How can I find out whether an item in my cache has expired?

You can verify using the NCache Monitor 'Cache Count' counter, PerfMon Counters, or by programmatically using cache.Contains or cache.Count after the interval.

What is necessary for Default Expiration to be enabled?

It must be enabled through the NCache Management Center or config.ncconf. If it is not configured in either of these, using the API for default expiration will not function.

Sliding Expiration in Cache

Sliding Expiration in NCache is a dynamic memory management strategy used to retain frequently accessed data while automatically removing data that has not been used for a specific period of time. By resetting the Time-to-Live (TTL) upon each access, it minimizes memory overhead for inactive datasets and guarantees optimal cache hits for active sessions.

Sliding Expiration ensures that if the data is accessed within the specified time interval, the Time-to-Live (TTL) for that specific cache entry is reset. This prevents that particular cache entry from expiring and helps keep the data fresh during high application activity. For example, a session is added with 10 minutes expiration. The session is accessed at the 6th minute, so the life of the session in the cache will be extended by another 10 minutes.

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, CacheItem, CacheItemAttributes, Contains, Count, Expiration, Insert, UpdateAttributes.

Install either of the following NuGet packages in your .NET client application:
- Enterprise: Install-Package Alachisoft.NCache.SDK -Version 4.9.1.0
- Professional: Install-Package Alachisoft.NCache.Professional.SDK -Version 4.9.1.0
Create a new Console Application.
Make sure that the data being added is serializable.
Add NCache References by locating %NCHOME%\NCache\bin\assembly\4.0 and adding Alachisoft.NCache.Web and Alachisoft.NCache.Runtime as appropriate.
Include the Alachisoft.NCache.Web.Caching namespace in your application.
To learn more about the NCache Legacy API, please download the NCache 4.9 documents available as a .zip file on the Alachisoft Website.

How to Configure Sliding Expiration via API

You can add or update the cache items to set expiration using the Insert method. The CacheItem is a custom class provided by NCache that can be used to add data to the cache and also lets you set additional metadata associated with an object of this class. This metadata defines the properties of the item like expiration, dependencies, and more.

The following example sets the value of the Expiration property to Sliding Expiration of 5 minutes for a CacheItem and adds it to the cache if the key does not exist - and updates the item if it exists. This CacheItem will then be removed from the cache after 5 minutes.

Tip

You can monitor/verify expiration through:

"Cache Count" Counter in the NCache Monitor or PerfMon Counters.
Using cache.Contains after the expiration interval has elapsed.
Using cache.Count before and after specifying the expiration.

// Precondition: Cache is already connected
// Get product from database against given product ID
Product product = FetchProductFromDB(1001);

// Generate a unique cache key for this product
string key = $"Product:{product.ProductID}";

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

// Resetting TTL to optimize memory
var expiration = new Expiration(ExpirationType.Sliding, TimeSpan.FromMinutes(5));

cacheItem.Expiration = expiration;

// Add/Update item with expiration
cache.Insert(key, cacheItem);

// Precondition: Cache is already connected
// Get product from database against given product ID
Product product = fetchProductFromDB(1001);

// Generate a unique cache key for this product
String key = "Product:" + product.getProductID();

// Create a new CacheItem for this product
CacheItem cacheItem = new CacheItem(product);

// Set Expiration type and TimeSpan
Expiration expiration = new Expiration(ExpirationType.Sliding, TimeSpan.FromMinutes(5));

cacheItem.setExpiration(expiration);

// Add/Update item in cache with expiration
cache.insert(key, cacheItem);

# Precondition: Cache is already connected
# Get product from database
product = fetch_product_from_db()

# Generate a unique cache key for this product
key = "Product:" + product.get_product_id()

# Create a new CacheItem for this product
cache_item = ncache.CacheItem(product)

# Set Expiration type and TimeSpan
expiration = ncache.Expiration(ncache.ExpirationType.SLIDING, ncache.TimeSpan.from_minutes(5))

cache_item.set_expiration(expiration)

# Add/Update item in cache with expiration
cache.insert(key, cache_item)

// This is an async method
// Precondition: Cache is already connected
// Get product from database against given product ID
var product = this.fetchProductFromDB(1001);

// Generate a unique cache key for this product
var key = "Product:" + product.getProductID();

