Component model – Any software application requires a component model that provides structure and coherence to an application. The component model used by DAF is JavaBeans, managed in a component container called the Nucleus (read about it in below section).
Data access model – All recent Web applications require information access and an ability to manipulate data. DAF’s data access and manipulation model is called the Data Anywhere Architecture.
Messaging model – Responsive applications require a messaging architecture that allows events to be fired and appropriate actions to execute on the occurrence of those events elsewhere in the system. The JMS messaging model implemented by DAF is managed by a service called the Patch Bay.
Although not classified as a ‘main pillar,’ the user interface (UI) programming model by which these previously mentioned elements can be used is also important, and an area where ATG has innovated ahead of the general market. DAF uses JavaBeans as the primary, lightweight, component model. These JavaBean components are configured and linked together by .properties files within Nucleus. The DAF application framework can also be run on all major J2EE application servers (JBoss, WebLogic, WebSphere etc.).
Nucleus is a ‘light weight’, yet feature-rich component model. It adheres to the “Inversion of Control” design pattern, whereby software components are discrete entities coupled together by the Nucleus container, rather than through direct reference. The services and structure provided by Nucleus makes building Java applications much simpler than when starting with the base set of Java and J2EE services. It promotes good interface-based programming principles and helps application developers take a modular approach, resulting in more modularized, maintainable, and understandable applications.
Nucleus is DAF’s component namespace for building applications from JavaBeans. Nucleus allows the assembly of applications through simple configuration files that specify what components are used by the application, what properties they should have, and how components hook together. Nucleus itself provides no application-specific functionality, since it is only a container in which components live, discover, and interact with each other. It is the collection of components that make up the functionality of an overall application. Nucleus organizes these application components into a hierarchical namespace.
A lot of what makes Nucleus special is encapsulated in the following core areas:
Component creation and administration – Nucleus provides a simple way to write new components. It is a simple process to take any Java object and allow it to act as a component in Nucleus. Nucleus takes on the task of creating and initializing components. A very useful aspect of Nucleus is that applications don’t need to contain code to create instances of components. Instead, components can be created and administered through configuration files that specify the component and the initial values of its properties. If needed, administrators can alter the properties of ‘live’ components within the application. The component instances are then initialized automatically at start up time, rather than programmatically created. Nucleus employs a ‘lazy instantiation’ policy for creating components. One component is only created at the point it is referenced by another.
Component layering and combination – Nucleus provides a convenient way to modify and extend component properties by organizing configuration files into layers. This layering allows application developers to add new components or override the settings of existing ones without modifying the configuration files shipped by ATG. Nucleus automatically combines the layers at application start-up. These layers are organized into ‘modules’ so the associated Java class files can be maintained with the configuration files, simplifying application maintenance and upgrade.
Component scoping – To further increase its usefulness as a component model for Web applications, Nucleus makes it very easy for application developers to set the scope of their components. The scope can be set to ‘global’, ‘session’, or ‘request’,. Nucleus takes care of how the components are managed so that developers do not have to do any specific coding.
Nucleus includes a large number of out-of-the-box generalized service components that can be used in many ways within an application. Service components include TCP Request Services, Scheduler Services, ID Generation Services, Resource Pools, Queues, Email Senders and Listeners, and many more. Each Nucleus service has a unique Nucleus name. For example, the default javax.sql.DataSource component is located at /atg/dynamo/service/jdbc/
It is clear that emerging models typically tackle one or the other element of DAF but not everything. Hibernate tackles the data access problem, Struts tackles the UI development problem, etc. It is often left to the application developer to figure out how these different initiatives are used together.
