Scale your Cassandra cluster
This topic explains how to add and remove Cassandra nodes in a Kubernetes cluster, as well as insights into the underlying operations that occur with scaling.
NoteK8ssandra currently only supports a single-datacenter cluster.
- A Kubernetes environment.
- Helm is installed.
- K8ssandra is installed and running in Kubernetes - see the Quick starts.
Create a cluster
Suppose we install K8ssandra as follows:
helm install my-k8ssandra k8ssandra/k8ssandra -f k8ssandra-values.yaml
k8ssandra-values.yaml has these properties:
cassandra: clusterName: my-k8ssandra datacenters: - name: dc1 size: 3
helm install command will result in the creation of a
CassandraDatacenter object with the size set to 3. The cass-operator deployment that’s installed by K8ssandra will in turn create the underlying StatefulSet that has 3 Cassandra pods.
Add nodes by updating the size property of the datacenter. Example values file:
cassandra: clusterName: my-k8ssandra datacenters: - name: dc1 size: 4
Apply the changes with
helm upgrade my-k8ssandra k8ssandra/k8ssandra -f k8ssandra-values.yaml
Another way to upgrade your K8ssandra cluster is by passing in a
--set parameter. Also include a
--reuse-values parameter so that Helm will reuse previous values (other than the one you’re overriding with each
--set parameter). Without
--reuse-values it’s easy to make a mistake if you have other, additional properties that you previously set.
helm upgrade my-k8ssandra k8ssandra/k8ssandra --reuse-values --set cassandra.datacenters\[0\].size=4,cassandra.datacenters\[0\].name=dc1
Underlying considerations when increasing size values
By default, cass-operator configures the Cassandra pods so that Kubernetes will not schedule multiple Cassandra pods on the same worker node. If you try to increase the cluster size beyond the number of available worker nodes, you may find that the additional pods do not deploy.
Look at this example output from
kubectl get pods with a test cluster whose size was increased to 6. Assume that this value is beyond the number of available worker nodes:
kubectl get pods
NAME READY STATUS RESTARTS AGE test-dc1-default-sts-0 2/2 Running 0 87m test-dc1-default-sts-1 2/2 Running 0 87m test-dc1-default-sts-2 2/2 Running 0 87m test-dc1-default-sts-3 2/2 Running 0 87m test-dc1-default-sts-4 2/2 Running 0 87m test-dc1-default-sts-5 2/2 Running 0 87m test-dc1-default-sts-6 0/2 Pending 0 3m6s
Notice that the
test-dc1-default-sts-6 pod has a status of
Pending. We can use
kubectl describe pod to get more details about the pod:
kubectl describe pod test-dc1-default-sts-6
... Events: Type Reason Age From Message ---- ------ ---- ---- ------- Warning FailedScheduling 3m22s (x51 over 73m) default-scheduler 0/6 nodes are available: 6 node(s) didn't match pod affinity/anti-affinity, 6 node(s) didn't satisfy existing pods anti-affinity rules.
The output reveals a
To resolve the mismatch between the configured size and the available nodes, consider the following option to set the
allowMultipleNodesPerWorker property to relax the constraint of only allowing one Cassandra pod per Kubernetes worker node.
Here is an updated k8ssandra-values.yaml with
cassandra: clusterName: my-k8ssandra allowMultipleNodesPerWorker: true # resources must be set when allowMultipleNodesPerWorker is true. resources: requests: cpu: 2 memory: 2Gi limits: cpu: 2 memory: 2Gi # It is not required to set the heap but is recommended. heap: size: 1024M newGenSize: 512M datacenters: - name: dc1 size: 3
When applied to the test cluster, this configuration updates the size property of the
CassandraDatacenter. Then cass-operator will in turn update the underlying
If you check the status of the
CassandraDatacenter object, there should be a
ScalingUp condition with its status set to
true. It should look like this:
kubectl get cassandradatacenter dc1 -o yaml
... status: cassandraOperatorProgress: Updating conditions: - lastTransitionTime: "2021-03-30T22:01:48Z" message: "" reason: "" status: "True" type: ScalingUp ...
After the new nodes are up and running,
nodetool cleanup should run on all of the nodes except the new ones to remove keys and data that no longer belong to those nodes. There is no need to do this manually. The cass-operator deployment, which again is installed with K8ssandra, automatically runs
nodetool cleanup for you.
Just like with adding nodes, removing nodes is simply a matter of changing the configured
size property. The cass-operator does a few things when you decrease the datacenter size.
Underlying considerations when lowering size values
First, cass-operator checks that the remaining nodes have enough capacity to handle the increased storage capacity. If cass-operator determines that there is insufficient capacity, it will log a message. Example:
Not enough free space available to decommission. my-k8ssandra-dc1-default-sts-3 has 12345 free space, but 67891 is needed.
The reported units are in bytes.
The cass-operator deployment will also add a condition to the
CassandraDatacenter status. Example:
status: conditions: - lastTransitionTime: "2021-03-30T22:01:48Z" message: "Not enough free space available to decommission. my-k8ssandra-dc1-default-sts-3 has 12345 free space, but 67891 is needed." reason: "NotEnoughSpaceToScaleDown" status: "False" type: Valid ...
Next, cass-operator runs
nodetool decommission on the node to be removed. This step is done automatically on your behalf.
Lastly, the pod is terminated.
NoteThe StatefulSet controller manages the deletion of Cassandra pods. It deletes one pod at a time, in reverse order with respect to its ordinal index. This means for example that
my-k8ssandra-dc1-default-sts-3will be deleted before
Explore other K8ssandra tasks.
See the Reference topics for information about K8ssandra Helm charts, and a glossary.
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