Accumulators

This is an advanced topic. You should have already written a Custom Resource and be familiar with:

• The built-in Chef Infra template resource
• Cookbook execution with_run_context

Overview

An accumulator is a programming pattern that gathers multiple values together.

In the context of Custom Resources, the accumulator pattern collects a set of properties from Custom Resources, and then applies the collection to a resource.

The steps for setting up an accumulator pattern are:

• Declare a resource
• Edit the resource
• Add both resources to the :root run_context
• Delay the actions on the resources

Example Accumulator: Samba

Samba Linux tool systems for supporting sharing resources in a network with Windows systems. Samba uses a single file for configuration. To share more than one directory using this configuration file, you need to write multiple samba_share resources.

The accumulator pattern lets you split out each configuration section into its own resource.

• Share Resource 1
• Share Resource 2

samba_share 'first_share' do
  comment 'exported share 1'
  path '/srv/export'
  guest_ok true
  printable false
  write_list ['test_user_1']
  create_mask '0644'
  directory_mask '0775'
  options 'inherit permissions' => 'yes'
end

samba_share 'second_share' do
  comment 'exported share 2'
  path '/srv/export_2'
  guest_ok false
  printable false
  write_list ['test_user_2']
  create_mask '0644'
  directory_mask '0775'
  create_directory false
end

Example

The following example shows how to:

• Setup the template resource
• Set the action to :nothing
• Set the delayed_action to :create.
• Add the template resource to the root run context, which allows the samba_share resource to find it.
• Use edit_resource to find the template
• Use edit_resource to add new variables to the existing collection

Note: This uses the ||= Ruby method to add a new Hash if one does not exist already.

# action for the samba_server custom resource
action :create do
  package 'samba'

  # We need to force both the server template and the
  # share templates into the root context to find each other
  with_run_context :root do
    template new_resource.config_file do
      source 'smb.conf.erb'
      owner 'root'
      group 'root'
      mode '0644'
      cookbook 'samba'
      variables(samba_options: new_resource.options)
      action :nothing
      delayed_action :create
    end
  end
end


# action for the samba_share custom resource
action :add do
  with_run_context :root do
    edit_resource(:template, new_resource.config_file) do |new_resource|
      variables[:shares] ||= {}
      variables[:shares][new_resource.share_name] ||= {}
      variables[:shares][new_resource.share_name]['comment'] = new_resource.comment
      variables[:shares][new_resource.share_name]['path'] = new_resource.path
    end
  end
end

Further Reading

• here
• DNSimple
• HAProxy