Propose control plane resize feature.

Spec to move resize logic into driver to allow CAPI drivers to optionally implement this, if they choose. Change-Id: I3d25c010b38134dabf64185bd38dbb3acb8b1dd8
2024-01-11 15:00:12 +13:00 · 2024-01-11 15:00:12 +13:00 · 4dde4d5af1
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+Magnum Control Plane resize
+===========================
+
+This is a proposal to implement support for resizing the number
+of control plane nodes.
+
+
+Problem Description
+-------------------
+
+Currently, no driver in Magnum supports resizing the number of control plane nodes.
+
+Cluster API supports modifying the control plane node count to any odd number such
+that etcd's raft consensus can be established and isn't wasteful of nodes.
+
+In practice, this means 1, 3, 5 and 7 nodes are supported, though any odd number
+may be acceptable[1].
+
+
+Use Cases
+---------
+
+Below are some of the use cases that this change addresses:
+
+1. User creates a cluster with a single Control Plane node, and then
+   updates their requirement to have an HA control plane.
+2. User has a HA cluster and wants to save resources by reducing to 1.
+3. Moving between 3 and 5 control plane nodes may be a good choice for users
+   as their availability needs change. 
+
+All of these actions currently require re-creation of the cluster.
+
+
+Proposed Change
+---------------
+
+The proposed change includes:
+
+1. Move the logic of `default-master` resize away from the Conductor and into the 
+   driver.
+2. The driver may choose to validate the new size, and only support odd numbered
+   sizes such as 1, 3, 5 and 7.
+3. No existing drivers in Magnum will support this resize action. The first drivers
+   to use this will be out of tree Cluster API drivers.
+
+Check sections 'Data Model Impact' and 'REST API Impact' for more details.
+
+
+Alternatives
+------------
+
+The alternative is to continue requiring users to re-create clusters when
+they have a need for differing control plane sizes.
+
+
+Data Model Impact
+-----------------
+
+There is no data model impact.
+
+The database already stores the number of control plane nodes.
+
+
+REST API Impact
+---------------
+
+The REST API does not need to change.
+
+This feature will be supported and implemented using the existing resize API
+under `POST /v1/clusters/{cluster_ident}/actions/resize` by providing a
+`nodegroup` parameter of `default-master` and the requested size.
+
+This resize API currently does not permit any changes to the `default-master`
+nodegroup.
+
+
+Security Impact
+---------------
+
+None
+
+
+Notifications Impact
+--------------------
+
+None
+
+
+Developer Impact
+----------------
+
+Driver developers will have the option of supporting this resize action.
+
+The default will be backwards compatible in that it is not supported.
+
+
+Implementation
+--------------
+
+Assignee(s)
+-----------
+
+dalees
+
+
+Milestones
+----------
+Target Milestone for completion:
+  Caracal
+
+
+Work Items
+----------
+
+* Move resize validation logic from Conductor and move into driver.
+* Define a default implementation for all drivers. The default will be Not Supported, which
+  is consistent with the current behaviour.
+* Implement the resize action in one or more drivers. The assignee intends to implement this in
+  the CAPI Helm driver, and inform other out-of-tree developers of this optional feature.
+
+Dependencies
+------------
+
+None
+
+References
+----------
+
+[1] https://etcd.io/docs/v3.5/faq/#what-is-failure-tolerance