GEN.LIB - An Approach to Object Persistence
by Jaroslav Pokorny
GEN.LIB is a library managing a persistency of application domain
objects using relational database servers. It is also able to
present rows of external (by other application maintained) tables
as C++ objects. Thus, GEN.LIB is, in general, a part of the client
application, which provides an interface between two different
paradigms: a bridge between object oriented paradigm on side of
the client, and a relational database server on the other side.
GEN.LIB has been developed as a part of the European ADDORE project
(Application specific Depository of Object ORiented Environment)
organized under the COPERNICUS programme. The ADDORE Consortium
involved the Charles University, Czech Republic and the companies
DCIT, Czech Republic, IQSOFT, Hungary, and Objectif Technologie,
France. The project started in January 1995 and has been completed
by the end of 1997.
GEN.LIB represents a stand-alone library usable without the rest
of the ADDORE project. Its primary purpose was to implant the
Rumbaugh’s OMT methodology of OO analysis and design into the
environment of building business applications. The GEN.LIB part
of the projects provides a support for persistence of C++ application
objects and some possibilities to use relational tuples as objects
in C++ programs. Moreover, some higher non-procedural querying
capabilities are available in GEN.LIB. Concerning research motivations,
we followed three well-known approaches to object persistence:
- Object/Relational Data Managers - they map objects directly to
relational tables and manage objects.
- Relational Wrapper Libraries - they map objects to database objects,
which are linked to relational database. The relational wrapper
detects a change in the contents of an object and automatically
generates the SQL to make the changes in the linked relational
database. Similarly, it detects changes to relational database
and moves that information back into the local objects.
- Object/Relational Databases - they store information using objects
or relations, and they let developers access the data using either
method.
The GEN.LIB addresses many of above trends. The library gives
at least a possibility:
- to handle persistent and transient objects via C++ programs
- to conceive rows of relations as C++ objects
- to use high-level query facilities in C++ programs
- to use many of database-oriented features on the level of C++
programming.
In order to respect the ADOORE project objectives we should accept
a set of common general requirements that increase the quality
of the library. These requirements focus on the database aspects
of GEN.LIB and are listed below:
- GEN.LIB provides general relational database management facilities:
multiple sessions opened simultaneous on different and independent
RDBMSs, multiple connections simultaneously opened on the database
server, full support of the transaction handling, full support
of all RDBMS features, resource allocation/de-allocation
- GEN.LIB is a library of components that provides transparent storage/retrieval
services of the objects written in the target programming language
(C++) in the relational database(s); this feature includes: automatic
and transparent SQL generation and execution, enhanced memory
management, transparent manipulation of the objects independent
on the fact if the copy of the instance is in memory or not.
- GEN.LIB allows the execution of SQL treatment on the server side
(for large set of data manipulation)
- GEN.LIB provides both object and set of objects access and manipulation
facilities: mapping strategies for the storage of the native C++
objects in the relational database, mapping strategies for the
representing data stored in the relational database using C++
objects, several kinds of obtaining sets of objects using various
selection criteria, support for standard binary relations (one
to one, one to many, many to many), retrieving objects related
to given instance, etc., various locking strategies
- GEN.LIB handles the object’s instances in the memory of the client
application: uniqueness of each object of the domain layer, format
transformation between objects attributes and tables’ column
- GEN.LIB supports DB errors handling: error capture, exception
raising, strong default error handling (reconnection procedure,
non-fatal error distinction, ...)
- GEN.LIB provides journalising facilities: DB statement journalising,
DB error journalising, GEN.LIB internal error journalising, debugging
information tracing (DB command/statement, main classes’ methods
calls).
The prototype has been implemented on SUN stations connected with
ORACLE server (Version 7). The library is fully functional. This
fact is confirmed by using the library in WISE system (a workflow
simulator and editor) implemented in DCIT. We must emphasise that
the above set of functionality constitutes a compromise between
all the needs of the future applications that will be developed
using GEN.LIB. As a consequence, an important requirement of GEN.LIB
was its flexibility: the analysis of the library as well as its
design took into account the possible and reasonable extensions
foreseen by its future users.
More information available on website: http://kocour.ms.mff.cuni.cz/adoore.html
Please contact:
Jaroslav Pokorny - CRCIM / Charles University
Tel: +420 2 21 91 42 65
E-mail: pokorny@ksi.ms.mff.cuni.cz
