This section provides brief descriptions of the four logical tiers shown in Figure 2–4. The descriptions refer to application components implemented using the Java 2 Platform, Enterprise Edition (J2EETM platform) component model. However, other distributed component models, such as CORBA, also support this architecture.
Client tier. The client tier consists of application logic accessed directly by an end user through a user interface. The logic in the client tier could include browser-based clients, Java components running on a desktop computer, or Java 2 Platform, Micro Edition (J2METM platform) mobile clients running on a handheld device.
Presentation tier. The presentation tier consists of application logic that prepares data for delivery to the client tier and processes requests from the client tier for delivery to back-end business logic. The logic in the presentation tier typically consists of J2EE components such as Java Servlet components or JSP components that prepare data for delivery in HTML or XML format or that receive requests for processing. This tier might also include a portal service that can provide personalized, secure, and customized access to business services in the business service tier.
Business service tier. The business service tier consists of logic that performs the main functions of the application: processing data, implementing business rules, coordinating multiple users, and managing external resources such as databases or legacy systems. Typically, this tier consists of tightly coupled components that conform to the J2EE distributed component model, such as Java objects, EJB components, or message-driven beans. Individual J2EE components can be assembled to deliver complex business services, such as an inventory service or tax calculation service. Individual components and service assemblies can be encapsulated as loosely coupled web services within a service oriented architecture model and that conform to Simple Object Access Protocol (SOAP) interface standards. Business services can also be built as standalone servers, such as an enterprise calendar server or messaging server.
Data tier. The data tier consists of services that provide persistent data used by business logic. The data can be application data stored in a database management system or it can be resource and directory information stored in a Lightweight Directory Access Protocol (LDAP) data store. The data services can also include data feeds from external sources or data accessible from legacy computing systems.