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The reliable way to test an Ansible role on an Ubuntu development machine is to use Molecule with a disposable target managed by a supported driver such as Podman. Install Ansible, Molecule, and the driver required by your project; initialize a scenario inside the role repository; apply the role in converge.yml; assert the resulting state in verify.yml; then run molecule test. Molecule performs dependency installation, cleanup, instance creation, convergence, idempotence testing, verification, and final destruction in a repeatable sequence.
Ubuntu is the host operating system in this guide. The system being tested can be an Ubuntu container, virtual machine, cloud instance, API, database, network device, or another target reachable by Ansible. Choose the target and driver according to the role’s support policy rather than assuming that an Ubuntu host requires an Ubuntu target.
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Molecule is an Ansible testing framework for developing and testing collections, playbooks, and roles. It supplies a scenario structure, a driver interface for creating test systems, and commands for applying and checking a role. A scenario can use containers, virtual machines, cloud infrastructure, APIs, databases, network devices, or any other system that Ansible can reach.
A normal role test answers several different questions:
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- Can the role be applied to a clean target?
- Does a second application make no unintended changes (idempotence)?
- Did the target reach the required state?
- Are resources cleaned up even after a failure?
The complete default lifecycle is dependency, destroy, create, converge, idempotence, verify, and a final destroy. The scenario can change that order, but changing it should be a deliberate testing decision.
Prerequisites on Ubuntu
Install the tools using your project policy
The official role tutorial treats Ansible, Molecule, and Podman as prerequisites. On Ubuntu, select an installation method that matches the Python and Ansible policy used by your repository. Depending on the project, that may be a distribution package, a virtual environment, or an isolated command-line installation. The important requirement is not a particular package command; it is that the versions installed on your machine are compatible with the role, Molecule, and the selected driver.
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ansible --versionmolecule --versionpodman --versionwhen Podman is your driver
Also confirm that your user can run the selected driver, create its resources, and access any registry or cloud credentials required by the scenario. A role that only manages files and services may need no cloud credentials, while a scenario targeting external infrastructure may require authentication and network access.
Keep host and target requirements separate
Your Ubuntu workstation needs the Molecule command and its Python dependencies. The target needs whatever the role manages: a package manager, a service manager, Python support for Ansible modules, or other prerequisites. If the target is a minimal container, the image may not contain all of those components. Put bootstrap work in the scenario’s creation or preparation steps, or select an image that already satisfies the role’s documented prerequisites.
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Recommended role repository layout
Work from the root of the role, not from the parent directory that contains several roles. A conventional repository looks like this:
my_role/
├── tasks/
│ └── main.yml
└── molecule/
└── default/
├── molecule.yml
├── requirements.yml
├── inventory/
│ └── hosts.yml
├── create.yml
├── converge.yml
├── verify.yml
└── destroy.ymlThe role’s implementation remains under directories such as tasks/, handlers/, templates/, and defaults/. The molecule/ directory contains tests and is separate from those task files. Each scenario is a self-contained directory. default is merely the conventional first scenario name; you can add directories such as ubuntu, upgrade, or cloud when the role needs different inputs or platforms.
Initialize the default scenario
- Change to the role directory:
cd my_role. - Initialize the conventional scenario:
molecule init scenario default. - Inspect the generated files and commit them with the role when the scenario is ready for collaboration.
The generated files are a starting point, not a complete test. Review the driver, platform definition, inventory, lifecycle sequence, and role path before the first run. Keep the scenario’s role lookup unambiguous: the project configuration should make the role under test discoverable without relying on a developer’s current directory or a globally installed copy.
Declare driver collections
If the driver is distributed as an Ansible collection, declare it in molecule/default/requirements.yml. The official example uses the containers.podman collection. A requirements file can contain the collection declaration required by your chosen driver; do not add unrelated collections simply because they are available. Run the scenario’s dependency phase after changing this file so the declared collection is installed in the environment used by Molecule.
Configure the scenario
molecule.yml controls the driver, platforms, inventory, and action sequence. At minimum, make sure it identifies the selected driver, defines the target platform or platforms, points Ansible at the scenario inventory, and includes the actions you want to run. A minimal structural example is:
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dependency:
name: galaxy
driver:
name: podman
platforms:
- name: instance
groups:
- molecule
provisioner:
name: ansible
inventory:
hosts: inventory/hosts.yml
scenario:
test_sequence:
- dependency
- destroy
- create
- converge
- idempotence
- verify
- destroyThe exact platform fields are driver-specific. Add the image, connection, privilege, and other settings required by the driver and approved by your project. There is no single Ubuntu image tag or universal combination that is correct for every role. Keep the molecule host group consistent with the plays in your converge and verify files.
