Helpers for writing layered+reactive OpenStack Charms
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Helpers for building layered, reactive OpenStack charms.

Support and discussions

We use the openstack-charmers mailing-lists for developer and user discussions, you can find and subscribe here:

If you prefer live discussions, some of us also hang out in #juju on

Bug reports

Bug reports can be filed at

Using charms.openstack

charms.openstack provides a module charms_openstack which is included in layer-openstack’s wheelhouse.txt. It is provides the fundamental functionality required of most OpenStack charms.

The main classes that the module provides are:

  • :class:OpenStackRelationAdapter
  • :class:RabbitMQRelationAdapter
  • :class:DatabaseRelationAdapter
  • :class:ConfigurationAdapter
  • :class:OpenStackRelationsAdapter
  • :class:OpenStackCharm

Key features of charms.openstack

The main features that charms.openstack provides are:

  • a base OpenStackCharm that provides:
    • The ability to specify the OpenStack release that the charm works with.
    • The list of packages to install on the charm.
    • The ports that the charm exposes.
    • The keystone service type (if applicable)
    • A mapping of config files to services to restart if the configuration changes.
    • The required relations for the charm (workload status)
    • The sync command that the database (if associated) will need for its schema.
    • a default install that gets the packages, installs them, and sets the appropriate workload status.
    • A configuration file renderer (using the relation adapters) to write the configuration files for the service being managed.
    • A workload status helper (assess_status()) that checks the state of interfaces, the services, and ports, and sets the workload status. This is automatically provided for the update-status hook in the layer-openstack layer.

How to leverage charms.openstack classes

Using OpenStackCharm

OpenStackCharm() and the related classes provide a powerful framework to build an OpenStack charm on. There are two approaches to writing charms that support multiple OpenStack releases. Note that determining the release is up to the charm author, and can be signalled to OpenStackCharm in two ways.

  1. Write a single OpenStackCharm derived class that uses self.release to determine what functionality to exhibit depending on the release. In this case, there is no need to register multiple charms and provide a chooser to determine which class to use.

  2. Write muliple OpenStackCharm derived classes which map to each difference in charm functionality depending on the release, and register a chooser function using the @register_os_release_selector decorator.


class LibertyCharm(OpenStackCharm):
    release = 'liberty'

class MitakaCharm(OpenStackCharm):
    release = 'mitaka'

def choose_release():
    """Determine the release based on the python-keystonemiddleware that is
    return ch_utils.os_release('python-keystonemiddleware')

This will automatically select LibertyCharm for a liberty release and MitakaCharm for the mitaka release. Note, that it will also set release on the OpenStackCharm instance via the __init__() method, so that the instance knows what the charm is.

If only a single charm class is needed, the __init__() method of the class can be used to determine the release instead:

class TheCharm(OpenStackCharm):
    release = 'liberty'

    def __init__(release=None, *args, **kwargs):
        if release is None:
            release = ch_utils.os_release('python-keystonemiddleware')
        super(TheCharm, self).__init__(release=release, *args, **kwargs)

If the release selector function is registered, then the overridden __init__() method is not needed as the release will be passed into the default __init__() method. However, there may be other functionality that the charm author needs to include in the initialiser.

Note that using os_release() can typically be used to determine the release of OpenStack.

Using the relation adapter classes - OpenStackRelationAdapter

The relation adapter classes adapt a reactive interface for use in the rendering functions. Their pricipal use is to provide an iterator of the attributes declared in the assessors attribute of the instance.

A reactive BaseRelation derived instance has an auto_accessors attribute which declares the variables that the relation has. These are copied into the accessors attribute of the OpenStackRelationAdapter class, and additional attributes can be added as part of class instantiation.

Note that the accessor properties are dynamic, in that they call the underlying relation property when they are accessed.

The purpose of the OpenStackRelation class is for the instance to be used as part of configuration file rendering, as an instance of an OpenStackRelation class can be passed to the render function, and the iterator will provide the key value pairs to the template processor.

A derived OpenStackRelation class can provide additional computed properties as required. e.g. the RabbitMQRelationAdapter implementation:

class RabbitMQRelationAdapter(OpenStackRelationAdapter):
    Adapter for the RabbitMQRequires relation interface.

    interface_type = "messaging"

    def __init__(self, relation):
        add_accessors = ['vhost', 'username']
        super(RabbitMQRelationAdapter, self).__init__(relation, add_accessors)

    def host(self):
        Hostname that should be used to access RabbitMQ.
            return self.private_address

    def hosts(self):
        Comma separated list of hosts that should be used
        to access RabbitMQ.
        hosts = self.relation.rabbitmq_hosts()
        if len(hosts) > 1:
            return ','.join(hosts)
            return None

Note that the additional accessors vhost and username are provided in the overridden __init__() method.

The ConfigurationAdapter

The ConfigurationAdapter class simply provides snapshot of the configuration opentions for the current charm, such that they can be accessed as attributes of an instance of the class. e.g. rather than config('vip') then user can use

The benefit, is that a derived version of ConfigurationAdapter can be provided that has computed properties that can be used like static properties on the instance. The ConfigurationAdapter, or derived class, is used with the OpenStackRelationAdapters class (not the plural …Adapters) class that brings together all of the relations into one place.

