Poise-Derived Cookbook

A Chef cookbook for defining lazily evaluated node
attributes.

Quick Start

To create a lazily evaluated node attribute:

# attributes/default.rb
default['mycookbook']['version'] = '1.0'
default['mycookbook']['url'] = lazy 'https://example.com/myapp-%{mycookbook.version}.zip'

# recipes/default.rb
node.override['mycookbook']['version'] = '2.0'

poise_archive '/srv/myapp' do
  source node['mycookbook']['url']
end

This attribute can now be used anywhere a normal string can be used and will
lazily evaluate to the derived value.

Requirements

• Chef >= 12.3

Block Attributes

When simple format strings are not enough, you can use the block form of lazy
attributes:

# attributes/default.rb
default['mycookbook']['memory_percent'] = 50
default['mycookbook']['command_options'] = lazy {
  "-m #{node['memory']['total'].to_i * node['mycookbook']['memory_percent'] / 100.0}"
}

If the block returns something other than a string, it will be run through to_s.

Template Overrides

If an attribute's default value is lazy'd, any overrides to it will be treated
as the string form of a lazy attribute. This allows overriding the template in
a role, environment, or policy:

# attributes/default.rb
default['mycookbook']['version'] = '1.0'
default['mycookbook']['url'] = lazy 'https://example.com/myapp-%{mycookbook.version}.zip'

# recipes/default.rb
node.override['mycookbook']['url'] = 'https://myapp.com/%{mycookbook.version}.tgz'

poise_archive '/srv/myapp' do
  source node['mycookbook']['url']
end

Why Do I Need This?

Cookbook attribute files are, at heart, plain Ruby code. This has led many
cookbook authors to use the naive approach to having the value of one attribute
be used as part of the value of another:

# attributes/default.rb
default['mycookbook']['version'] = '1.0'
default['mycookbook']['url'] = lazy "https://example.com/myapp-#{node['mycookbook']['version']}.zip"

The problem with this is that if a wrapper cookbook wants to change the value
of node['mycookbook']['version'], by the time it sets the new version the URL
has already been baked so further changes to the version won't affect it. One
solution here is to never use one attribute in another, but this is often
ungainly. poise-derived provides a middle ground where the interpolation can
be expressed cleanly and with minimal impact to the recipe code that uses it.

Sponsors

Development sponsored by Bloomberg.

The Poise test server infrastructure is sponsored by Rackspace.

License

Copyright 2016, Noah Kantrowitz

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.