1. March 31, 200391.3913 R McFadyen1 Persistent Objects Persistent objects are objects that continue to exist after the application program terminates Persistence is provided by a DBMS ACID property “D” Two main categories OODBMS RDBMS Sections Intro, 34.1, 2, 4, 5, 6, 7, 8, 9, 19

2. March 31, 200391.3913 R McFadyen2 Persistent Objects To use an RDBMS, we require an object to relational mapping service persistence service is a subsystem within the Technical Services Layer mismatch between record-oriented and object-oriented representations of data recommendation : buy, not build provides functions such as store, retrieve, modify, commit, rollback

3. March 31, 200391.3913 R McFadyen3 Key Ideas Mapping There are two schemas: object and relational. We need a mapping that translates from one to the other. Object Identity Records and objects have a unique identifier Materialization/Dematerialization need to be able to translate the non-object representation to an object representation, and vice versa Lazy Materialization Some objects are not materialized all-at-once; part of the object may be done first, and other parts later on

4. March 31, 200391.3913 R McFadyen4 Patterns Representing Objects as Tables Object Identifier Representing Object Associations as Tables one-to-one one-to-many many-to-many many-to-many with an Association Class Aggregation and Composition Superclass/Subclass

5. March 31, 200391.3913 R McFadyen5 Each class becomes a table Representing Objects as Tables pattern define a table for each persistent object class Some mappings are straight forward We require 1NF tables ClassAn objectThe table Many of these patterns mentioned come from a paper “Crossing Chasms: A Pattern Language for Object-RDBMS” published in 1996 in Pattern Languages of Program Design

6. March 31, 200391.3913 R McFadyen6 > stereotype We can indicate tables in UML using the stereotype: > Note you can use UML for designing relational databases

7. March 31, 200391.3913 R McFadyen7 OIDs The OID will help prevent the system from instantiating the same object twice and to help find an instance Objects have an OID that is used as the PK in a relational table. Each object is uniquely mapped to a row in the table Object Identifier Pattern assign an OID to each record and object

8. March 31, 200391.3913 R McFadyen8 Façade Persistence subsystem will have a Façade that provides access to its services façade Requesting a ProductSpecification with a specific OID

9. March 31, 200391.3913 R McFadyen9 Representing Relationships in Tables Representing Object Associations as Tables pattern one-to-one place an OID FK in one or both tables representing the objects in the association or, create an associative table that records the OIDs of each object in the association or, could merge the two classes into one table Consider StoreRegister houses 11

10. March 31, 200391.3913 R McFadyen10 Representing Relationships in Tables Representing Object Associations as Tables pattern one-to-many create an associative table that records the OIDs of each object in relationship or, could use a Foreign Key on the Many side Consider ProductCatalogProductSpecification contains 11..*

11. March 31, 200391.3913 R McFadyen11 Representing Relationships in Tables Representing Object Associations as Tables pattern many-to-many create an associative table that records the OIDs of each object in relationship CompanyPerson employs **

12. March 31, 200391.3913 R McFadyen12 Representing Relationships in Tables Representing Object Associations as Tables pattern many-to-many AND with an Association Class create an associative table that records the OIDs of each object in relationship and attributes of the association class CompanyPerson employs ** Employment

13. March 31, 200391.3913 R McFadyen13 Representing Relationships in Tables Representing Object Associations as Tables pattern many-to-many AND with an Association Class create an associative table that records the OIDs of each object in relationship... Person marries * * Marriage

14. March 31, 200391.3913 R McFadyen14 Representing Relationships in Tables Representing Object Associations as Tables pattern Aggregation and Composition SaleSalesLineItem contains 1..* ProgrammeCourse contains 1..* * Treat as 1-to-many and many-to-many associations (Aside: Note that relational integrity rules/triggers may need to be defined.)

15. March 31, 200391.3913 R McFadyen15 Representing Relationships in Tables Representing Object Associations as Tables pattern Superclass/Subclass Payment CreditPaymentCashPaymentChequePayment 3 basic choices: all classes represented as separate tables, only the ‘leaf’ classes, or one table for all Others are possible (see 3902)

16. March 31, 200391.3913 R McFadyen16 OR Mapping - Example When we map this structure to a Relational Database, we are creating an object-to-relational mapping. We need to define the relational database schema and specify which class(es) map to which relation(s), and whether the associations are mapped to relations or to foreign keys. Suppose we have the following class model: Program name type description Course name credit description * * Offering slot room contains * offer

17. March 31, 200391.3913 R McFadyen17 Example - recall patterns for O-R mappings O-R Mapping The Representing Objects as Tables pattern says that each class will be represented in the RDb by a separate table (relation). The Object Identifier pattern tells us that each table representing a class will have the OID as the PK. The Representing Object Associations as Tables pattern gives us several options: use an association table to represent the association (for 1-1 and 1-m) we can choose to use a Foreign Key for a 1-1 we can choose to merge the two classes into one table etc

18. March 31, 200391.3913 R McFadyen18 Example - mapping ClassMaps to Relation Program Course AssociationMaps to Relation/FKey containsTable: Contains offerFK in Offering (refers to Course) Offering O-R Mapping, for the given class model we could decide: Course Program

19. March 31, 200391.3913 R McFadyen19 Example - mapping Relational Schema Program Course Offering nametypedescription slotroom namecreditdescription Contains pOID oOID cOID pOID cOID Illustrates the structure of the relations PKs are underlined. FKs are indicated via dashed lines. cOID

20. March 31, 200391.3913 R McFadyen20 Example - mapping Table structure with some sample records Program Course nametypedescription C Programming3This course introduces … pOID 256 3-yearR123 4-yearR...334 Java Programming3...333 Systems Analysis3...456 namecreditdescriptioncOID A 3-year programme of studies in Business Computing

21. March 31, 200391.3913 R McFadyen21 Example - application models Note that these relational structures are very different from the class structures that application programs would use. For example, a course object will contain, within it, its offerings: public class Course { private Hashtable Offerings = new Hashtable(); … course_offering = new Offering (…) Offerings.put (…, course_offering); } Part of the complexity of a system that uses objects and tuples/rows in an RDb is the translation from/to the other. A collection of course offerings Adding a course offering to the collection

22. March 31, 200391.3913 R McFadyen22 Object to Relational Mapping Products EJB-related products providing object to relational mapping services CoCoBase TopLink enJin

23. March 31, 200391.3913 R McFadyen23 enJin: the following is from: http://www.versant.com/products/enjin/whitepapers/WP_Benchmark-011022r4-W.pdf “… This object-to-relational bottleneck is often referred to as an 'impedance mismatch', since in order to store data in an RDBMS, objects must be broken down into flat table structures, and to retrieve objects, they must be built up from these RDBMS 'flat' structures. This white paper examines the performance characteristics of a product that has been designed specifically to overcome this object-to- relational impedance mismatch, and which successfully improves both the response times and transaction throughput of applications deployed on J2EE application servers such as IBM's WebSphere. Object to Relational Mapping Products

24. March 31, 200391.3913 R McFadyen24 TopLink a commercial product that provides object to relational mapping capability “In an enterprise Java environment, one of the most formidable challenges is storing business objects and components in a relational database (RDB). Oracle TopLink provides developers with the flexibility to map objects and Enterprise Java Beans to a relational database schema with minimal impact on ideal application design or database integrity. The result — developers focused on addressing business needs rather than building infrastructure. Object to Relational Mapping Products