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.
Prerequisites
- Kubernetes cluster with the K8ssandra operators deployed:
- If you haven’t already installed a
K8ssandraClusterusing K8ssandra Operator, see the local install topic.
- If you haven’t already installed a
Create a cluster
Suppose we have the below K8ssandraCluster object, stored in a k8ssandra.yaml file:
apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
name: my-k8ssandra
spec:
cassandra:
serverVersion: "4.0.5"
datacenters:
- metadata:
name: dc1
size: 3
storageConfig:
cassandraDataVolumeClaimSpec:
storageClassName: standard
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 5Gi
We can create this cluster with the following command:
kubectl apply -f k8ssandra.yaml
The above definition will result in the creation of a CassandraDatacenter object named dc1 with
the size set to 3.
kubectl get cassandradatacenter dc1
Output:
NAME AGE
dc1 3m35s
The cass-operator deployment that’s installed by K8ssandra will in turn create the underlying
StatefulSet that has 3 Cassandra pods:
kubectl get statefulsets -l cassandra.datastax.com/datacenter=dc1
Output:
NAME READY AGE
my-k8ssandra-dc1-default-sts 3/3 7m35s
Add nodes
Add nodes by updating the size property of the K8ssandraCluster spec:
apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
name: my-k8ssandra
spec:
cassandra:
serverVersion: "4.0.5"
datacenters:
- metadata:
name: dc1
size: 4 # change this from 3 to 4
storageConfig:
cassandraDataVolumeClaimSpec:
storageClassName: standard
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 5Gi
We can then update the cluster by running kubectl apply again:
kubectl apply -f k8ssandra.yaml
Alternatively, we can also patch the existing object with a kubectl patch command:
kubectl patch k8c my-k8ssandra --type='json' -p='[{"op": "replace", "path": "/spec/cassandra/datacenters/0/size", "value": 4}]'
Underlying considerations when scaling up
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.
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 K8ssandraCluster whose size was
increased to 6. Assume that this value is beyond the number of available worker nodes:
kubectl get pods
Output:
NAME READY STATUS RESTARTS AGE
my-k8ssandra-dc1-default-sts-0 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-1 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-2 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-3 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-4 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-5 2/2 Running 0 87m
my-k8ssandra-dc1-default-sts-6 0/2 Pending 0 3m6s
Notice that the my-k8ssandra-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 my-k8ssandra-dc1-default-sts-6
Output:
...
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 FailedScheduling event.
To resolve the mismatch between the configured size and the available nodes, consider setting the
softPodAntiAffinity property to true in order to relax the constraint of only allowing one
Cassandra pod per Kubernetes worker node. This is useful in test/dev scenarios to minimise the
number of nodes required, but should not be done in production clusters.
Here is an updated K8ssandraCluster with softPodAntiAffinity:
apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
name: my-k8ssandra
spec:
cassandra:
serverVersion: "4.0.5"
datacenters:
- metadata:
name: dc1
size: 6
softPodAntiAffinity: true
# Resources must be specified for each Cassandra node when using softPodAntiAffinity
resources:
requests:
cpu: 1
memory: 2Gi
limits:
cpu: 2
memory: 2Gi
# It is also recommended to set the JVM heap size
config:
jvmOptions:
heap_initial_size: 1G
heap_max_size: 1G
storageConfig:
cassandraDataVolumeClaimSpec:
storageClassName: standard
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 5Gi
When applied, this configuration updates the size property of the CassandraDatacenter to 6. Then
cass-operator will in turn update the underlying StatefulSet, allowing more than one Cassandra
node to sit on the same worker node.
Conditions and events when scaling up
If you describe the CassandraDatacenter object, there should be a ScalingUp condition with its
status set to true. You should also see one or more ScalingUpRack events, one per rack; and
finally, you should also see StartedCassandra events, one per pod:
```bash
kubectl describe cassandradatacenter dc1
Output:
...
Status:
Cassandra Operator Progress: Updating
Conditions:
Last Transition Time: 2022-09-16T11:39:35Z
Message:
Reason:
Status: True
Type: ScalingUp
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal ScalingUpRack 8m45s cass-operator Scaling up rack default
Normal StartedCassandra 2m11s cass-operator Started Cassandra for pod my-k8ssandra-dc1-default-sts-1
...
You can wait for the scale up operation to be finished by checking the value of the ScalingUp
condition. This can also be done with the following kubectl wait command:
kubectl wait --for=condition=ScalingUp=false cassandradatacenter dc1 --timeout=30m
Remove nodes
Just like with adding nodes, removing nodes is simply a matter of changing the configured size
property. Then cass-operator does a few things when you decrease the datacenter size (see below).
Underlying considerations when scaling down
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.
Note
The 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 thatmy-k8ssandra-dc1-default-sts-3 will be deleted before
my-k8ssandra-dc1-default-sts-2.
Conditions and events when scaling down
If you describe the CassandraDatacenter object, there should be a ScalingDown condition with its
status set to true. You should also see one or more ScalingDownRack events, one per rack; and
finally, for each pod being decommissioned, you should also see one LabeledPodAsDecommissioning
and one DeletedPvc event:
kubectl describe cassandradatacenter dc1
Output:
...
