynchronously through an AsyncEventListener
for example. Using an ExecutorService
to delegate that execution to a different thread is possible, but considered an anti-pattern, since it breaks the fail-fast and happens-before concept of this callback.
Cache Listeners
A CacheListener
is an event handler that synchronously responds to events after modifications occurred in the system. The main use cases for a CacheListener
are synchronous write-behind and notifications, triggering actions after certain modifications occur on a region or on the system. It can handle cache events related to entries (EntryEvent
) and regions (RegionEvent
) but events can be processed in a different order than the order they’re applied in the region.
Basic rules:
- You can install multiple
CacheListener
in the same region. - When multiple listeners are installed, they will be executed in the same order they’re registered in the system. One at a time.
- For partitioned regions, only the listeners in the event’s primary node will process the event.
- For replicated regions, all nodes with the listener installed will process the event.
- For local regions, only local listener (if defined) will process the event.
- For long running or batch processing consider using an
AsynchronousEventListener
. - Being a synchronous callback, it will block the current execution until it completes.
Available events and callbacks:
Code Block | ||
---|---|---|
| ||
afterCreate(EntryEvent<K,V> event) - Handles the event of new key being added to a region afterDestroy(EntryEvent<K,V> event) - Handles the event of an entry being destroyed afterInvalidate(EntryEvent<K,V> event) - Handles the event of an entry's value being invalidated afterRegionClear(RegionEvent<K,V> event) - Handles the event of a region being cleared afterRegionCreate(RegionEvent<K,V> event) - Handles the event of a region being created afterRegionDestroy(RegionEvent<K,V> event) - Handles the event of a region being destroyed afterRegionInvalidate(RegionEvent<K,V> event) - Handles the event of a region being invalidated afterRegionLive(RegionEvent<K,V> event) - Handles the event of a region being live after receiving the marker from the server afterUpdate(EntryEvent<K,V> event) - Handles the event of an entry's value being modified in a region |
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 using the Distributed Lock service
- Avoid calling Region methods, particularly on non-colocated partitioned regions.
- Avoid calling functions through
FunctionService
, since they can cause distributed deadlocks. - 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 where 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()
orEntryEvent.getOldValue
can result in deserializations unless PDX andread-serialized=true
are used.- Operations inside a
CacheListener
or aCacheWriter
are thread-safe and entries are locked for the current thread.
When using transactions:
CacheWriter
should not start transactions;CacheWriter
andCacheListener
will receive all individual operations part of a transaction, unlike their transactional counterpartsTransactionWriter
andTransactionListener
;- When a rollback or commit happens, a
CacheWriter
can only be notified by aTransactionWriter
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 aCacheWriter
- With global transactions,
EntryEvent.getTransactionId()
will return the current internal transaction ID - Use the same transactional data source and make sure it’s 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
orTransactionWriter
for handling transaction events, but do notice that they’re cache-wide handlers.