RDD (Replaceable Data Driver) has been created by the architects of CA-Clipper compiler. The idea is brilliantly simple - a set of functions has been defined by means of which an application access the data and execute operations on indexes. There have also been defined a method of registering new RDD drivers in an application and how such an application selects a driver which it wants to use at a given moment.
Since Clipper version 5, applications access the data by the means of well documented functions of RDD drivers. RDD drivers conventionally available in CA-Clipper 5.3 are DBFNTX and DBFCDX. They both work on the data stored in DBF files. DBFNTX driver creates *.NTX indexes, while DBFCDX may use structural *.CDX indexes.
Thanks to RDD drivers idea, an application may in an identical way refer to different data - the whole responsibility for the correct realization of data access lies on RDD driver. Due to imperfections of the genuine DBFNTX and DBFCDX drivers, many companies have created commercial RDDs.
RDD drivers improvement can be divided into several types:
- Drivers working on common DBF data provided by a file server. The working idea is similar to the one in the genuine DBFNTX and DBFCDX drivers, but improved algorithms made the data access faster and data and index damage rarer. Six RDD driver by Vista Software may be an example here.
- Client-server drivers working on the data stored in DBF by the means of a centralized server processing application commands. These drivers indeed increase data and index security. Advantage Database Server by Extended Systems may be an example here.
- Client-server drivers working on-line on the data stored in SQL management systems. A solution connecting two worlds - the world of xBase based on ISAM concept (Indexed Sequential Access Method), which is oriented towards record-after-record processing, and the world of relational database with SQL language, oriented towards database sets processing and declarational programming. These drivers work is to translate the xBase orders into SQL orders. The main representative of this group of drivers is Mediator created by OTC S.A..
Also have a look at: What should you pay attention to while selecting RDD driver cooperating with SQL database?
Of course, there have been created other drivers to work on other types of data, such as text sets or specialized forms. These are specialist solutions, whose usage is limited.