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6: Legacy Connectivity
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EXERCISE 6-2 Implementing Data Validation and Referential Integrity Contraints
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These essay questions will help develop the ability to articulate and describe the JEE concepts and components used in parts 2 and 3 of the exam.
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Question As an architect integrating a JEE system with an existing EIS database system, where should data validation and referential integrity constraints be implemented Answer This is a difficult call. The practical aspects of the decision revolve around the following:
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n How much will it ultimately cost n How much is already invested in the database application n How long is it expected to be functional
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If the DBMS is relational and were implemented after the mid 1980s, it is typically best to use the DBMS functionality to enforce value and referential integrity. Sybase, Oracle, SQL Server, DB2, and Informix to mention the most popular DBMSs have had these abilities for many years. These DBMSs include declarative value and referential integrity constraint features, integrated with the Data Description Language (DDL), and they provide built-in declarative triggers to handle cascading actions required for referential integrity, such as deleting all item rows in a canceled order. Implementing these in the enterprise bean layer would duplicate logic, making maintenance difficult. Any change to the database constraints would require making the change to enterprise beans and to the database. The architectural benefits and capabilities maintaining data integrity constraints in the database layer include the following:
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n Facilitated use by multiple applications
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If for some reason multiple applications are responsible for maintaining database integrity, every application creates an opportunity for bugs that would violate that integrity. Furthermore, other applications that may want to access the database are relieved of the duty of maintaining integrity constraints. They still must, of course, deal with error conditions that result if they violate those constraints.
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n Centralization
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If the constraints are maintained only in the database, the database is the one place where data can be considered consistent by definition. If data inconsistencies exist, either the integrity constraints are incorrect or the design has flaws. Simple value and integrity constraints, such as primary keys, simple foreign keys, uniqueness, value range checking, and so on, are reasonably portable. Database vendors that offer database constraints features have invested a great deal of time and money in ensuring that those features operate correctly and efficiently.
n Portability
n Performance and reliability
The drawbacks of using the DBMS built-in database integrity constraints mechanisms and the EJB can include the following:
n Possible duplication of logic
Enterprise beans generally need reasonable data to perform properly. Therefore, most well-designed enterprise beans do a reasonably good job of checking data values and existence constraints. Database integrity violation errors usually indicate a bug or a problem with the design. Nevertheless, the logic enforcing value and referential integrity is necessarily duplicated. Changing the integrity rules in the database will usually also entail changes to the code, and keeping the two synchronized can be a problem. While simple value and referential integrity constraints are fairly portable, databases differ in coverage and syntax for more involved mechanisms such as composite foreign keys, database triggers, and procedural triggers. Procedural triggers in particular are portability concerns, because, when offered, they are often written in the database vendor s product-specific proprietary language. For example, Sybase Transact SQL is very different from the Oracle PL/SQL procedure language. Because database constraint and trigger configuration are performed with the database vendor s tools, such constraints are maintained outside of the JEE server framework. Because the data model constraints are specified not in the deployment descriptor but in the persistence layer, such constraints are not part of a JEE server deployment. They must therefore be managed separately, complicating deployment and maintenance and providing another possible avenue for system flaws.
n Potential nonportability of DBMS constraints
n Database definition and configuration is uncontrolled
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Another option is to use the EJB to handle constraints. Referential integrity constraints can be implemented in the EJB tier. The constraints required for an application may not be available in the DBMS chosen. The data model may have constraint requirements that cannot be satisfied using the DBMS constraint language. Such constraints can be implemented in the EJB tier. EJB-tier constraint management also provides portability, since the enterprise beans will operate identically in JEE-branded containers. Constraints in the EJB tier can also be controlled by way of environment settings in the application deployment descriptor, centralizing constraint management and making it controllable at deploy time. Yet another option is to implement constraints in both the EJB and database tiers and configuring the constraint implementation at deploy time. This strategy is useful especially when an application must be portable to many different databases, and you want consistent behavior across vendors while optimizing performance by using each database s full power. You could also create a persistence server for the EIS tier. Constraints should be expressed in a declarative constraint language provided by the database vendor. In their absence, the implementation should choose to wrap a layer of integrity management software around the database API. The EIS tier of your application can be an API that you create to wrap the database. Your application accesses the data store only through that server. This application of the decorator design pattern can provide a solution that is portable across databases, is declaratively configurable, and provides a consistent behavior across various clients. As a great deal of design, construction, validation, and maintenance are required, it should be the solution where ultimate flexibility and portability is required. Finally, commercial transaction processing (TP) monitors provide the benefits of the persistence server just described. TP monitors can provide scalability and availability. Typically, they work with multiple database vendors. You avoid vendor lock-in by wrapping calls to the monitor in DAO classes.
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