Make your charm configurable

Important

This document is part of a series, and we recommend you follow it in sequence. However, you can also jump straight in by checking out the code from the previous chapter:

git clone https://github.com/canonical/operator.git
cd operator/examples/k8s-1-minimal

A charm might have a specific configuration that the charm developer might want to expose to the charm user so that the latter can change specific settings during runtime.

As a charm developer, it is thus important to know how to make your charm configurable.

This can be done by defining a charm configuration in a file called charmcraft.yaml and then adding configuration event handlers (‘hooks’) in the src/charm.py file.

In this part of the tutorial you will update your charm to make it possible for a charm user to change the port on which the workload application is available.

Define the configuration options

To begin with, let’s define the options that will be available for configuration.

In the charmcraft.yaml file you created earlier, define a configuration option, as below. The name of your configurable option is going to be server-port. The default value is 8000 – this is the value you’re trying to allow a charm user to configure.

config:
  options:
    server-port:
      default: 8000
      description: Default port on which FastAPI is available
      type: int

Define a configuration class

Open your src/charm.py file, and add a configuration class that matches the configuration you added in charmcraft.yaml:

@dataclasses.dataclass(frozen=True, kw_only=True)
class FastAPIConfig:
    """Configuration for the FastAPI demo charm."""

    server_port: int = 8000
    """Default port on which FastAPI is available."""

    def __post_init__(self):
        """Validate the configuration."""
        if self.server_port == 22:
            raise ValueError('Invalid port number, 22 is reserved for SSH')

Then, still in src/charm.py, add import dataclasses in the imports at the top of the file.

We’ll use CharmBase.load_config to create an instance of your config class from the Juju config data. This allows IDEs to provide hints when we are accessing the configuration, and static type checkers are able to validate that we are using the config option correctly.

Define the configuration event handlers

Open your src/charm.py file.

In the __init__ function, add an observer for the config_changed event and pair it with an _on_config_changed handler:

framework.observe(self.on.config_changed, self._on_config_changed)

Now, define the handler, as below. Since configuring something like a port affects the way we call our workload application, we need to update our Pebble configuration.

def _on_config_changed(self, _: ops.ConfigChangedEvent) -> None:
    self._update_layer_and_restart()

We’ll define _update_layer_and_restart shortly.

Caution

A charm does not know which configuration option has been changed. Thus, make sure to validate all the values. This is especially important since multiple values can be changed in one call. Using a config class simplifies this, as all validation should be done when the config object is created.

In the __init__ function, add a new attribute to define a container object for your workload:

# See 'containers' in charmcraft.yaml.
self.container = self.unit.get_container('demo-server')

Create a new method, as below. This method will get the current Pebble layer configuration and compare the new and the existing service definitions – if they differ, it will update the layer and restart the service.

def _update_layer_and_restart(self) -> None:
    """Define and start a workload using the Pebble API.

    You'll need to specify the right entrypoint and environment
    configuration for your specific workload. Tip: you can see the
    standard entrypoint of an existing container using docker inspect
    Learn more about interacting with Pebble at
        https://documentation.ubuntu.com/ops/latest/reference/pebble/
    Learn more about Pebble layers at
        https://documentation.ubuntu.com/pebble/how-to/use-layers/
    """
    # Learn more about statuses at
    # https://documentation.ubuntu.com/juju/3.6/reference/status/
    self.unit.status = ops.MaintenanceStatus('Assembling Pebble layers')
    try:
        config = self.load_config(FastAPIConfig)
    except ValueError as e:
        logger.error('Configuration error: %s', e)
        self.unit.status = ops.BlockedStatus(str(e))
        return
    try:
        self.container.add_layer('fastapi_demo', self._get_pebble_layer(config.server_port), combine=True)
        logger.info("Added updated layer 'fastapi_demo' to Pebble plan")

        # Tell Pebble to incorporate the changes, including restarting the
        # service if required.
        self.container.replan()
        logger.info(f"Replanned with '{self.pebble_service_name}' service")

        self.unit.status = ops.ActiveStatus()
    except (ops.pebble.APIError, ops.pebble.ConnectionError) as e:
        logger.info('Unable to connect to Pebble: %s', e)
        self.unit.status = ops.MaintenanceStatus('Waiting for Pebble in workload container')

When the config is loaded as part of creating the Pebble layer, if the config is invalid (in our case, if the port is set to 22), then a ValueError will be raised. The _update_layer_and_restart method handles that by logging the error and setting the status of the unit to blocked, letting the Juju user know that they need to take action.

