Counter Behavior and Usage in Cache

A counter is a single long-value data structure stored in the cache. You can increment, decrement, lock, and have events registered against it. For example, a counter can be used to store the views every time a video is viewed on a social media site. It can also be used to store subscriptions against a website where people can subscribe or unsubscribe.

NCache further enhances this counter by providing NCache-specific features such as Groups, Tags, Expiration, Locking, Dependencies, and more. For example, you can specify a counter against the group SubscriptionDetails which can contain customer objects as well as the subscription counter.

Behavior

Null is not a supported value type.
Counters are named. Hence, you need to provide a unique cache key for a counter.

Prerequisites

To learn about the standard prerequisites required to work with all NCache client-side features, please refer to the given page on Client-Side API Prerequisites.
For API details, refer to: ICache, ICounter, IDataTypeManager, CreateCounter, SetValue, Increment, IncrementBy, Decrement, DecrementBy, GetCounter, ICollectionManager, RegisterNotification, DataTypeDataNotificationCallback, EventType, DataTypeEventDataFilter, Lock, Unlock.

Create Counter

The following code sample shows how a counter can be created in the cache using CreateCounter against the cache key SubscriptionCounter.

Tip

You can also configure searchable attributes such as Groups/Tags/Named Tags and invalidation attributes such as Expiration/Eviction/Dependency while creating a data structure.

// Precondition: Cache must be connected

// Specify unique cache key for counter
string key = "SubscriptionCounter";

// Set initial value of counter
long initialValue = 15;

// Create counter
ICounter counter = cache.DataTypeManager.CreateCounter(key, initialValue);

// Precondition: Cache must be connected

// Specify unique cache key for counter
String key = "SubscriptionCounter";

// Set initial value of counter
long initialValue = 15;

// Create counter
Counter counter = cache.getDataStructuresManager().createCounter(key, initialValue);

# Precondition: Cache must be connected
# Specify unique cache key for counter
key = "SubscriptionCounter"

# Set initial value of counter
initial_value = 15

# Create counter
manager = cache.get_data_structures_manager()
counter = manager.create_counter(key, initial_value)

// This is an async method
// Precondition: Cache must be connected

// Specify unique cache key for counter
var key = "SubscriptionCounter";

// Set initial value of counter
var initialValue = 15;

// Create counter
var manager = await this.cache.getDataStructuresManager();
var counter = await manager.createCounter(key, initialValue);

Fetch and Update the Value of the Counter

You can update the value of a counter after its creation. The following code sample fetches a counter from the cache using GetCounter and sets its value to another value, using SetValue. It then increments its value using Increment or IncrementBy and decrements its value using Decrement or DecrementBy.

// Unique key for counter
string key = "SubscriptionCounter";

// Get counter against key
ICounter retrievedCounter = cache.DataTypeManager.GetCounter(key);

// Set value of counter to 100
retrievedCounter.SetValue(100);

// Increment value
long newValue =  retrievedCounter.Increment();

// Decrement value
newValue =  retrievedCounter.Decrement();

// Increment value by number of 10
newValue =  retrievedCounter.IncrementBy(10);

// Decrement value by number of 5
newValue =  retrievedCounter.DecrementBy(5);

// Precondition: Cache is already connected

// Unique key for counter
String key = "SubscriptionCounter";

// Get counter against key
Counter retrievedCounter = cache.getDataStructuresManager().getCounter(key);

// Set Value of counter to 100
retrievedCounter.setValue(100);

// Increment value
long newValue =  retrievedCounter.increment();

// Decrement value
newValue =  retrievedCounter.decrement();

// Increment value by number of 10
newValue =  retrievedCounter.incrementBy(10);

// Decrement value by number of 5
newValue =  retrievedCounter.decrementBy(5);

