Policy Types

updated: None

Policy Types

Policy Types¶

Concept¶

A Policy Type is an abstract specification of the rules to be checked and/or enforced when certain Action is performed on a cluster that contains nodes of certain Profile Type.

A registry of policy types is built in memory when the Senlin engine (senlin-engine) is started. In future, Senlin will allow users to provide additional policy type implementations as plug-ins to be loaded dynamically.

A policy type implementation dictates which fields are required, which fields are optional and sometimes the constraints on field values. When a policy is created by referencing this policy type, the fields are assigned with concrete values. For example, a policy type senlin.policy.deletion conceptually specifies the properties required:

criteria: String # valid values - OLDEST_FIRST, YOUNGEST_FIRST, RANDOM
destroy_after_deletion: Boolean
grace_period: Integer
reduce_desired_capacity: Boolean

The speci of a policy object of this policy type may look like following:

type: senlin.policy.deletion
version: 1.0
properties:
  criteria: OLDEST_FIRST
  distroy_after_deletion: True
  grace_period: 120
  reduce_desired_capacity: True

Listing Policy Types¶

Senlin server comes with some built-in policy types. You can check the list of policy types using the following command:

$ senlin policy-type-list
+--------------------------------+
| name                           |
+--------------------------------+
| senlin.policy.batch            |
| senlin.policy.deletion         |
| senlin.policy.health           |
| senlin.policy.loadbalance      |
| senlin.policy.region_placement |
| senlin.policy.scaling          |
| senlin.policy.zone_placement   |
+--------------------------------+

The output is a list of policy types supported by the Senlin server.

Showing Policy Details¶

Each Policy Type has a schema for its spec (i.e. specification) that describes the names and types of the properties that can be accepted. To show the schema of a specific policy type along with other properties, you can use the following command:

$ senlin policy-type-show senlin.policy.deletion
name: senlin.policy.deletion
schema:
  criteria:
    constraints:
    - constraint:
      - OLDEST_FIRST
      - OLDEST_PROFILE_FRIST
      - YOUNGEST_FIRST
      - RANDOM
      type: AllowedValues
    default: RANDOM
    description: Criteria used in selecting candidates for deletion
    readonly: false
    required: false
    type: String
  destroy_after_deletion:
    default: true
    description: Whether a node should be completely destroyed after
      deletion. Default to True
    readonly: false
    required: false
    type: Boolean
  grace_period:
    default: 0
    description: Number of seconds before real deletion happens.
    readonly: false
    required: false
    type: Integer
  reduce_desired_capacity:
    default: false
    description: Whether the desired capacity of the cluster should be
      reduced along with the deletion. Default to False.
    readonly: false
    required: false
    type: Boolean

Here, each property has the following attributes:

default: the default value for a property when not explicitly specified;
description: a textual description of the use of a property;
readonly: a boolean indicating whether a property is read only;
required: whether the property must be specified. Such kind of a property usually doesn’t have a default value.
type: one of String, Integer, Boolean, Map or List.

The default output from the policy-type-show command is in YAML format. You can choose to show the spec schema in JSON format by specifying the the -F json option as shown below:

$ senlin policy-type-show -F json senlin.policy.deletion

For information on how to manage the relationship between a policy and a cluster, please refer to Cluster-Policy Bindings.