Load Balancing Policy V1.0¶

S1: CLUSTER_SCALE_IN¶

When scaling in a cluster, there may and may not be a scaling policy attached to the cluster. The loadbalance policy has to cope with both cases. The loadbalance policy first attempts to get the number of nodes to remove then it tries to get the candidate nodes for removal.

It will first check if there is a “deletion” key in the action’s data field. If it successfully finds it, it means there are other policies already helped decide the number of nodes to remove, even the candidate nodes for removal. If the “deletion” key is not found, it means the policy has to figure out the deletion count itself. It first checks if the action has an input named “count”. The count value will be used if found, or else it will assume the count to be 1.

When the policy finds that the candidate nodes for removal have not yet been chosen, it will try a random selection from all cluster nodes.

After the policy has figured out the candidate nodes for removal, it invokes the LBaaS API to remove the candidates from the load balancer. If any of the removal operation fails, the scale in operation fails before node removal actually happens.

When all candidates have been removed from the load balancer, the scale in operation continues to delete the candidate nodes.

S2: CLUSTER_DEL_NODES¶

When deleting specified nodes from a cluster, the candidate nodes are already provided in the action’s inputs property, so the loadbalance policy just iterate the list of candidate nodes to update the load balancer. The load balancer side operation is identical to that outlined in scenario S1.

S3: CLUSTER_RESIZE that Shrinks a Cluster¶

For a cluster resize operation, the loadbalance policy is invoked BEFORE the operation is attempting to remove any nodes from the cluster. If there are other policies (such as a scaling policy or a deletion policy) attached to the cluster, the number of nodes along with the candidate nodes might have already been decided.

The policy first checks the “deletion” key in the action’s data field. If it successfully finds it, it means there are other policies already helped decide the number of nodes to remove, even the candidate nodes for removal. If the “deletion” key is not found, it means the policy has to figure out the deletion count itself. In the latter case, the policy will try to parse the inputs property of the action and see if it is about to delete nodes from the cluster. If the action is indeed about removing nodes, then the policy gets what it wants, i.e. the count value. If the action is not about deleting nodes, then the action passes the policy check directly.

After having figured out the number of nodes to delete, the policy may still need to decide which nodes to remove, i.e. the candidates. When no other policy has made a decision, the loadbalance policy randomly chooses the specified number of nodes as candidates.

After the candidates is eventually selected, the policy proceeds to update the load balancer as outlined in scenario S1.

S4: CLUSTER_SCALE_OUT¶

The policy may be checked AFTER a scale out operation is performed on the cluster. After new nodes have been created into the cluster, the loadbalance policy needs to notify the load balancer about the new members added. When the loadbalance policy is checked, there may and may not be other policies attached to the cluster. So the policy will need to check both cases.

It first checks if there is a “creation” key in the action’s data field. If the “creation” key is not found, it means the operation has nothing to do with the loadbalance policy. For example, it could be a request to resize a cluster, but the result is about removal of existing nodes instead of creation of new nodes. In this case, the policy checking aborts immediately.

When new nodes are created, the operation is expected to have filled the action’s data field with data that looks like the following example:

{
  "creation": {
    "count": 2,
    "nodes": [
      "4e54e810-6579-4436-a53e-11b18cb92e4c",
      "e730b3d0-056a-4fa3-9b1c-b1e6e8f7d6eb",
    ]
  }
}

The “nodes” field in the creation map always contain a list of node IDs for the nodes that have been created. After having get the node IDs, the policy proceeds to add these nodes to the load balancer (recorded in the policy binding data) by invoking the LBaaS API. If any update operation to the load balancer fails, the policy returns with an error message. If a node has been successfully added to the load balancer, the engine will record the load balancer IDs into the node’s data field.

S5: CLUSTER_ADD_NODES¶

When a CLUSTER_ADD_NODES operation is completed, it will record the IDs of the nodes into the creation property of the action’s data field. The logic to update the load balancer and the logic to update the data field of individual nodes are identical to that described in scenario S4.

S6: CLUSTER_RESIZE that Expands a Cluster¶

When a CLUSTER_RESIZE operation is completed and the operation results in some new nodes created and added to the cluster, it will record the IDs of the nodes into the creation property of the action’s data field. The logic to update the load balancer and the logic to update the data field of individual nodes are identical to that described in scenario S4.