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Database and Information Systems

Database and Information Systems. Beyond RDBMS Lectures based on material from Phil Trinder (HW) , Connelly & Begg textbook Monica Farrow G30 email : monica@macs.hw.ac.uk. Topics. Impedance mismatch Persistence OR Mapping Frameworks Limitations of RDBMS OODBMS ORDBMS.

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Database and Information Systems

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  1. Database and Information Systems Beyond RDBMS Lectures based on material from Phil Trinder (HW) , Connelly & Begg textbook Monica Farrow G30 email : monica@macs.hw.ac.uk DBIS: Beyond RDBMS

  2. Topics • Impedance mismatch • Persistence • OR Mapping Frameworks • Limitations of RDBMS • OODBMS • ORDBMS DBIS: Beyond RDBMS

  3. Basic Object Relational Mapping Programming code and SQL e.g. ODBC/JDBC DBIS: Beyond RDBMS

  4. Introduction • The situation is that : • Relational databases are highly commercially successful (Oracle, DB2, SQL Server, etc) • Most databases are relational • SQL is not computationally complete • Most programming languages are now object-oriented • Therefore many applications use a RDBMS as their data store while using an object-oriented programming language for development • This requires some form of SQL within the programming language DBIS: Beyond RDBMS

  5. Flattening • For anything other than extremely simple systems, it is likely that good OO program design requires : • Converting the ResultSet which is returned by the query into an object • Using the functionality provided by the object’s class to process the results • Objects must be mapped to tables in the database (‘flattened’) and vice versa • It is estimated that as much as 30% of programming effort and code space is expended on this type of conversion DBIS: Beyond RDBMS

  6. Impedance mismatch • This is the term used for the complexity introduced by constructing programs using these two different systems, relational databases and object-oriented programming • http://service-architecture.com/object-oriented-databases/articles/impedance_mismatch.html • http://www.agiledata.org/essays/impedanceMismatch.html DBIS: Beyond RDBMS

  7. Impedance mismatch • Constructing programs this way is arduous and error-prone • Relational model uses set operations, Java has OO model with iteration, one record at a time • Data structures need to be constructed from the relations before manipulations, and flattened before storage • Relational Domains may differ from class types • The relational model does not support inheritance • ‘Many to many’ relationships are unlikely to map directly to classes • SQL and Java cannot be simply combined • SQL queries cannot be always be statically checked and may produce errors DBIS: Beyond RDBMS

  8. Persistence • One of the most critical tasks that applications have to perform is to save and restore data • Persistence is the storage of data from an application’s working memory so that it can be restored when the application is run again DBIS: Beyond RDBMS

  9. Object-Relational Mapping Frameworks • Much effort has been put in recently to making object-relational mapping more convenient • The framework handles the mapping of the objects to relational database tables where they are actually stored • Selected objects are initially marked as being persistent • thereafter, changes in those objects are transparently changed in the database • the programmer does not have to write code specifically to update the database • Examples • Hibernate, Castor, OJB, JDO…. DBIS: Beyond RDBMS

  10. OR Mapping Frameworks contd • Using a framework provides: • Transparent persistence • Object classes do not contain db details • The programmer can work only with objects • no SQL statements in the code • Could be used with different database • Easier to code • Mapping of objects to database tables is defined, usually in XML descriptor files • An extra ‘persistance layer’ is automatically inserted between the compilation and execution DBIS: Beyond RDBMS

  11. OR Mapping Frameworks DBIS: Beyond RDBMS

  12. RDBMS advantages • There are advantages in using a Relational Database Management System • The relational database has a strong theoretical foundation which supported the development of SQL • Simple • Suitable for Online Transaction Processing, which is used by many business applications • Supports data independence DBIS: Beyond RDBMS

  13. Advanced Database Applications • Many advanced database applications are unsuitable for the relational model • Computer-Aided Design/Manufacturing (CAD/CAM) • Computer-Aided Software Engineering (CASE) • Network Management Systems • Office Information Systems (OIS) and Multimedia Systems • Digital Publishing • Geographic Information Systems (GIS) • Interactive and Dynamic Web sites • Other applications with complex and interrelated objects and procedural data. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  14. RDBMS Limitations • Impedance Mismatch (discussed) • Entity may be fragmented into many relations, needing joins (which is costly) during query processing • Semantic overloading • Only one construct (the relation) for modelling relations AND relationships • Poor at navigational access from record to record • Good for access by specifying criteria • Not enough support for integrity and business constraints • Referential integrity not always supported • Business constraints may need specifying within DBMS or application DBIS: Beyond RDBMS

