Dictionary Behavior and Usage in Cache

A dictionary is a general-purpose data structure for storing a group of objects. It is also called a Hash, Map, and HashMap. It is an unordered data structure, which provides a mapping of string keys against the values. For example, a dictionary can be used to store the information of all the products in a superstore against their productID as the dictionary entry key.

NCache further enhances the dictionary datatype by providing NCache-specific features such as Groups, Tags, Expiration, Locking, Dependencies, and more against it.

Note

In Java, a Map corresponds to a Dictionary in .NET.

Behavior

A dictionary value can be of any primitive type or custom object.
A dictionary of CacheItem and nested dictionaries are not yet supported.
Dictionaries are named. Hence, you need to provide a unique cache key for each dictionary.
A dictionary key can only be of string type.
Duplicate dictionary keys are not allowed.

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, IDistributedDictionary, IDataTypeManager, CreateDictionary, IDictionary, GetDictionary, ICollectionManager, RegisterNotification, DataTypeDataNotificationCallback, EventType, DataTypeEventDataFilter, Lock, Unlock.

Create Dictionary and Add Data

The following code sample shows how a dictionary is created in the cache against the cache key ProductDictionary and then data is added to the dictionary.

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.

.NET
Java

// Precondition: Cache must be connected

// Specify unique cache key for dictionary
string key = "ProductDictionary";

// Create dictionary of Product type
IDistributedDictionary<string, Product> dictionary = cache.DataTypeManager.CreateDictionary<string, Product>(key);

// Adding products to dictionary
Product[] products = FetchProducts();
foreach(var product in products)
{
    // Add products
    string productKey = $"Product:{product.ProductID}";
    dictionary.Add(productKey, product);
}

// Precondition: Cache must be connected

// Specify unique cache key for map
String key = "ProductMap";

// Create Map of Product type
DistributedMap<String, Product> map = cache.getDataStructuresManager().createMap(key, Product.class);

// Adding products to map
Product[] products = fetchProducts();
for (var product : products) {
    // Add products
    String productKey = "Product:" + product.getProductID();
    map.put(productKey, product);
}

Note

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

Fetch Dictionary from Cache

You can fetch a dictionary from the cache which takes a cache key as a parameter. This key is the name of the dictionary, which is specified during the dictionary creation.

Warning

If the item being fetched is not of dictionary type, a Type mismatch exception is thrown.

.NET
Java

// Dictionary with this key already exists in cache
string key = "ProductDictionary";

// Get dictionary and show items of dictionary
IDistributedDictionary<string, Product> retrievedDictionary = cache.DataTypeManager.GetDictionary<string, Product>(key);
if (retrievedDictionary != null)
{
    foreach (var item in retrievedDictionary)
    {
        // Perform operations
    }
}
else
{
    // Dictionary does not exist
}

// Precondition: Cache is already connected

// Map with this key already exists in cache
String key = "ProductMap";

// Get map and show items of map
DistributedMap<String, Product> retrievedMap = cache.getDataStructuresManager().getMap(key, Product.class);
if (retrievedMap != null) {
    for (var item : retrievedMap) {
        // Perform operations
    }
} else {
    // Map does not exist
}

Fetch Values of Specific Dictionary Keys

You can get values of only dictionary entries using Get. The following code sample uses the dictionary instance from the example for adding data to dictionary and gets values against the keys specified.

Note

If no value exists against the key specified, null is returned.

.NET
Java

// Dictionary exists in cache
// Create list of keys to fetch corresponding values
var keys = new List<string>();
keys.Add("Product:1001");
keys.Add("Product:1002");
keys.Add("Product:1003");

// Get values against keys
// "dictionary" instance was created while creating dictionary
ICollection<Product> values = dictionary.Get(keys);
foreach (var value in values)
{
    // Perform operations
}

// Precondition: Cache is already connected

// Map exists in cache
// Create list of keys to fetch corresponding values
var keys = new ArrayList<String>();
keys.add("Product:1001");
keys.add("Product:1002");
keys.add("Product:1003");

// Get values against keys
// "map" instance was created while creating map
Collection<Product> values = map.get(keys);
for (var value : values) {
    // Perform operations
}

Insert Data to Existing Dictionary

You can insert data into an existing dictionary using Insert. The following code sample fetches new products from the data source and inserts them into a dictionary that already exists within the cache.

Note

If the keys already exist, this will overwrite the values in the dictionary.

