List Behavior and Usage in Cache

A list is an unordered data structure where data can be added or removed from the list. For example, lists maintain the products added to a cart for an e-commerce website. Let's suppose a user adds the products Umbrella, Green Apples, and Coffee to the cart. Before making the transaction, the product Green Apples is removed, and a new product Pears is added. This is possible because you can update the list from any point within itself.

NCache further enhances the list data structure by providing NCache-specific features such as Groups, Tags, Expiration, Locking, Dependencies, and more. In this scenario, the company wants the cart list to be maintained only as long as the session is active. Hence, expiration can be associated with each list created that is equal to the session timeout value.

Behavior

A list can be of any primitive type or custom object.
A list of CacheItem and nested lists are not yet supported.
Lists can be directly accessed by index.
Lists are named. Hence, you need to provide a unique cache key for a list.
Null is not a supported value type.
Duplicate values are supported.

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, IDistributedList, IDataTypeManager, CreateList, AddRange, InsertAtHead, GetList, RemoveRange, IList, RegisterNotification, DataTypeDataNotificationCallback, EventType, DataTypeEventDataFilter, Lock, Unlock.

Create a List and Add the Data

The following code sample shows how a list of Product types can be created in the cache using CreateList against the cache key ProductList. Products are added to the list using Add, and then a new range of products is added to the list using AddRange.

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 list
string key = "ProductList";

// Create list of Product type
IDistributedList<Product> list = cache.DataTypeManager.CreateList<Product>(key);

// Get products to add to list
Product[] products = FetchProducts();

foreach (var product in products)
{
    // Add products to list
    list.Add(product);
}
// Get new products
Product[] newProducts = FetchNewProducts();

// Append list of new Products to existing list
list.AddRange(newProducts);

// Precondition: Cache must be connected

// Specify unique cache key for list
String key = "ProductList";

// Create a list of Product type
DistributedList<Product> list = cache.getDataStructuresManager().createList(key, Product.class);

// Get products to add to list
Product[] products = fetchProducts();

for (var product : products) {
    // Add products to list
    list.add(product);
}

// Get new products
Product[] newProducts = fetchNewProducts();

// Append list of new Products to existing list
list.addRange(Arrays.asList(newProducts));

# Precondition: Cache must be connected
# Specify unique cache key for list
key = "ProductList"

# Create list
manager = cache.get_data_structures_manager()

product_list = manager.create_list(key, Product)

# Get products to add to list
products = fetch_products()

for product in products:
    # Add products to list
    product_list.add(product)

# Get new products
new_products = fetch_new_products()

# Append list of new Products to existing list
product_list.add_range(new_products)

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

// Specify unique cache key for list
var key = "ProductList";

// Create list
var manager = await this.cache.getDataStructuresManager();
var list = await manager.createList(key, ncache.JsonDataType.Object);

// Get products to add to list
var products = this.fetchProducts();

for (var product in products) {
// Add products to list
list.add(product);
}
// Get new products
var newProducts = this.fetchNewProducts();

// Append list of new Products to existing list
list.addRange(newProducts);

Note

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

Update Items in List

You can update lists and items in them using indexes as the lists can be accessed via index. The following code sample updates a value in an existing list (created in the previous example) using the index. It then gets a sale item and adds it to the first index of the list using InsertAtHead.

// "list" is created in previous example
// Update value of index with updated product
Product updatedProduct = GetUpdatedProductByID(11);
list[11] = updatedProduct;

// Get product on sale to insert at head of List
Product saleProduct = FetchSaleItem();
list.InsertAtHead(saleProduct);

// Precondition: Cache is already connected

// "list" is created in previous example
// Update value of index with updated product
Product updatedProduct = getUpdatedProductByID(11);
list.add(11, updatedProduct);

// Get product on sale to insert at head of List
Product saleProduct = fetchSaleItem();
list.insertAtHead(saleProduct);

# Precondition: Cache is already connected
# "product_list" is created in previous example
# Update value of index with updated product
updated_product = get_updated_product_by_id(1011)
product_list .add(updated_product, 1011)

# Get product on sale to insert at head of List
sale_product = fetch_sale_item()
product_list .insert_at_head(sale_product)

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

// "list" is created in previous example
// Update value of index with updated product
var updatedProduct = this.getUpdatedProductByID(11);
list.add(11, updatedProduct);

// Get product on sale to insert at head of List
var saleProduct = this.fetchSaleItem();
list.insertAtHead(fetchSaleItem);

Fetch List from Cache

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

Warning

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

// List with this key already exists in cache
string key = "ProductList";

// Get list and show items of list
IDistributedList<Product> retrievedList = cache.DataTypeManager.GetList<Product>(key);

if (retrievedList != null)
{
    foreach (var item in retrievedList)
    {
        // Perform operations
    }
}
else
{
    // List does not exist
}

// Precondition: Cache is already connected

// List with this key already exists in cache
String key = "ProductList";

// Get list and show items of list
DistributedList<Product> retrievedList = cache.getDataStructuresManager().getList(key, Product.class);

if (retrievedList != null) {
    for (var item : retrievedList) {
        // Perform operations
    }
} else {
    // List does not exist
}

# Precondition: Cache is already connected
# List with this key already exists in cache
key = "ProductList"

# Get list and show items of list
manager = cache.get_data_structures_manager()
retrieved_list = manager.get_list(key, Product)

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

// This is an async method
// Precondition: Cache is already connected
// List with this key already exists in cache
var key = "ProductList";

// Get list and show items of list
var manager = await this.cache.getDataStructuresManager();
var retrievedList = await manager.getList(key, ncache.JsonDataType.Object);

if (retrievedList != null) {
for (var item in retrievedList) {
    // Perform operations
}
} else {
// List does not exist
}

Remove Items from List

Tip

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

Individual items or a given range of items can be removed from a list. The following code sample removes an individual item using Remove and a range of items for the expired products using RemoveRange.

