List Behavior and Usage in Cache

Note

This feature is only available in NCache Enterprise Edition.

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 List and Add Data

The following code sample shows how a list of Product type 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.

try
{
    // Pre-condition: 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);
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    if(ex.ErrorCode == NCacheErrorCodes.KEY_ALREADY_EXISTS)
    {
        // The specified key already exists in cache,
        // Either remove the existing object from cache
        // Or specify another key
    }
    else if (ex.ErrorCode == NCacheErrorCodes.CACHEITEM_IN_DATA_STRUCTURES)
    {
        // Data structures cannot be of CacheItem type
        // CacheItems cannot be added in data structures
    }
    else
    {
        // Exception can occur due to:
        // Connection Failures
        // Operation Timeout
        // Operation performed during state transfer
    }
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}
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.

try
{
    // Pre-condition: Cache is already connected

    // "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);
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    // Exception can occur due to:
    // Connection Failures
    // Operation Timeout
    // Operation performed during state transfer
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}

Fetch List from Cache

You can fetch a list from the cache using GetList that 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. 

try
{
    // Pre-condition: Cache is already connected

    // 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
    }
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    if (ex.ErrorCode == NCacheErrorCodes.NOT_A_LIST)
    {
        // Item being fetched is not of List type;
        // Cache key corresponds to an item of different data structure
    }
    else
    {
        // NCache specific exception
        // Exception can occur due to:
        // Connection Failures
        // Operation Timeout
        // Operation performed during state transfer
    }
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}

Remove Items from List

Tip

To remove the whole list from 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.

try
{
    // Pre-condition: Cache is already connected

    // 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);
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    if (ex.ErrorCode == NCacheErrorCodes.NOT_A_LIST)
    {
        // Item being fetched is not of List type;
        // Cache key corresponds to an item of different data structure
    }
    else
    {
        // NCache specific exception
        // Exception can occur due to:
        // Connection Failures
        // Operation Timeout
        // Operation performed during state transfer
    }
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}

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. 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

try
{
    // 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
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    // Exception can occur due to:
    // Connection Failures
    // Operation Timeout
    // Operation performed during state transfer
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}

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().

try
{
    // Pre-conditions: Cache is already connected
    // 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();
}
catch (OperationFailedException ex)
{
    // NCache specific exception
    // Exception can occur due to:
    // Connection Failures
    // Operation Timeout
    // Operation performed during state transfer
}
catch (Exception ex)
{
    // Any other generic exception like ArgumentNullException or ArgumentException
}

Additional Resources

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

See Also

Queue Behavior and Usage in Cache
Sets Behavior and Usage in Cache
Dictionary Behavior and Usage in Cache
Counter Behavior and Usage in Cache