k3s/docs/adrs/servicelb-ccm.md

Move the ServiceLB load-balancer controller into the K3s cloud provider

Date: 2022-09-29

Status

Accepted

Context

K3s includes a stub cloud-provider the implements just enough node lifecycle functionality (the cloudprovider.Instances interface) to get node addresses set properly, and clear the Uninitialized taint that is added to nodes when they first join the cluster. The cloud-provider interface also has extension points for load-balancer controllers, but we did not implement these, in favor of running a standalone ServiceLB controller that is directly hooked into the core Wrangler controllers.

Because it doesn't make use of the existing load-balancer interface, the ServiceLB controller must implement all the logic to watch Services, ensure that it's only handling services of the correct type and state, manage finalizers, and so on. This would all be handled by core Kubernetes code if we implemented the cloudprovider.LoadBalancer interface.

Decision

We will move the ServiceLB code into the cloud-controller, as a backend for the LoadBalancer interface implementation. Existing behavior for disabling node lifecycle functionality will be retained, such that users can still use ServiceLB alongside other cloud-controller-managers that handle node lifecycle. Support for customizing ServiceLB behavior via node labels will be retained.

Consequences

• K3s uses less resources when ServiceLB is disabled, as several core controllers are no longer started unconditionally.
• The --disable-cloud-controller flag now disables the CCM's cloud-node and cloud-node-lifecycle controllers that were historically the only supported controllers.
• The --disable=servicelb flag now disables the CCM's service controller.
• If the cloud-controller and servicelb are both disabled, the cloud-controller-manager is not run at all.