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Duck Soup Chef

An evolving simplest possible explanation of how Chef works for us.

Explain Like I'm 5

We define machine states as Roles i.e: 'redis-server', 'ow-media-capture'. Roles are JSON files in /roles composed of Attributes and Recipes. Attributes are the arguments of Recipes i.e: a recipe installs nginx and sets it to listen on a port specified by an attribute.

Recipes belong to Cookbooks which are git submodules of our repository.

Tell Me More

Roles

We specify roles for our server with json files in the ./roles directory of our chef repo.

A role consists only of run lists and attributes:

  • Run lists are arrays of recipes. Recipes are defined in cookbooks which reside in the ./cookbooks directory (our home-brewed cookbooks are in ./site-cookbooks). We generally pull cookbooks from the community, and define our run lists with a selection of their recipes.

      # Example run_list for our node role
  # Installs nodejs and npm
  ...
  "run_list": [
  	"recipe[nodejs]",
  	"recipe[nodejs::npm]"
  ]

    recipe[nodejs] directs Chef to /cookbooks/nodejs/recipes/default.rb

  • Attributes are key-value data that describe the system state. Recipes supply default attributes, which can be overridden by role or node override attributes. Finally, automatic attributes represent the state of the target machine during a chef run.

    In order of increasing precedence:

    (lowest) default -> normal -> override -> automatic (highest)

    In our node role, we override the version of node to install (The cookbook's defaults are available in /cookbooks/name/attributes/default.rb):

      # Example override attributes for our node role
  ...
  "override_attributes": {
      "nodejs": {
        "version": "0.8.20",
        "npm": "1.2.11"
      }
    }

Example Node.js Role

	# /roles/nodejs.json
	# Example role which specifies installing node and npm
	{
	  "name": "nodejs",
	  "description": "Node and npm",
	  "json_class": "Chef::Role",
	  "default_attributes": {
	  },
	  "override_attributes": {
	    "nodejs": {
	      "version": "0.8.20",
	      "npm": "1.2.11"
	    }
	  },
	  "chef_type": "role",
	  "run_list": [
	    "recipe[nodejs]",
	    "recipe[nodejs::npm]"
	  ]
	}

Resources and Providers

Resources describe a fundamental piece of work i.e: "install package X". Providers execute resources based on the target system state i.e: "apt-get install X". Resources are a key part of a recipe.

Templates

Templates are stored in a cookbook's /templates/default/. Note you can have a subdir within /templates/ for each system type if that affects the template. In example, a template directory might look like:

	templates/
		host-foo.example.com
		ubuntu-8.04
		ubuntu
		default

Templates are written in Erubis, a ruby-like templating language. Let's say we have an example template in /templates/default/config.yaml.erb

	Process:
	  first: <%= @first_var %>
	  …
	  last: <%= @last_var %>

In a recipe, you would render this template as config.yaml with the template resource:

	template "/destination/of/template/config.yaml" do
	  source 'config.yaml.erb'  # filename within /templates/*/
	  variables({
	  :first_var => a_variable,
	  …
	  :last_var => another_variable
	  })
	end