Expose operational tasks via actions

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-3-postgresql

A charm should ideally cover all the complex operational logic within the code, to help avoid the need for manual human intervention.

Unfortunately, that is not always possible. As a charm developer, it is thus useful to know that you can also expose charm operational tasks to the charm user by defining special methods called actions.

This can be done by adding an actions section in your charmcraft.yaml file and then adding action event handlers to the src/charm.py file.

In this part of the tutorial we will follow this process to add an action that will allow a charm user to view the current database access points and, if set, also the username and the password.

Define the actions

Open the charmcraft.yaml file and add to it a block defining an action, as below. As you can see, the action is called get-db-info and it is intended to help the user access database authentication information. The action has a single parameter, show-password; if set to True, it will show the username and the password.

actions:
  get-db-info:
    description: Fetches database authentication information
    params:
      show-password:
        description: Show username and password in output information
        type: boolean
        default: False

Define an action class

Open your src/charm.py file, and add an action class that matches the definition you used in charmcraft.yaml:

@dataclasses.dataclass(frozen=True, kw_only=True)
class GetDbInfoAction:
    """Fetches database authentication information."""

    show_password: bool
    """Show username and password in output information."""

We’ll use ActionEvent.load_params to create an instance of your config class from the Juju action event. This allows IDEs to provide hints when we are accessing the action parameter, and static type checkers are able to validate that we are using the parameter correctly.

Define the action event handlers

Open the src/charm.py file.

In the charm __init__ method, add an action event observer, as below. As you can see, the name of the event consists of the name defined in the charmcraft.yaml file (get-db-info) and the word action.

# Events on charm actions that are run via 'juju run'.
framework.observe(self.on.get_db_info_action, self._on_get_db_info_action)

Now, define the action event handler, as below: First, read the value of the parameter defined in the charmcraft.yaml file (show-password). Then, use the fetch_postgres_relation_data method (that we defined in a previous chapter) to read the contents of the database relation data and, if the parameter value read earlier is True, add the username and password to the output. Finally, use event.set_results to attach the results to the event that has called the action; this will print the output to the terminal.

If we are not able to get the data (for example, if the charm has not yet been integrated with the postgresql-k8s application) then we use the fail method of the event to let the user know.

def _on_get_db_info_action(self, event: ops.ActionEvent) -> None:
    """Return information about the integrated database.

    This method is called when "get_db_info" action is called. It shows information about
    database access points by calling the `fetch_postgres_relation_data` method and creates
    an output dictionary containing the host, port, if show_password is True, then include
    username, and password of the database.

    If the PostgreSQL charm is not integrated, the output is set to "No database connected".

    Learn more about actions at https://documentation.ubuntu.com/ops/latest/howto/manage-actions/
    """
    params = event.load_params(GetDbInfoAction, errors='fail')
    db_data = self.fetch_postgres_relation_data()
    if not db_data:
        event.fail('No database connected')
        return
    output = {
        'db-host': db_data.get('db_host', None),
        'db-port': db_data.get('db_port', None),
    }
    if params.show_password:
        output.update(
            {
                'db-username': db_data.get('db_username', None),
                'db-password': db_data.get('db_password', None),
            }
        )
    event.set_results(output)

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

Next, test that the basic action invocation works:

juju run demo-api-charm/0 get-db-info

It might take a few seconds, but soon you should see an output similar to the one below, showing the database host and port:

Running operation 1 with 1 task
  - task 2 on unit-demo-api-charm-0

Waiting for task 2...
db-host: postgresql-k8s-primary.welcome-k8s.svc.cluster.local
db-port: "5432"

Now, test that the action parameter (show-password) works as well by setting it to True:

juju run demo-api-charm/0 get-db-info show-password=True

The output should now include the username and the password:

Running operation 3 with 1 task
  - task 4 on unit-demo-api-charm-0

Waiting for task 4...
db-host: postgresql-k8s-primary.welcome-k8s.svc.cluster.local
db-password: RGv80aF9WAJJtExn
db-port: "5432"
db-username: relation_id_4

Congratulations, you now know how to expose operational tasks via actions!

Write unit tests

Let’s add a test to check the behaviour of the get_db_info action that we just set up. Our test sets up the context, defines the input state with a relation, then runs the action and checks whether the results match the expected values:

def test_get_db_info_action():
    ctx = testing.Context(FastAPIDemoCharm)
    relation = testing.Relation(
        endpoint='database',
        interface='postgresql_client',
        remote_app_name='postgresql-k8s',
        remote_app_data={
            'endpoints': 'example.com:5432',
            'username': 'foo',
            'password': 'bar',
        },
    )
    container = testing.Container(name='demo-server', can_connect=True)
    state_in = testing.State(
        containers={container},
        relations={relation},
        leader=True,
    )

    ctx.run(ctx.on.action('get-db-info', params={'show-password': False}), state_in)

    assert ctx.action_results == {
        'db-host': 'example.com',
        'db-port': '5432',
    }

Since the get_db_info action has a parameter show-password, let’s also add a test to cover the case where the user wants to show the password:

def test_get_db_info_action_show_password():
    ctx = testing.Context(FastAPIDemoCharm)
    relation = testing.Relation(
        endpoint="database",
        interface="postgresql_client",
        remote_app_name="postgresql-k8s",
        remote_app_data={
            "endpoints": "example.com:5432",
            "username": "foo",
            "password": "bar",
        },
    )
    container = testing.Container(name="demo-server", can_connect=True)
    state_in = testing.State(
        containers={container},
        relations={relation},
        leader=True,
    )

    ctx.run(ctx.on.action("get-db-info", params={"show-password": True}), state_in)

    assert ctx.action_results == {
        "db-host": "example.com",
        "db-port": "5432",
        "db-username": "foo",
        "db-password": "bar",
    }

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.