Now, crucially, update the _get_pebble_layer method to make the layer definition dynamic, as shown below. This will replace the static port 8000 with the port passed to the method.

def _get_pebble_layer(self, port: int) -> ops.pebble.Layer:
    """Pebble layer for the FastAPI demo services."""
    command = ' '.join(
        [
            'uvicorn',
            'api_demo_server.app:app',
            '--host=0.0.0.0',
            f'--port={port}',
        ]
    )
    pebble_layer: ops.pebble.LayerDict = {
        'summary': 'FastAPI demo service',
        'description': 'pebble config layer for FastAPI demo server',
        'services': {
            self.pebble_service_name: {
                'override': 'replace',
                'summary': 'fastapi demo',
                'command': command,
                'startup': 'enabled',
            }
        },
    }
    return ops.pebble.Layer(pebble_layer)

As you may have noticed, the new _update_layer_and_restart method looks like a more advanced variant of the existing _on_demo_server_pebble_ready method. Remove the body of the _on_demo_server_pebble_ready method and replace it a call to _update_layer_and_restart like this:

def _on_demo_server_pebble_ready(self, _: ops.PebbleReadyEvent) -> None:
    self._update_layer_and_restart()

Validate your charm

First, repack and refresh your charm:

charmcraft pack
juju refresh \
  --path="./demo-api-charm_ubuntu-22.04-amd64.charm" \
  demo-api-charm --force-units --resource \
  demo-server-image=ghcr.io/canonical/api_demo_server:1.0.1

Now, check the available configuration options:

juju config demo-api-charm

Our newly defined server-port option is there. Let’s try to configure it to something else, e.g., 5000:

juju config demo-api-charm server-port=5000

Now, let’s validate that the app is actually running and reachable on the new port by sending the HTTP request below, where 10.1.157.74 is the IP of our pod and 5000 is the new application port:

curl 10.1.157.74:5000/version

You should see JSON string with the version of the application: {"version":"1.0.0"}

Let’s also verify that our invalid port number check works by setting the port to 22 and then running juju status:

juju config demo-api-charm server-port=22
juju status

As expected, the application is indeed in the blocked state:

Model        Controller  Cloud/Region        Version  SLA          Timestamp
welcome-k8s  microk8s    microk8s/localhost  3.6.8    unsupported  18:19:24+01:00

App             Version  Status   Scale  Charm           Channel  Rev  Address         Exposed  Message
demo-api-charm           blocked      1  demo-api-charm             1  10.152.183.215  no       Invalid port number, 22 is reserved for SSH

Unit               Workload  Agent  Address      Ports  Message
demo-api-charm/0*  blocked   idle   10.1.157.74         Invalid port number, 22 is reserved for SSH

Congratulations, you now know how to make your charm configurable!

Before continuing, reset the port to 8000 and check that the application is in active status:

juju config demo-api-charm server-port=8000
juju status

Write unit tests

Since we added a new feature to configure server-port and use it in the Pebble layer dynamically, we should write tests for the feature.

First, we’ll add a test that sets the port in the input state and asserts that the port is used in the service’s command in the container layer:

def test_config_changed():
    ctx = testing.Context(FastAPIDemoCharm)
    container = testing.Container(name='demo-server', can_connect=True)
    state_in = testing.State(
        containers={container},
        config={'server-port': 8080},
        leader=True,
    )
    state_out = ctx.run(ctx.on.config_changed(), state_in)
    command = (
        state_out.get_container(container.name)
        .layers['fastapi_demo']
        .services['fastapi-service']
        .command
    )
    assert '--port=8080' in command

In _on_config_changed, we specifically don’t allow port 22 to be used. If port 22 is configured, we set the unit status to blocked. So, we can add a test to cover this behaviour by setting the port to 22 in the input state and asserting that the unit status is blocked:

def test_config_changed_invalid_port():
    ctx = testing.Context(FastAPIDemoCharm)
    container = testing.Container(name='demo-server', can_connect=True)
    state_in = testing.State(
        containers={container},
        config={'server-port': 22},
        leader=True,
    )
    state_out = ctx.run(ctx.on.config_changed(), state_in)
    assert state_out.unit_status == testing.BlockedStatus(
        'Invalid port number, 22 is reserved for SSH'
    )

Run tox -e unit to check that all tests pass.

Review the final code

For the full code, see our example charm for this chapter.