On the ASP.NET tier, we have a listener class containing a callback method to handle the notification when the cache item gets invalidated. The callback method RemovedCallback is registered by using a delegate function. The callback method onRemove declaration must have the same signature as the CacheItemRemovedCallback delegate declaration.
Dim onRemove As CacheItemRemovedCallback = Nothing
onRemove = New CacheItemRemovedCallback(AddressOf RemovedCallback)
The definition for the listener event handler method RemovedCallback responsible for handling the notification from the database trigger is illustrated below. When the cache item gets invalidated, data is retrieved from the database by using the database method call getRecordFromdatabase(). The parameter "key" refers to the index location for the item removed from the cache. The parameter "value" refers to the data object removed from the cache. The parameter "CacheItemRemovedReason" specifies the reason causing the data item to be removed from the cache.
PublicSub RemovedCallback(ByVal key AsString, ByVal value AsObject,
ByVal reason As CacheItemRemovedReason)
Dim Source As DataView
Source = getRecordFromdatabase()
Cache.Insert("employeeTable
", Source, New
System.Web.Caching.CacheDependency("d:\download\tblemployee.txt"),
Cache.NoAbsoluteExpiration,
Cache.NoSlidingExpiration,
CacheItemPriority.Normal, onRemove)
EndSub
The method getRecordFromdatabase() is responsible for querying the database table Employee and it returns a DataView object reference. It makes use of a stored procedure called getEmployee to abstract the SQL for retrieving the data from the Employee table. The method expects a parameter called p_empid representing the primary key for the Employee table.
PublicFunction getRecordFromdatabase (ByVal p_empid As Int32) As DataView
Dim con As OracleConnection = Nothing
Dim cmd As OracleCommand = Nothing
Dim ds As DataSet = Nothing
Try
con = getDatabaseConnection(
"UserId=scott;Password=tiger;Data
Source=testingdb;")
cmd = New OracleCommand("Administrator.getEmployee", con)
cmd.CommandType =
CommandType.StoredProcedure
cmd.Parameters.Add(New OracleParameter("employeeId",
OracleDbType.Int64)).Value = p_empid
Dim param AsNew OracleParameter("RC1", OracleDbType.RefCursor)
cmd.Parameters.Add(param).Direction = ParameterDirection.Output
Dim myCommand AsNew OracleDataAdapter(cmd)
ds = New DataSet
myCommand.Fill(ds)
Dim table As DataTable = ds.Tables(0)
Dim index As Int32 = table.Rows.Count
Return ds.Tables(0).DefaultView
Catch ex As Exception
ThrowNew Exception("Exception in Database Tier
Method
getRecordFromdatabase () " + ex.Message, ex)
Finally
Try
cmd.Dispose()
Catch ex As Exception
Finally
cmd = Nothing
EndTry
Try
con.Close()
Catch ex As Exception
Finally
con = Nothing
EndTry
EndTry
EndFunction
The function getDatabaseConnection accepts a connectionstring as an argument and returns an OracleConnection object reference.
PublicFunction getDatabaseConnection(ByVal strconnection as string) As
OracleConnection
Dim con As Oracle.DataAccess.Client.OracleConnection = Nothing
Try
con = New Oracle.DataAccess.Client.OracleConnection
con.ConnectionString =
strconnection
con.Open()
Return con
Catch ex As Exception
ThrowNew Exception("Exception in Database Tier
Method
getOracleConnection() " + ex.Message, ex)
EndTry
EndFunction
