Cluster/DC Level Settings

Keeping your CRDs short with cluster-level settings

Important: The following documentation applies only to K8ssandra Operator 1.5.0 and higher.

Most of the properties in a K8ssandraCluster object can be provided at two levels: either globally, in which case they will apply to all datacenters in the cluster; or at the datacenter level, in which case they will apply only to the datacenter in question.

This is particularly useful for multi-dc clusters, since, instead of repeating the same configuration for each datacenter, we can define a default configuration at cluster level, and only repeat at datacenter level the properties that we want to override for that specific datacenter.

Let’s have a look at a K8ssandraCluster manifest that defines a cluster with three datacenters:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  cassandra:
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3

Now let’s suppose that we want to customize the storage configuration for the cluster. We can do that by providing a storageConfig property for each datacenter:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  cassandra:
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
        # dc-level configuration for dc1 goes here
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: standard
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 5Gi
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
        # dc-level configuration for dc2 goes here
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: standard
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 5Gi
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3
        # dc-level configuration for dc3 goes here
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: standard
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 5Gi

That’s admittedly verbose, especially since the storage configuration is the same for all datacenters. Luckily, we can simplify the manifest above by moving the storageConfig property to the cluster level:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  cassandra:
    # cluster-level configuration goes here (applies to all datacenters unless overridden)
    storageConfig:
      cassandraDataVolumeClaimSpec:
        storageClassName: standard
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 5Gi
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3

Now, if we want to override the storage configuration for a specific datacenter, we can do that by providing a storageConfig property at the datacenter level for just that datacenter, and by providing only the properties that we want to override:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  cassandra:
    storageConfig:
      cassandraDataVolumeClaimSpec:
        storageClassName: standard
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 5Gi
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: premium
            # rest of the properties are inherited from the cluster-level configuration

The same technique can be used to customize other properties, such as the serverImage, serverVersion, racks, etc. Here is a more elaborate example:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  cassandra:
    serverVersion: "3.11.11"
    softPodAntiAffinity: true
    mgmtAPIHeap: 64Mi
    resources:
      requests:
        cpu: 1000m
        memory: 1Gi
    storageConfig:
      cassandraDataVolumeClaimSpec:
        storageClassName: standard
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 5Gi
    config:
      cassandraYaml:
        concurrent_reads: 4
        concurrent_writes: 4
        commitlog_directory: "/etc/commitlog"
      jvmOptions:
        heap_initial_size: 512M
        heap_max_size: 512M
    networking:
      hostNetwork: true
    racks:
      - name: rack1
        nodeAffinityLabels:
          "topology.kubernetes.io/zone": zone1
      - name: rack2
        nodeAffinityLabels:
          "topology.kubernetes.io/zone": zone2
      - name: rack3
        nodeAffinityLabels:
          "topology.kubernetes.io/zone": zone3
    containers:
      - name: my-sidecar
        image: my-sidecar:latest
        env:
          - name: MY_SIDECAR_VAR
            value: "my-sidecar-value"
        volumeMounts:
          - mountPath: /var/lib/my-sidecar
            name: my-sidecar-data
    extraVolumes:
      volumes:
        - name: my-sidecar-data
          emptyDir: { }
      pvcs:
        - name: commitlog-vol
          mountPath: "/etc/commitlog"
          pvcSpec:
            storageClassName: standard
            accessModes:
              - ReadWriteOnce
            resources:
              requests:
                storage: 100Mi
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3
        # dc-level configuration for dc3 goes here (only the properties that we want to override)
        serverVersion: "3.11.12"
        softPodAntiAffinity: false
        mgmtAPIHeap: 128Mi
        resources:
          # requests are inherited from the cluster-level configuration
          limits:
            cpu: 2000m
            memory: 4Gi
        storageConfig:
          cassandraDataVolumeClaimSpec:
            storageClassName: premium
        config:
          cassandraYaml:
            # rest of the configuration is inherited from the cluster-level configuration
            commitlog_directory: "/var/lib/cassandra/commitlog"
          jvmOptions:
            # rest of the JVM options are inherited from the cluster-level configuration
            heap_max_size: 1024M
        networking:
          hostNetwork: false
        racks:
          - name: rack3
            nodeAffinityLabels:
              "topology.kubernetes.io/region": europe
          - name: rack2
            nodeAffinityLabels:
              "topology.kubernetes.io/region": europe
          - name: rack1
            nodeAffinityLabels:
              "topology.kubernetes.io/region": europe
        containers:
          - name: my-sidecar
            image: my-sidecar:v.1.2.3
            env:
              - name: MY_SIDECAR_VAR
                valueFrom:
                  secretKeyRef:
                    name: my-sidecar-secret
                    key: my-sidecar-secret-key
        extraVolumes:
          pvcs:
            - name: commitlog-vol
              mountPath: "/var/lib/cassandra/commitlog"
              pvcSpec:
                storageClassName: premium
                resources:
                  requests:
                    storage: 100Mi

Let’s break down the above manifest: first off, we notice that dc1 and dc2 do not have any configuration overrides: they inherit from the cluster-level configuration as is; dc3, on the other hand, has a few overrides.