# Precondition: Cache is already connected
# Unique key for counter
key = "SubscriptionCounter"

# Get counter against key
manager = cache.get_data_structures_manager()
retrieved_counter = manager.get_counter(key)

# Increment value of retrievedCounter on new subscription
retrieved_counter.increment()

// This is an async method
// Precondition: Cache is already connected
// Unique key for counter
var key = "SubscriptionCounter";

// Get counter against key
var manager = await this.cache.getDataStructuresManager();
var retrievedCounter = await manager.getCounter(key);

// Set Value of counter to 100
retrievedCounter.setValue(100);

// Increment value
var newValue =  retrievedCounter.increment();

// Decrement value
newValue =  retrievedCounter.decrement();

// Increment value by number of 10
newValue =  retrievedCounter.incrementBy(10);

// Decrement value by number of 5
newValue =  retrievedCounter.decrementBy(5);

Note

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

Event Notifications on Counters

You can register cache events, key-based events, and data structure events on a data structure such as a counter. For behavior, refer to feature wise behavior.

The following code sample registers a cache event of ItemAdded and ItemUpdated as well as registers an event for ItemAdded and ItemUpdated on the counter in the cache. Once a counter is created in the cache, an ItemAdded cache-level event is fired. However, once an item is added to the counter, an ItemAdded data structure event is fired, and an ItemUpdated cache level event is fired.

Register Event on Counter Created

// Unique cache key for counter
string key = "SubscriptionCounter";

// Set initial value of counter
long initialValue = 15;

// Create counter
ICounter counter = cache.DataTypeManager.CreateCounter(key, initialValue);

// Register ItemAdded, ItemUpdated, ItemRemoved events on counter created
// DataTypeNotificationCallback is callback method specified
counter.RegisterNotification(DataTypeDataNotificationCallback, EventType.ItemAdded |
        EventType.ItemUpdated | EventType.ItemRemoved,
        DataTypeEventDataFilter.Data);

// Perform operations

// Precondition: Cache is already connected

// Unique cache key for counter
String key = "SubscriptionCounter";

// Set initial value of counter
long initialValue = 15;

// Create counter
Counter counter = cache.getDataStructuresManager().createCounter(key, initialValue);

// Create EnumSet of event types
EnumSet<EventType> enumSet = EnumSet.of(com.alachisoft.ncache.runtime.events.EventType.ItemAdded,
        EventType.ItemUpdated, EventType.ItemRemoved);

// Register ItemAdded, ItemUpdated, ItemRemoved events on queue created
// dataChangeListener is the specified callback method
DataStructureDataChangeListener dataChangeListener = dataStructureListener.onDataStructureChanged(collectionName, args);
counter.addChangeListener(dataChangeListener, enumSet, DataTypeEventDataFilter.Data);

// Perform operations

def datastructure_callback_function(collection_name, collection_event_args):
    # Perform Operations
    print("Event Fired for " + str(collection_name))

    # Precondition: Cache is already connected
    # Unique cache key for counter
    key = "SubscriptionCounter"

    # Set initial value of counter
    initial_value = 15

    # Create counter
    counter = cache.get_data_structures_manager().create_counter(key, initial_value)

    # Register ItemAdded, ItemUpdated, ItemRemoved events on hashset created
    events_list = [ncache.EventType.ITEM_ADDED, ncache.EventType.ITEM_UPDATED, ncache.EventType.ITEM_REMOVED]

    counter.add_change_listener(datastructure_callback_function, events_list, ncache.DataTypeEventDataFilter.DATA)

    # Perform operations

// This is an async method
// Precondition: Cache is already connected

// Unique cache key for counter
var key = "SubscriptionCounter";

// Set initial value of counter
var initialValue = 15;

// Create counter
var counter = await this.cache.getDataStructuresManager().createCounter(key, initialValue);

// Create EnumSet of event types
var enumSet = enumSet.of(
ncache.EventType.ItemAdded,
ncache.EventType.ItemUpdated,
ncache.EventType.ItemRemoved
);