Apply the role in converge.yml
converge.yml is the playbook that applies the role under test. Target the scenario’s molecule group and include the role explicitly. The role guide’s pattern is:
---
- name: Converge
hosts: molecule
gather_facts: false
tasks:
- name: Apply the role under test
ansible.builtin.include_role:
name: my_roleReplace my_role with the role name as Ansible resolves it in your repository. Set gather_facts to true when the role needs facts; leaving it false makes a test faster and exposes roles that accidentally depend on facts they never requested. Pass representative variables in the play or through scenario variables, and include separate plays when the role must be exercised under more than one input set.
Keep convergence focused on setup and application. Assertions belong in verify.yml, where a failure clearly means that the resulting state is wrong rather than that the role could not be applied.
Assert the end state in verify.yml
A verification playbook inspects the target after convergence and fails with a useful message when the expected state is absent. For a role that creates a marker file, an Ansible-native check can be written as:
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---
- name: Verify
hosts: molecule
gather_facts: false
tasks:
- name: Read the role marker file
ansible.builtin.stat:
path: /tmp/molecule_role_marker
register: marker
- name: Confirm the role created the marker file
ansible.builtin.assert:
that:
- marker.stat.exists
- marker.stat.isregUse checks that represent the contract of the role, not its implementation details. Examples include a service being enabled and running, a configuration file containing the required setting, a user having the intended attributes, a package being present at an acceptable version, or a file having the expected owner and mode. Register a module result once and assert several properties from it when that makes failures easier to interpret.
Assertions should be deterministic and safe to run repeatedly. Avoid checks that depend on wall-clock time, random identifiers, an administrator’s personal environment, or an external service unless those dependencies are the subject of the scenario.
Run the lifecycle
| Command | Use it for | What it does not guarantee |
|---|---|---|
molecule test |
End-to-end validation | It cannot compensate for an incorrect scenario or weak assertions. |
molecule converge |
Fast development iterations | It does not run verification or necessarily destroy the instance. |
molecule verify |
Re-running assertions against an existing instance | It does not create a missing instance. |
molecule create |
Creating the test target for inspection | It does not apply the role. |
molecule list |
Seeing scenario instances and their state | It does not diagnose the cause of a failed task. |
molecule login |
Opening an interactive session in the instance | It is unavailable if creation failed or the instance was destroyed. |
molecule destroy |
Removing instances and stale resources | It does not preserve evidence unless you collect it first. |
During normal development, use molecule converge to apply a change, inspect the result, and then use molecule verify for focused assertions. Before committing, run molecule test. That command resolves dependencies, removes stale resources, creates a clean target, converges the role, applies it again for idempotence, runs verification, and performs final cleanup according to the configured sequence.
Understand the idempotence phase
Molecule’s idempotence step applies the converge play a second time and checks whether Ansible reports unintended changes. A failure usually indicates that a task rewrites a file, recreates a resource, changes permissions, or otherwise performs work on every run. Investigate the task’s module arguments, file content, ownership, notifications, and conditionals. Some tasks are intentionally non-idempotent or report changes for external reasons; if so, document the reason and isolate that behavior rather than blindly disabling the check.
Debug a failing test without losing the target
- Run the failing command with Molecule’s
--debugoption and use the installed tool’s supported verbosity controls when Ansible output is insufficient. - Run
molecule listto determine whether the failure occurred during creation, convergence, verification, or cleanup. - If the instance was destroyed, recreate it with
molecule createand repeat the smallest failing phase. - Use
molecule loginto inspect files, packages, services, users, mounts, and logs inside the target. - Re-run
molecule convergeafter correcting the role or scenario, then runmolecule verify. - Finish with
molecule destroywhen the investigation is complete.
When a full test fails and you need the failed instance to remain available, run molecule test --destroy=never. This is particularly useful for failures in verify.yml: the role has already converged, so you can inspect the exact state that caused the assertion to fail. Remember that retaining resources can consume disk, memory, cloud quota, or money; destroy them explicitly after collecting evidence.
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Diagnose by lifecycle stage
- Dependency failure: check collection names, connectivity to the configured collection source, and whether the environment uses the intended Python interpreter.
