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Testing Ansible roles with Molecule
Testing Ansible roles with Molecule
Learn how to automate your verifications using Python.
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Test techniques play an important role in software development, and this is no different when we are talking about Infrastructure as Code (IaC).Developers are always testing, and constant feedback is necessary to drive development. If it takes too long to get feedback on a change, your steps might be too large, making errors hard to spot. Baby steps and fast feedback are the essence of TDD (test-driven development). But how do you apply this approach to the development of ad hoc playbooks or roles?
When you're developing an automation, a typical workflow would start with a new virtual machine. I will use Vagrant to illustrate this idea, but you could use libvirt, Docker, VirtualBox, or VMware, an instance in a private or public cloud, or a virtual machine provisioned in your data center hypervisor (oVirt, Xen, or VMware, for example).
When deciding which virtual machine to use, balance feedback speed and similarity with your real target environment.
The minimal start point with Vagrant would be:
vagrant init centos/7 # or any other box
Then add Ansible provisioning to your Vagrantfile:
config.vm.provision "ansible" do |ansible|
ansible.playbook = "playbook.yml"
In the end, your workflow would be:
- Edit playbook.
vagrant sshto verify VM state.
- Repeat steps 2 to 4.
Occasionally, the VM should be destroyed and brought up again (
vagrant destroy -f; vagrant up) to increase the reliability of your playbook (i.e., to test if your automation is working end-to-end).
Although this is a good workflow, you're still doing all the hard work of connecting to the VM and verifying that everything is working as expected.
When tests are not automated, you'll face issues similar to those when you do not automate your infrastructure.
I will focus on Testinfra, as it is written in Python and is the default verifier for Molecule. The idea is pretty simple: Automate your verifications using Python:
nginx = host.package("nginx")
nginx = host.service("nginx")
In a development environment, this script would connect to the target host using SSH (just like Ansible) to perform the above verifications (package presence/version and service state):
py.test --connection=ssh --hosts=server
In short, during infrastructure automation development, the challenge is to provision new infrastructure, execute playbooks against them, and verify that your changes reflect the state you declared in your playbooks.
What can Testinfra verify?
- Infrastructure is up and running from the user's point of view (e.g., HTTPD or Nginx is answering requests, and MariaDB or PostgreSQL is handling SQL queries).
- OS service is started and enabled
- A process is listening on a specific port
- A process is answering requests
- Configuration files were correctly copied or generated from templates
- Virtually anything you do to ensure that your server state is correct
What safeties do these automated tests provide?
- Perform complex changes or introduce new features without breaking existing behavior (e.g., it still works in RHEL-based distributions after adding support for Debian-based systems).
- Refactor/improve the codebase when new versions of Ansible are released and new best practices are introduced.
What we've done with Vagrant, Ansible, and Testinfra so far is easily mapped to the steps described in the Four-Phase Test pattern—a way to structure tests that makes the test objective clear. It is composed of the following phases: Setup, Exercise, Verify, and Teardown:
Setup: Prepares the environment for the test execution (e.g., spins up new virtual machines):
Exercise: Effectively executes the code against the system under test (i.e., Ansible playbook):
Verify: Verifies the previous step output:
Teardown: Returns to the state prior to Setup:
The same idea we used for an ad hoc playbook could be applied to role development and testing, but do you need to do all these steps every time you develop something new? What if you want to use containers, or an OpenStack, instead of Vagrant? What if you'd rather use Goss than Testinfra? How do you run this continuously for every change in your code? Is there a more simple and fast way to develop our playbooks and roles with automated tests?
Molecule helps develop roles using tests. The tool can even initialize a new role with test cases:
molecule init role –role-name foo
Molecule is flexible enough to allow you to use different drivers for infrastructure provisioning, including Docker, Vagrant, OpenStack, GCE, EC2, and Azure. It also allows the use of different server verification tools, including Testinfra and Goss.
Its commands ease the execution of tasks commonly used during development workflow:
lint- Executes yaml-lint, ansible-lint, and flake8, reporting failure if there are issues
syntax- Verifies the role for syntax errors
create- Creates an instance with the configured driver
prepare- Configures instances with preparation playbooks
converge- Executes playbooks targeting hosts
idempotence- Executes a playbook twice and fails in case of changes in the second run (non-idempotent)
verify- Execute server state verification tools (testinfra or goss)
destroy- Destroys instances
test- Executes all the previous steps
logincommand can be used to connect to provisioned servers for troubleshooting purposes.
Step by step
How do you go from no tests at all to a decent codebase being executed for every change/commit?
virtualenv tool creates isolated environments, while
virtualenvwrapper is a collection of extensions that facilitate the use of
These tools prevent dependencies and conflicts between Molecule and other Python packages in your machine.
sudo pip install virtualenvwrapper
Install Molecule with the Docker driver:
pip install molecule ansible docker
Generate a new role with test scenarios:
molecule init role -r role_name
or for existing roles:
molecule init scenario -r my-role
All the necessary configuration is generated with your role, and you need only write test cases using Testinfra:
testinfra_hosts = testinfra.utils.ansible_runner.AnsibleRunner(
jboss = host.service('wildfly')
socket = host.socket('tcp://0.0.0.0:8080')
command = """curl --digest -L -D - http://localhost:9990/management \
cmd = host.run(command)
assert 'HTTP/1.1 200 OK' in cmd.stdout
This example test case for a Wildfly role verifies that OS service is enabled, a process is listening in port 8080, and authentication is properly configurated.
Coding these tests is straightforward, and you basically need to think about an automated way to verify something.
You are already writing tests when you log into a machine targeted by your playbook, or when you build verifications for your monitoring/alerting systems. This knowledge will contribute to building something with the Testinfra API or using a system command.
Continuously executing your Molecule tests is simple. The example above works for TravisCI with the Docker driver, but it could be easily adapted for any CI server and any infrastructure drivers supported by Molecule.
- sudo apt-get -qq update
- pip install molecule
- pip install docker
- molecule test
Visit Travis CI for sample output.