Attribute Arrays

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Attributes are typically defined in cookbooks, recipes, roles, and environments. These attributes are rolled-up to the node level during a Chef Infra Client run. A recipe can store attribute values using a multi-level hash or array.

For example, a group of attributes for web servers might be:

override_attributes(
  :apache => {
    :listen_ports => [ 80 ],
    :prefork => {
      :startservers => 20,
      :minspareservers => 20,
      :maxspareservers => 40
    }
  }
)

But what if all of the web servers are not the same? What if some of the web servers required a single attribute to have a different value? You could store these settings in two locations, once just like the preceding example and once just like the following:

override_attributes(
  :apache => {
    :listen_ports => [ 80 ],
    :prefork => {
      :startservers => 30,
      :minspareservers => 20,
      :maxspareservers => 40
    }
  }
)

But that is not efficient, especially because most of them are identical. The deep merge capabilities of Chef Infra Client allows attributes to be layered across cookbooks, recipes, roles, and environments. This allows an attribute to be reused across nodes, making use of default attributes set at the cookbook level, but also providing a way for certain attributes (with a higher attribute precedence) to be applied only when they are supposed to be.

For example, a role named baseline.rb:

name "baseline"
description "The most basic role for all configurations"
run_list "recipe[baseline]"

override_attributes(
  :apache => {
    :listen_ports => [ 80 ],
    :prefork => {
      :startservers => 20,
      :minspareservers => 20,
      :maxspareservers => 40
    }
  }
)

and then a role named web.rb:

name 'web'
description 'Web server config'
run_list 'role[baseline]'

override_attributes(
  :apache => {
    :prefork => {
      :startservers => 30
    }
  }
)

Both of these files are similar because they share the same structure. When an attribute value is a hash, that data is merged. When an attribute value is an array, if the attribute precedence levels are the same, then that data is merged. If the attribute value precedence levels in an array are different, then that data is replaced. For all other value types (such as strings, integers, etc.), that data is replaced.

For example, the web.rb references the baseline.rb role. The web.rb file only provides a value for one attribute: :startservers. When Chef Infra Client compares these attributes, the deep merge feature will ensure that :startservers (and its value of 30) will be applied to any node for which the web.rb attribute structure should be applied.

This approach will allow a recipe like this:

include_recipe 'apache2'
Chef::Log.info(node['apache']['prefork'].to_hash)

and a run_list like this:

run_list/web.json
{
  "run_list": [ "role[web]" ]
}

to produce results like this:

[Tue, 16 Aug 2011 14:44:26 -0700] INFO:
         {
           "startservers"=>30,
           "minspareservers"=>20,
           "maxspareservers"=>40,
           "serverlimit"=>400,
           "maxclients"=>400,
           "maxrequestsperchild"=>10000
         }

Even though the web.rb file does not contain attributes and values for minspareservers, maxspareservers, serverlimit, maxclients, and maxrequestsperchild, the deep merge capabilities pulled them in.

Attribute Array Logic

The following sections show how the logic works for using deep merge to perform substitutions and additions of attributes.

Substitution

The following examples show how the logic works for substituting an existing string using a hash:

role_or_environment 1 { :x => '1', :y => '2' }
+
role_or_environment 2 { :y => '3' }
=
{ :x => '1', :y => '3' }

For substituting an existing boolean using a hash:

role_or_environment 1 { :x => true, :y => false }
+
role_or_environment 2 { :y => true }
=
{ :x => true, :y => true }

For substituting an array with a hash:

role_or_environment 1 [ '1', '2', '3' ]
+
role_or_environment 2 { :x => '1' , :y => '2' }
=
{ :x => '1', :y => '2' }

When items cannot be merged through substitution, the original data is overwritten.

Addition

The following examples show how the logic works for adding a string using a hash:

role_or_environment 1 { :x => '1', :y => '2' }
+
role_or_environment 2 { :z => '3' }
=
{ :x => '1', :y => '2', :z => '3' }

For adding a string using an array:

role_or_environment 1 [ '1', '2' ]
+
role_or_environment 2 [ '3' ]
=
[ '1', '2', '3' ]

For adding a string using a multi-level hash:

role_or_environment 1 { :x => { :y => '2' } }
+
role_or_environment 2 { :x => { :z => '3' } }
=
{ :x => { :y => '2', :z => '3' } }

For adding a string using a multi-level array:

role_or_environment 1 [ [ 1, 2 ] ]
+
role_or_environment 2 [ [ 3 ] ]
=
[ [ 1, 2 ], [ 3 ] ]