How to add an action to a charm

Actions are methods defined by a charm developer designed to be invoked by an administrator against a deployed charm. They are commonly used to expose maintenance or operations tasks, such as creating a snapshot of a database, adding a user to a system or dumping debug information.

Actions are defined in the actions.yaml file. As with configuration, each action is defined as a top-level key of a YAML map. The name of the key is the name of the action, and corresponds to a map of fields that define the action.

Action names are validated to ensure they do not collide with Python keywords, and that they are valid identifiers:

• Identifiers contain only alphanumeric characters or underscores
• First character must not be a digit
• Action names may also include hyphens

Each action should define a description, and can optionally define some parameters in a YAML map named params, which is a JSON Schema transformed into a YAML map. Parameters can contain nested JSON schema.

Contents:

• Example action definitions
  • Simple action example
  • Complex action example
• Action handling
  • Action handling example

Example action definitions

The following illustrate some example action definitions:

Simple action example

The following shows a simple example of an actions.yaml file, defining three actions named pause, resume, and snapshot. The snapshot action takes a single string parameter named outfile:

pause:
  description: Pause the database.
resume:
  description: Resume a paused database.
snapshot:
  description: Take a snapshot of the database.
  params:
    outfile:
      type: string
      description: The filename to write to.

Complex action example

The following example showcases a more complex configuration file that uses a nested schema to define detailed options. It also mandates that the action should not run if extra parameters are provided (additionalProperties: false) and makes the filename field mandatory:

pause:
  description: Pause the database.
resume:
  description: Resume a paused database.
snapshot:
  description: Take a snapshot of the database.
  params:
    filename:
      type: string
      description: The name of the snapshot file.
    compression:
      type: object
      description: The type of compression to use.
      properties:
        kind:
          type: string
          enum: [gzip, bzip2, xz]
        quality:
          description: Compression quality
          type: integer
          minimum: 0
          maximum: 9
  required: [filename]
  additionalProperties: false

An administrator would invoke this action like so:

$ juju run <unit> snapshot filename=out.tar.gz compression.kind=gzip

Action handling

Actions are surfaced as events that a charm can observe, similarly to lifecycle events. Action names are parsed by the Charmed Operator Framework and surfaced as events named <action_name>_action.

Action handlers are passed an ActionEvent as their first parameter, and thus have access to the action parameter values through the <event>.params construct. In addition to the params dict, the ActionEvent class provides three convenience methods to the developer:

• <event>.fail(message=""): Report to the administrator that the action has failed, optionally providing a message indicating why the failure occured.
• <event>.log(message): Log a message for the administrator (available while the action is running).
• <event>.set_results(results): Report the result of the action, where results is a dictionary.

Messages are displayed to administrators in real time when they run juju run-action with the --wait flag. For more information on working with actions as an administrator see Juju OLM | How to manage actions.

Action handling example

For the following actions.yaml:

grant-admin-role:
  description: >
    Grant the "system_admin" role to a user. The user will need to log out and
    log back in to realise their permissions upgrade.
  params:
    user:
      type: string
      description: The user to grant "system_admin" role to.
  required: [user]

A charm developer could handle the action invocation like so:

class ActionsCharm(CharmBase):
    def __init__(self, *args):
        super().__init__(*args)
        # ...
        self.framework.observe(self.on.grant_admin_role_action, self._on_grant_admin_role_action)
        # ...

    def _on_grant_admin_role_action(self, event):
        """Handle the grant-admin-role action."""
        # Fetch the user parameter from the ActionEvent params dict
        user = event.params["user"]
        # Do something useful with it
        cmd = ["/usr/bin/myapp", "roles", "system_admin", user]
        # Set a log message for the action
        event.log(f"Running this command: {' '.join(cmd)}")
        granted = subprocess.run(cmd, capture_output=True)
        if granted.returncode != 0:
            # Fail the action if there is an error
            event.fail(
                f"Failed to run '{' '.join(cmd)}'. Output was:\n{granted.stderr.decode('utf-8')}"
            )
        else:
            # Set the results of the action
            msg = f"Ran grant-admin-role for user '{user}'"
            event.set_results({"result": msg})

Last updated 2 months ago.