## Zone Placement Policy V1.0

updated: 2020-10-17 18:54

# Zone Placement Policy V1.0

This policy is designed to make sure the nodes in a cluster are distributed across multiple availability zones according to a specified scheme.

## Spec

### Latest Version

1.0

### Available Versions

`Version` | `Status` | `Supported Since`
--- | --- | ---
`1.0` | `EXPERIMENTAL` | `2016.04`
`1.0` | `SUPPORTED` | `2016.10`

### Applicable Profile Types

This policy is designed to handle the following profile types:

- os.nova.server-1.0

### Policy Triggers

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

`Action` | `Phase`
--- | ---
`CLUSTER_RESIZE` | `BEFORE`
`CLUSTER_SCALE_IN` | `BEFORE`
`CLUSTER_SCALE_OUT` | `BEFORE`
`NODE_CREATE` | `BEFORE`

### Properties

zones
  List properties:
    name
      Name of an availability zone.
    weight
      Weight of the availability zone (default is 100).

## 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_SCALE_OUT`: an action that carries an optional integer value named `count` in its `inputs`.

- `CLUSTER_RESIZE`: an action that accepts a map as its input parameters in its `inputs` property, such as “`adjustment_type`”, “`number`” etc.

- `NODE_CREATE`: an action originated directly from a RPC request. Such an action will have a node object associated with it, which becomes the one to be handled by this policy.

The policy will be checked BEFORE any of the above mentioned actions is executed. Because the same policy implementation is used for covering both the cases of scaling out a cluster and the cases of scaling in, the zone placement policy need to parse the inputs in different scenarios.

The placement policy can be used independently, with and without other polices attached to the same cluster. So the policy needs to understand whether there are policy decisions from other policies (such as a scaling policy).

When the policy is checked, it will first attempt to get the proper `count` input value, which may be an outcome from other policies or the inputs for the action. For more details, check the scenarios described in following sections.

## Scenarios

### S1: `CLUSTER_SCALE_IN`

The placement policy first checks if there are policy decisions from other policies by looking into the `deletion` field of the action’s `data` property. If there is such a field, the policy attempts to extract the `count` value from the `deletion` field. If the `count` value is not found, 1 is assumed to be the default.

If, however, the policy fails to find the `deletion` field, it tries to find if there is a `count` field in the action’s `inputs` property. If the answer is true, the policy will use it, or else it will fall back to assume 1 as the default count.

After the policy has find out the `count` value (i.e. number of nodes to be deleted), it validates the list of availability zone names provided to the policy. If for some reason, none of the provided names passed the validation, the policy check fails with the following data recorded in the action’s `data` property:

    {
      "status": "ERROR",
      "reason": "No availability zone found available.",
    }

With the list of availability zones known to be good and the map of node distribution specified in the policy spec, senlin engine continues to calculate a distribution plan that best matches the desired distribution. If there are nodes that cannot be fit into the distribution plan, the policy check fails with an error recorded in the action’s `data`, as shown below:

    {
      "status": "ERROR",
      "reason": "There is no feasible plan to handle all nodes."
    }

If there is a feasible plan to remove nodes from each availability zone, the policy saves the plan into the `data` property of the action as exemplified below:

    {
      "status": "OK",
      "deletion": {
        "count": 3,
        "zones": {
          "nova-1": 2,
          "nova-2": 1
        }
      }
    }

This means in total, 3 nodes should be removed from the cluster. Among them, 2 nodes should be selected from availability zone “`nova-1`” and the rest one should be selected from availability zone “`nova-2`”.

NOTE: When there is a deletion policy attached to the same cluster. That deletion policy will be evaluated after the zone placement policy and it is expected to rebase its candidate selection on the zone distribution enforced here. For example, if the deletion policy is tasked to select the oldest nodes for deletion, it will adapt its behavior to select the oldest nodes from each availability zone. The number of nodes to be chosen from each availability zone would be based on the output from this placement policy.

### S2: `CLUSTER_SCALE_OUT`

The placement policy first checks if there are policy decisions from other policies by looking into the `creation` field of the action’s `data` property. If there is such a field, the policy attempts to extract the `count` value from the `creation` field. If the `count` value is not found, 1 is assumed to be the default.

If, however, the policy fails to find the `creation` field, it tries to find if there is a `count` field in the action’s `inputs` property. If the answer is true, the policy will use it, or else it will fall back to assume 1 as the default node count.

After the policy has find out the `count` value (i.e. number of nodes to be created), it validates the list of availability zone names provided to the policy and extracts the current distribution of nodes among those availability zones.

If for some reason, none of the provided names passed the validation, the policy check fails with the following data recorded in the action’s `data` property:

    {
      "status": "ERROR",
      "reason": "No availability zone found available.",
    }

The logic of generating a distribution plan is almost identical to what have been described in scenario S1, except for the output format. When there is a feasible plan to accommodate all nodes, the plan is saved into the `data` property of the action as shown in the following example:

    {
      "status": "OK",
      "creation": {
        "count": 3,
        "zones": {
          "nova-1": 1,
          "nova-2": 2
        }
      }
    }

This means in total, 3 nodes should be created into the cluster. Among them, 2 nodes should be created at availability zone “`nova-1`” and the left one should be created at availability zone “`nova-2`”.

### S3: `CLUSTER_RESIZE`

The placement policy first checks if there are policy decisions from other policies by looking into the `creation` field of the action’s `data` property. If there is such a field, the policy extracts the `count` value from the `creation` field. If the `creation` field is not found, the policy tries to find if there is a `deletion` field in the action’s `data` property. If there is such a field, the policy extracts the `count` value from the `creation` field. If neither `creation` nor `deletion` is found in the action’s `data` property, the policy proceeds to parse the raw inputs of the action.

The output from the parser may indicate an invalid combination of input values. If that is the case, the policy check fails with the action’s `data` set to something like the following example:

    {
      "status": "ERROR",
      "reason": <error message from the parser.>
    }

If the parser successfully parsed the action’s raw inputs, the policy tries again to find if there is either `creation` or `deletion` field in the action’s `data` property. It will use the `count` value from the field found as the number of nodes to be handled.

When the placement policy finds out the number of nodes to create (or delete), it proceeds to calculate a distribution plan. If the action is about growing the size of the cluster, the logic and the output format are the same as that have been outlined in scenario S2. Otherwise, the logic and the output format are identical to that have been described in scenario S1.

### S4: `NODE_CREATE`

When handling a `NODE_CREATE` action, the zone placement policy needs to process the single node associated with the action, i.e. the node to be created. If, however, the node is referencing a profile whose spec contains a `availability_zone` property, it means the requesting user has a preferred availability zone for the new node. In this case, the placement policy will return directly without choosing availability zone for the node.

If the profile spec doesn’t have `availability_zone` specified, the placement policy will proceed to do an evaluation of the current zone distribution followed by a calculation of distribution plan so that the new node will be deployed in a proper availability zone. These logics and the output format are identical to that in scenario S2.

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?