25 Using Map Events

You can use map event listeners to receive cache events and events from any class in Coherence that implements the ObservableMap interface.

This chapter includes the following sections:

Overview of Map Events

Coherence provides cache events using the JavaBean Event model.
The implementation allows applications to receive the events when and where they are needed, regardless of where the changes are actually occurring in the cluster. Developers that are familiar with the JavaBean model should have no difficulties working with events, even in a complex cluster.

Note:

Coherence also includes the live event programming model. Live events provide support for common event types and can be used instead of map events. See Using Live Events.

This section includes the following topics:

Listener Interface and Event Object

In the JavaBeans Event model, there is an EventListener interface that all listeners must extend. Coherence provides a MapListener interface, which allows application logic to receive events when data in a Coherence cache is added, modified or removed.

An application object that implements the MapListener interface can sign up for events from any Coherence cache or class that implements the ObservableMap interface, simply by passing an instance of the application's MapListener implementation to a addMapListener() method.

The MapEvent object that is passed to the MapListener carries all of the necessary information about the event that has occurred, including the source (ObservableMap) that raised the event, the identity (key) that the event is related to, what the action was against that identity (insert, update or delete), what the old value was and what the new value is.

Understanding Event Guarantees

The partitioned cache service guarantees that under normal circumstances an event is delivered only once. However, there are two scenarios that could break this guarantee:

  • A catastrophic cluster failure that caused the data loss (for example, simultaneous crash of two machines holding data). In this case, the PARTITION_LOST event is emitted to all registered PartitionListener instances on the server side.

  • Client disconnect. In this case, the MEMBER_LEFT event is emitted to all registered MemberListener instances on the client side.

Caches and Classes that Support Events

All Coherence caches implement ObservableMap; in fact, the NamedCache interface that is implemented by all Coherence caches extends the ObservableMap interface. That means that an application can sign up to receive events from any cache, regardless of whether that cache is local, partitioned, near, replicated, using read-through, write-through, write-behind, overflow, disk storage, and so on.

Note:

Regardless of the cache topology and the number of servers, and even if the modifications are being made by other servers, the events are delivered to the application's listeners.

In addition to the Coherence caches (those objects obtained through a Coherence cache factory), several other supporting classes in Coherence also implement the ObservableMap interface:

  • ObservableHashMap

  • LocalCache

  • OverflowMap

  • NearCache

  • ReadWriteBackingMap

  • AbstractSerializationCache, SerializationCache, and SerializationPagedCache

  • WrapperObservableMap, WrapperConcurrentMap, and WrapperNamedCache

Signing Up for All Events

To sign up for events, pass an object that implements the MapListener interface to an addMapListener method on the ObservableMap interface.The following example illustrates a sample MapListener implementation that prints each event it receives.
/**
* A MapListener implementation that prints each event as it receives
* them.
*/
public static class EventPrinter
        extends Base
        implements MapListener
    {
    public void entryInserted(MapEvent evt)
        {
        out(evt);
        }

    public void entryUpdated(MapEvent evt)
        {
        out(evt);
        }

    public void entryDeleted(MapEvent evt)
        {
        out(evt);
        }
    }

Using this implementation, you can print all events from any given cache (since all caches implement the ObservableMap interface):

cache.addMapListener(new EventPrinter());

To be able to later remove the listener, it is necessary to hold on to a reference to the listener:

Listener listener = new EventPrinter();
cache.addMapListener(listener);
m_listener = listener; // store the listener in a field

The listener can then be removed:

Listener listener = m_listener;
if (listener != null)
    {
    cache.removeMapListener(listener);
    m_listener = null; // clean up the listener field
    }

Each addMapListener method on the ObservableMap interface has a corresponding removeMapListener method. To remove a listener, use the removeMapListener method that corresponds to the addMapListener method that was used to add the listener.

Using an Inner Class as a MapListener

You can use the AbstractMapListener base class when creating an inner class to use as a MapListener or when implementing a MapListener that only listens to one or two types of events (inserts, updates or deletes).The following example, prints out only the insert events for the cache.
cache.addMapListener(new AbstractMapListener()
    {
    public void entryInserted(MapEvent evt)
        {
        out(evt);
        }
    });

Another helpful base class for creating a MapListener implementation is the MultiplexingMapListener, which routes all events to a single method for handling. Since only one method must be implemented to capture all events, the MultiplexingMapListener can also be very useful when creating an inner class to use as a MapListener:

public static class EventPrinter
        extends MultiplexingMapListener
    {
    public void onMapEvent(MapEvent evt)
        {
        out(evt);
        }
    }

Using Lambda Expressions to Add Map Listeners

A lambda expression can be used to add MapListener<K, V> implementations.The following example uses a lambda expression to add the SimpleMapListener<K, V> implementation that is delivered with Coherence. The implementation delegates to an appropriate event handler based on the event type.

