Readiness and Liveness Cluster Checks

When a cluster is registered with Sveltos, Sveltos attempts to connect to its API server. A successful connection, results in the cluster's status being set to Ready. This status is a prerequisite for Sveltos to deploy add-ons and applications to clusters.

SveltosCluster Readiness Checks

While basic API server connectivity is a good initial indicator, it is not always a comprehensive measure of a cluster's readiness condition. In many cases, a cluster might be reachable via its API server, but be in a transitional state. For example, the control plane might be running, but the worker nodes, responsible for running workloads, might still be joining the cluster. As a result, deploying applications to such a cluster could lead to failures. Sveltos needs a more nuanced approach to determine when a cluster is in a stable and operational state. To address these limitations, Sveltos provides a mechanism for defining custom readiness checks for each registered cluster. The readiness checks allow administrators to specify the same conditions that must be met before Sveltos considers a cluster Ready.

The below YAML configuration defines a SveltosCluster resource that includes a readiness check. The check verifies the existence of at least one worker node in the cluster by iterating through all nodes and confirming that at least one node lacks the node-role.kubernetes.io/control-plane label, which is assumed to be absent on worker nodes in this particular setup.

apiVersion: lib.projectsveltos.io/v1beta1
kind: SveltosCluster
metadata:
  name: staging
  namespace: default
spec:
  kubeconfigName: clusterapi-workload-sveltos-kubeconfig
  readinessChecks:
  - name: worker-node-check
    condition: |-
      function evaluate()
        hs = {}
        hs.pass = false

        for _, resource in ipairs(resources) do
          if  not (resource.metadata.labels and resource.metadata.labels["node-role.kubernetes.io/control-plane"]) then
            hs.pass = true
          end
        end

        return hs
      end

    resourceSelectors:
    - group: ""
      kind: Node
      version: v1

SveltosCluster Liveness Checks

In addition to readiness checks, Sveltos employs liveness checks. While readiness checks determine if a cluster is initially ready for deployments, the liveness checks ensure the cluster remains healthy over time. Once a cluster is marked as Ready (readiness checks), Sveltos periodically connects to its API server and evaluates the defined liveness checks. Only when all liveness checks pass, Sveltos consider the cluster as Healthy.

The syntax for the liveness checks is identical to that of the readiness checks. They are defined within the SveltosCluster specification using the livenessChecks field.