add managing of services using Hiera

rhel8/software.md

@@ -44,6 +44,7 @@ base::pkg_group::java:
 
 ### Install Latest Package Version
 
+Puppet by default only checks if a package is installed and only installs it if missing. 
 To ensure that always the latest available package version is installed, append the `:latest` tag to the package name in the package group:
 
 ```
@@ -96,6 +97,87 @@ Then `yum_client::package_groups` is the Hiera list to contain the package group
 
 Please migrate them to the equivalent `base::*` list. You can have both of them available at the same time, with possibly the same content.
 
+## Managing Services with Systemd
+
+Hiera can also be used to manage services and to automate reoccuring tasks with timers.
+
+### Enabling/Starting a Service 
+
+If the software already comes with an systemd unit file, then it is sufficient to just enable it in Hiera by using the `base::services` key:
+
+```
+base::services:
+    netdata:
+        enable: true
+```
+The key inside is the `systemd` service name without the `.service` suffix. 
+
+### Disabling/Stopping a Service
+
+To stop and disable an already running service, disable it in the `base::services` Hiera key with `enable: false`:
+```
+base::services:
+    netdata:
+        enable: false
+```
+
+### Systemd Timers
+To have custom executables run regulary on given time/interval, you may use the `base::timers` Hiera key:
+
+```
+base::timers:
+    'timer_test':
+        description: 'test timers'
+        command: '/usr/bin/logger foo'
+        on_calendar: '*:*:10'
+        persistence:  false
+```
+
+For each time following keys are mandatory
+
+- `description` for a short explaination what it is about
+- `command` for the command to run
+- `on_calendar`defining when it should run using the [`systemd` calendar event format](https://www.freedesktop.org/software/systemd/man/systemd.time.html#Calendar%20Events)
+
+Optional is
+- `persistence` which signals if the timer should run immediately after boot when the node was switched of on the last suggested run time (default is `false`)
+
+### Manage Services with Custom Unit Files
+
+It is also possible to provide a full systemd unit file if there is none already. For this define the different secions and their content with subkeys below the `options` key as in below example:
+
+```
+# The following service stops users from accessing the node
+# before the home directory is mounted
+base::services:
+    'wait_for_home':
+        enable: true
+        options:
+            Unit:
+                Before: 'systemd-user-sessions.service'
+            Install:
+                WantedBy: 'multi-user.target'
+                RequiredBy: 'multi-user.target'
+            Service:
+                Type: 'oneshot'
+                ExecStart: '/opt/pli/libexec/waitformount -m /das/home'
+                RemainAfterExit: 'true'
+```
+
+### Enhance a Service with a Dropin Unit File
+It is possible to fine-tune already existing `systemd` unit files with dropins. These are placed as `.conf` files in `/etc/systemd/system/service_name.service.d`.
+
+With the `dropin: true` parameter the content of the `options` parameter is now written into the according dropin directory:
+```
+base::services:
+    'name_of_enhanced_service':
+        enable: true
+        dropin: true
+        options:
+           ...
+```
+If there are multiple dropins, you might also name them individually with the `dropin_name` parameter.
+
 ## Management of Package Repositories
 
 ### Package Repository Lists
@@ -222,7 +304,7 @@ A small list of packages managed by the Linux Team.
 - lightdm-gtk v2.0.8-3.pli, a patched lightdm-gtk-greeter ([SRPM](https://git.psi.ch/linux-infra/lightdm-gtk-rpm), [PR](https://github.com/Xubuntu/lightdm-gtk-greeter/pull/121)) which allows to limit the presented keyboard layouts
 - Code Beamer Office pluging v9.5.0 managed by Gilles Martin
 - storecli 007.2007.0000.0000 managed by Marc Caubet Serrabou
-- pam_single_kcm_cache PAM Module managed by Konrad Bucheli
+- [pam_single_kcm_cache PAM Module](https://github.com/paulscherrerinstitute/pam_single_kcm_cache) managed by Konrad Bucheli
 - [nvidia-detect](http://elrepo.org/tiki/nvidia-detect) copied over from ElRepo to make it generally available
 
 ### Package Repositories made Available by other PSI Groups