  15. RDBMS Limitations continued • Homogeneous data structure • All rows must contain the same columns • All columns must come from same domain • Values must be atomic • Now some RDBMS permit Binary Large Objects (BLOBs), suitable for multimedia or other unstructured object • Limited operations • SQL does not permit new business-specific operations • E.g. intersection of lines, mathematical formulae • SQL cannot perform recursive queries • Not designed for long transactions – 2-phase locking is unsuitable • Difficult to alter the schema DBIS: Beyond RDBMS

  16. Example of complex data structure : Bill of Material Cake BasePart Icing Mixture Water Sugar Eggs Fruit ConsistsOf CompositePart Complex data structures can be more easily represented in OOPL (graph) © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  17. Persistence through Object Serialization • Simple persistence method which provides a program the ability to read or write a whole object to and from a stream of bytes • Allows Java objects to be encoded into a byte stream suitable for streaming to a file on disk or over a network. The bytes can be rebuilt into a live object in the future. • The class must implement the Serializable interface (java.io.Serializable) • Simple, but does not support DBMS features such as multi-users, partial retrieval, transactions, recovery etc DBIS: Beyond RDBMS

  18. Persistence through Object Serialization DBIS: Beyond RDBMS

  19. Using Databases • There are essentially three approaches which have been developed for the management of object storage in databases: • the Object-Oriented Database Management System (OODBMS) • Extend OO programming language with persistence • the Object-Relational Database Management System (ORDBMS) • Extend relational system with object features • Object Relational Mapping • Already discussed DBIS: Beyond RDBMS

  20. Object Oriented Database Management Systems OODBMS http://service-architecture.com/object-oriented-databases/articles/index.html http://www.odbms.org/downloads.html DBIS: Beyond RDBMS

  21. Object Data Model DBIS: Beyond RDBMS

  22. OODBMS Manifesto • To establish agreement on what an OODBMS is, (Atkinson et al. 1990) propose the following 13 rules: • Object characteristics • Complex objects must be supported. • Object identity must be supported.. • Encapsulation must be supported.. • Types and classes must be supported. • Types and classes must be able to inherit from their ancestors. • Dynamic binding must be supported • The DML must be computationally complete. • The set of data types must be extensible. DBIS: Beyond RDBMS

  23. OODBMS Manifesto contd • DBMS characteristics • Data persistence must be provided • The DBMS must be capable of managing very large databases • The DBMS must accept concurrent users. • The DBMS must be capable of recovery from hardware and software failures. • The DBMS must provide a simple way of querying data. DBIS: Beyond RDBMS

  24. The ODMG Proposed Standard • One of the crucial factors in the commercial success of RDBMSs is the relative standardisation of the data model • The Object Data Management Group (ODMG) was formed by a group of industry representatives to define a proposed standard for the object data model. • It includes an Object Definition Language (ODL) and Object Query Language (OQL) • It is still far from being as widely recognised as the relational database standards. DBIS: Beyond RDBMS

  25. Object Identity • In an OODb, an object is identified by an object identifier (oid), and objects are related by storing the oid of one object in another. • The crucial distinction between oids and addresses is that an oid is not related to the object’s physical location, whereas an address is. • The object’s physical location changes between main memory and the database, managed by the DBMS • The object identifier is not visible to the user or database programmer DBIS: Beyond RDBMS

  26. Using an OODBMS • The programmer defines the persistent data • Subsequently, they do not have to know about • Format of data storage • Transfer of persistant data between main memory and database • This happens transparently and is managed by the OODBMS • The programmer can start and commit transactions DBIS: Beyond RDBMS

