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Show / Hide Table of Contents
  • Programmer's Guide
  • Setting Up Development Environment
    • .NET
      • Client API Prerequisites
      • Server-side API Prerequisites
    • Java
      • Client API Prerequisites
      • Server-side API Prerequisites
    • Python
      • Client API Prerequisites
    • Node.js
      • Client API Prerequisites
  • Client Side API Programming
    • Error Handling
    • Troubleshooting
    • Cache Keys and Data
    • How to Connect to Cache
    • Basic Operations - An Overview
      • Add Data
      • Update/Insert Data
      • Retrieve Data
      • Remove Data
    • Groups
      • Overview
      • Add/Update Data with Groups
      • Retrieve Data with Groups
      • Remove Data with Group
      • Search Group Data Using SQL
      • Delete Group Data Using SQL
    • Tags
      • Overview
      • Add/Update Data with Tags
      • Retrieve Data with Tags
      • Remove Data with Tags
      • Search Tag Data Using SQL
      • Delete Tag Data Using SQL
    • Named Tags
      • Overview
      • Add/Update Data with Named Tags
      • Remove Data with Named Tags
      • Search Data with Named Tags Using SQL
      • Delete Data with Named Tags Using SQL
    • Expirations
      • Overview
      • Absolute Expiration
      • Sliding Expiration
    • Data Dependency
      • Key Dependency
      • Multi-Cache Dependency
    • Dependency on Database
      • SQL Server
      • Oracle
      • OleDB with Polling
      • CLR Procedures in SQL Server
    • Dependency on External Source
      • File Dependency
      • Custom Dependency
      • Aggregate Dependency
    • Locks
      • Types of Locking
      • Pessimistic Locking
      • Optimistic Locking
    • SQL Query
      • Overview
      • Define Indexes Programmatically
      • Query with ExecuteReader and ExecuteScalar
      • Delete Data with ExecuteNonQuery
      • SQL Reference
    • LINQ Query
      • Overview
      • LINQ Query for Objects
      • LINQ Reference
    • Data Structures
      • Overview
      • List
      • Queue
      • Set
      • Dictionary
      • Counter
      • Invalidation Attributes
      • Searchable Attributes
      • Query on Data Structures
      • Remove from Data Structure
    • Events
      • Cache Level Events
      • Item Level Events
      • Management Level Events
    • Pub/Sub Messaging
      • Overview
      • Topics
      • Publish Messages
      • Subscribe to a Topic
      • Pub/Sub Events
    • Continuous Query
      • Overview
      • Use Continuous Query
    • Stream Processing
      • Add/Update Stream Data
      • Retrieve Stream Data
    • JSON
      • Overview
      • Use JSON Objects
      • Query JSON Data
    • Security API
      • Login with Credentials
    • Management API
    • Clear Cache
    • Error Logging
    • Location Affinity
  • Server-side API Programming
    • Loader and Refresher
      • Overview
      • Implement Loader and Refresher
      • Components of Loader/Refresher
    • Data Source Providers
      • Read-through
        • Implement Read-through
        • Use Read-through
      • Write-through
        • Implement Write-through
        • Use Write-through
        • Use Write-behind
    • Custom Dependency
      • Implement Extensible Dependency
      • Implement Bulk Extensible Dependency
      • Implement Notify Extensible Dependency
    • Bridge Conflict Resolver
    • Entry Processor
      • Overview
      • Implement Entry Processor
    • MapReduce
      • Overview
      • Implement MapReduce
      • Use MapReduce
    • MapReduce Aggregator
      • Overview
      • Implement and Use Aggregator
    • Compact Serialization
  • Client Side Features
    • ASP.NET Core Caching
      • Session Storage
        • Session Provider
        • IDistributedCache
        • Sessions Usage
        • Multi-site Session Provider
        • Session Sharing with ASP.NET
      • SignalR
        • NCache Extension for SignalR Core
      • Response Caching
        • Configure and Use
        • Configure with IDistributedCache
      • Data Caching
        • NCache API
        • IDistributedCache API
      • Data Protection Provider
        • Configure
    • Java Web App Caching
      • Web Sessions
        • Overview
        • Configure App
          • Add Maven Dependencies
          • Deploy Application
        • Multi-site Sessions
    • Node.js App Caching
      • Web Sessions
    • ASP.NET Caching Benefits and Overview
      • ASP.NET Session State Provider Properties
      • Multi-region ASP.NET Session State Provider Configuration
      • Session Sharing between ASP.NET and ASP.NET Core
      • ASP.NET SignalR Backplane
        • NCache Extension for SignalR
      • ASP.NET View State Caching
        • View State Content Optimization Configuration
        • Group View State with Sessions
        • Limit View State Caching
        • Page Level Grouping
      • ASP.NET Output Cache
        • Output Caching Provider Overview
        • Output Cache with Custom Hooks
  • .NET Third Party Integrations
    • Entity Framework (EF) Core
      • Installation
      • Configure
      • EF Core Extension Methods
        • Extension Methods
        • Cache Handle
        • Caching Options
        • Query Deferred API
      • Logging in EF Core
    • Entity Framework EF 6
      • EF Second Level Cache
      • EF Caching Resync Provider
      • EF Caching Configuration File
    • NHibernate
      • Second Level Cache
      • Query Caching
      • Synchronize Database with Cache
    • Debug NCache Providers in Visual Studio
  • Java Third Party Integrations
    • Hibernate
      • Second Level Cache
      • Configure Cacheable Objects and Regions
      • Configure Application
      • Query Caching
    • Spring
      • Overview
      • Use NCache with Spring
        • Configure Generic Provider
        • Configure JCache Spring Caching Provider
        • Configure Caching Declaration
        • Configure Spring Sessions
    • JCache API
      • CRUD Operations
      • Expiration
      • Events
  • Third-Party Wrappers
    • AppFabric to NCache Migration
      • AppFabric API vs. NCache API
      • Configuration Differences Between AppFabric and NCache
      • Migrating from an AppFabric Application to NCache
    • Redis to NCache Migration
      • Redis to NCache Migration
    • Memcached Wrapper
      • Memcached Gateway Approach
      • Memcached Client Plugin for .NET