For example, in dc3 the serverVersion is different, the softPodAntiAffinity property has been turned off, and the mgmtAPIHeap property (heap size for the management-api agent) is twice as large as in other datacenters. Also, host networking has been disabled.

Overriding also applies to complex properties. Consider the resources property in dc3: its requests section will be inherited from the cluster-level configuration, but a new limits section is also defined. Thus, the resulting definition of the resources property in dc3 will be:

resources:
  requests:
    cpu: 1000m
    memory: 1Gi
  limits:
    cpu: 2000m
    memory: 4Gi

The same happens with the config property: cassandraYaml settings and jvmOptions defined at cluster level are merged with the dc-specific ones, and the resulting definition of the config property in dc3 will be:

config:
  cassandraYaml:
    concurrent_reads: 4
    concurrent_writes: 4
    commitlog_directory: "/var/lib/cassandra/commitlog"
  jvmOptions:
    heap_initial_size: 512M
    heap_max_size: 1024M

But there’s more: the racks property has been overridden as well. Racks are merged by name, which means that when two racks with the same name appear in a list of racks at both cluster and dc levels, they are merged together. Here, racks in dc3 will inherit the nodeAffinityLabels from the corresponding cluster-level configuration, but each one will be given one more label: topology.kubernetes.io/region. Thus, the resulting definition for each rack in dc3 will be:

racks:
  - name: rack1
    nodeAffinityLabels:
      "topology.kubernetes.io/zone": zone1
      "topology.kubernetes.io/region": europe
  - name: rack2
    nodeAffinityLabels:
      "topology.kubernetes.io/zone": zone2
      "topology.kubernetes.io/region": europe
  - name: rack3
    nodeAffinityLabels:
      "topology.kubernetes.io/zone": zone3
      "topology.kubernetes.io/region": europe

Note in our example above that you don’t need to enter the racks in the same order in both cluster and dc levels: again, the racks will be merged by name, regardless of the order in which they appear in the list.

A few other objects are also merged in a similar way when they appear in both cluster and dc levels: containers, extraVolumes.volumes, and extraVolumes.pvcs are some other examples. Inside a container section, all volumeMounts are merged by identical mount paths.

Back to our example, in dc3, the my-sidecar container has been modified: the image has been changed, and the MY_SIDECAR_VAR environment variable has been changed to read its value from a secret. Also, the extraVolumes.pvcs section has been modified: the commitlog-vol PVC has been modified to use a different storage class and to request a different amount of storage. The resulting definition of both the containers and extraVolumes properties in dc3 will be as follows:

containers:
  - name: my-sidecar
    image: my-sidecar:v.1.2.3
    env:
      - name: MY_SIDECAR_VAR
        valueFrom:
          secretKeyRef:
            name: my-sidecar-secret
            key: my-sidecar-secret-key
        volumeMounts:
          - mountPath: /var/lib/my-sidecar
            name: my-sidecar-data
extraVolumes:
  volumes:
    - name: my-sidecar-data
      emptyDir: { }
  pvcs:
    - name: commitlog-vol
      mountPath: "/var/lib/cassandra/commitlog"
      pvcSpec:
        storageClassName: premium
        accessModes:
          - ReadWriteOnce
        resources:
          requests:
            storage: 100Mi

The same merging rules apply to other components of your cluster, namely, Stargate and Telemetry.

Let’s consider another example:

apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
  name: cluster1
spec:
  # cluster-level Stargate configuration goes here
  stargate:
    size: 1
    resources:
      requests:
        cpu: 1000m
        memory: 1Gi
  # cluster-level Reaper configuration goes here
  reaper:
    resources:
      requests:
        cpu: 1000m
        memory: 1Gi
  # cluster-level Medusa configuration goes here
  medusa:
    storageProperties:
      bucketName: my-bucket
      prefix: my-prefix
  cassandra:
    # cluster-level Telemetry configuration goes here
    telemetry:
      prometheus:
        enabled: true
    datacenters:
      - metadata:
          name: dc1
        k8sContext: data-plane1
        size: 3
      - metadata:
          name: dc2
        k8sContext: data-plane2
        size: 3
      - metadata:
          name: dc3
        k8sContext: data-plane3
        size: 3
        # dc-level Stargate configuration goes here
        stargate:
          size: 2
          resources:
            limits:
              cpu: 2000m
              memory: 2Gi
        # dc-level Telemetry configuration goes here
        telemetry:
          prometheus:
            enabled: false
        # Reaper and Medusa configuration overrides are not allowed at dc level!

In the above example, the stargate section in dc3 will be merged with the cluster-level stargate section; and likewise for the telemetry sections. However, the reaper and medusa sections are not allowed to be overridden at dc level, as these components must have exactly the same configuration for the entire cluster.


Last modified June 12, 2024: Release v1.17.0 (8804770)