Monday, 12 October 2015

Functional Map API: Listeners

We continue with the blog series on the experimental Functional Map API which was released as part of Infinispan 8.0.0.Final. In this blog post we'll be focusing on how to listen for Functional Map events. For reference, here are the previous entries in the series:
  1. Functional Map Introduction
  2. Working with single entries
  3. Working with multiple entries
The first thing to notice about Functional Map listeners is that they only send events post-event, so that means the events are received after the event has happened. In contrast with Infinispan Cache listeners, there are no pre-event listener invocations. The reason pre-events are not available is because listeners are meant to be an opportunity to find out what has happened, and having pre-events can sometimes hint as if the listener was able to alter the execution of the operation, for which the listener is not really suited. If interested in pre-events or potentially altering the execution, plugging custom interceptors is the recommended solution.

Functional Map offers two type of event listeners: write-only operation listeners and read-write operation listeners.

Write-Only Listeners

Write listeners enable users to register listeners for any cache entry write events that happen in either a read-write or write-only functional map.

Listeners for write events cannot distinguish between cache entry created and cache entry modify/update events because they don’t have access to the previous value. All they know is that a new non-null entry has been written. However, write event listeners can distinguish between entry removals and cache entry create/modify-update events because they can query what the new entry’s value via ReadEntryView.find() method.

Adding a write listener is done via the WriteListeners interface which is accessible via both ReadWriteMap.listeners() and WriteOnlyMap.listeners() method. A write listener implementation can be defined either passing a function to onWrite(Consumer<ReadEntryView<K, V>>) method, or passing a WriteListener implementation to add(WriteListener<K, V>) method. Either way, all these methods return an AutoCloseable instance that can be used to de-register the function listener. Example and expected output:

Read-Write Listeners

Read-write listeners enable users to register listeners for cache entry created, modified and removed events, and also register listeners for any cache entry write events. Entry created, modified and removed events can only be fired when these originate on a read-write functional map, since this is the only one that guarantees that the previous value has been read, and hence the differentiation between create, modified and removed can be fully guaranteed.

Adding a read-write listener is done via the ReadWriteListeners interface which is accessible via ReadWriteMap.listeners() method. If interested in only one of the event types, the simplest way to add a listener is to pass a function to either onCreate, onModify or onRemove methods. Otherwise, if interested in multiple type of events, passing a ReadWriteListener implementation via add(ReadWriteListener<K, V>) is the easiest. As with write-listeners, all these methods return an AutoCloseable instance that can be used to de-register the listener.

Here's an example of adding a ReadWriteListener that handles multiple type of events:

Closing Notes

More listener event types are yet to be implemented for Functional API, such as expiration events or passivation/activation events. We are capturing this future work and other improvements under the ISPN-5704 issue.

We'd love to hear from you on how you are finding this new API. To provide feedback or report any problems with it, head to our user forums and create a post there.

In next blog post in the series, we'll be looking into how to pass per-invocation parameters to tweak operations.


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