| --- |
| title: Application, Parent and Membership |
| layout: website-normal |
| toc: ../guide_toc.json |
| categories: [use, guide, defining-applications] |
| --- |
| |
| All entities have a ***parent*** entity, which creates and manages it, with one important exception: *applications*. |
| Application entities are the top-level entities created and managed externally, manually or programmatically. |
| |
| Applications are typically defined in Brooklyn as an ***application descriptor***. |
| This is a Java class specifying the entities which make up the application, |
| by extending the class ``AbstractApplication``, and specifying how these entities should be configured and managed. |
| |
| All entities, including applications, can be the parent of other entities. |
| This means that the "child" is typically started, configured, and managed by the parent. |
| For example, an application may be the parent of a web cluster; that cluster in turn is the parent of web server processes. |
| In the management console, this is represented hierarchically in a tree view. |
| |
| A parallel concept is that of ***membership***: in addition to one fixed parent, |
| and entity may be a ***member*** of any number of special entities called ***groups***. |
| Membership of a group can be used for whatever purpose is required; |
| for example, it can be used to manage a collection of entities together for one purpose |
| (e.g. wide-area load-balancing between locations) even though they may have been |
| created by different parents (e.g. a multi-tier stack within a location). |
