## Making Your Cluster Elastic

updated: 2020-10-17 18:54

# Making Your Cluster Elastic

## Creating Receivers

Suppose you want a cluster to scale out by one node each time an event occurs, you can create a receiver for this task:

    $ openstack cluster receiver create --type webhook --cluster mycluster \
        --action CLUSTER_SCALE_OUT so_receiver_1
    +------------+---------------------------------------------------+
    | Field      | Value                                             |
    +------------+---------------------------------------------------+
    | action     | CLUSTER_SCALE_OUT                                 |
    | actor      | {                                                 |
    |            |   "trust_id": "b2b8fd71c3d54f67ac14e5851c0117b8"  |
    |            | }                                                 |
    | channel    | {                                                 |
    |            |   "alarm_url": "<WEBHOOK_URL>"                    |
    |            | }                                                 |
    | cluster_id | 30d7ef94-114f-4163-9120-412b78ba38bb              |
    | created_at | 2017-02-08T02:08:13Z                              |
    | domain_id  | None                                              |
    | id         | 5722a2b0-1f5f-4a82-9c08-27da9982d46f              |
    | location   | None                                              |
    | name       | so_receiver_1                                     |
    | params     | {}                                                |
    | project_id | 36d551c0594b4cc99d1bbff8bf202ec3                  |
    | type       | webhook                                           |
    | updated_at | None                                              |
    | user_id    | 9563fa29642a4efdb1033bf8aab07daa                  |
    +------------+---------------------------------------------------+

The command above creates a receiver named `so_receiver_1` which can be used to initiate a `CLUSTER_SCALE_OUT` action on the cluster `my_cluster`. From the output of this command, you will find an `alarm_url` value from the `channel` property. This will be the URL for you to trigger the scaling operation.

Note

You are expected to treat the `alarm_url` value as a secret. Any person or software which knows this value will be able to trigger the scaling operation on your cluster. This may not be what you wanted.

The default type of receiver would be “`webhook`”. You may choose to create a “`message`” type of receiver if you have the zaqar messaging service installed. For more details, please refer to Receiver.

## Triggering Scaling

Once you have received a channel from the created receiver, you can use it to trigger the associated action on the specified cluster. The simplest way to do this is to use the curl command as shown below:

    $ curl -X POST <WEBHOOK_URL>

Once the above request is received by the senlin-api, your cluster will be scaled out by one node. In other words, a new node is created into the cluster.

## Creating Scaling Policies

Senlin provides some builtin policy types to control how a cluster will be scaled when a relevant request is received. A scaling request can be a simple `CLUSTER_SCALE_OUT` or `CLUSTER_SCALE_IN` action which can accept an optional `count` argument; it can be a more complex `CLUSTER_RESIZE` action which can accept more arguments for fine-tuning the scaling behavior.

In the absence of such arguments (which is not uncommon if you are using a 3rd party monitoring software which doesn’t have the intelligence to decide each and every argument), you can always use scaling policies for this purpose.

Below is a sample YAML file (`examples/policies/scaling_policy.yaml`) used for creating a scaling policy object:

    type: senlin.policy.scaling
    version: 1.0
    properties:
      event: CLUSTER_SCALE_IN
      adjustment:
        type: CHANGE_IN_CAPACITY
        number: 2
        min_step: 1
        best_effort: True
        cooldown: 120

To create a policy object, you can use the following command:

    $ openstack cluster policy create \
      --spec-file examples/policies/scaling_policy.yaml \
      policy1
    +------------+--------------------------------------+
    | Field      | Value                                |
    +------------+--------------------------------------+
    | created_at | 2016-12-08T02:41:30.000000           |
    | data       | {}                                   |
    | domain_id  | None                                 |
    | id         | 3ca962c5-68ce-4293-9087-c73964546223 |
    | location   | None                                 |
    | name       | policy1                              |
    | project_id | 36d551c0594b4cc99d1bbff8bf202ec3     |
    | spec       | {                                    |
    |            |   "version": 1.0,                    |
    |            |   "type": "senlin.policy.scaling",   |
    |            |   "properties": {                    |
    |            |     "adjustment": {                  |
    |            |       "min_step": 1,                 |
    |            |       "cooldown": 120,               |
    |            |       "best_effort": true,           |
    |            |       "number": 1,                   |
    |            |       "type": "CHANGE_IN_CAPACITY"   |
    |            |     },                               |
    |            |     "event": "CLUSTER_SCALE_IN"      |
    |            |   }                                  |
    |            | }                                    |
    | type       | senlin.policy.scaling-1.0            |
    | updated_at | None                                 |
    | user_id    | 9563fa29642a4efdb1033bf8aab07daa     |
    +------------+--------------------------------------+

The next step to enforce this policy on your cluster is to attach the policy to it, as shown below:

    $ openstack cluster policy attach --policy policy1 mycluster
    Request accepted by action: 89626141-0999-4e76-9795-a86c4cfd531f

    $ openstack cluster policy binding list mycluster
    +-----------+-------------+---------------------------+------------+
    | policy_id | policy_name | policy_type               | is_enabled |
    +-----------+-------------+---------------------------+------------+
    | 3ca962c5  | policy1     | senlin.policy.scaling-1.0 | True       |
    +-----------+-------------+---------------------------+------------+

In future, when your cluster is about to be scaled in (no matter the request comes from a user or a software or via a receiver), the scaling policy attached will help determine 1) how many nodes to be removed, 2) whether the scaling operation should be done on a best effort basis, 3) for how long the cluster will not respond to further scaling requests, etc.

For more information on using scaling policy, you can refer to Scaling Policy.

updated: 2020-10-17 18:54

Except where otherwise noted, this document is licensed under Creative Commons Attribution 3.0 License. See all OpenStack Legal Documents.

questions?