[I] Apache Celix Features (celix)

["pnoltes (via GitHub)" <gi...@apache.org>]

pnoltes opened a new issue, #623:
URL: https://github.com/apache/celix/issues/623

   # Apache Celix Features
   
   Add support for Apache Celix Features, inspired by Apache Karaf Features. This will allow users to select bundles and link the required libraries for the executable/container using a single entity.
   
   
   # Background
   
   [Apache Karaf has a concept of "feature"](https://karaf.apache.org/manual/latest/provisioning) to make it easier to add a selection of bundles.
   
   > Apache Karaf provides a simple and flexible way to provision applications.
   In Apache Karaf, the application provisioning is an Apache Karaf "feature".
   A feature describes an application as:
   > - a name
   > - a version
   > -  a optional description (eventually with a long description)
   > - a set of bundles
   > - optionally a set configurations or configuration files
   > - optionally a set of dependency features
   
   # Proposed Solution
   
   For Apache Celix, introducing a feature concept would be beneficial, especially for specifying additional linking requirements for the executable/container running the feature-based selected bundles. This is because some bundles depend on system-installed libraries rather than private bundle libraries.
   
   Take the civetweb::civetweb library, for instance. Linking to this library is necessary if an executable/container uses the http_admin:
   
   ```cmake
       #examples/celix-examples/http_example/CMakeLists.txt 
       ...
       target_link_libraries(http_example PRIVATE Celix::http_admin_api)
       ...
   ```
   
   With the Apache Celix Features concept, this could be arranged by the feature if it supports library configuration. A feature could then be defined in CMake as:
   
   ```cmake
   add_celix_feature(celix_http_admin
      NAME "Apache Celix HTTP Admin"
      DESCRIPTION "Adds the Apache Celix HTTP Admin bundle so that the HTTP whiteboard pattern can be used to pick up HTTP and/or websocket services."
      VERSION "1.0.0"
      BUNDLES_LEVEL_3 Celix::http_admin
      EXE_LINKING Celix::http_admin_api
   ```
   
   Usage might look like:
   
   ```cmake
   add_celix_container(my_container 
     FEATURES celix_http_admin
   )
   # or 
   # celix_container_features(my_container celix_http_admin)
   ```
   
   # Downsides
   
   - The proposed solution relies on CMake, which further tightens the coupling between Apache Celix and CMake.
   - The add_celix_feature calls should either be "installable" or provided in the CelixConfig.cmake (or a Use.cmake file) to ensure they are usable downstream.
   
   


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