MapListener<ContactId, Contact> listener = new SimpleMapListener<ContactId,
   Contact>().addInsertHandler((event) -> System.out.println("\ninserted:\n" +
   event.getNewValue()));

cache.addMapListener(listener);

Configuring a MapListener For a Cache

If a listener should always be on a particular cache, then place it into the cache configuration using the <listener> element and the listener is automatically added to the cache.See listener.

Signing Up For Events On Specific Identities

You can sign up for events that occur against specific identities (keys).For example, to print all events that occur against the Integer key 5:
cache.addMapListener(new EventPrinter(), new Integer(5), false);

Thus, the following code would only trigger an event when the Integer key 5 is inserted or updated:

for (int i = 0; i < 10; ++i)
    {
    Integer key   = new Integer(i);
    String  value = "test value for key " + i;
    cache.put(key, value);
    }

Filtering Events

You can use filters to listen to particular events.The following example adds a listener to a cache with a filter that allows the listener to only receive delete events.
// Filters used with partitioned caches must be 
// Serializable, Externalizable or ExternalizableLite
public class DeletedFilter
        implements Filter, Serializable
    {
    public boolean evaluate(Object o)
        {
        MapEvent evt = (MapEvent) o;
        return evt.getId() == MapEvent.ENTRY_DELETED;
        }
    }

cache.addMapListener(new EventPrinter(), new DeletedFilter(), false);

Note:

Filtering events versus filtering cached data:

When building a filter for querying, the object that is passed to the evaluate method of the Filter is a value from the cache, or - if the filter implements the EntryFilter interface - the entire Map.Entry from the cache. When building a filter for filtering events for a MapListener, the object that is passed to the evaluate method of the filter is of type MapEvent. See Listening to Queries.

If you then make the following sequence of calls:

cache.put("hello", "world");
cache.put("hello", "again");
cache.remove("hello");

The result would be:

CacheEvent{LocalCache deleted: key=hello, value=again}

Using Lite Events

You can use lite events if an event should only include new values. By default, Coherence provides both the old and the new value as part of an event. Consider the following example:
MapListener listener = new MultiplexingMapListener()
    {
    public void onMapEvent(MapEvent evt)
        {
        out("event has occurred: " + evt);
        out("(the wire-size of the event would have been "
            + ExternalizableHelper.toBinary(evt).length()
            + " bytes.)");
        }
    };
cache.addMapListener(listener);

// insert a 1KB value
cache.put("test", new byte[1024]);

// update with a 2KB value
cache.put("test", new byte[2048]);

// remove the 2KB value
cache.remove("test");

The output from running the test shows that the first event carries the 1KB inserted value, the second event carries both the replaced 1KB value and the new 2KB value, and the third event carries the removed 2KB value:

event has occurred: CacheEvent{LocalCache added: key=test, value=[B@a470b8}
(the wire-size of the event would have been 1283 bytes.)
event has occurred: CacheEvent{LocalCache updated: key=test, old value=[B@a470b8, new value=[B@1c6f579}
(the wire-size of the event would have been 3340 bytes.)
event has occurred: CacheEvent{LocalCache deleted: key=test, value=[B@1c6f579}
(the wire-size of the event would have been 2307 bytes.)

When an application does not require the old and the new value to be included in the event, it can indicate that by requesting only "lite" events. When adding a listener, you can request lite events by using a addMapListener method that takes an additional boolean fLite parameter:

cache.addMapListener(listener, (Filter) null, true);

Note:

Obviously, a lite event's old value and new value may be null. However, even if you request lite events, the old and the new value may be included if there is no additional cost to generate and deliver the event. In other words, requesting that a MapListener receive lite events is simply a hint to the system that the MapListener does not have to know the old and new values for the event.

Listening to Queries

The same filters that are used to query a cache can listen to events from a cache.

This section includes the following topics:

Overview of Listening to Queries

All Coherence caches support querying by any criteria. When an application queries for data from a cache, the result is a point-in-time snapshot, either as a set of identities (keySet) or a set of identity/value pairs (entrySet). The mechanism for determining the contents of the resulting set is referred to as filtering, and it allows an application developer to construct queries of arbitrary complexity using a rich set of out-of-the-box filters (for example, equals, less-than, like, between, and so on), or to provide their own custom filters (for example, XPath).

For example, in a trading system it is possible to query for all open Order objects for a particular trader:

NamedCache mapTrades = ...
Filter filter = new AndFilter(new EqualsFilter("getTrader", traderid),
                              new EqualsFilter("getStatus", Status.OPEN));
Set setOpenTrades = mapTrades.entrySet(filter);

To receive notifications of new trades being opened for that trader, closed by that trader or reassigned to or from another trader, the application can use the same filter:

// receive events for all trade IDs that this trader is interested in
mapTrades.addMapListener(listener, new MapEventFilter(filter), true);

The MapEventFilter converts a query filter into an event filter.

