The Charmed Operator Framework

This Charmed Operator Framework simplifies operator development for model-driven application management.

Operators emerged from the Kubernetes community; an operator is software that drives lifecycle management, configuration, integration and daily actions for an application. Operators simplify software management and operations. They capture reusable app domain knowledge from experts in a software component that can be shared.

This project extends the operator pattern to enable charmed operators, not just for Kubernetes but also operators for traditional Linux or Windows application management.

Operators use a Charmed Operator Lifecycle Manager (Charmed OLM) to coordinate their work in a cluster. The system uses Golang for concurrent event processing under the hood, but enables the operators to be written in Python.

Simple, composable operators

Operators should 'do one thing and do it well'. Each operator drives a single microservice and can be composed with other operators to deliver a complex application.

It is better to have small, reusable operators that each drive a single microservice very well. The operator handles instantiation, scaling, configuration, optimisation, networking, service mesh, observability, and day-2 operations specific to that microservice.

Operator composition takes place through declarative integration in the OLM. Operators declare integration endpoints, and discover lines of integration between those endpoints dynamically at runtime.

Pure Python operators

The framework provides a standard Python library and object model that represents the application graph, and an event distribution mechanism for distributed system coordination and communication.

The OLM is written in Golang for efficient concurrency in event handling and distribution. Operators can be written in any language. We recommend this Python framework for ease of design, development and collaboration.

Better collaboration

Operator developers publish Python libraries that make it easy to integrate your operator with their operator. The framework includes standard tools to distribute these integration libraries and keep them up to date.

Development collaboration happens at Charmhub.io where operators are published along with integration libraries. Design and code review discussions are hosted in the Charmhub discourse. We recommend the Open Operator Manifesto as a guideline for high quality operator engineering.

Event serialization and operator services

Distributed systems can be hard! So this framework exists to make it much simpler to reason about operator behaviour, especially in complex deployments. The Charmed OLM provides operator services such as provisioning, event delivery, leader election and model management.

Coordination between operators is provided by a cluster-wide event distribution system. Events are serialized to avoid race conditions in any given container or machine. This greatly simplifies the development of operators for high availability, scale-out and integrated applications.

Model-driven Operator Lifecycle Manager

A key goal of the project is to improve the user experience for admins working with multiple different operators.

We embrace model-driven operations in the Charmed Operator Lifecycle Manager. The model encompasses capacity, storage, networking, the application graph and administrative access.

Admins describe the application graph of integrated microservices, and the OLM then drives instantiation. A change in the model is propagated to all affected operators, reducing the duplication of effort and repetition normally found in operating a complex topology of services.

Administrative actions, updates, configuration and integration are all driven through the OLM.

Getting started

A package of operator code is called a charmed operator or “charm. You will use charmcraft to register your operator name, and publish it when you are ready. There are more details on how to get a complete development environment setup over in the documentation

Charmed Operators written using the Charmed Operator Framework are just Python code. The goal is to feel natural for somebody used to coding in Python, and reasonably easy to learn for somebody who is not a pythonista.

The dependencies of the operator framework are kept as minimal as possible; currently that's Python 3.5 or greater, and PyYAML (both are included by default in Ubuntu's cloud images from 16.04 on).

For a brief intro on how to get started, check out the Hello, World! section of the documentation!

Testing your charmed operators

The operator framework provides a testing harness, so you can check your charmed operator does the right thing in different scenarios, without having to create a full deployment. pydoc3 ops.testing has the details, including this example:

harness = Harness(MyCharm)
# Do initial setup here
relation_id = harness.add_relation('db', 'postgresql')
# Now instantiate the charm to see events as the model changes
harness.begin()
harness.add_relation_unit(relation_id, 'postgresql/0')
harness.update_relation_data(relation_id, 'postgresql/0', {'key': 'val'})
# Check that charm has properly handled the relation_joined event for postgresql/0
self.assertEqual(harness.charm. ...)

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

You can deep dive into the API docs if that's your thing.

Operator Framework development

To work in the framework itself you will need Python >= 3.5. Linting, testing, and docs automation is performed using tox.

The following are likely to be useful during development:

# Run linting and unit tests
tox

# Run tests, specifying whole suite or specific files
tox -e unit
tox -e unit test/test_charm.py

# Format the code using isort
tox -e fmt

# Generate a local copy of the Sphinx docs in docs/_build
tox -e docs

For more in depth debugging, you can enter any of tox's created virtualenvs provided they have been run at least once:

# Enter the unit testing virtualenv
source .tox/unit/bin/activate

# Enter the linting virtualenv
source .tox/lint/bin/activate

For improved performance on the tests, ensure that you have PyYAML installed with the correct extensions:

apt-get install libyaml-dev
pip install --force-reinstall --no-cache-dir pyyaml