- Create failure: inspect the driver, image or platform definition, permissions, registry access, and container or VM runtime.
- Converge failure: read the Ansible task error first, then verify inventory group names, role discovery, variables, privilege escalation, and target prerequisites.
- Idempotence failure: identify the task that reports changes on the second run and correct its state handling.
- Verify failure: inspect the target directly and decide whether the role or the assertion misstates the intended contract.
- Destroy failure: remove orphaned resources with the driver’s own tools and then correct the scenario cleanup configuration.
Test more than one Ubuntu or input combination
Create additional scenario directories when the role has materially different inputs, operating systems, drivers, or deployment assumptions. Each scenario normally owns its own resources and creates and destroys them independently. This keeps failures isolated, at the cost of additional setup time.
For a matrix, vary one meaningful axis at a time: the target image, connection driver, privilege model, or role variables. The exact image tags and supported combinations must come from your project’s compatibility policy; Molecule’s documentation does not establish a universal Ubuntu image matrix. Record the tested combinations in project documentation and keep them aligned with the versions supported by the role.
Use shared state deliberately
With shared_state: true, the default scenario owns resource creation and destruction. Other scenarios reuse that instance for their converge, idempotence, and verify phases. Shared state can reduce expensive provisioning, but it couples scenarios and allows one scenario’s changes to affect another. Use it only when the scenarios are intentionally testing components on the same base instance. Otherwise, let every scenario create a clean target.
Choose the right test approach
| Decision axis | Disposable container | Virtual machine or external target |
|---|---|---|
| Isolation | Fast, local, and easy to recreate; kernel-level behavior may differ from a host. | Closer to a full operating system or production topology; provisioning is slower and more complex. |
| Cost | Usually limited to local compute and storage. | May consume hypervisor, cloud, licensing, or network resources. |
| Feedback speed | Well suited to repeated converge and verify loops. |
Useful when the role depends on system services, kernels, or topology unavailable in a container. |
| State ownership | Normally scenario-local and disposable. | Must be carefully isolated or explicitly shared to avoid changing real systems. |
| Best assertions | Files, packages, users, services, permissions, and configuration. | Those checks plus boot behavior, networking, storage, system integration, or provider-specific effects. |
The driver is an implementation choice, not the test itself. Keep the role contract and assertions identical where possible, then add platform-specific preparation only when the target genuinely differs.
Common mistakes and fixes
- Running Molecule outside the role: change into the role root so the scenario and role path resolve together.
- Using a different inventory group: make the group in
hosts.ymlmatchhosts: moleculein both playbooks. - Assuming the generated scenario is complete: review every generated file, especially the driver, platform, inventory, and test sequence.
- Putting assertions in converge: move end-state checks to
verify.ymlso application and verification failures remain distinct. - Testing only a reused instance: run the full
molecule testlifecycle before committing to expose missing creation-time prerequisites. - Ignoring idempotence: treat second-run changes as a role defect or document a narrowly justified exception.
- Leaving failed infrastructure running: use
--destroy=neveronly while investigating, then destroy the instance and any external resources. - Hard-coding an unapproved platform image: select an image and tag supported by the repository’s compatibility policy instead of assuming one Ubuntu release represents all deployments.
- Overly weak verification: assert the role’s externally visible contract, including ownership, mode, service state, and configuration values where those are part of the contract.
- Overly brittle verification: avoid asserting incidental implementation details that can change without changing the role’s promised behavior.
A practical workflow
- Install and verify Ansible, Molecule, and the selected driver on Ubuntu using the versions approved by the project.
- Initialize
molecule/defaultfrom the role root. - Declare required driver collections in
requirements.yml. - Configure the driver, platform, inventory, role path, and lifecycle sequence in
molecule.yml. - Write a focused
converge.ymlthat includes the role under test. - Write deterministic assertions in
verify.yml. - Use
molecule convergewhile developing andmolecule verifyfor rapid checks on an existing instance. - Run
molecule testfrom a clean state before merging. - When a test fails, preserve the instance with
--destroy=never, inspect it withmolecule login, and clean up explicitly. - Add separate scenarios for genuinely different platforms or inputs, using shared state only when resource reuse is intentional.
This workflow gives an Ubuntu-based developer a repeatable, inspectable test for both role application and the state the role promises to produce, without confusing the host operating system with the system being managed.
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