1. OO databases 1 Object Oriented databases

2. OO databases 2 Developing OODBMS - motivation motivation more and more application areas require systems that offer support (implement) for both traditional language programming and database capabilities

3. OO databases 3 Traditional programming languages procedural control data abstraction functional abstraction

4. OO databases 4 Persistent programming languages definition: a language that provides the ability to preserve data across successive executions of a program (or even by many different programs) the user can make volatile data permanent in a transparent manner (no translations are required, such as transforming data structures in file structures) impedance mismatch - disappears; however, if some data is available, then it is only available for a specific language no other features beyond persistence are available such as transaction management, concurrency control and recovery

5. OO databases 5 Database systems support a data model provide data persistence provide data sharing (includes concurrent access) provide a higher degree of reliability (includes recovery control) support scalability support security and integrity suitable for distribution

6. OO databases 6 Database programming languages incorporate features beyond persistence transaction management, concurrency control, data recovery database programming DB language (e.g. SQL) - computationally incomplete therefore, a DB language is usually embedded into a high level programming language SQL92 can be embedded in C, Pascal, Fortran, Ada... communication: set of variables in the host language + call to DBMS routines (a pre-processor translates SQL statements into calls)

7. OO databases 7 Drawbacks of existing approaches to database programming impedance mismatch the programmer has to write code to translate between the host languages data model and the DBs language data model additional type-checking while manipulated in the host language the data objects are strongly typed; they are then stored in a traditional DBMS; they can be accessed from another language, not strongly typed! when retrieved in the host language type checks must be performed read and write data on persistent storage it is always the programmers responsibility to decide WHEN to read and write on secondary space (for instance, when some data is required the programmer should not have to specify whether this data is to come from secondary space or from the main memory - this should be transparent at the level of the language)

8. OO databases 8 Cause of drawbacks 2 level storage model persistent memory conceptual / external SQL transformations + type checking application program main or virtual memory physical

9. OO databases 9 Possible solution single level storage model (pointer swizzling - OID - next slide) persistent memory application program main or virtual memory

10. OO databases 10 OO concepts classes and instances object identity; object identifier OID not a pointer, but a logical identifier (an object can exist at different addresses, but the OID remains the same through the life time of the object) hierarchies attributes methods and messages abstraction encapsulation information hiding polymorphism and dynamic binding (overloading)

11. OO databases 11 OO DBMS - Khoshafian and Abnous, 1990 OO DBMS = object orientation + database capabilities

12. OO databases 12 OO DBMS - Kim 1991 OODM (data model) a logical data model that captures the semantics of objects supported in OO programming languages NO UNIQUE DATA MODEL OODB (database) a persistent and sharable collection of objects defined by an OODM OODBMS the manager of an OODB

13. OO databases 13 OO DBMS- Zdonic and Maier definition through features provided database functionality support object identity provide encapsulation support objects with complex state

14. OO databases 14 OO DBMS - Parsaye, 1989 definition through features provided high level query language + optimisation capabilities support for persistent and atomic transactions support for complex object storage + mechanisms of efficient access OODBMS = OO system + the above characteristics

15. OO databases 15 Strategies for developing OO DBMS (languages) extend an existing OO language with DB capabilities provide OO DBMS libraries in the first approach the language is extended, whereas in the second approach a set of libraries is provided embed OO DB constructs into a conventional language extend an existing DB language with OO capabilities SQL3 (Object SQL) - supported by both camps develop a novel data model / database language

16. OO databases 16 Issues in OO DBMSs transactions versioning schema evolution

17. OO databases 17 Transactions support for long (duration) transaction required locking for long term - unsatisfactory in OO DBMSs - unit of transaction = object coarser granularity may be required for efficiency solutions versions advanced transaction models e.g. nested transactions

18. OO databases 18 Versioning solutions to long term transactions also, required per se versioning get a copy of current version in the private work space - check out establish a given object as the current version - check in merge distinct versions delete / archive obsolete versions interrogate about version history of an object

19. OO databases 19 Schema evolution database schema must be easily modifiable e.g. for design applications changes needed changes to class definition (modify definitions of attributes or methods) changes to the inheritance hierarchy changes to the set of classes (adding / deleting classes) schema invariant rules e.g. resolution of conflicts e.g. precedence of subclasses over superclasses (definitions in subclass take precedence)

20. OO databases 20 Advantages enriched modelling capabilities extensibility removal of impedance mismatch more expressive query language support for schema evolution support for long duration transactions applicability to advanced database applications improved performance

21. OO databases 21 Disadvantages lack of a universal data model lack of experience lack of standards query optimisation compromises encapsulation locking at object level may have repercussions on performance high complexity lack of support for views lack of support for security

22. OO databases 22 Towards standardisation Object Oriented Datanase Systems Manifesto (1989) 13 mandatory features proposed the Object Model proposed by the Object Database Management Group ODMG 2.0 adopted in September 1997 de facto standard