The ops library

The ops library is a Python framework (available on PyPI) for developing and testing Juju charms in a consistent way, using standard Python constructs to allow for clean, maintainable, and reusable code.

A charm is an operator -- business logic encapsulated in a reusable software package that automates every aspect of an application's life.

Charms written with ops support Kubernetes using Juju's "sidecar charm" pattern, as well as charms that deploy to Linux-based machines and containers.

Charms should do one thing and do it well. Each charm drives a single application and can be integrated with other charms to deliver a complex system. A charm handles creating the application in addition to scaling, configuration, optimisation, networking, service mesh, observability, and other day-2 operations specific to the application.

The ops library is part of the Charm SDK (the other part being Charmcraft). Full developer documentation for the Charm SDK is available at https://juju.is/docs/sdk.

To learn more about Juju, visit https://juju.is/docs/olm.

Pure Python

The framework provides a standardised Python object model that represents the application graph, as well as an event-handling mechanism for distributed system coordination and communication.

The latest version of ops requires Python 3.8 or above.

Juju itself is written in Go for efficient concurrency even in large deployments. Charms can be written in any language, however, we recommend using Python with this framework to make development easier and more standardised. All new charms at Canonical are written using it.

Getting started

A package of operator code is called a charmed operator or simply "charm". You'll use charmcraft to register your charm name and publish it when you are ready. You can follow one of our charming tutorials to get started writing your first charm.

Testing your charms

The framework provides a testing harness, so you can ensure that your charm does the right thing in different scenarios, without having to create a full deployment. Our API documentation has the details, including this example:

class TestCharm(unittest.TestCase):
    def test_foo(self):
        harness = Harness(MyCharm)
        self.addCleanup(harness.cleanup)  # always clean up after ourselves

        # Instantiate the charm and trigger events that Juju would on startup
        harness.begin_with_initial_hooks()

        # Update charm config and trigger config-changed
        harness.update_config({'log_level': 'warn'})

        # Check that charm properly handled config-changed, for example,
        # the charm added the correct Pebble layer
        plan = harness.get_container_pebble_plan('prometheus')
        self.assertIn('--log.level=warn', plan.services['prometheus'].command)

Talk to us

If you need help, have ideas, or would just like to chat with us, reach out on the Charmhub Mattermost.

We also pay attention to the Charmhub Discourse.

And of course you can deep dive into the API reference.

Development of the framework

See HACKING.md for details on dev environments, testing, and so on.