Getting Ready › Overview Guide › The CA AppLogic® Application Model › Applications

Applications

A CA AppLogic® application consists of an application descriptor, one or more application volumes, two singletons named Top and Main respectively, and a local catalog that contains classes used only in this application. The application may contain additional singletons if you define them, and those will be referenced in the application descriptor the same way as Top and Main are.

Most applications define one or more application volumes. These volumes are named objects, and their names are unique within the application. Whenever you use an appliance that defines a volume placeholder, you configure it with the name of one of your application volumes.

The Main singleton is an assembly that contains the top-level structure of the application. For obvious reasons, each application has one of those. The Top singleton is the topmost assembly in the application. It contains the single instance of Main, and configures it with the application settings - instance settings that apply to the application as a whole.

Note:

• The application structure makes it possible to treat the whole application as a single appliance.
• When you define appliance classes in your application, their place in the application structure depends on the type of the class. Singletons are included only in the application descriptor; they cannot exist outside of an application, and cannot be included in catalogs. All other classes specific to your application are placed in the application catalog (sometimes named also the local catalog). Every application has an application catalog, even if it is empty.
• Singletons do not exist outside of an application and cannot be included in catalogs.