The OpenStackRelationAdapters class

The OpenStackRelationAdapters class joins together the relation adapter classes, with the ConfigurationAdapter (or derived) class, and works like a charmhelpers OSRenderConfig instance to the rendering functions in charmhelpers.

Thus an instance of the OpenStackRelationAdapters (or derived) class is used in the charmhelpers.core.templating.render() function to provide the variables needed to render templates.

The OpenStackRelationAdapters class can be subclassed (derived) with additional custom OpenStackRelationAdapter classes (to map to particular relations) using the relation_adapters class property:

class MyRelationAdapters(OpenStackRelationAdapters):

    relation_adapters = {
       'my-relation': MyRelationAdapter,

This enables custome relation adapters to be mapped to particular relations such that custom functionality can be implemented for a particular reactive relationship.

HighAvailability Support

To be completed.

OpenStacke Upgrade via config

An OpenStack principle charm has an ‘openstack-origin’ configuration option. This is used to setup the package source for a charm. If a user updates this option to point at a package repository then the charm can be configured to automatically upgrade. This is achieved with the following steps:

  1. Add hook to reactive handler
def config_changed(args):
  1. Define the package for the charm to monitor and a mapping of OpenStack releases to package versions.
class TheCharm(OpenStackCharm):

    release_pkg = 'pkg-name'
    package_codenames = {
        'pkg-name': collections.OrderedDict([
            ('2', 'mitaka'),
            ('3', 'newton'),
            ('4', 'ocata'),

Workload status

OpenStack charms support the concept of workload status which helps to inform a user of the charm of the current state of the charm. The following workload statuses are supported:

  • unknown - The charm doesn’t support workload status. This should not be used for charms that DO support workload status.
  • active - The unit under the charms control is fully configuration and available for use.
  • maintenance - the unit is installing, or doing something of that nature.
  • waiting - The unit is waiting for a relation to become available. i.e. the relation is not yet complete in that some data is missing still.
  • blocked - a relation is not yet connected, or some other blocking condition.
  • paused - (Not yet availble) - the unit has been put into the paused state.

The default is for charms to support workload status, and the default installation method sets the status to maintenance with an install message.

If the charm is not going to support workload status, and this is not recommended, then the charm author will need to override the install() method of OpenStackCharm derived class to disable setting the maintenance state, and override the assess_status() method to a NOP.

The assess_status() method on OpenStackCharm provides a helper to enable the charm author to provide workload status. By default:

  • The actual assessment of status is deferred until the all of the reactive handlers have had a chance to execute (according to their conditions), just before the charm hook exits. The real assess_status() method is actually _assess_status() and the assess_status() method simply sets up an atexit() hook to defer the operation. This means that you can call assess_status() multiple times BUT it will actually only be invoked at the end of the charm hook execution. If you need to actually run assess_status() at the point in the handler, then call _assess_status().
  • The install method provides the maintenance status.
  • The layer-openstack layer provides a hook for update-status which calls the assess_status() function on the charm class.
  • The _assess_status() method uses various attributes of the class to provide a default mechanism for assessing the workload status of the charm/unit.

The latter is extremely useful for determining the workload status. The _assess_status() method does the following checks:

  1. The unit checks if it is paused. (Not yet available as a feature).
  2. The unit checks the relations to see if they are connected and available.
  3. The unit checks custom_assess_status_check()
  4. The unit checks that the services are running and ports are open.

Checking of relations

The assess_status function checks that the relations named in the class attribute required_relations are connected and available. It does this using the convention of:

  • A connected relation has the {relation}.connected state set.
  • An available relation has the {relation}.available state set.

This is a convention that the interfaces (e.g. interface-keystone, etc.) use. interface-keystone sets identity-service.connected when it has a connection with keystone, and identity-service.available when the connection is completed and all information transferred.

That if required_relations is ['identity-service'], then the assess_status() function will check for identity-service.connected and identity-service.available states.

If the charm author requires additional states to be checked for an interface, then the method states_to_check should be overridden in the derived class and additional states, the status and error message provided. See the code for further details.


def states_to_check():
    states = super(MyCharm, self).states_to_check()
        ("some-relation.available.ssl", "waiting", "'some-relation' incomplete"))
    return states

The custom_assess_status_check() method

If the charm author needs to do additional status checking, then the custom_assess_status_check() method should be overridden in the derived class. The return value from the method is:

  • (None, None) - the unit is fine.
  • status, message - the unit’s workload status is not active.

Not checking services are running

By default, the _assess_status() method checks that the services declared in the class attribute services (list of strings) are checked to ensure that they are running. Additionally, the ports declared in the class attribute api_ports are also checked for being listened on.

However, if the services check is not required, then the derived class should overload the check_running_services() method and return None, None.

Additionally, if the services running check is required, but the ports should not be checked, then the ports_to_check method can be overridden and return an empty list [].

Using assess_status()

The assess_status() method should be used on any hook or state method where the unit’s status may have changed. e.g. interfaces connecting or becoming available, configuration changes, etc.


def setup_amqp_req(amqp):¬
    """Use the amqp interface to request access to the amqp broker using our
    local configuration.