MACROMEDIA 38000382 - JRun - Mac Getting Started Manual page 31

The business logic of a website. The middle tier contains both the
Middle tier
presentation logic (web tier) and the business rules (EJB tier) that define a website.
You use JRun to implement applications for the middle tier.
Makes application functionality available on the web. It accesses data
Web tier
and business functionality from other tiers and often encapsulates some user
interaction control. In web service scenarios, other applications take the place of
the client programs, accessing a J2EE application through its web tier.
Provides portable, scalable, available, and high-performance access to
EJB tier
enterprise data and business rules. It offers object persistence and access to
business logic implemented as EJBs. Other tiers access server-side enterprise bean
instances using the Java Remote Method Invocation (RMI) API.
Integrates a J2EE application with other enterprise information systems.
Data tier
The data tier provides data storage and other information services to the J2EE
application. Databases, enterprise resources planning systems, mainframe transaction
processors, legacy systems, and enterprise integration technologies usually reside in
the data tier. Other tiers access the data tier with a driver supporting the JDBC API,
and access non-database EIS resources with J2EE connector extensions, CORBA, or
vendor-specific proprietary protocols.
The following figure shows a three-tier application model:
This three-tier architecture offers the following advantages to website developers:
The tiers, or even components of the tiers, can be distributed across multiple
hardware systems to improve system scalability and performance.
The middle tier shields clients from the complexities of accessing the enterprise data
store.
EJBs provide a secure, component solution that can be reused by multiple
applications to share access to enterprise data. EJBs can be accessed by web
applications, or web clients can access them directly.
The component architecture lets you distribute application development across your
development group. For example, JSP developers are typically concerned with
information presentation rather than with implementing business rules. Conversely,
EJB developers are concerned with data access and manipulation, but not with
presentation.
Enterprise application architecture 17