charm-ops-sunbeam/howto-pebble-handler.rst

How-To Write a pebble handler

A pebble handler sits between a charm and a container it manages. A pebble handler presents the charm with a consistent method of interaction with the container. For example the charm can query the handler to check config has been rendered and services started. It can call the execute method to run commands in the container or call write_config to render the defined files into the container.

Common Pebble handler changes

ASO provides a pebble handler base classes which provide the starting point for writing a new handler. If the container runs a service then the ServicePebbleHandler should be used. If the container does not provide a service (perhaps its just an environment for executing commands that affact other container) then PebbleHandler should be used.

import container_handlers

class MyServicePebbleHandler(container_handlers.ServicePebbleHandler):
    """Manage MyService Container."""

The handlers can create directories in the container once the pebble is available.

@property
def directories(self) -> List[sunbeam_chandlers.ContainerDir]:
    """Directories to create in container."""
    return [
        sunbeam_chandlers.ContainerDir(
            '/var/log/my-service',
            'root',
            'root'),

In addition to directories the handler can list configuration files which need to be rendered into the container. These will be rendered as templates using all available contexts.

def default_container_configs(
    self
) -> List[sunbeam_core.ContainerConfigFile]:
    """Files to render into containers."""
    return [
        sunbeam_core.ContainerConfigFile(
            '/etc/mysvc/mvsvc.conf',
            'root',
            'root')]

If a service should be running in the conainer the handler specifies the layer describing the service that will be passed to pebble.

def get_layer(self) -> dict:
    """Pebble configuration layer for MyService service."""
    return {
        "summary": "My service",
        "description": "Pebble config layer for MyService",
        "services": {
            'my_svc': {
                "override": "replace",
                "summary": "My Super Service",
                "command": "/usr/bin/my-svc",
                "startup": "disabled",
            },
        },
    }

Advanced Pebble handler changes

By default the pebble handler is the observer of pebble events. If this behaviour needs to be altered then setup_pebble_handler method can be changed.

def setup_pebble_handler(self) -> None:
    """Configure handler for pebble ready event."""
    pass

Or perhaps it is ok for the pebble handler to observe the event but a different reaction is required. In this case the method associated with the event can be overridden.

def _on_service_pebble_ready(
   self, event: ops.charm.PebbleReadyEvent
) -> None:
   """Handle pebble ready event."""
   container = event.workload
   container.add_layer(self.service_name, self.get_layer(), combine=True)
   self.execute(["run", "special", "command"])
   logger.debug(f"Plan: {container.get_plan()}")
   self.ready = True
   self._state.pebble_ready = True
   self.charm.configure_charm(event)

Configuring Charm to use custom pebble handler

The charms get_pebble_handlers method dictates which pebble handlers are used.

class MyCharmCharm(NeutronOperatorCharm):

    def get_pebble_handlers(self) -> List[sunbeam_chandlers.PebbleHandler]:
        """Pebble handlers for the service."""
        return [
            MyServicePebbleHandler(
                self,
                'my-server-container',
                self.service_name,
                self.container_configs,
                self.template_dir,
                self.openstack_release,
                self.configure_charm,
            )
        ]