Status:
Cassandra Operator Progress: Updating
Conditions:
Last Transition Time: 2022-09-16T11:51:20Z
Message:
Reason:
Status: True
Type: ScalingDown
Events:
Type Reason Age From Message
---- ------ ---- ---- -------
Normal LabeledPodAsDecommissioning 82m cass-operator Labeled node as decommissioning my-k8ssandra-dc1-default-sts-1
Normal ScalingDownRack 82m cass-operator Scaling down rack rack1
Normal DeletedPvc 80m cass-operator Claim Name: server-data-my-k8ssandra-dc1-default-sts-1
...
You can wait for the scale down operation to be finished by checking the value of the ScalingDown
condition. This can also be done with the following kubectl wait command:
kubectl wait --for=condition=ScalingDown=false cassandradatacenter dc1 --timeout=30m
Scaling a multi-rack datacenter
The above K8ssandraCluster example has a single rack. When multiple racks are present, the
operator will always bootstrap new nodes and decommission existing nodes in a strictly deterministic
order that strives to achieve balanced racks.
Scaling up with multiple racks
When scaling up, the operator will add new nodes to the racks with the fewest nodes. If two racks have the same number of nodes, the operator will start with the rack that comes first in the datacenter definition.
Let’s take a look at an example. Suppose we create a 4-node K8ssandraCluster with 3 racks:
apiVersion: k8ssandra.io/v1alpha1
kind: K8ssandraCluster
metadata:
name: my-k8ssandra
spec:
cassandra:
serverVersion: "4.0.5"
datacenters:
- metadata:
name: dc1
size: 4
racks:
- name: rack1
- name: rack2
- name: rack3
When this cluster is created, the target topology according to the rules above will be as follows:
rack1should have 2 nodesrack2should have 1 noderack3should have 1 node
To achieve this, the operator will assign each rack its own StatefulSet:
kubectl get statefulsets
Output:
NAME READY AGE
my-k8ssandra-dc1-rack1-sts 2/2 7m21s
my-k8ssandra-dc1-rack2-sts 1/1 7m21s
my-k8ssandra-dc1-rack3-sts 1/1 7m21s
When all the pods in all StatefulSets are ready to be started, the operator will then bootstrap the Cassandra nodes rack by rack, in this exact order:
rack1:my-k8ssandra-dc1-rack1-sts-0gets bootstrapped first and becomes a seed;rack2:my-k8ssandra-dc1-rack2-sts-0gets bootstrapped next and becomes a seed;rack3:my-k8ssandra-dc1-rack3-sts-0gets bootstrapped next.rack1:my-k8ssandra-dc1-rack1-sts-1gets bootstrapped last.
This is indeed the most balanced topology we could have achieved with 4 nodes and 3 racks, and the bootstrap order makes it so that no rack could have 2 nodes bootstrapped before the others.
Now let’s scale up to, say, 8 nodes. The operator will bootstrap 4 new nodes, exactly in the following order:
rack2will scale up from 1 to 2 nodes andmy-k8ssandra-dc1-rack2-sts-1gets bootstrapped;rack3will scale up from 1 to 2 nodes andmy-k8ssandra-dc1-rack3-sts-1gets bootstrapped;rack1will scale up from 2 to 3 nodes andmy-k8ssandra-dc1-rack1-sts-2gets bootstrapped;rack2will scale up from 2 to 3 nodes andmy-k8ssandra-dc1-rack2-sts-2gets bootstrapped.
When the scale up operation is complete, the target topology will be as follows:
rack1will have 3 nodes, among which 1 seed;rack2will have 3 nodes, among which 1 seed;rack3will have 2 nodes.
The StatefulSets will be updated accordingly:
kubectl get statefulsets
Output:
NAME READY AGE
my-k8ssandra-dc1-rack1-sts 3/3 19m
my-k8ssandra-dc1-rack2-sts 3/3 19m
my-k8ssandra-dc1-rack3-sts 2/2 19m
This is again the most balanced topology we could have achieved with 8 nodes and 3 racks.
Scaling down with multiple racks
When scaling down, the operator will remove nodes from the racks with the most nodes. If two racks have the same number of nodes, the operator will start with the rack that comes last in the datacenter definition.
Now let’s scale down the above cluster to 3 nodes. The operator will decommission 5 nodes, exactly in the following order:
rack2will scale down from 3 to 2 nodes andmy-k8ssandra-dc1-rack2-sts-2gets decommissioned;rack1will scale down from 3 to 2 nodes andmy-k8ssandra-dc1-rack1-sts-2gets decommissioned;rack3will scale down from 2 to 1 node andmy-k8ssandra-dc1-rack3-sts-1gets decommissioned;rack2will scale down from 2 to 1 node andmy-k8ssandra-dc1-rack2-sts-1gets decommissioned;rack1will scale down from 2 to 1 node andmy-k8ssandra-dc1-rack1-sts-1gets decommissioned.
When the scale down operation is complete, the cluster will reach the following topology:
rack1will have 1 node, among which 1 seed;rack2will have 1 node, among which 1 seed;rack3will have 1 node.
The StatefulSets will be updated accordingly:
kubectl get statefulsets
Output:
NAME READY AGE
my-k8ssandra-dc1-rack1-sts 1/1 30m
my-k8ssandra-dc1-rack2-sts 1/1 30m
my-k8ssandra-dc1-rack3-sts 1/1 30m
Next steps
- Explore other K8ssandra Operator tasks.
- See the Reference topics for information about K8ssandra Operator Custom Resource Definitions (CRDs) and the single K8ssandra Operator Helm chart.
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