The following diagram illustrates the principals of a read transformation.
A read transformation is defined on a data view, by using the dpconf command as follows:
$ dpconf add-virtual-transformation -h host -p port -D bindDN / view-name read action attr-name parameters
Imagine an organization has a legacy application whose function is to display person entries. The application does not support entries that do not contain a mail attribute. The physical data source has been upgraded and the email attribute no longer exists for person entries (e-mail addresses are constructed using other attributes).
The transformation required here is to add the mail attribute during the search response. This transformation changes the entry that is read from the database and adds a mail attribute whose value is firstname.lastname@example.org. No reverse transformation is required and the physical data is not changed.
Note that with the above transformation, the mail attribute makes no sense in a search request filter. Search request filters must contain physical attributes.