This chapter discusses lightweight resource providers (LWRPs). They are a way to enhance the functionality of chef to provide new integrations which are not provided for out-of-box in chef.

Lightweight Resource Providers

A resource is something that defines the action that needs to be taken and a provider is something that executes that action.

To implement the functionality of a resource and provider in a recipe which is not an inbuilt resource provider, you can create your own LWRPs. You can use custom Ruby codes and inbuilt chef resources to create an LWRP.

LWRPs help in achieving the core objective of chef which is idempotence. One can achieve this state using scripts or recipes by leveraging “if”-type constructs to choose when to run the script; however, this process becomes fairly complex to manage.

LWRPs help us in achieving idempotence for complex and scalable infrastructure to provide reliable configuration management.

LWRPs are loaded from files that are saved in the following cookbook subdirectories:

The name of the cookbook and the name of the files in the resources/ and providers/ subdirectories determine the naming patterns of LWRPs.

For example, if a cookbook named example was downloaded to the chef repository, it would be located under /cookbooks/example/. If that cookbook contained two resources and two providers, the following files would be part of the resources/ directory:

And the following files would be part of the providers/ directory:

Chef-Maintained LWRPs

Chef maintains a number of LWRPs. We will discuss some of the important LWRPs maintained by the Opscode community. These are available in cookbooks. If we need to use them, we need to download the cookbook from the community, upload the cookbook to our chef server, and then use them in our custom recipes.

Creating an LWRP

This section will demonstrate how to create an LWRP. We will create an LWRP to download Wordpress setup, extract it to the desired location, and then delete the downloaded file.

1. The first step is to create a cookbook in which we will create the resource and provider. Create a cookbook named wp_setup using the knife.  We execute the following command from a workstation:

   knife cookbook create wp_setup

2. The command creates the files and directories that are required for developing a cookbook (see Figure 10-1). The cookbook will be created in the local chef repository.

   

   Figure 10-1. Directory structure

3. The resource directory is where we create our resource. Create a default.rb file in the resource directory. In the default.rb, write the following code:

   actions :extract, :remove
   attribute :wp_url, :kind_of=> String, default: "https://wordpress.org/latest.tar.gz"
   attribute :wp_path, :kind_of=> String, default: "/var/www"

   In this code sample, we are defining the following:

   • Actions: this defines the actions that need to be taken in this particular resource. In this case, we are defining two actions: extract and remove.
   • Attribute: here we define the attributes that will be passed to the resource. In this example, we are passing an attribute called wp_url, which is kind_of string and is a default parameter. The value of this parameter is the download location of the Wordpress web site tar file.

   A default attribute is specified in the resource file so that if an attribute is not specified in the recipe or attribute file, chef client will take the default value from the resource file.

4. The next step after defining the resource is to map the actions that we have defined to actual execution code which is defined in the provider.

   When the chef client identifies a custom resource, it will look for the related actions method in the provider.

   Create a default.rb file under the folder named providers of wp_setup cookbook.

   In this file, we define the actions to be performed.

   Create methods for the custom actions specified in the resource file.

   def whyrun_supported?
     true
   end

   We use the whyrun method if we want chef client to tell the changes that would be applied to the node without actually applying them when it is run in whyrun mode. Its value can be true or false. For this example, we will set this value to true. To run the chef client in whyrun mode, run Chef-client –W.

   Now we need to define the code that will be executed on calling of actions that we defined in the resources. First we will define the extract action. We will use Ruby and inbuilt chef resources to define the complete set of actions.

   In this method we are using the File.exists? method to check if there is an existing Wordpress folder to achieve idempotency.

   Chef::Log class is used to log entries in the log file.

   A new_resource.updated_by_last_action method notifies the LWRP if the node has successfully updated. True or false is passed as an argument to this method to notify the execution of LWRP.

   The converge_by is a wrapper method used when the chef client runs in whyrun mode and displays a message about that block of code.

   if ::File.exists?("#{new_resource.wp_path}/wordpress")
   
      Chef::Log.info "#{ @new_resource } already exists - nothing to do."
      new_resource.updated_by_last_action(false)
    
   else
    
   converge_by("Downloading wordpress file") do

   The remote_file is an inbuilt chef resource that downloads the file from a certain URL (uniform resource locator) to the node.

   remote_file "#{new_resource.wp_path}/wordpress.tar.gz" do
   
      source "#{new_resource.wp_url}"
      action :create
    
   end

   Bash is also an inbuilt chef resource that is used to run bash scripts. We are using the bash resource to extract the downloaded Wordpress file.

   bash "extracting wordpress" do

       code <<-EOH
       cd /var/www
       mkdir wordpress
       tar -xvf wordpres.tar.gz wordpress
       EOH
    
   end

   The following snippet shows the whole code clubbed together:

   action :extract do
    
    if ::File.exists?("#{new_resource.wp_path}/wordpress")
   
      Chef::Log.info "#{ @new_resource } already exists - nothing to do."
      new_resource.updated_by_last_action(false)
    
   else
    
   converge_by("Downloading wordpress file") do
    
   remote_file "#{new_resource.wp_path}/wordpress.tar.gz" do
   
      source "#{new_resource.wp_url}"
      action :create
    
   end
    
   bash "extracting wordpress" do
   
      code <<-EOH
      cd /var/www
      mkdir wordpress
      tar -xvf wordpres.tar.gz wordpress
      EOH
   
      end
   
     end
   
     new_resource.updated_by_last_action(true)
   
     end
    
   end

   Action: remove: This method contains the inbuilt file resource which will delete the Wordpress installation file.

   Chef::Log class is used to log entries in the log file.

   The new_resource.updated_by_last_action method notifies the LWRP if the node has successfully updated. True or false is passed as argument to this method to notify the execution of LWRP.

   converge_by is a wrapper method used when chef client runs in whyrun mode and displays a message about that block of code.

   File is an inbuilt chef resource that has an action delete, which will delete any file.

   action :remove do
    
    unless ::File.exists?("#{new_resource.wp_path}/wordpress.tar.gz")
   
     Chef::Log.info "#{ @new_resource } file deleted - nothing to do."
     new_resource.updated_by_last_action(false)
   
    else
   
    converge_by("removing the compressed wordpress file") do
   
     file "#{new_resource.wp_path}/wordpress.tar.gz" do
     action :delete
   
     end
   
     directory "/root/chefdemo" do
   
      action :create
   
     end
   
    end
   
    new_resource.updated_by_last_action(true)
   
    end
   
   end

5. You can use the resource name in the recipe once the resource and provider are in place. Since we created the resource and provider in default.rb files, the resource name will be the cookbook’s name. If the resource and provider have a name other than default.rb, that name has to be appended to the cookbook name while using it in the recipe.

   syntax: cookbookname_resourcename
   wp_setup "extracting wordpress" do
   wp_path node[:wordpress][:path]
   wp_url node[:wordpress][:url]
   action :extract
   end
    
   wp_setup "deleting the installation file" do
   wp_path node[:wordpress][:path]
   action :remove
   end

6. The attributes wp_path and wp_url are passed using the attribute file.

   default[:wordpress][:url] = "https://wordpress.org/latest.tar.gz"
   default[:wordpress][:path] = "/var/www"

7. Upload the cookbook to chef server using knife (see Figure 10-2). Knife commands are executed from a workstation.

   knife cookbook upload wp_setup

   

   Figure 10-2. Uploading the cookbook

8. Run the chef client on the node (see Figure 10-3).

   

   Figure 10-3. Running chef client

9. Confirm the chef client run (see Figure 10-4) by browsing the Wordpress directory under the /var/www folder.