Cache Level Event Notifications

The Cache Level Event notifications are fired when data is added, updated, or removed from the cache by clients, Cache Loader, Backing Source, etc. These notifications allow applications to receive callbacks for cache-wide data changes. By default, the Cache Level Events are disabled (except for the cache cleared operation) and can be enabled through the NCache Management Center.

The Cache Level Event can be registered using the RegisterCacheNotification by specifying the implemented callback, the required EventType, and the EventDataFilter (None, Metadata, or DataWithMetadata). Here, we describe how you can register and unregister the Cache Level Events.

Note

The application will not be able to receive events unless it registers itself against the cache using the specific event registration API call.

Prerequisites

Implement Callback for Event Notifications

You can implement a callback for events where the EventType is specified according to the user's logic for ItemAdded, ItemUpdated, and ItemRemoved events. The following example implements a callback method for cache notifications with certain event types.

public void OnCacheDataModification(string key, CacheEventArg args)
{
    switch (args.EventType)
    {
        case EventType.ItemAdded:
            Console.WriteLine($"Item with Key '{key}' has been added to cache '{args.CacheName}'");
            break;

        case EventType.ItemUpdated:
            Console.WriteLine($"Item with Key '{key}' has been updated in the cache '{args.CacheName}'");

            // Item can be used if EventDataFilter is DataWithMetadata or Metadata
            if (args.Item != null)
            {
                Product updatedProduct = args.Item.GetValue<Product>();
                Console.WriteLine($"Updated Item is a Product having name '{updatedProduct.ProductName}', price '{updatedProduct.UnitPrice}', and quantity '{updatedProduct.QuantityPerUnit}'");
            }
            break;

        case EventType.ItemRemoved:
            Console.WriteLine($"Item with Key '{key}' has been removed from the cache '{args.CacheName}'");
            break;
    }
}
Note

To ensure the operation is fail-safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.

Register Cache Notifications

To register Cache Level Notifications, a target method is created that can have multiple callbacks. The method contains an EventType and EventDataFilter. The EventType is adjusted according to the type of operation the user wants to receive notifications for. The EventDataFilter can have one of the three possible values:

  • None
  • MetaData
  • DataWithMetadata

The events will be notified to their appropriate listeners and handled according to the user's implementation. The following example demonstrates how to create a method that registers callbacks for cache notifications using specific event types.

Important

The EventDataFilter must be carefully set to avoid unnecessary network bandwidth consumption.

Note

If the EventDataFilter is not specified, the EventDataFilter.None is set automatically.

// Precondition: Cache is already connected

public void RegisterCacheNotificationsForAllOperations()
{
    // Create CacheDataNotificationCallback object
    var dataNotificationCallback = new CacheDataNotificationCallback(OnCacheDataModification);

    // Register cache notification with "ItemAdded", "ItemUpdated", and "ItemRemoved" EventType
    // EventDataFilter "None" means only keys will be returned
    CacheEventDescriptor eventDescriptor = cache.MessagingService.RegisterCacheNotification(dataNotificationCallback, EventType.ItemAdded | EventType.ItemUpdated | EventType.ItemRemoved, EventDataFilter.None);

    if (eventDescriptor.IsRegistered)
    {
        Console.WriteLine("Cache level notifications registered successfully");
    }
}

Unregister Cache Notifications

The previously registered Cache Level Event Notifications can be unregistered when they are no longer required by using the UnRegisterCacheNotification method. Using this method, the CacheEventDescriptor also needs to be specified to unregister the notifications. For Java and Node.js, use the remove notification listener API. The following example shows how to unregister notifications using this method.

// Precondition: Cache is already connected

// Unregister Notifications using the EventDescriptor
cache.MessagingService.UnRegisterCacheNotification(eventDescriptor);
Note

Using Cache Level Events might affect the performance of your application since it fires notifications for all specified operations performed on the entire cache data set. Hence, using Item Level Events is the recommended approach to avoid this problem.

Additional Resources

NCache provides a sample application for Cache Level Event Notifications on GitHub.

See Also

.NET: Alachisoft.NCache.Runtime.Events namespace.
Java: com.alachisoft.ncache.events namespace.
Python: ncache.runtime.caching.events class.
Node.js: EventCacheItem class.