## Deletion Policy V1.1

updated: 2020-10-17 18:54

# Deletion Policy V1.1

The deletion policy is designed to be enforced when a cluster’s size is to be shrunk.

## Spec

### Latest Version

1.1

### Available Versions

`Version` | `Status` | `Supported Since`
--- | --- | ---
`1.0` | `SUPPORTED` | `2016.04`
`1.1` | `SUPPORTED` | `2018.01`

### Applicable Profile Types

This policy is designed to handle the following profile types:

- ANY

### Policy Triggers

This policy is triggered by the following actions during the respective phases:

`Action` | `Phase`
--- | ---
`CLUSTER_DEL_NODES` | `BEFORE`
`CLUSTER_RESIZE` | `BEFORE`
`CLUSTER_SCALE_IN` | `BEFORE`
`NODE_DELETE` | `BEFORE`

### Properties

criteria
  Criteria used in selecting candidates for deletion
destroy_after_deletion
  Whether a node should be completely destroyed after deletion. Default to True
grace_period
  Number of seconds before real deletion happens.
reduce_desired_capacity
  Whether the desired capacity of the cluster should be reduced along the deletion. Default to True.
hooks
  timeout
    Number of seconds before actual deletion happens.
  type
    Type of lifecycle hook
  params
    queue
      Zaqar queue to receive lifecycle hook message
    url
      Url sink to which to send lifecycle hook message

## Actions Handled

The policy is capable of handling the following actions:

- `CLUSTER_SCALE_IN`: an action that carries an optional integer value named `count` in its `inputs`.

- `CLUSTER_DEL_NODES`: an action that carries a list value named `candidates` in its `inputs` value.

- `CLUSTER_RESIZE`: an action that carries various key-value pairs as arguments to the action in its `inputs` value.

- `NODE_DELETE`: an action that has a node associated with it. This action has to be originated from a RPC request directly so that it will be processed by the deletion policy. The node ID associated with the action obviously become the ‘candidate’ node for deletion.

The policy will be checked BEFORE any of the above mentioned actions is executed.

## Scenarios

Under different scenarios, the policy works by checking different properties of the action.

### S1: `CLUSTER_DEL_NODES`

This is the simplest case. An action of `CLUSTER_DEL_NODES` carries a list of UUIDs for the nodes to be removed from the cluster. The deletion policy steps in before the actual deletion happens so to help determine the following details:

- whether the nodes should be destroyed after being removed from the cluster;

- whether the nodes should be granted a grace period before being destroyed;

- whether the `desired_capacity` of the cluster in question should be reduced after node removal.

After the policy check, the `data` field is updated with contents similar to the following example:

    {
      "status": "OK",
      "reason": "Candidates generated",
      "deletion": {
         "count": 2,
         "candidates": ["<node-id-1>", "<node-id-2"],
         "destroy_after_deletion": true,
         "grace_period": 0
      }
    }

### S2: `CLUSTER_SCALE_IN` without Scaling Policy

When the request is about scaling in the target cluster, the Senlin engine expects that the action carries a `count` key in its `inputs`. If the `count` key doesn’t exist, it means the requester has no idea (or he/she doesn’t care) the number of nodes to remove. The decision is left to the scaling policy (if any) or to the Senlin engine.

When there is no scaling policy attached to the cluster, Senlin engine takes the liberty to assume that the expectation is to remove 1 node from the cluster. This is equivalent to the case when `count` is specified as `1`.

The policy then continues evaluate the cluster nodes to select `count` victim node(s) based on the `criteria` property of the policy. Finally it updates the action’s `data` field with the list of node candidates along with other properties, as described in scenario S1.

### S3: `CLUSTER_SCALE_IN` with Scaling Policy

If there is a scaling policy attached to the cluster, that policy will yield into the action’s `data` property some contents similar to the following example:

    {
      "deletion": {
         "count": 2
      }
    }

The senlin engine will use value from the `deletion.count` field in the `data` property as the number of nodes to remove from cluster. It selects victim nodes from the cluster based on the `criteria` specified and then updates the action’s `data` property along with other properties, as described in scenario S1.

