Search Tag Data in Cache with SQL Query
NCache provides object queries through which you can search and delete result sets based on the criteria given to the query. To retrieve the data according to your specified criteria, NCache provides you with an extension of SQL query. It lets you search the data in your cache based on the criteria of the requirement. A special keyword $Tag$
is used to specify that the condition under consideration uses tags. The SQL query with the searching criteria is executed using ExecuteReader
. The ExecuteReader
processes the query on the server-side and then sends the result in chunks (as a dictionary containing keys and values) to the client in the tabular form to the ICacheReader
type of instance.
You can query an item having a specific tag as explained below.
Prerequisites to Search Tag Data in Cache Using SQL Query
- 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.
- Custom classes/searchable attributes must be indexed as explained in Configuring Query Indexes.
- For API details, refer to: ICache, ICacheReader, QueryCommand, ExecuteReader, SearchService.
Search Data in Cache with One Tag
Using object queries, a single tag can be used to retrieve all the items associated with that tag.
Syntax
The following example retrieves all the items associated with the tag VIP Customers using the SQL query.
Note
Use the fully-qualified name (FQN) of the class Customer, e.g., Data.Customer.
// Precondition: Cache is already connected
// Create a query for search
// Use the Fully Qualified Name (FQN) of your own custom class
string query = "Select CustomerID,ContactName FROM Alachisoft.NCache.Samples.Data.Customer WHERE $Tag$ = ?";
// Use QueryCommand for query execution
var queryCommand = new QueryCommand(query);
queryCommand.Parameters.Add("$Tag$", "VIP Customers");
// Executing Query
ICacheReader reader = cache.SearchService.ExecuteReader(queryCommand);
// Read results if result set is not empty
if (reader.FieldCount > 0)
{
while (reader.Read())
{
// Get the value of the result set
string customerID = reader.GetValue<string>("CustomerID");
string contactName = reader.GetValue<string>("ContactName");
Console.WriteLine($"Customer '{contactName}' having ID '{customerID}' is a VIP Customer.");
}
}
else
{
Console.WriteLine($"No VIP Customers found");
}
Warning
Providing a Null
tag value for the query will throw an ArgumentNullException
or NullPointerException
.
In order to get more detail on NCache queries, please refer to the SQL Reference for NCache section.
Note
To ensure the operation is fail-safe, it is recommended to handle any potential exceptions within your application, as explained in Handling Failures.
Additional Resources
NCache provides sample application for Tags on GitHub.
See Also
.NET: Alachisoft.NCache.Runtime.Caching namespace.
Java: com.alachisoft.ncache.runtime.caching namespace.
Node.js: Tag class.
Python: ncache.runtime.caching class.