## Cluster-Policy Bindings

updated: 2020-10-17 18:54

# Cluster-Policy Bindings

## Concept

A Policy object can be attached to at least one Cluster at the same time. A cluster at any time can have more than one Policy objects attached to it.

After a policy object is attached to a cluster, you can still enable or disable it or update some properties of the policy object.

## Listing Policies Attached to a Cluster

The openstack cluster command provides a sub-command policy binding list to list policy objects that are attached to a cluster. You can provide the name, the ID or the “short ID” of a cluster as the identifier to reference a cluster. For example, the command below lists the policies attached to the cluster `webservers`:

    $ openstack cluster policy binding list webservers

### Sorting the List

You can specify the sorting keys and sorting direction when list cluster policies, using the option `--sort`. The `--sort` option accepts a string of format `key1[:dir1],key2[:dir2],key3[:dir3]`, where the keys used are properties of the policy bound to a cluster and the dirs can be one of `asc` and `desc`. When omitted, Senlin sorts a given key using `asc` as the default direction.

For example, the following command line sorts the policy bindings using the `enabled` property in descending order:

    $ openstack cluster policy binding list --sort enabled:desc c3

When sorting the list of policies, `enabled` is the only key you can specify for sorting.

### Filtering the List

The openstack cluster command also supports options for filtering the policy list at the server side. The option `--filters` can be used for this purpose. For example, the following command filters clusters by the `is_enabled` field:

    $ openstack cluster policy binding list --filters enabled=True c3
    +-----------+-------------+---------------------------+------------+
    | policy_id | policy_name | policy_type               | is_enabled |
    +-----------+-------------+---------------------------+------------+
    | 0705f0f4  | up01        | senlin.policy.scaling-1.0 | True       |
    +-----------+-------------+---------------------------+------------+

The option `--filters` accepts a list of key-value pairs separated by semicolon (`;`), where each key-value pair is expected to be of format `<key>=<value>`. The only key that can be used for filtering as of today is `enabled`.

## Attaching a Policy to a Cluster

Senlin permits policy objects to be attached to clusters and to be detached from clusters dynamically. When attaching a policy object to a cluster, you can customize the policy properties for the particular cluster. For example, you can specify whether the policy should be enabled once attached.

The following options are supported for the command openstack cluster policy attach:

- `--enabled`: a boolean indicating whether the policy to be enabled once attached.

For example, the following command attaches a policy named `up01` to the cluster `c3`, When a policy is attached to a cluster, it is enabled by default. To keep it disabled, the user can use the parameter `--enabled False`. For example:

    $ openstack cluster policy attach --policy up01 --enabled False c3

Note that most of the time, Senlin doesn’t allow more than one policy of the same type to be attached to the same cluster. This restriction is relaxed for some policy types. For example, when working with policies about scaling, you can actually attach more than one policy instances to the same cluster, each of which is about a specific scenario.

For the identifiers specified for the cluster and the policy, you can use the name, the ID or the “short ID” of an object. The Senlin engine will try make a guess on each case. If no entity matches the specified identifier or there are more than one entity matching the identifier, you will get an error message.

## Showing Policy Properties on a Cluster

To examine the detailed properties of a policy object that has been attached to a cluster, you can use the openstack cluster policy binding show command with the policy identifier and the cluster identifier specified. For example:

    $ openstack cluster policy binding show --policy dp01 c3
    +--------------+--------------------------------------+
    | Field        | Value                                |
    +--------------+--------------------------------------+
    | cluster_name | c3                                   |
    | data         | None                                 |
    | id           | 2b7e9294-b5cd-470f-b191-b18f7e672495 |
    | is_enabled   | True                                 |
    | location     | None                                 |
    | name         | None                                 |
    | policy_id    | 239d7212-6196-4a89-9446-44d28717d7de |
    | policy_name  | dp01                                 |
    | policy_type  | senlin.policy.deletion-1.0           |
    +--------------+--------------------------------------+

You can use the name, the ID or the “short ID” of a policy and/or a cluster to name the objects.

## Updating Policy Properties on a Cluster

Once a policy is attached to a cluster, you can request its property on this cluster be changed by using the command openstack cluster policy binding update. Presently, you can only specify the `enabled` property to be updated.

For example, the following command disables a policy on the specified cluster:

    $ openstack cluster policy binding update \
        --enabled False --policy dp01 \
        mycluster

The Senlin engine will perform validation of the arguments in the same way as that for the policy attach operation. You can use the name, the ID or the “short ID” of an entity to reference it, as you do with the policy attach operation as well.

## Detach a Policy from a Cluster

Finally, to remove the binding between a specified policy object from a cluster, you can use the openstack cluster policy detach command as shown below:

    $ openstack cluster policy detach --policy dp01 mycluster

This command will detach the specified policy from the specified cluster. You will use the option `--policy` to specify the policy.

updated: 2020-10-17 18:54

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