Software Architecture and J2EE Patterns

Model-View-Controller (MVC) separates application into three distinct components: Model (data), View (presentation), and Controller (logic/controls). This separation allows independent development and modification of each component. Publish-Subscribe and Observer-Observable handle notifications, while Reader-Writer manages concurrent access - only MVC separates data, presentation, and control.

The Data Access Object (DAO) layer is specifically responsible for managing CRUD operations (Create, Read, Update, Delete) on stored data. It provides an abstraction interface between the application and persistent storage. Value Objects carry data, Presentation Layer handles display, and Deployment Layer manages infrastructure - only DAO handles data persistence operations.

Persistence means data survives beyond the execution of the program that created it. Objects can be stored to disk or database and reconstructed later, allowing data to exist between program runs. Thread support, error handling, and other options are unrelated to the core concept of persistence.

For a web-based price comparison site, JSP (presentation) and Servlets (controller logic) are sufficient. EJBs add unnecessary complexity for what appears to be a simple web application without distributed transaction or complex business logic requirements. Applets are deprecated client-side technology.

The statement 'EJB tier does not host system-level services... they are hosted on the EIS tier' is NOT true. The EJB container (within EJB tier) actually PROVIDES system-level services like transaction management, security, concurrency control, pooling, and lifecycle management. EIS tier handles enterprise information systems like databases and legacy systems.

EJBContext.getCallerPrincipal() is the standard EJB API method to retrieve the caller's identity (as a java.security.Principal object) within enterprise bean code. This works in both session and entity beans. getIdentity(), getCallerIdentity(), and getUserId() are not standard EJBContext methods.

EJB Containers run WITHIN EJB Servers. The server provides the runtime environment (JVM, networking, resources), while the container manages EJB lifecycle, transactions, security, and other services. Multiple containers can run within a single server. The server is the broader hosting environment.

Three-tier architecture consists of: 1) Presentation Tier (Client Interface), 2) Business Logic Tier (application logic), 3) Data Storage Tier (databases/persistence). 'Security' is a cross-cutting concern, not a distinct tier - it applies across all three tiers (authentication at presentation, authorization at business logic, data encryption at storage).

In requirements engineering, functional requirements specify what a system must do while non-functional requirements specify how the system performs those functions. Security is a classic non-functional requirement at the architectural level because it defines quality attributes like confidentiality and integrity. A checking account rule represents a functional requirement at the design level because it specifies specific business logic the system must implement.