Code extensibility

Let us extend the discussion from the previous section and devise a plan for extending DataTableADODB class, which is a part of the Storage library devloped for this project. Consider Fig. 1. The goal is to extend functionality provided by the DataTableADODB class, while preserving backward compatibility. First, DataTableADODB can be extended via new methods or via etxtensions to existing methods without breaking compatibility. This new functionality needs to be exposed on the interface. Changing IDataTableStorage, however, will break compatibility, so the new functionality must be exposed via a separate new interface IDataTableStorageV2, which should also expose all functionality provided by IDataTableStorage. Now, each DataTableXXX class may implement IDataTableStorageV2 in addition to IDataTableStorage (if, e.g., DataTableCSV class does not provide functionality exposed via IDataTableStorageV2, it does not have to implement it).

Figure 1. DataTable class diagram

IDataTableStorage is related to two other classes, DataTableFactory and DataTableManager (Fig. 1). Based on the discussion above, several approaches are available for extending DataTableFactory. For example, a second abstract factory can be defined:

DataTableFactory.cls

Implements IDataTableStorage
Implements IDataTableStorageV2

'''' Default factory for IDataTableStorage
Public Function Create(ByVal Model As DataTableModel, _
                       ByVal ConnectionString As String, _
                       ByVal TableName As String) As IDataTableStorage
    Dim Instance As DataTableADODB
    Set Instance = New DataTableADODB
    Instance.Init Model, ConnectionString, TableName
    Set Create = Instance
End Function

'''' Default factory for IDataTableStorageV2
Public Function CreateV2(ByVal Model As DataTableModel, _
                         ByVal ConnectionString As String, _
                         ByVal TableName As String) As IDataTableStorageV2
    Dim Instance As DataTableADODB
    Set Instance = New DataTableADODB
    Instance.Init Model, ConnectionString, TableName
    Set Create = Instance
End Function

and the CreateInstance method should be appropriately implemented on individual interfaces.

DataTableManager incorporates IDataTableStorage as a private field. DataTableManagerV2 incorporating IDataTableStorageV2 should be introduced. While it may not be immediatly necessary, IDataTableManagerV2 should also be introduced (which might be identical initially). In this case, DataTableManager is left unchanged, and DataTableManagerV2 would implement IDataTableManagerV2 only.