.NET
Java

// Dictionary with this key already exists in cache
string key = "ProductDictionary";

// Get dictionary to insert more values
IDistributedDictionary<string, Product> retrievedDictionary = cache.DataTypeManager.GetDictionary<string, Product>(key);

// Create dictionary of new products to be added
IDictionary<string, Product> newProducts = new Dictionary<string, Product>();
Product[] products = FetchProducts();
foreach (var product in products)
{
    // Add new products
    string productKey = $"Product:{product.ProductID}";
    newProducts.Add(productKey, product);
}
// Append dictionary entries to existing dictionary
retrievedDictionary.Insert(newProducts);

// Precondition: Cache is already connected

// Map with this key already exists in cache
String key = "ProductMap";

// Get map to insert more values
DistributedMap<String, Product> retrievedMap = cache.getDataStructuresManager().getMap(key, Product.class);

// Create map of new products to be added
DistributedMap<String, Product> newProducts = cache.getDataStructuresManager().getMap(key, Product.class);
Product[] products = fetchProducts();
for (var product : products) {

    // Add new products
    String productKey = "Product:" + product.getProductID();
    newProducts.put(productKey, product);
}

// Append map entries to existing map
retrievedMap.insert(newProducts);

Remove Items from Dictionary

Note

If the key specified to be removed does not exist, nothing is returned. You can verify the number of keys returned using the return type of Remove.

Items can be removed from a dictionary against a given collection of keys. The following code sample removes the dictionary entities for the expired products using Remove.

Tip

To remove the whole dictionary from the cache, refer to the Remove Data Structures from Cache page.

.NET
Java

// Dictionary with this key already exists in cache
string key = "ProductDictionary";

// Get dictionary and show items of dictionary
IDistributedDictionary<string, Product> retrievedDictionary = cache.DataTypeManager.GetDictionary<string,  Product>(key);

// Create list of keys to remove
List<string> keysToRemove = FetchExpiredProducts();

// Number of keys removed is returned
int itemsRemoved = retrievedDictionary.Remove(keysToRemove);

// Precondition: Cache is already connected

// Map with this key already exists in cache
String key = "ProductMap";

// Get map and show items of map
DistributedMap<String, Product> retrievedMap = cache.getDataStructuresManager().getMap(key, Product.class);

// Create list of keys to remove
List<String> keysToRemove = fetchExpiredProducts();

// Number of keys removed is returned
int itemsRemoved = retrievedMap.remove(keysToRemove);

Event Notifications on Dictionary

You can register cache events, key-based events, and data structure events on a data structure such as a dictionary. 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 dictionary in the cache. Once a dictionary is created in the cache, an ItemAdded cache-level event is fired. However, once an item is added to the dictionary, an ItemAdded data structure event is fired, and an ItemUpdated cache level event is fired.

Register Event on Dictionary Created

.NET
Java

// Unique cache key for dictionary
string key = "ProductDictionary";

// Create dictionary of Product type
IDistributedDictionary<string, Product> dictionary = cache.DataTypeManager.CreateDictionary<string, Product>(key);

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

// Perform operations

// Precondition: Cache is already connected

// Unique cache key for map
String key = "ProductMap";

// Create map of type Product
DistributedMap<String, Product> map = cache.getDataStructuresManager().createMap(key, Product.class);

// 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);
map.addChangeListener(dataChangeListener, enumSet, DataTypeEventDataFilter.Data);

// Perform operations

Specify Callback for Event Notification

.NET
Java

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;
        }
    }
};

Locking Dictionary

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

.NET
Java

// Dictionary exists with key "ProductDictionary"
// Cache Key
string key = "ProductDictionary";

// Get dictionary
IDistributedDictionary<string, Product> dictionary = cache.DataTypeManager.GetDictionary<string, Product>(key);

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

dictionary.Unlock();

// Preconditions: Cache is already connected

// Map exists with key "ProductMap"
// Cache Key
String key = "ProductMap";

// Get dictionary
DistributedMap<String, Product> map = cache.getDataStructuresManager().getMap(key, Product.class);

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

Additional Resources

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

Dictionary Behavior and Usage in Cache

Note

Behavior

Prerequisites

Create Dictionary and Add Data

Tip

Note

Fetch Dictionary from Cache

Warning

Fetch Values of Specific Dictionary Keys

Note

Insert Data to Existing Dictionary

Note

Remove Items from Dictionary

Note

Tip

Event Notifications on Dictionary

Register Event on Dictionary Created

Specify Callback for Event Notification

Locking Dictionary

Additional Resources

See Also

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