Extensible Dependency Usage and Implementation

Note

This feature is only available in NCache Enterprise Edition.

Kindly refer to Custom Cache Dependencies to get acquainted with all cache dependency custom methods provided by NCache.

In custom dependency, the users can have their own dependencies scenarios. In this way, items can be expired from cache in a number of flexible ways where expiration logic meets the user’s business requirements. NCache provides an abstract class ExtensibleDependency that is base for all dependencies.

Step 1: Implement ExtensibleDependency Class

Pre-Requisites

The class must be implemented as a Class Library (.dll) in Visual Studio. This will be deployed on NCache cluster.
To override the ExtensibleDependency class, include the following assembly in your application:
- Alachisoft.NCache.Runtime.Dependencies
The class implementing ExtensibleDependency interface must be marked as Serializable, along with any parameters the process code might be taking in.

Let's suppose there is a grocery store which has a big collection of those products which customers often buy. These products can be cached from the database to check their units in stock. The dependency polls the Northwind database and expires all the Products for which units in stock are less than 100.

[Serializable]
public class Dependency : ExtensibleDependency
{
    private string _connString;
    private int _productID;

    public override bool Initialize()
    {
        return true;
    }

    public Dependency(int productID, string connString)
    {
        _connString = connString;
        _productID = productID;
    }

    internal bool DetermineExpiration()
    {
        if (GetAvailableUnits(_productID) < 100)
            return true;
        return false;
    }

    internal int GetAvailableUnits(int productID)
    {
        SqlDataReader reader = null;
        var connection = new SqlConnection(_connString);
        connection.Open();

        int availableUnits = -1;
        try
        {
            SqlCommand cmd = connection.CreateCommand();
            cmd.CommandText = "Select UnitsInStockFrom Products where ProductID=" + productID;

            reader = cmd.ExecuteReader();

            if (reader.Read())
            {
                availableUnits = Convert.ToInt32(reader["UnitsInStock"].ToString());
            }

            reader.Close();
            return availableUnits;
        }
        catch (Exception)
        {
            return availableUnits;
        }
    }

    public override bool HasChanged
    {
        get
        {
            return DetermineExpiration();
        }
    }
    // This class is to be deployed on NCache
}

Step 2: Deploy Implementation on Cache

Deploy this class and any dependent assemblies on NCache by referring to Deploy Providers in Administrator’s Guide for help.

Step 3: Use Extensible Dependency

Once extensible dependency class has been implemented and deployed, it is ready to be used in your application. The following code shows how to add data into the cache using the Insert() with extensible dependency:

Pre-Requisites for Using Extensible Dependency

To utilize the Insert API, include the following namespaces in your application:
- Alachisoft.NCache.Client
- Alachisoft.NCache.Runtime.Dependencies
- Alachisoft.NCache.Runtime.Exceptions
The ExtensibleDependency class being used must be deployed on cache.
The application must be connected to cache before performing the operation.
Cache must be running.
To ensure the operation is fail safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.

try
{   // Specify the connection string
    string connectionString = ConfigurationManager.AppSettings["connectionstring"];

    // Fetch the product to be added to the cache
    Product product = FetchProductFromDB(1001);

    // Specify the unique key of the item
    string key = $"Product:{product.ProductID}";

    // Create a cacheItem
    var cacheItem = new CacheItem(product);

    //Creating extensible dependency
    Dependency dependency = new Dependency(1001, connectionString);
    cacheItem.Dependency = dependency;

    // Add cacheItem to the cache with dependency
    cache.Insert(key, cacheItem);
}
catch (OperationFailedException ex)
{
    // Exception can occur due to:
    // Connection Failures
    // Operation Timeout
    // Operation performed during state transfer
}
catch (Exception ex)
{
    // Any generic exception like ArgumentNullException or ArgumentException
}

Additional Resources

NCache provides sample application for extensible dependency on GitHub.