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.

Prerequisites

  • .NET
  • Java
  • Python
  • 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.
  • 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: Cache, DistributedMap, getDataStructuresManager, createMap, getMap, EventType, getEventType, DataStructureDataChangeListener, onDataStructureChanged, DataStructureEventArg, DataTypeEventDataFilter, getCollectionItem, lock, unlock.
  • 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: Cache, DistributedDictionary, DataStructureManager, get_data_structures_manager, create_dictionary, get_dictionary, get_iterator, get_event_type, DataTypeEventDataFilter, EventDataFilter.

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
  • Python
// 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);
}
# Precondition: Cache must be connected
# Specify unique cache key for dictionary
key = "ProductDictionary"

# Create dictionary of Product type
dictionary = cache.get_data_structures_manager().create_dictionary(key, Product)

# Adding products to dictionary
products = fetch_products()

for product in products:
    # Add products
    product_key = "Product:" + product.get_product_id()

    dictionary.put(product_key, 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
  • Python
// 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
}
# Precondition: Cache is already connected
# Dictionary with this key already exists in cache
key = "ProductDictionary"

# Get dictionary and show items of dictionary
retrieved_dictionary = cache.get_data_structures_manager().get_dictionary(key, Product)

if retrieved_dictionary is not None:
    for item in retrieved_dictionary.get_iterator():
        # Perform operations
        print(item)
else:
    # Dictionary does not exist
    print("Dictionary not found")

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
  • Python
// 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
}
# Precondition: Cache is already connected
# Dictionary exists in cache
# Create list of keys to fetch corresponding values
keys = [
    "Product:1001",
    "Product:1002",
    "Product:1003"
]

# Get values against keys
# "dictionary" instance was created while creating dictionary
values = dictionary.get(keys)

for value in values:
    # Perform operations
    print(value)

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
  • Python
// 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);
# Pre-condition: Cache is already connected
# Dictionary with this key already exists in cache
key = "ProductDictionary"

# Get dictionary to insert more values
retrieved_dictionary = cache.get_data_structures_manager().get_dictionary(key, Product)

# Create dictionary of new products to be added
new_products = {}

products = fetch_products()

for product in products:
    # Add new products
    product_key = "Product:" + product.get_product_id()

    new_products[product_key] = product

# Append dictionary entries to existing dictionary
retrieved_dictionary.insert(new_products)

# You can verify using len(retrieved_dictionary)

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
  • Python
// 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);
# Precondition: Cache is already connected
# Dictionary with this key already exists in cache
key = "ProductDictionary"

# Get dictionary and show items of dictionary
retrieved_dictionary = cache.get_data_structures_manager().get_dictionary(key, Product)

# Create list of keys to remove
keys_to_remove = fetch_expired_products_ids()

# Number of keys removed is returned
items_removed = retrieved_dictionary.remove(keys_to_remove)

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
  • Python
// 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
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 dictionary
    key = "ProductDictionary"

    # Create dictionary
    product_dictionary = cache.get_data_structures_manager().create_hashset(key, int)

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

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

    # Perform operations

Specify Callback for Event Notification

  • .NET
  • Java
  • Python
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)

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.

See Also

.NET: Alachisoft.NCache.Client.DataTypes namespace.
Java: com.alachisoft.ncache.client.datastructures namespace.
Python: ncache.client.datastructures class.

In This Article
  • Behavior
  • Prerequisites
  • Create Dictionary and Add Data
  • Fetch Dictionary from Cache
  • Fetch Values of Specific Dictionary Keys
  • Insert Data to Existing Dictionary
  • Remove Items from Dictionary
  • Event Notifications on Dictionary
    • Register Event on Dictionary Created
    • Specify Callback for Event Notification
  • Locking Dictionary
  • Additional Resources
  • See Also

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