add feature metadata documentation

docs/feature-metadata.md

@@ -0,0 +1,142 @@
+# Feature Metadata
+
+Users want to enable abstract features, which means the deployment needs to know how to translate a feature name to a set of changes (configuration files, services, etc).
+
+The metadata is a Hash with the feature name as the key and the feature definition as the value.
+The feature definition itself is again a Hash with the various properties of the feature.
+
+```yaml
+feature_one:
+  property1: value_of_property1
+  property2: value_of_property2
+  …
+feature_two:
+  property1: another_value_of_property1
+  property2: another_value_of_property2
+  …
+…
+```
+
+The following properties are defined:
+* `description` (_String_): A human-readable description of the feature, can be used in documentation/help output.
+* `internal` (_Boolean_): Whether the feature is user visible (shows up in documentation/help) or internal (just to perform additional configuration without user interaction).
+* `foreman` (_Hash_): How this feature should be applied on the "main" system that offers the main user interaction via UI/API.
+  * `plugin_name` (_String_): The name of the Foreman plugin to be enabled (via `FOREMAN_ENABLED_PLUGINS`).
+     If `roles/foreman/tasks/feature/{{ foreman_plugin }}.yaml` exists, it will be executed to perform any plugin-specific setup.
+      * **FIXME**: task file not implemented yet
+  * `role` (_String_): The name of the Ansible role to be executed if the feature to be placed on the main system but is not implemented as a Foreman plugin.
+* `foreman_proxy` (_Hash_): How this feature should be applied to a secondary system that is connected using the Proxy API to the main system.
+  * `plugin_name` (_String_): The name of the Foreman Proxy plugin to be enabled (by deploying `roles/foreman_proxy/templates/settings.d/{{ foreman_proxy_plugin }}.yml.j2` to `/etc/foreman-proxy/settings.d`).
+    If `roles/foreman/tasks/feature/{{ foreman_proxy_plugin }}.yaml` exists, it will be executed to perform any plugin-specific setup.
+  * `role` (_String_): The name of the Ansible role to be executed if the feature is not implemented as a Foreman Proxy plugin.
+* `hammer` (_String_): The name of the Hammer plugin to be enabled.
+  * **FIXME**: Not implemented, right now we use the same list as Foreman plugins, but needs modification for foreman-tasks and friends
+* `dependencies` (_Array_ of _String_): List of features that are automatically enabled when the user requests this feature. Usually will point at features with `internal: true`.
+
+Properties can be omitted.
+
+## Examples
+
+### REX + Dynflow
+
+```yaml
+dynflow:
+  internal: true
+  foreman_proxy: dynflow
+
+remote_execution:
+  description: Foreman Remote Execution support
+  foreman: foreman_remote_execution
+  foreman_proxy: remote_execution_ssh
+  dependencies:
+    - dynflow
+```
+
+The `remote_execution` feature will enable the `foreman_remote_execution` plugin for Foreman and the `remote_execution_ssh` and `dynflow` plugins for Foreman Proxy.
+The `dynflow` feature is hidden from the user and only present so that `remote_execution` can pull it in.
+
+### RH Cloud + Katello
+```yaml
+katello:
+  description: Katello
+  foreman: katello
+
+rh_cloud:
+  description: Foreman RH Cloud
+  foreman: foreman_rh_cloud
+  dependencies:
+    - katello
+```
+
+The `rh_cloud` feature can't be enabled unless the `katello` feature is also present in the deployment.
+
+### Katello + tasks
+```yaml
+foreman-tasks:
+  foreman: foreman-tasks
+  hammer: foreman_tasks
+
+katello:
+  description: Katello
+  foreman: katello
+  dependencies:
+    - foreman-tasks
+```
+
+The `foreman-tasks` feature is automatically enabled when the user requests Katello, thus also gaining the Hammer integration for tasks which would be missing if we'd only let the Ruby gem dependency pull in `foreman-tasks`.
+
+### another example
+
+```yaml
+dynflow:
+  internal: true
+  foreman_proxy:
+    plugin_name: dynflow
+
+foreman-tasks:
+  foreman:
+    plugin_name: foreman-tasks
+  hammer: foreman_tasks
+  dependencies:
+    - dynflow
+
+katello:
+  description: Katello
+  foreman:
+    plugin_name: katello
+  dependencies:
+    - foreman-tasks
+
+rh_cloud:
+  description: Foreman RH Cloud
+  foreman:
+    plugin_name: foreman_rh_cloud
+  dependencies:
+    - katello
+
+remote_execution:
+  description: REX
+  foreman:
+    plugin_name: foreman_remote_execution
+  foreman_proxy:
+    plugin_name: smart_proxy_remote_execution_ssh
+  dependencies:
+    - dynflow
+
+openscap:
+  description: OpenSCAP
+  foreman:
+    plugin_name: foreman_openscap
+  foreman_proxy:
+    plugin_name: smart_proxy_openscap
+
+iop:
+  description: IoP
+  foreman:
+    role: iop_core
+
+container_gateway:
+  description: gw is a proxy-only plugin
+  foreman_proxy:
+    plugin_name: smart_proxy_container_gateway
+```