// Register ItemAdded, ItemUpdated, ItemRemoved events on queue created
// dataChangeListener is the specified callback method
var dataChangeListener = dataStructureListener.onDataStructureChanged(
collectionName,
args
);

counter.addChangeListener(
dataChangeListener,
enumSet,
ncache.DataTypeEventDataFilter.Data
);

// Perform operations

Specify Callback for Event Notification

private void DataTypeDataNotificationCallback(string collectionName, DataTypeEventArg collectionEventArgs)
{
    switch (collectionEventArgs.EventType)
    {
        case EventType.ItemAdded:
            // Item has been added to the collection
        break;
        case EventType.ItemUpdated:
            if (collectionEventArgs.CollectionItem != null)
            {
                // Item has been updated in the collection
                // Perform operations
            }
        break;
        case EventType.ItemRemoved:
            // Item has been removed from the collection
        break;
    }
}

DataStructureDataChangeListener dataStructureListener = new DataStructureDataChangeListener() {
    @Override
    public void onDataStructureChanged(String collection, DataStructureEventArg dataStructureEventArg) {
        switch (dataStructureEventArg.getEventType()) {
            case ItemAdded:
                // Item has been added to the collection
            break;
            case ItemUpdated:
                if (dataStructureEventArg.getCollectionItem() != null) {
                    //Item has been updated in the collection
                    // perform operations
                }
            break;
            case ItemRemoved:
                //Item has been removed from the collection
            break;
        }
    }
};

def datastructure_callback_function(collection_name: str, collection_event_args: DataStructureEventArg):
    if collection_event_args.get_event_type() is ncache.EventType.ITEM_ADDED:
        # Item has been added to the collection
        print("Item added in " + collection_name)

    elif collection_event_args.get_event_type() is ncache.EventType.ITEM_UPDATED:
        # Item has been updated in the collection
        print("Item updated in " + collection_name)

    elif collection_event_args.get_event_type() is ncache.EventType.ITEM_REMOVED:
        # Item has been removed from the collection
        print("Item removed from " + collection_name)

dataStructureListener = new ncache.DataStructureDataChangeListener();
{
  function onDataStructureChanged(collection, dataStructureEventArg) {
    switch (dataStructureEventArg.getEventType()) {
       case ncache.EventType.ItemAdded:
        //Item has been added to the collection
       break;
       case ncache.EventType.ItemUpdated:
        if (dataStructureEventArg.getCollectionItem() != null) {
          //Item has been updated in the collection
          // perform operations
        }
       break;
       case ncache.EventType.ItemRemoved:
        //Item has been removed from the collection
       break;
    }
  }
}

Locking Counter

Counters can be explicitly locked and unlocked to ensure data consistency. The following code sample creates a counter and locks it for a period of 10 seconds using Lock and then unlocks it using Unlock.

.NET
Java

// Counter exists with key "SubscriptionCounter"
// Cache Key
string key = "SubscriptionCounter";

// Get counter against key
ICounter counter = cache.DataTypeManager.GetCounter(key);

// Lock counter for 10 seconds
bool isLocked = counter.Lock(TimeSpan.FromSeconds(10));
if (isLocked)
{
    // Counter is successfully locked for 10 seconds
    // Unless explicitly unlocked
}
else
{
    // Counter is not locked because either:
    // Counter is not present in the cache
    // Counter is already locked
}

counter.Unlock();

// Preconditions: Cache is already connected

// Counter exists with key "SubscriptionCounter"
// Cache Key
String key = "SubscriptionCounter";

// Get counter against key
Counter counter = cache.getDataStructuresManager().getCounter(key);

// Lock counter for 10 seconds
boolean isLocked = counter.lock(TimeSpan.FromSeconds(10));
if (isLocked) {
    // Counter is successfully locked for 10 seconds
    // Unless explicitly unlocked
} else {
    // Counter is not locked because either:
    // Counter is not present in the cache
    // Counter is already locked
}
counter.unlock();

Additional Resources

NCache provides a sample application for the counter data structure on GitHub.