In Infrastructure, configuration management is all about how well one configures the hosts. In general, all the configurations are done using the configuration files. Chef uses templates to be able to fill the configuration file with dynamic values.
Chef provides templates as a resource which can be used in the recipe. Configuration files’ dynamic values can be retrieved from data bags, attributes or even calculate them by passing them into the template.

How to Use It?

Step 1 − Add the template to the recipe.
vipin@laptop:~/chef-repo $ subl cookbooks/<Cookbook Name>/recipes/default.rb
template '/tmp/message' do
source 'Test.erb'
variables(
hi: 'Tesing',
world: 'Welt',
from: node['fqdn']
)
end
Step 2 − Add ERB Template file.
vipin@laptop:~/chef-repo $ subl cookbooks/<Cookbook Name>/templates/default/test.erb
<%- 4.times do %>
<%= @hi %>, <%= @world %> from <%= @from %>!
<%- end %>
Step 3 − Upload the modified cookbook to Chef server.
vipin@laptop:~/chef-repo $ knife cookbook upload <Cookbook Name>
Uploading my_cookbook [0.1.0]
Run Chef Client on your node:
user@server:~$ sudo chef-client
...TRUNCATED OUTPUT...
[2017-01-14T20:41:21+00:00] INFO: Processing template[/tmp/
message] action create (my_cookbook::default line 9)
[2017-01-14T20:41:22+00:00] INFO: template[/tmp/message] updated
content
Step 4 − Validate the content of the uploaded file.
user@server:~$ sudo cat /tmp/message
Hallo, Welt from vagrant.vm!
Hallo, Welt from vagrant.vm!
Hallo, Welt from vagrant.vm!
Hallo, Welt from vagrant.vm!

Workflow

Chef uses Erubis as its template language. It allows embedding pure Ruby code inside special symbols in the templates.
• <%= %> is used if you want to print the value of a variable or Ruby expression into the generated file.
• <%- %> is used if you want to embed Ruby logic into your template file. We use it to loop our expression four times.

Chef – Dynamically Configuring Recipes

Attributes are the key components for dynamically configuring cookbooks. Attributes enable the authors to make the cookbook configurable. By overriding default values set in cookbooks, the user can inject their own values.

Step 1 − Create a default file for cookbook attributes and add a default attribute to it.
vipin@laptop:~/chef-repo $ subl cookbooks/my_cookbook/attributes/default.rb
default['my_cookbook']['message'] = 'hello world!'
Step 2 − Define the attribute inside the recipe.
vipin@laptop:~/chef-repo $ subl cookbooks/<Cookbook Name>/recipes/default.rb
message = node['my_cookbook']['message']
Chef::Log.info("** Saying what I was told to say: #{message}")
Step 3 − Uploading the modified cookbook.
vipin@laptop:~/chef-repo $ knife cookbook upload my_cookbook
Uploading my_cookbook [0.1.0]
Step 4 − Running Chef-Client of the defined node.
user@server:~$ sudo chef-client
...TRUNCATED OUTPUT...
[2013-01-13T20:48:21+00:00] INFO: ** Saying what I was told to
say: hello world!
...TRUNCATED OUTPUT...

Working Method

Chef loads all attributes from the attribute file before it executes them. The attributes are stored with the node object. One can access all the attributes stored with the node object within recipes and retrieve their current values.
Chef has a restricted structure starting from the default being the lowest, then comes normal (which is aliased with the set) and then overrides. The attribute level set in the recipe has precedence over the same level set in an attribute file.

Overriding Attribute at the Node and Environment Level

Attribute defined in roles or environment have the highest precedence.

Step 1 − Create a role.
vipin@laptop:~/chef-repo $ subl roles/german_hosts.rb
name "german_hosts"
description "This Role contains hosts, which should print out
their messages in German"
run_list "recipe[my_cookbook]"
default_attributes "my_cookbook" => { "message" => "Hallo Welt!" }
Step 2 − Upload the role to Chef server.
vipin@laptop:~/chef-repo $ knife role from file german_hosts.rb
Updated Role german_hosts!
Step 3 − Assign the role to a node.
vipin@laptop:~/chef-repo $ knife node edit server
"run_list": [
"role[german_hosts]"
]
Saving updated run_list on node server
Step 4 − Run the Chef-Client.
user@server:~$ sudo chef-client
...TRUNCATED OUTPUT...
[2013-01-13T20:49:49+00:00] INFO: ** Saying what I was told to
say: Hallo Welt!
...TRUNCATED OUTPUT...

So, this brings us to the end of blog. This Tecklearn ‘Concept of Templates and Dynamically Configuring Recipes’ blog helps you with commonly asked questions if you are looking out for a job in DevOps. If you wish to learn Chef and build a career in DevOps domain, then check out our interactive, Continuous Delivery using Chef Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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