Easy Resource Load And Converge

With the introduction of action on resources, it becomes useful to have a blessed way to get the actual value of the resource. This proposal adds load_current_value and converge_if_changed to help with this purpose, enabling:

Motivation

As a Chef resource writer,
I want to be able to read the current value of my resource at converge time,
so that it is easy to tell the difference between current and desired value.

As a Chef resource writer,
I want a converge model that compares current and desired values for me,
So that the easiest converge to write is the most correct one.

Specification

load_current_value: in-place resource load

When using action, one needs a way to load the actual system value of the resource, so that it can be compared to the desired value and a decision made as to whether to change anything.

When the resource writer defines load_current_value on the resource class, it can be called to load the real system value into the resource. Before any action runs, this will be used by load_current_resource to load the resource. action will do some important work before calling the new method:

  1. Create a new instance of the resource with the same name.
  2. Copy all non-desired-state values from the desired resource into the new instance.
  3. Call load_current_value on the new instance.
class File < Chef::Resource
  property :path, name_attribute: true
  property :mode, default: 0666
  property :content

  load_current_value do
    current_value_does_not_exist! unless File.exist?(path)
    mode File.stat(path).mode
    content IO.read(path)
  end

  action :create do
    converge_if_changed do
      File.chmod(mode, path)
      IO.write(path, content)
    end
  end
end

file '/x.txt' do
  # Before the change, the above code would have modified `mode` to be `0666`.
  # After, it leaves `mode` alone.
  content 'Hello World'
end

Non-existence

To appropriately handle actual value loading, the user needs a way to specify that the actual value legitimately does not exist (rather than simply not filling in the object and getting nils in it). If load_current_value raises Chef::Exceptions::ActualValueDoesNotExist, the new resource will be discarded and current_resource becomes nil. The current_value_does_not_exist! method can be called to raise this.

NOTE: The alternative was to have users return false if the resource does not exist; but I didn't want users to be forced into the ceremony of a trailing true line.

  load_current_value do
    # Check for existence before doing anything else.
    current_value_does_not_exist! if !File.exist?(path)

    # Set "mode" on the resource.
    mode File.stat(path).mode
  end

The block will also be passed the original (desired) resource as a parameter, in case it is needed.

Inheritance

super in load_current_value! will call the superclass's load_current_value! method.

Handling Multi-Key Resources

The new resource is created with all properties copied over except desired state properties (properties in ResourceClass.state_properties). This means name, and properties with identity: true or desired_state: false are copied over. Normal property and attribute are not.

class DataBagItem < Chef::Resource
  # Copied
  attribute :item_name, name_attribute: true
  attribute :data_bag_name, identity: true
  attribute :recursively_delete, desired_state: false
  # Not copied:
  attribute :data
  def load_current_value!
    data Chef::DataBagItem.new(data_bag_name, item_name).data
  end
end

converge_if_changed: automatic test-and-set

The new converge_if_changed do ... end syntax is added to actions, which enables a lot of help for resource writers to make safe, effective resources. It performs several key tasks common to nearly every resource (which are often not done correctly):

class File < Chef::Resource
  property :path, name_attribute: true
  property :content

  load_current_value do
    current_value_does_not_exist! unless File.exist?(path)
    content IO.read(path)
  end

  action :create do
    converge_if_changed do
      IO.write(path, content)
    end
  end
end

Side-by-side: new and old

Here is a sample converge_if_changed statement from a hypothetical FooBarBaz resource with properties foo, bar and baz:

converge_if_changed do
  if current_resource
    FooBarBaz.update(new_resource.id, new_resource.foo, new_resource.bar, new_resource.baz)
  else
    FooBarBaz.create(new_resource.id, new_resource.foo, new_resource.bar, new_resource.baz)
  end
end

This is what you would have to write to do the equivalent:

if current_resource
  # We're updating; look for properties that the user wants to change (do the "test" part of test-and-set)
  differences = []
  if (new_resource.property_is_set?(:foo) && new_resource.foo != current_resource.foo)
    differences << "foo = #{new_resource.foo}"
  end
  if (new_resource.property_is_set?(:bar) && new_resource.bar != current_resource.bar)
    differences << "bar = #{new_resource.bar}"
  end
  if (new_resource.property_is_set?(:baz) && new_resource.baz != current_resource.baz)
    differences << "baz = #{new_resource.baz}"
  end

  if !differences.empty?
    converge_by "updating FooBarBaz #{new_resource.id}, setting #{differences.join(", ")}" do
      FooBarBaz.create(new_resource.id, new_resource.foo, new_resource.bar, new_resource.baz)
    end
  end

else
  # If the current resource doesn't exist, we're definitely creating it
  converge_by "creating FooBarBaz #{new_resource.id} with foo = #{new_resource.foo}, bar = #{new_resource.bar}, baz = #{new_resource.baz}" do
    FooBarBaz.update(new_resource.id, new_resource.foo, new_resource.bar, new_resource.baz)
  end
end

Desired value = actual value

The easiest way to write a resource must be the most correct one.

There is a subtle pitfall when updating a resource, where the user has set some values, but not all. One can easily end up writing a resource which will overwrite perfectly good system properties with their defaults, which can cause instability. If the user does not specify a property, it is generally preferable to preserve its existing value rather than overwrite it.

To prevent this, referencing the bare property in an action will now yield the actual value if loadcurrentvalue succeeded, and the default value if we are creating a new resource (if load_current_value raised ActualValueDoesNotExist).

class File < Chef::Resource
  property :path, name_attribute: true
  property :mode, default: 0666
  property :content

  load_current_value do
    current_value_does_not_exist! unless File.exist?(path)
    mode File.stat(path).mode
    content IO.read(path)
  end

  action :create do
    converge_if_changed do
      File.chmod(mode, path)
      IO.write(path, content)
    end
  end
end

file '/x.txt' do
  # Before the change, the above code would have modified `mode` to be `0666`.
  # After, it leaves `mode` alone.
  content 'Hello World'
end

There will be times when the old behavior of overwriting with defaults is desired. The resource writer can still find out whether mode was set with property_is_set?(:mode), and can still access the default value with new_resource.mode if it is not set.

There are no backwards-compatibility issues with this because it only applies to action, which has not been released yet.

Compound Resource Convergence

Some resources perform several different (possibly expensive) operations depending on what is set. converge_if_changed :attribute1, :attribute2, ... do allows the user to target different groups of changes based on exactly which attributes have changed:

class File < Chef::Resource
  property :path, name_attribute: true
  property :mode
  property :content

  load_current_value do
    current_value_does_not_exist! unless File.exist?(path)
    mode File.stat(path).mode
    content IO.read(path)
  end

  action :create do
    converge_if_changed :mode do
      File.chmod(mode, path)
    end
    converge_if_changed :content do
      IO.write(path, content)
    end
  end
end

Copyright

This work is in the public domain. In jurisdictions that do not allow for this, this work is available under CC0. To the extent possible under law, the person who associated CC0 with this work has waived all copyright and related or neighboring rights to this work.