The MapEventFilter has several very powerful options, allowing an application listener to receive only the events that it is specifically interested in. More importantly for scalability and performance, only the desired events have to be communicated over the network, and they are communicated only to the servers and clients that have expressed interest in those specific events. For example:

// receive all events for all trades that this trader is interested in
nMask = MapEventFilter.E_ALL;
mapTrades.addMapListener(listener, new MapEventFilter(nMask, filter), true);

// receive events for all this trader's trades that are closed or
// re-assigned to a different trader
nMask = MapEventFilter.E_UPDATED_LEFT | MapEventFilter.E_DELETED;
mapTrades.addMapListener(listener, new MapEventFilter(nMask, filter), true);

// receive events for all trades as they are assigned to this trader
nMask = MapEventFilter.E_INSERTED | MapEventFilter.E_UPDATED_ENTERED;
mapTrades.addMapListener(listener, new MapEventFilter(nMask, filter), true);

// receive events only fornew trades assigned to this trader
nMask = MapEventFilter.E_INSERTED;
mapTrades.addMapListener(listener, new MapEventFilter(nMask, filter), true);

Filtering Events Versus Filtering Cached Data

When building a Filter for querying, the object that is passed to the evaluate method of the Filter is a value from the cache, or if the Filter implements the EntryFilter interface, the entire Map.Entry from the cache. When building a Filter for filtering events for a MapListener, the object that is passed to the evaluate method of the Filter is of type MapEvent.

The MapEventFilter converts a Filter that is used to do a query into a Filter that is used to filter events for a MapListener. In other words, the MapEventFilter is constructed from a Filter that queries a cache, and the resulting MapEventFilter is a filter that evaluates MapEvent objects by converting them into the objects that a query Filter would expect.

Using Synthetic Events

You can choose to monitor synthetic events.Events usually reflect the changes being made to a cache. For example, one server is modifying one entry in a cache while another server is adding several items to a cache while a third server is removing an item from the same cache, all while fifty threads on each and every server in the cluster is accessing data from the same cache! All the modifying actions produces events that any server within the cluster can choose to receive. We refer to these actions as client actions, and the events as being dispatched to clients, even though the "clients" in this case are actually servers. This is a natural concept in a true peer-to-peer architecture, such as a Coherence cluster: Each and every peer is both a client and a server, both consuming services from its peers and providing services to its peers. In a typical Java Enterprise application, a "peer" is an application server instance that is acting as a container for the application, and the "client" is that part of the application that is directly accessing and modifying the caches and listening to events from the caches.

Some events originate from within a cache itself. There are many examples, but the most common cases are:

  • When entries automatically expire from a cache;

  • When entries are evicted from a cache because the maximum size of the cache has been reached;

  • When entries are transparently added to a cache as the result of a Read-Through operation;

  • When entries in a cache are transparently updated as the result of a Read-Ahead or Refresh-Ahead operation.

Each of these represents a modification, but the modifications represent natural (and typically automatic) operations from within a cache. These events are referred to as synthetic events.

When necessary, an application can differentiate between client-induced and synthetic events simply by asking the event if it is synthetic. This information is carried on a sub-class of the MapEvent, called CacheEvent. Using the previous EventPrinter example, it is possible to print only the synthetic events:

public static class EventPrinter
        extends MultiplexingMapListener
    {
    public void onMapEvent(MapEvent evt)
        {
        if (evt instanceof CacheEvent && ((CacheEvent) evt).isSynthetic())
            {
            out(evt);
            )
        }
    }

Listening to Backing Map Events

You can listen to events for the map that backs a cache (partitioned, replicated, near, continuously-query, read-through/write-through and write-behind).

This section includes the following topics:

Overview of Listening to Backing Map Events

For some advanced use cases, it may be necessary to "listen to" the "map" behind the "service". Replication, partitioning and other approaches to managing data in a distributed environment are all distribution services. The service still has to have something in which to actually manage the data, and that something is called a "backing map".

Backing maps can be configured. If all the data for a particular cache should be kept in object form on the heap, then use an unlimited and non-expiring LocalCache (or a SafeHashMap if statistics are not required). If only a small number of items should be kept in memory, use a LocalCache. If data are to be read on demand from a database, then use a ReadWriteBackingMap (which knows how to read and write through an application's DAO implementation), and in turn give the ReadWriteBackingMap a backing map such as a SafeHashMap or a LocalCache to store its data in.

