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

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);
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);

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
}

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);

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

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;
    }
}

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.

// 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();

Additional Resources

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

See Also

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