  27. Advantages of OODBMSs • 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. DBIS: Beyond RDBMS

  28. Disadvantages of OODBMSs • Lack of Universal Data Model. • Lack of Experience. • Lack of Standards. • Query Optimization compromises Encapsulation. • Object Level Locking may impact Performance. • Complexity. • Lack of Support for Views. • Lack of Support for Security. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  29. When to use • In general, RDBMSs are probably more suitable for databases with a variety of query and user interface requirements • i.e. most mainstream business applications • OODBMSs are appropriate for applications with complex, irregular data, where data access will follow predictable patterns • e.g CAD/CAM systems, manufacturing databases DBIS: Beyond RDBMS

  30. OODBMS Users • The Chicago Stock Exchange - managing stock trades • CERN in Switzerland - large scientific data sets • Radio Computing Services – automating radio stations (library, newsroom, etc) • Adidas – content for web site and CD-ROM catalogue • Federal Aviation Authority – passenger and baggage traffic simulation • Electricite de France – managing overhead power lines DBIS: Beyond RDBMS

  31. Object Relational Database Management Systems ORDBMS DBIS: Beyond RDBMS

  32. Market Share • RDBMSs currently dominant database technology with estimated sales $6 - $10 billion per year ($25 billion with tools sales included). • OODBMS market still small, but still finds new applications areas. • Some analysts expect OODBMS market to grow at a faster rate than total database market, but unlikely to overtake relational systems. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  33. ORDBMSs • Vendors of RDBMSs conscious of threat and promise of OODBMS. • Agree that RDBMSs not currently suited to advanced database applications, and added functionality is required. • Reject claim that extended RDBMSs will not provide sufficient functionality or will be too slow to cope adequately with new complexity. • Can remedy shortcomings of relational model by extending model with OO features. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  34. ORDBMSs - Features • OO features being added include: • user-extensible types • encapsulation • inheritance • polymorphism • dynamic binding of methods • complex objects including non-1NF objects • object identity © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  35. ORDBMSs - Features • However, there is no single extended relational model • All models: • share basic relational tables and query language • all have some concept of ‘object’ • some can store methods (or procedures or triggers) • Some analysts predict ORDBMS will have 50% larger share of market than RDBMS. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  36. The Object Relational Model DBIS: Beyond RDBMS

  37. SQL:2003 - New OO Features • Type constructors for row types and reference types • User-defined types (distinct types and structured types) that can participate in supertype/subtype relationships • User-defined procedures, functions, methods, and operators • Type constructors for collection types (arrays, sets, lists, and multisets) • Support for large objects – BLOBs and CLOBs • Recursion © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  38. ORDB Example - Oracle User defined type CREATE TYPE Name AS OBJECT ( first_name CHAR (15), last_name CHAR (15), middle_initial CHAR (1); MEMBER PROCEDURE initialize,; Code to define operations – here simply a class constructor CREATE TYPE BODY Name AS MEMBER PROCEDURE initialize IS BEGIN first_name := NULL; last_name := NULL; middle_initial := NULL; END initialize; END; DBIS: Beyond RDBMS

  39. ORDB Example contd - Oracle Using the new type in a table CREATE TABLE person( person_ID NUMBER; person_name Name, PRIMARY KEY (person_ID)); DBIS: Beyond RDBMS

  40. Stonebraker’s View © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  41. Advantages of ORDBMSs • Resolves many of known weaknesses of RDBMS • Reuse and sharing: • reuse comes from ability to extend server to perform standard functionality centrally; • gives rise to increased productivity both for developer and end-user. • Preserves significant body of knowledge and experience gone into developing relational applications. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  42. Disadvantages of ORDBMSs • Complexity • Increased costs • Proponents of relational approach believe simplicity and purity of relational model are lost • Some believe RDBMS is being extended for what will be a minority of applications • OO purists not attracted by extensions either • SQL now extremely complex. © Pearson Education Limited 1995, 2005 DBIS: Beyond RDBMS

  43. Data modelling comparison ORDBMS/OODBMS DBIS: Beyond RDBMS

  44. Data sharing comparison ORDBMS/OODBMS DBIS: Beyond RDBMS

  45. DBIS: Beyond RDBMS

  46. Summary • Relational databases are widely established • An application needs programming capability as well as just a relational database • There is an impedance mismatch between the objects in an object-oriented program and relations in the database • OR Mapping frameworks handle the mapping from object to relation • Object-oriented databases occupy a niche in the market. The data in the application persists from one run to the next. • Many relational databases now have been extended to support OOP concepts DBIS: Beyond RDBMS

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