docs(pivotgrid): add information about state mode

Author: CecoMilchev

controls/pivotgrid/caching.md

@@ -12,15 +12,15 @@ position: 11
 
 
 
-**RadPivotGrid**’s aggregate calculations and grouping are driven by a powerful data engine which produces a special pivot view model. The latter feeds the aggregate values into the final output by the control. At times, however, when the data to be aggregated is quite large, the creation of the pivot view model can get pretty demanding in terms of CPU resources. In order to avoid the recalculation of large data that does not change very often, RadPivotGrid allows the caching of the pivot view model into the session state.
+**RadPivotGrid**’s aggregate calculations and grouping are driven by a powerful data engine which produces a special pivot view model. The latter feeds the aggregate values into the final output by the control. At times, however, when the data to be aggregated is quite large, the creation of the pivot view model can get pretty demanding in terms of CPU resources. In order to avoid the recalculation of large data that does not change very often, **RadPivotGrid** allows the caching of the pivot view model into the session state.
 
-## 
+##
 
 Caching is enabled through the **EnableCaching** property of the control. For large sets of data using it will result in considerable speed up of any operations that require the rebinding of the pivot grid.
 
 However, there are trade-offs that should be carefully considered before opting for the employment of this feature:
 
-* **Memory consumption** is very likely to increase significantly if the web page where RadPivotGrid is placed on experiences intense traffic.
+* **Memory consumption** is very likely to increase significantly if the web page where the **RadPivotGrid** is placed in experiences intense traffic.
 
 * There is evidently **no guarantee the data to be displayed will be up-to-date.**
 
@@ -30,6 +30,25 @@ Note that in case you need to remove the model from the session you just should
 Session[RadPivotGrid1.UniqueID] = null;
 ````
 
+Caching is not supported when using <code>sessionState</code> modes requiring the object to be Serializable, e.g.:
+````ASP.NET
+<sessionState mode="SQLServer">
+</sessionState>
+````
 
+The stored state object of type *Telerik.Web.UI.PivotGrid.Core.ViewModels.PivotViewModel* is not defined as [Serializable] in its implementation.
 
-
+````C#
+...
+internal class PivotViewModel : IPivotViewModel
+{
+    private IDataProvider dataProvider;
+    private IGroupDescription valuesDescription;
+
+    public PivotViewModel()
+    {
+        this.GrandTotalText = PivotLocalizationManager.GrandTotal;
+        this.ValuesGroupText = PivotLocalizationManager.Values;
+...
+````
+Therefore, when using SQLServer mode the *SessionState* will not be saved.