Policy Types

Policy Types

A policy policy is a set of rules that are checked and enforced. The checking can be done before or after an action’s execution or both. Policies are of different policy types, each of which is designed to make sure that a cluster’s behavior follows certain patterns or complies with certain restrictions.

When released, Senlin comes with some built-in policy types to meet the requirements found in some typical use cases. However, the distributors or the users can always augment their collection of policy types by implementing their own ones.

Policy type implementations are managed as Senlin plugins. The plan is to have Senlin engine support dynamical loading of plugins from user specified modules and classes. Currently, this can be achieved by adding new senlin.policies entries in the entry_points section in the setup.cfg file, followed by a reinstall of the Senlin service, i.e. sudo pip install command.

The Base Class Policy

The base class Policy provides some common logics regarding the following operations:

  • The initialization of the spec_data property, based on the spec_schema definition and the spec input.
  • The serialization and deserialization of a policy object into/from database.
  • The serialization and deserialization of a policy object into/from a dict.
  • The default validation operation for the spec_data property.
  • Default implementations for the following methods which are to be overridden by a policy type implementation:
    • attach(cluster_id, action): a method that will be invoked when a policy object of this type is attached to a cluster.
    • detach(cluster_id, action): a method that will be invoked when a policy object of this type is detached from a cluster.
    • pre_op(cluster_id, action): a method that will be invoked before an action is executed;
    • post_op(cluster_id, action): a method that will be invoked after an action is executed.

The VERSIONS Property

Each policy type class has a VERSIONS class property that documents the changes to the policy type. This information is returned when users request to list all policy types supported.

The VERSIONS property is a dict with version numbers as keys. For each specific version, the value is list of support status changes made to the policy type. Each change record contains a status key whose value is one of EXPERIMENTAL, SUPPORTED, DEPRECATED or UNSUPPORTED, and a since key whose value is of format yyyy.mm where yyyy and mm are the year and month of the release that bears the change to the support status. For example, the following record indicates that the specific policy type was introduced in April, 2016 (i.e. version 1.0 release of Senlin) as an experimental feature; later, in October, 2016 (i.e. version 2.0 release of Senlin) it has graduated into a mature feature supported by the developer team.

  '1.0': [
          "status": "EXPERIMENTAL",
          "since": "2016.04"
          "status": "SUPPORTED",
          "since": "2016.10"

Providing New Policy Types

Adding new policy type implementations is an easy task with only a few steps to follow.

Develop A New Policy Type

The first step for adding a new policy type is to create a new file containing a subclass of Policy. Then you will define the spec schema for the new policy type in a Python dictionary named spec_schema.

Defining Spec Schema

Each key in this dictionary represents a property name; the value of it is an object of one of the schema types listed below:

  • String: A string property.
  • Boolean: A boolean property.
  • Integer: An integer property.
  • List: A property containing a list of values.
  • Map: A property containing a map of key-value pairs.

For example:

spec_schema = {
  'destroy_after_delete': schema.Boolean(
    'Boolean indicating whether object will be destroyed after deletion.',

If a property value will be a list, you can further define the type of items the list can accept. For example:

spec_schema = {
  'criteria': schema.List(
    'Criteria for object selection that will be evaluated in order.',
    schema=schema.String('Name of a criterion'),

If a property value will be a map of key-value pairs, you can define the schema of the map, which is another Python dictionary containing definitions of properties. For example:

spec_schema = {
  'strategy': schema.Map(
    'Strategy for dealing with servers with different states.',
      'inactive': 'boot',
      'deleted': 'create',
      'suspended': 'resume',

When creating a schema type object, you can specify the following keyword arguments to gain a better control of the property:

  • default: a default value of the expected data type;
  • required: a boolean value indicating whether a missing of the property is acceptable when validating the policy spec;
  • constraints: a list of Constraint objects each of which defines a constraint to be checked. Senlin currently only support AllowedValues constraint.

Applicable Profile Types

Not all policy types can be used on all profile types. For example, a policy about load-balancing is only meaningful for objects that can handle workloads, or more specifically, objects that expose service access point on an IP port.

You can define what are the profile types your new policy type can handle by specifying the PROFILE_TYPE property of your policy type class. The value of PROFILE_TYPE is a list of profile type names. If a policy type is designed to handle all profile types, you can specify a single entry ANY as the value. See profile types for profile type related operations.

Policy Targets

A policy type is usually defined to handle certain operations. The rules embedded in the implementation may need to be checked before the execution of an action or they may need to be enforced after the execution of the action. When an action is about to be executed or an action has finished execution, the Senlin engine will check if any policy objects attached to a cluster is interested in the action. If the answer is yes, the engine will invoke the pre_op function or the post_op function respectively, thus giving the policy object a chance to adjust the action’s behavior.

You can define a TARGET property for the policy type implementation to indicate the actions your policy type want to subscribe to. The TARGET property is a list of tuple (WHEN, ACTION). For example, the following property definition indicates that the policy type is interested in the action CLUSTER_SCALE_IN and CLUSTER_DEL_NODES. The policy type wants itself be consulted before these actions are performed.

class MyPolicyType(Policy):
  TARGET = [

When the corresponding actions are about to be executed, the pre_op function of this policy object will be invoked.

Passing Data Between Policies

Each policy type may decide to send some data as additional inputs or constraints for the action to consume. This is done by modifying the data property of an Action object (see action).

A policy type may want to check if there are other policy objects leaving some policy decisions in the data property of an action object.

Senlin allows for more than one policy to be attached to the same cluster. Each policy, when enabled, is supposed to check a specific subset of cluster actions. In other words, different policies may get checked before/after the engine executes a specific cluster action. This design is effectively forming a chain of policies for checking. The decisions (outcomes) from one policy sometimes impact other policies that are checked later.

To help other developers to understand how a specific policy type is designed to work in concert with others, we require all policy type implementations shipped with Senlin accompanied by a documentation about:

  • the action data items the policy type will consume, including how these data will impact the policy decisions.
  • the action.data items the policy type will produce, thus consumable by any policies downstream.

For built-in policy types, the protocol is documented below:

Registering The New Policy Type

For Senlin service to be aware of and thus to make use of the new policy type you have just developed, you will register it to the Senlin service. Currently, this is done through a manual process shown below. In future, Senlin will provide dynamical loading support to policy type plugins.

To register a new plugin type, you will add a line to the setup.cfg file that can be found at the root directory of Senlin code base. For example:

senlin.policies =
    ScalingPolicy = senlin.policies.scaling_policy:ScalingPolicy
    MyCoolPolicy = <path to the policy module>:<policy class name>

Finally, save that file and do a reinstall of the Senlin service, followed by a restart of the senlin-engine process.

$ sudo pip install -e .

Now, when you do a openstack cluster policy type list, you will see your policy type listed along with other existing policy types.

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