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 RemoveRange.

// List with this key already exists in cache
string key = "ProductList";

// Get list to remove items
IDistributedList<Product> retrievedList = cache.DataTypeManager.GetList<Product>(key);

// Get range of out of stock products to be removed
List<Product> outOfStockProducts = FetchOutOfStockProducts();

// Remove each item individually from retrievedList
foreach(Product prod in outOfStockProducts)
{
    retrievedList.Remove(prod);
}

// Get range of discontinued products to be removed
List<Product> discontinuedProducts = FetchDiscontinuedProducts();

// Remove this range from retrievedList
// Number of keys removed is returned
int itemsRemoved = retrievedList.RemoveRange(discontinuedProducts);

// Precondition: Cache is already connected

// List with this key already exists in cache
String key = "ProductList";

// Get list to remove items
DistributedList<Product> retrievedList = cache.getDataStructuresManager().getList(key, Product.class);

// Get range of out of stock products to be removed
List<Product> outOfStockProducts = fetchOutOfStockProducts();

// Remove an individual item from retrieved list
for(Product prod : outOfStockProducts)
{
    retrievedList.remove(prod);
}

// Get range of discontinued products to be removed
List<Product> discontinuedProducts = fetchDiscontinuedProducts();

// Remove this range from retrievedList
// Number of keys removed is returned
int itemsRemoved = retrievedList.removeRange(discontinuedProducts);

# Precondition: Cache is already connected
# List with this key already exists in cache
key = "ProductList"

# Get list to remove items
manager = cache.get_data_structures_manager()
retrieved_list = manager.get_list(key, Product)

# Get range of expired products to be removed
items_to_remove = fetch_expired_products()

# Remove this range from retrievedList
# Number of keys removed is returned
items_removed = retrieved_list.remove_range(collection=items_to_remove)

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

// List with this key already exists in cache
var key = "ProductList";

// Get list to remove items
var manager = await this.cache.getDataStructuresManager();
var retrievedList = await manager.getList(key, ncache.JsonDataType.Object);

// Get range of out of stock products to be removed
var outOfStockProducts = this.fetchOutOfStockProducts();

// Remove an individual item from retrieved list
for(var prod in outOfStockProducts){
    await retrievedList.remove(prod);
}

// Get range of discontinued json datatype products to be removed
var discontinuedProducts = this.fetchDiscontinuedProducts();

// Remove this range from retrievedList
// Number of keys removed is returned
var itemsRemoved = await retrievedList.removeRange(discontinuedProducts);

Event Notifications on Lists

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

The following code sample registers a cache event of ItemAdded and ItemUpdated. It also registers an event for ItemAdded and ItemUpdated on the list in the cache. Once you create a list in the cache, an ItemAdded cache-level event is fired. However, once you add an item to the list, an ItemAdded data structure event, along with an ItemUpdated cache level event is fired. A DataTypeEventDataFilter is specified to quantify the amount of information returned upon an event execution. Events, thus registered, then provide the user with the information based on these data filters.

Register Event on List Created

// Pre-condition: Cache is connected

// Unique cache key for list
string key = "ProductList";

// Create list of Product type
IDistributedList<Product> list = cache.DataTypeManager.CreateList<Product>(key);

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

// Perform operations

// Precondition: Cache is already connected

// Unique cache key for list
String key = "ProductList";

// Create list of product type
DistributedList<Product> list = cache.getDataStructuresManager().createList(key, Product.class);

// Register ItemAdded, ItemUpdated, ItemRemoved events on list created
// dataChangeListener is the specified callback method
DataStructureDataChangeListener dataChangeListener = dataStructureListener.onDataStructureChanged(collectionName, args);
EnumSet<EventType> enumSet = EnumSet.of(com.alachisoft.ncache.runtime.events.EventType.ItemAdded,
        EventType.ItemUpdated, EventType.ItemRemoved);
list.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 list
    key = "ProductList"

    # Create list
    product_list = cache.get_data_structures_manager().create_list(key, Product)

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

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

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

// Unique cache key for list
var key = "ProductList";

// Create list
var list = await this.cache
.getDataStructuresManager()
.createList(key, ncache.JsonDataType.Object);

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

var enumSet = enumSet.of(
ncache.EventType.ItemAdded,
ncache.EventType.ItemUpdated,
ncache.EventType.ItemRemoved
);

list.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 Lists

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

.NET
Java

// List exists with key "ProductList"
string key = "ProductList";

// Get list
IDistributedList<Product> list = cache.DataTypeManager.GetList<Product>(key);

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

list.Unlock();

// Preconditions: Cache is already connected
// List exists with key "ProductList"
String key = "ProductList";

// Get list
DistributedList<Product> list = cache.getDataStructuresManager().getList(key, Product.class);

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

Additional Resources

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

List Behavior and Usage in Cache

Behavior

Prerequisites

Create a List and Add the Data

Tip

Note

Update Items in List

Fetch List from Cache

Warning

Remove Items from List

Tip

Note

Event Notifications on Lists

Register Event on List Created

Specify Callback for Event Notification

Locking Lists

Additional Resources

See Also

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