Some backing maps are observable. The events coming from these backing maps are not usually of direct interest to the application. Instead, Coherence translates them into actions that must be taken (by Coherence) to keep data synchronous and properly backed up, and it also translates them when appropriate into clustered events that are delivered throughout the cluster as requested by application listeners. For example, if a partitioned cache has a LocalCache as its backing map, and the local cache expires an entry, that event causes Coherence to expire all of the backup copies of that entry. Furthermore, if any listeners have been registered on the partitioned cache, and if the event matches their event filter(s), then that event is delivered to those listeners on the servers where those listeners were registered.

In some advanced use cases, an application must process events on the server where the data are being maintained, and it must do so on the structure (backing map) that is actually managing the data. In these cases, if the backing map is an observable map, a listener can be configured on the backing map or one can be programmatically added to the backing map. (If the backing map is not observable, it can be made observable by wrapping it in an WrapperObservableMap.)

Each backing map event is dispatched once and only once. However, multiple backing map events could be generated from a single put. For example, if the entry from put has to be redistributed, then distributed events (deleted from original node, and inserted in a new node) are created. In this case, the backing map listener is called multiple times for the single put.

Lastly, backing map listeners are always synchronous; they are fired on a thread that is doing the modification operation while holding the synchronization monitor for the backing map itself. Often times for internal backing map listeners, events are not processed immediately, but are queued and processed later asynchronously.

Producing Readable Backing MapListener Events from Distributed Caches

Backing MapListener events are returned from replicated caches in readable Java format. However, backing MapListener events returned from distributed caches are in internal Coherence format. The Coherence Incubator Common project provides an AbstractMultiplexingBackingMapListener class that enables you to obtain readable backing MapListener events from distributed caches. See https://coherence.java.net/ to download the Coherence Common libraries.

To produce readable backing MapListener events from distributed caches:

  1. Implement the AbstractMultiplexingBackingMapListener class.
  2. Register the implementation in the <listener> section of the backing-map-scheme in the cache-config file.
  3. Start the cache server application file and the client file with the cacheconfig Java property:
    -Dcoherence.cacheconfig="cache-config.xml"
    

The AbstractMultiplexingBackingMapListener class provides an onBackingMapEvent method which you can override to specify how you would like the event returned.

The following listing of the VerboseBackingMapListener class is a sample implementation of AbstractMultiplexingBackingMapListener. The onBackingMapEvent method has been overridden to send the results to standard output.

import com.tangosol.net.BackingMapManagerContext;
import com.tangosol.util.MapEvent;


public class VerboseBackingMapListener extends AbstractMultiplexingBackingMapListener {

        public VerboseBackingMapListener(BackingMapManagerContext context) {
                super(context);
        }
        
        @Override
        protected void onBackingMapEvent(MapEvent mapEvent, Cause cause) {
                
                System.out.printf("Thread: %s Cause: %s Event: %s\n",
                Thread.currentThread().getName(), cause, mapEvent);
        }
}

The following example demonstrates setting the <listener> element in a distributed cache scheme and identifies the VerboseBackingMapListener implementation as being of type com.tangosol.net.BackingMapManagerContext.

<distributed-scheme>
   <scheme-name>my-dist-scheme</scheme-name>
   <service-name>DistributedCache</service-name>
      <backing-map-scheme>
         <read-write-backing-map-scheme>
            <internal-cache-scheme>
               <local-scheme>
                  <high-units>0</high-units>
                  <expiry-delay>0</expiry-delay>
               </local-scheme>
            </internal-cache-scheme>
            <cachestore-scheme>
               <class-scheme>
                  <class-name>CustomCacheStore</class-name>
                  <init-params>
                     <init-param>
                        <param-type>java.lang.String</param-type>
                        <param-value>{cache-name}</param-value>
                     </init-param>
                  </init-params>
               </class-scheme>
            </cachestore-scheme>
            <listener> 
               <class-scheme>
                  <class-name>VerboseBackingMapListener</class-name>
                     <init-params>
                        <init-param>
                           <param-type>com.tangosol.net.BackingMapManagerContext
                           </param-type>
                           <param-value>{manager-context}</param-value>
                        </init-param>
                     </init-params>
                  </class-scheme>
            </listener> 
         </read-write-backing-map-scheme>                                            
      </backing-map-scheme>
   <autostart>true</autostart>
</distributed-scheme>

Using Synchronous Event Listeners

Some events are delivered asynchronously, so that application listeners do not disrupt the cache services that are generating the events. In some rare scenarios, asynchronous delivery can cause ambiguity of the ordering of events compared to the results of ongoing operations. To guarantee that cache API operations and the events are ordered as if the local view of the clustered system were single-threaded, a MapListener must implement the SynchronousListener marker interface.

One example in Coherence itself that uses synchronous listeners is the near cache, which can use events to invalidate locally cached data. That is, on a put operation into a near cache, the local copy and the distributed primary and backup copies of the entry are updated as a synchronous operation. Once this update is complete, asynchronous events are sent to all the other listening near caches. This invalidates the local copies so that the entries are retrieved from the back cache on the next get operation.