Class Chef::Mixin::WhyRun::ResourceRequirements
In: lib/chef/mixin/why_run.rb
Parent: Object

ResourceRequirements

ResourceRequirements provides a framework for making assertions about the host system‘s state. It also provides a mechanism for making assumptions about what the system‘s state might have been when running in why run mode.

For example, consider a recipe that consists of a package resource and a service resource. If the service‘s init script is installed by the package, and Chef is running in why run mode, then the service resource would fail when attempting to run `/etc/init.d/software-name status`. In order to provide a more useful approximation of what would happen in a real chef run, we want to instead assume that the service was created but isn‘t running. The logic would look like this:

    # Hypothetical service provider demonstrating why run assumption logic.
    # This isn't the actual API, it just shows the logic.
    class HypotheticalServiceProvider < Chef::Provider

      def load_current_resource
        # Make sure we have the init script available:
        if ::File.exist?("/etc/init.d/some-service"
          # If the init script exists, proceed as normal:
          status_cmd = shell_out("/etc/init.d/some-service status")
          if status_cmd.success?
            @current_resource.status(:running)
          else
            @current_resource.status(:stopped)
          end
        else
          if whyrun_mode?
            # If the init script is not available, and we're in why run mode,
            # assume that some previous action would've created it:
            log("warning: init script '/etc/init.d/some-service' is not available")
            log("warning: assuming that the init script would have been created, assuming the state of 'some-service' is 'stopped'")
            @current_resource.status(:stopped)
          else
            raise "expected init script /etc/init.d/some-service doesn't exist"
          end
        end
      end

    end

In short, the code above does the following:

  • runs a test to determine if a requirement is met: `::File.exist?("/etc/init.d/some-service"`
  • raises an error if the requirement is not met, and we‘re not in why run mode.
  • if we are in why run mode, print a message explaining the situation, and run some code that makes an assumption about what the state of the system would be. In this case, we also skip the normal `load_current_resource` logic
  • when the requirement is met, we run the normal `load_current_resource` logic

ResourceRequirements encapsulates the above logic in a more declarative API.

Examples

Assertions and assumptions should be created through the WhyRun#assert method, which gets mixed in to providers. See that method‘s documentation for examples.

Methods

action_blocked?   assert   events   new   run  

Classes and Modules

Class Chef::Mixin::WhyRun::ResourceRequirements::Assertion

Public Class methods

Public Instance methods

Check to see if a given action is blocked by a failed assertion

Takes the action name to be verified.

Define a new Assertion.

Takes a list of action names for which the assertion should be made.

Examples:

A File provider that requires the parent directory to exist:

  assert(:create, :create_if_missing) do |a|
    parent_dir = File.basename(@new_resource.path)
    a.assertion { ::File.directory?(parent_dir) }
    a.failure_message(Exceptions::ParentDirectoryDoesNotExist,
                      "Can't create file #{@new_resource.path}: parent directory #{parent_dir} doesn't exist")
    a.why_run("assuming parent directory #{parent_dir} would have been previously created"
  end

A service provider that requires the init script to exist:

  assert(:start, :restart) do |a|
    a.assertion { ::File.exist?(@new_resource.init_script) }
    a.failure_message(Exceptions::MissingInitScript,
                      "Can't check status of #{@new_resource}: init script #{@new_resource.init_script} is missing")
    a.why_run("Assuming init script would have been created and service is stopped") do
      @current_resource.status(:stopped)
    end
  end

A File provider that will error out if you don‘t have permissions do delete the file, *even in why run mode*:

  assert(:delete) do |a|
    a.assertion { ::File.writable?(@new_resource.path) }
    a.failure_message(Exceptions::InsufficientPrivileges,
                      "You don't have sufficient privileges to delete #{@new_resource.path}")
  end

A Template provider that will prevent action execution but continue the run in whyrun mode if the template source is not available.

  assert(:create, :create_if_missing) do |a|
    a.assertion { File::exist?(@new_resource.source) }
    a.failure_message Chef::Exceptions::TemplateError, "Template #{@new_resource.source} could not be found exist."
    a.whyrun "Template source #{@new_resource.source} does not exist. Assuming it would have been created."
    a.block_action!
  end

  assert(:delete) do |a|
    a.assertion { ::File.writable?(@new_resource.path) }
    a.failure_message(Exceptions::InsufficientPrivileges,
                      "You don't have sufficient privileges to delete #{@new_resource.path}")
  end

Run the assertion and assumption logic.

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