run puppet resource types only when file has changed

Question

I feel like this is a very common scenario but yet I haven't found a solution that meets my needs. I want to create something that works like below. (Unfortunately set_variable_only_if_file_was_changed doesn't exist)

file { 'file1':
      path => "fo/file1",
      ensure => present,
      source => "puppet:///modules/bar/file1",
      set_variable_only_if_file_was_changed  => $file_change = true
}

if $file_change{
    file{'new file1'
        ...
    }
    exec{'do something1'
        ...
    }
    package{'my package'
        ...
    }
} 

I have seen how exec can be suppressed using the refreshonly => true but with files it seems harder. I would like to avoid putting code into a .sh file and execute it through a exec.

Answer 1

Puppet does not have the feature you describe. Resources that respond to refresh events do so in a manner appropriate to their type; there is no mechanism to define custom refresh behavior for existing resources. There is certainly no way to direct which resources will be included in a node's catalog based on the results of applying other resources -- as your pseudocode seems to suggest -- because the catalog is completely built before anything in it is applied.

Furthermore, that is not nearly as constraining as you might suppose if you're coming to Puppet with a substantial background in scripting, as many people do. People who think in terms of scripting need to make a mental adjustment to Puppet, because Puppet's DSL is definitely not a scripting language. Rather, it is a language for describing the construction of a machine state description (a catalog). It is important to grasp that such catalogs describe details of the machine's target state, not the steps required to take it from its current state to some other state. (Even Exec s, a possible exception, are most effective when used in that way.) It is very rare to be unable to determine a machine's intended state from its identity and initial state. It is uncommon even for it to be difficult to do so.

How you might achieve the specific configuration you want depends on the details. Often, the answer lies in user-defined facts. For example, instead of capturing whether a given file was changed during catalog application, you provide a fact that you can interrogate during catalog building to evaluate whether that file will be changed, and adjust the catalog details appropriately. Sometimes you don't need such tests in the first place, because Puppet takes action on a given resource only when changes to it are needed.