Which resource metaparameter should be used to trigger the restart of a service after a configuration file change?

The metaparameter that effectively triggers the restart of a service after a configuration file change is the notify metaparameter. When a resource includes notify, it allows you to specify that if the resource is updated, it should notify another resource to take some action, such as restarting a service.

For example, if you have a file resource representing a configuration file and a service resource representing the service that relies on that configuration file, applying notify on the service within the configuration file resource means that whenever the configuration file changes (and the file resource is updated), Puppet will automatically trigger a refresh of the service. This ensures that the service always runs with the latest configuration.

While the other metaparameters, such as subscribe and refresh, also relate to resource dependencies and triggering actions, they serve slightly different purposes. The subscribe metaparameter triggers an action when the specified resource changes, essentially the opposite of notify. Refresh, on the other hand, causes the resource to be refreshed when the specified resource itself changes. The before metaparameter is used to indicate that a resource should be applied before another specified resource but does not inherently manage service restarts in response to changes.

In summary, using notify ensures that services are correctly restarted in