Zone Placement Policy¶

Properties¶

A typical spec for a zone placement policy is exemplified in the following sample:

type: senlin.policy.zone_placement
version: 1.0
properties:
  regions:
    - name: az_1
      weight: 100
    - name: az_2
      weight: 200

In this sample spec, two availability zones are provided, namely “az_1” and “az_2”. Each availability zone can have an optional “weight” attribute associated with it.

The “weight” value is to be interpreted as a relative number. The value assigned to one zone has to be compared to those assigned to other zones for an assessment. In the sample shown above, az_1 and az_2 are assigned weights of 100 and 200 respectively. This means that among every 3 nodes creation, one is expected to be scheduled to az_1 and the other 2 are expected to be scheduled to az_2. In other words, the chance for az_2 receiving a node creation request is twice of that for az_1.

The “weight” value has to be a positive integer, if specified. The default value is 100 for all zones whose weight is omitted.

Validation¶

When creating a zone placement policy, the Senlin engine validates whether the zone names given are all known to be usable availability zones by the Nova compute service. Do NOT pass in an invalid availability zone name and hope Senlin can create a zone for you.

Later on when the zone placement policy is triggered upon node creation or node deletion actions, it always validates if the provided availability zones are still valid and usable.

Node Distribution¶

After a zone placement policy is attached to a cluster and enabled, all future node creations (by cluster scaling for example) will trigger an evaluation of the policy. Similarly, a node deletion action will also trigger an evaluation of it because the policy’s goal is to maintain the node distribution based on the one computed from the weight distribution of all zones.

The zone placement policy will favor availability zones with highest weight values when selecting a zone for nodes to be created.