// Create a new CacheItem for this product
// You also need to specify the FQN (Fully Qualified Name) of the class
var cacheItem = new ncache.CacheItem(product,"FQN.Product");

// Set Expiration type and TimeSpan
var expiration = new ncache.Expiration(ncache.ExpirationType.Sliding, ncache.TimeSpan.FromMinutes(5));

cacheItem.setExpiration(expiration);

// Add/Update item in cache with expiration
await this.cache.insert(key, cacheItem);

// Using NCache Enterprise 4.9.1
// Precondition: Cache is already connected
Product product = FetchProductFromDB("1001");

// Generate a unique cache key for this product
string key = $"Product:{product.ProductID}";
CacheItem cacheItem = new CacheItem(product);

// Adding sliding expiration of 5 min through Add API
cache.Add(key, product, null, Cache.NoAbsoluteExpiration, new TimeSpan(0, 5, 0), CacheItemPriority.Default);

How to Add/Update Item with Configured Default Expirations

You can specify Sliding Expiration in the API using the DefaultSliding and DefaultSlidingLonger values configured in the NCache Management Center or config.ncconf. This provides the flexibility of changing the value externally if needed, without changing the code.

The following code associates the configured Default Expiration (DefaultSliding in this case) with an item, adds it to the cache if the key does not exist, or updates the existing item with the new expiration. If the item is not accessed within the configured time interval, the item will expire. Otherwise, the item's life will be extended by the same time interval.

Important

To enable Default Expiration, it is mandatory that you enable it through the NCache Management Center or config.ncconf. If you pass the API for default expiration and it is not configured through the NCache Management Center or config.ncconf, it will not work.

// Precondition: Cache is already connected
// Get product from database against given product ID
Product product = FetchProductFromDB(1001);

// Generate a unique cache key for this product
string key = $"Product:{product.ProductID}";

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

// Set Expiration TimeSpan
var expiration = new Expiration(ExpirationType.DefaultSliding);

cacheItem.Expiration = expiration;

// Resetting TTL to optimize memory
CacheItemVersion version = cache.Insert(key, cacheItem);

// Precondition: Cache is already connected
// Get product from database against given product ID
Product product = fetchProductFromDB(1001);

// Generate a unique cache key for this product
String key = "Product:" + product.getProductID();

// Create a cacheItem instance for product
CacheItem cacheItem = new CacheItem(product);

// Set Expiration type
Expiration expiration = new Expiration(ExpirationType.DefaultSliding);

cacheItem.setExpiration(expiration);

// Add/Update item into the cache with expiration
CacheItemVersion version = cache.insert(key, cacheItem);

# Precondition: Cache is already connected
# Get product from database against given product ID
product = fetch_product_from_db()

# Generate a unique cache key for this product
key = "Product:" + product.get_product_id()

# Create a new CacheItem for this product
cache_item = ncache.CacheItem(product)

# Set Expiration type and TimeSpan
expiration = ncache.Expiration(ncache.ExpirationType.DEFAULT_SLIDING)

cache_item.set_expiration(expiration)

# Add/Update item in cache with expiration
version = cache.insert(key, cache_item)

// Precondition: Cache is already connected
// This is an async method
// Get product from database against given product ID
var product = this.fetchProductFromDB(1001);

// Generate a unique cache key for this product
var key = "Product:" + product.getProductID();

// Create a cacheItem instance for product
// You also need to specify the FQN(Fully Qualified Name) of the class
var cacheItem = new ncache.CacheItem(product,"FQN.Product");

// Set Expiration type
var expiration = new ncache.Expiration(ncache.ExpirationType.DefaultSliding);

cacheItem.setExpiration(expiration);

// Add/Update item into the cache with expiration
var version = await this.cache.insert(key, cacheItem);

// Using NCache Enterprise 4.9.1
// Precondition: Cache is already connected

// Get product from database against given product ID
Product product = FetchProductFromDB(1001);

// Generate a unique cache key for this product
string key = $"Product:{product.ProductID}";
CacheItem cacheItem = new CacheItem(product);

// Adding Default Sliding expiration with configured value
cache.Add(key, product, null, Cache.NoAbsoluteExpiration, Cache.DefaultSliding, CacheItemPriority.Default);

Additional Resources

NCache provides a sample application for Sliding Expiration on GitHub.