Tyler Fitch

One of my customers had a question about using name/value pairs from his cookbook’s attributes as settings for a Resource.

TL;DR

The name/value pairs in a cookbook Resource object are more than just a combination of attribute + value. More precisely, they’re a method_name + value combination. So making the method_name come from a dynamic source requires you use send() method of a Ruby Object to invoke the method, not just output a String equal to what the cookbook would look like if everything was hard coded.

Long form:

Let’s start with the hardcoded cookbook recipe and see what that would look like.

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# if nothing was dynamic
iis_pool 'MyAppPool' do
  runtime_version '12'
  max_proc 4
  thirty_two_bit false
  action :config
end

Now to convert all those Resource attributes to be dynamic let’s convert them in to Attributes of the cookbook

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# attributes for the iis_pool
default['config']['setting']['runtime_version'] = '12'
default['config']['setting']['thirty_two_bit'] = false
default['config']['setting']['max_proc'] = 4

A first attempt at setting the attributes looked liked this, but it did not work

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# outputs the same as hardcoded recipe but does *not* work
iis_pool 'MyAppPool' do
  node['config']['setting'].each do |setting, value|
    "#{setting}" "#{value}"
  end
  action :config
end 

At first pass this all looked correct to me too. I was stumped until my brilliant co-worker Steve Danna (@SteveDanna) shared some great Ruby/Chef knowledge with me. The correct way to achieve the goal would be this

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# same result as hardcoded recipe but now driven by Attributes
iis_pool 'MyAppPool' do
  node['config']['setting'].each do |setting, value|
    send(setting, value)
  end
  action :config
end 

The key here is the send() method. http://ruby-doc.org/core-2.1.2/Object.html#method-i-send

When you do #{setting} #{value} then setting is just returned as a string and not calling the relevant setter method of the Resource object (iis_pool in this case).

I guess this surprised me, because when the cookbook is hardcoded you see the line thirty_two_bit false and it reads like two strings and not that thirty_two_bit is actually a method name being called.

Alas, now I know and you do too.