Ansible Roles Explained – Info Linux
Roles are a robust feature of Ansible that facilitate reuse and further promote modularization of configuration, but Ansible roles are often overlooked in lieu of straightforward playbooks for the task at hand. The good news is that Ansible roles are simple to get set up and allow for complexity when necessary. Join me as I go through the basics of how to set up and deploy a simple Ansible role.
The anatomy of an Ansible role
The concept of an Ansible role is simple; it is a group of variables, tasks, files, and handlers that are stored in a standardized file structure.
The most complicated part of a role is recalling the directory structure, but there is help. The built-in ansible-galaxy command has a subcommand that will create our role skeleton for us.
ansible-galaxy init to create a new role in your present working directory. You will see here that several directories and files are created within the new role:
The number of files and directories may appear intimidating, but they are fairly straightforward. Most directories contain a main.yml file; Ansible uses each of those files as the entry point for reading the contents of the directory (except for files, templates, and test). You have the freedom to branch your tasks and variables into other files within each directory. But when you do this, you must use the include directive in a directory’s main.yml to have your files utilized as part of the role. We will take a closer look at this after a brief rundown of each directory’s purpose.
The defaults directory is designated for variable defaults that take the lowest precedence. Put another way: If a variable is defined nowhere else, the definition given in defaults/main.yml will be used.
The files and templates directories serve a similar purpose. They contain affiliated files and Ansible templates (respectively) that are used within the role. The beautiful part about these directories is that Ansible does not need a path for resources stored in them when working in the role. Ansible checks them first. You may still use the full path if you want to reference files outside of the role, however, best practices suggest that you keep all of the role components together.
The handlers directory is used to store Ansible handlers. If you aren’t familiar, Ansible handlers are simply tasks that may be flagged during a play to run at the play’s completion. You may have as many or as few handlers as are needed for your role.
The meta directory contains authorship information which is useful if you choose to publish your role on galaxy.ansible.com. The meta directory may also be used to define role dependencies. As you may suspect, a role dependency allows you to require that other roles be installed prior to the role in question.
The README.md file is simply a README file in markdown format. This file is essential for roles published to galaxy.ansible.com and, honestly, the file should include a general description of how your role operates even if you do not make it publicly available.
The task directory is where most of your role will be written. This directory includes all the tasks that your role will run. Ideally, each logically related series of tasks would be laid out in their own files, and simply included through the main.yml file in the tasks directory.
The test directory contains a sample inventory and a test.yml playbook. This may be useful if you have an automated testing process built around your role. It can also be handy as you are constructing your role but use of it is not mandatory.
The last directory created is the vars directory. This is where you create variable files that define necessary variables for your role. The variables defined in this directory are meant for role internal use only. It is a good idea to namespace your role variable names, to prevent potential naming conflicts with variables outside of your role. For example, if you needed a variable named config_file in your baseline playbook, you may want to name your variable baseline_config_file, to avoid conflicts with another possible config_file variable defined elsewhere.
Creating a simple role
As stated earlier, Ansible roles can be as complex or as simple as you need. Sometimes, it is helpful to start simple and iterate into a more complex role as you shore up the base functionality. Let’s try that, and define a role called base_httpd that installs httpd with a simple configuration and a very simple website.
To get started, we will need to create our role. We could create each directory and file by hand, but it is far simpler to let ansible-galaxy do the grunt work for us by simply running
ansible-galaxy init base_httpd:
Next, we can create our simple web page in the files directory. For our academic purposes, we can create a file named index.html containing some tried and true sample text:
We will create a template of our httpd.conf file by copying an existing one from a fresh install of httpd. Let’s take this opportunity to define a couple of default variables in our role. We will do a default listening port of 80 and a LogLevel that will default to warn. We can do this by adding an entry to defaults/main.yml:
You will notice here the template file has the .j2 extension as custom, and I have used grep to highlight where we have customized the template by replacing the default values in httpd.conf with Ansible variables. Then I show where the variables are defined in defaults/main.yml. Using defaults instead of vars here is preferred, as it allows for later customization without having to change the actual role.
Now that we have our simple website and configuration, we will need to create the tasks to bring our webserver to life. For the sake of example, I will isolate our httpd setup to its own yaml file in tasks/httpd.yml and then include that file into the tasks/main.yml:
We use the yum, template, copy, and service modules to install, configure, and start our webserver. It is worth noting here that I reference the httpd.yml, httpd.conf.j2, and index.html files without full paths as they are stored within the role.
Deploying the role
Most of the hard work was completed when we constructed the role itself. Deployment of the role, by comparison, is easy. We only need to set up a simple playbook and pull in the role using the appropriate keyword:
We could easily add typical tasks after the role deployment, or we could also deploy additional roles by simply adding them to the list. Also, we can override the default variables we configured using the same variable names as shown below:
Ansible roles provide a robust way to organize deployment artifacts and tasks for the purposes of reuse and sharing. The tooling provided in ansible-galaxy makes setting up a new role as simple as ever. Anybody with a foundational understanding of writing Ansible playbooks can just as easily create an Ansible role.
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Article Prepared by Ollala Corp