### S4: `CLUSTER_RESIZE` without Scaling Policy

If there is no scaling policy attached to the cluster, the deletion policy won’t be able to find a `deletion.count` field in the action’s `data` property. It then checks the `inputs` property of the action directly and generates a `deletion.count` field if the request turns out to be a scaling-in operation. If the request is not a scaling-in operation, the policy check aborts immediately.

After having determined the number of nodes to remove, the policy proceeds to select victim nodes based on its `criteria` property value. Finally it updates the action’s `data` field with the list of node candidates along with other properties, as described in scenario S1.

### S5: `CLUSTER_RESIZE` with Scaling Policy

In the case there is already a scaling policy attached to the cluster, the scaling policy will be evaluated before the deletion policy, so the policy works in the same way as described in scenario S3.

### S6: Deletion across Multiple Availability Zones

When you have a zone placement policy attached to a cluster, the zone placement policy will decide in which availability zone(s) new nodes will be placed and from which availability zone(s) old nodes should be deleted to maintain an expected node distribution. Such a zone placement policy will be evaluated before this deletion policy, according to its builtin priority value.

When scaling in a cluster, a zone placement policy yields a decision into the action’s `data` property that looks like:

    {
      "deletion": {
         "count": 3,
         "zones": {
             "AZ-1": 2,
             "AZ-2": 1
         }
      }
    }

The above data indicate how many nodes should be deleted globally and how many nodes should be removed from each availability zone. The deletion policy then evaluates nodes from each availability zone to select specified number of nodes as candidates. This selection process is also based on the `criteria` property of the deletion policy.

After the evaluation, the deletion policy completes by modifying the `data` property to something like:

    {
      "status": "OK",
      "reason": "Candidates generated",
      "deletion": {
         "count": 3,
         "candidates": ["node-id-1", "node-id-2", "node-id-3"]
         "destroy_after_deletion": true,
         "grace_period": 0
      }
    }

In the `deletion.candidates` list, two of the nodes are from availability zone `AZ-1`, one of the nodes is from availability zone `AZ-2`.

### S6: Deletion across Multiple Regions

When you have a region placement policy attached to a cluster, the region placement policy will decide to which region(s) new nodes will be placed and from which region(s) old nodes should be deleted to maintain an expected node distribution. Such a region placement policy will be evaluated before this deletion policy, according to its builtin priority value.

When scaling in a cluster, a region placement policy yields a decision into the action’s `data` property that looks like:

    {
      "deletion": {
         "count": 3,
         "region": {
             "R-1": 2,
             "R-2": 1
         }
      }
    }

The above data indicate how many nodes should be deleted globally and how many nodes should be removed from each region. The deletion policy then evaluates nodes from each region to select specified number of nodes as candidates. This selection process is also based on the `criteria` property of the deletion policy.

After the evaluation, the deletion policy completes by modifying the `data` property to something like:

    {
      "status": "OK",
      "reason": "Candidates generated",
      "deletion": {
         "count": 3,
         "candidates": ["node-id-1", "node-id-2", "node-id-3"]
         "destroy_after_deletion": true,
         "grace_period": 0
      }
    }

In the `deletion.candidates` list, two of the nodes are from region `R-1`, one of the nodes is from region `R-2`.

### S7: Handling `NODE_DELETE` Action

If the action that triggered the policy checking is a `NODE_DELETE` action, the action has an associated node as its property. When the deletion policy has detected this action type, it will copy the policy specification values into the action’s `data` field although the `count` and `candidates` value are so obvious. For example:

    {
      "status": "OK",
      "reason": "Candidates generated",
      "deletion": {
         "count": 1,
         "candidates": ["node-id-1"]
         "destroy_after_deletion": true,
         "grace_period": 0
      }
    }

updated: 2020-10-17 18:54

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