Fall 2008 1 CIS 764 Database Systems Engineering L18.2 : Object Relational Mapping … ….Object...

Fall 2008 http://www.cis.ksu.edu 1

CIS 764 Database Systems Engineering

L18.2 : Object Relational Mapping …

….Object persistence



Objects are transient .. they last as long as run environment is alive (as long as a middleware session)

Object persistence is the automatic save/load of object state to some persistent storage form (e.g. persisting between sessions). Example storage: binary data files (e.g. Word doc) XML files (e.g. UML XMI) relational data base (using SQL) <<<< Object data base Note: first three save only the state, not the methods.

http://www.idt.mdh.se/kurser/cd5130/msl/2006lp4/reports/drafts/object_persistence.pdf



Object persistence

there are two issues:

the form (format) of persistent storage the mapping of scalar data values.

(e.g. Java and Oracle data types are not identical).



Objects to relational data base (using SQL) is Object-Relational-Mapping.

It can go both ways: objects -> tables tables -> objects (you used this in ADF assignment)

Which way do we want to go?



Which way do we want to go?

Ans1: Design at the conceptual level: ER conceptual domain model (class model)

Ans2: Build web applications for existing tables.

Ans2a: tuning the DB and sync the model



Impedance mismatch … refers to the difference in concepts between object models and table model.

There is (IMHO) a reasonable mapping of object concepts to tables; not so clear mapping backwards.

See following for discussions of mismatch: http://c2.com/cgi/wiki?ObjectRelationalMapping

http://en.wikipedia.org/wiki/Object-Relational_impedance_mismatch



Impedance mismatch … some issues:

* single persistent object may spans several tables

* objects "ownership“ ↔ relational * Objects have "getters"; expensive to submit several queries for single row; use Value object w “putAll” , “getAll” methods. * RDBMSs are faster on global queries; object faster for small scope (single objects). * inefficient to keep objects synchronized with database



One suggestion is to map to object database: not for 764 focus … inefficient for object persistence in 3 layer model

not likely transition for existing DB applications.



OR tools available for most language platforms and target DBMS:

http://en.wikipedia.org/wiki/Object-relational_mapping

http://en.wikipedia.org/wiki/List_of_object-relational_mapping_software



Toplink ( 1990 Smalltalk, ~ 1995 Java) purchased by Oracle in 2002 objects and beans to DB or XML (targets multiple DBs) integrated with JDeveloper; http://www.oracle.com/technology/products/ias/toplink/index.html

http://en.wikipedia.org/wiki/TopLink … donated for open source Eclipse project, … used for Sun EJB3 reference implemenation

…. You implicitly used it for ADF homework



JDO Java Data Objects (~2003) http://java.sun.com/products/jdo/ a standard interface-based Java model abstraction of persistence; applications are portable, and independent of the underlying database (provided there is a JDO mapping) can be used with POJO or with EJB; uses a persistence manager class;

but ... has not been a dominant technology



Hibernate http://www.hibernate.org/ (~2004) express queries in SQL extension (HQL) or native SQL or with an object-oriented Criteria and Example API; open source project ; entity manager for EJB3.0; generates a configuration file that defines the mapping key component of JBoss (http://www.jboss.com/products/index)

integrates with Eclipse http://www.myeclipseide.com/ContentExpress-display-ceid-61.html the major ORM tool .

(See also Gardner Hibernate slides)



NHibernate …. port of the core to the .NET Framework;

Suspend2 http://www.suspend2.net/ … Linux equivalent



Ruby-on-Rails http://www.rubyonrails.org/ a "full-stack" framework for developing DB web applications; follows the Model-View-Control pattern; includes Active Record persistence framework; an "agile programming" approach ... based on the Ruby language http://www.ruby-lang.org/en/ (~1995) “Active Record” name comes from the pattern for object that wraps a DB row ( e.g. an entity ejb).

See also Enterprise patterns <<<<<<<<<<<<<<<<<<<< http://www.martinfowler.com/eaaCatalog/index.html



Django http://www.djangoproject.com/ (~2005) a high-level Python Web DB framework includes ORM … configuration defined in Python code. Includes a sample app in the tutorial .



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Fall 2008 1 CIS 764 Database Systems Engineering L18.2 : Object Relational Mapping … ….Object...

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