Mapping of data source schema data types relating a server side database description into a set of OLE DB data types is rendered subject to user modification by provision of a dual Schema File at a client side terminal which contains an original unmodifiable copy and a modifiable working copy of the data source schema. When the server side database description is altered, user modifications to the modifiable copy are preserved as much as possible by performing a matching process on the client side in the course of generating new unmodifiable and modifiable schema files which comprise a new dual Schema File.