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+++ title = "Accumulators" gh_repo = "chef-web-docs"

[menu] [menu.infra] title = "Accumulators" identifier = "chef_infra/resources/custom_resources/accumulators" parent = "chef_infra/resources/custom_resources" weight = 20 +++

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

  • The built-in Chef Infra [template]({{< relref "resources/template" >}}) resource
  • Cookbook execution [with_run_context]({{< relref "infra_language/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.

{{< foundation_tabs tabs-id="tabs-panel-container" >}} {{< foundation_tab active="true" panel-link="id-1" tab-text="Share Resource 1">}} {{< foundation_tab panel-link="id-2" tab-text="Share Resource 2" >}} {{< /foundation_tabs >}}

{{< foundation_tabs_panels tabs-id="tabs-panel-container" >}} {{< foundation_tabs_panel active="true" panel-id="id-1" >}}

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

{{< /foundation_tabs_panel >}} {{< foundation_tabs_panel panel-id="id-2" >}}

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

{{< /foundation_tabs_panel >}} {{< /foundation_tabs_panels >}}

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 doesn't 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