Migrate a Cassandra cluster to K8ssandra

How to migrate an existing Cassandra cluster that’s running in Kubernetes to K8ssandra.

The strategy to perform this migration to K8ssandra focuses on a datacenter migration.

The environment

It’s assumed that the Cassandra cluster is running in the same Kubernetes cluster in which K8ssandra will run. The Cassandra cluster may have been installed with another operator (such as CassKop), a Helm chart (such as bitnami/cassandra), or by directly creating the YAML manfiests for the StatefulSets and any other objects that were created.

Check the replication strategies of keyspaces

Confirm that the user-defined keyspaces and each of the system_auth, system_distributed, and system_traces are using NetworkTopologyStrategy.

It is generally recommended to use a replication factor of 3. If your keyspaces currently use SimpleStrategy and assuming you have at least 3 Cassandra nodes, then you would change the replication factor for system_auth as follows:

ALTER KEYSPACE system_auth WITH replication = {'class': 'NetworkTopologyStrategy', 'dc1': 3}

dc1 is the name of the datacenter as defined in cassandra-rackdc.properties, which you can find in the /etc/cassandra/ directory of your Cassandra installation.

Changing the recommendation may result in a topology change; that is, a token ownership change.

Check the endpoint snitch

Make sure that GossipingPropertyFileSnitch is used, and not SimpleSnitch.

Client changes

Make sure client services are using a LoadBalancingPolicy that will route requests to the local datacenter. Also make sure your clients are using LOCAL_* and not QUORUM consistency levels.

Here is an example application.conf file for version 4.11 of the Java driver that configures the LoadBalancingPolicy and the default consistency level:

# application.conf

datastax-java-driver {
    basic.load-balancing-policy {
        local-datacenter = dc1

    basic.request {
        consistency = LOCAL_QUORUM

Install K8ssandra

Before installing K8ssandra, make a note of the IP addresses of the seed nodes of the current datacenter. You will use the following to configure the additionalSeeds property in the k8ssandra chart. Here is an example chart properties overrides file named k8ssandra-values.yaml that we can use to install k8ssandra:

# Note this only demonstrates usage of the additionalSeeds property.
# relrefer to the chart documentation for other properties you may want
# to configure.
  # The cluster name needs to match the cluster name in the original
  # datacenter.
  clusterName: cassandra
  - name: dc2
    size: 3
  # The following should be replaced with actual IP addresses or
  # hostnames of pods in the original datacenter.
  - <dc1-seed>
  - <dc1-seed>
  - <dc1-seed>

Install k8ssandra as follows. Replace my-k8ssandra with whatever name you prefer:

helm install my-k8ssandra k8ssandra/k8ssandra -f k8ssandra-values.yaml

Run nodetool status <keyspace-name> to verify that the nodes in the new datacenter are gossiping with the old datacenter. For the keyspace argument, you can specify a user-defined one or system_auth. Here is some example output:

nodetool status system_auth


Datacenter: dc1
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load        Tokens       Owns (effective)  Host ID                               Rack
UN  236 KiB     256          100.0%            f659967d-07b8-49b8-9ca8-bd02e2a58911  rack1
UN   318.03 KiB  256          100.0%            4b58ef5a-5578-4126-b1b6-4fb9a2d2cd40  rack1
UN   341.91 KiB  256          100.0%            380848f0-297b-47fd-9509-56d70835f410  rack1
Datacenter: dc2
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load        Tokens       Owns (effective)  Host ID                               Rack
UN  71.04 KiB   256          0.0%              0380fb66-2697-4a90-80a3-cf6f4b1f3476  default
UN  97.22 KiB   256          0.0%              1fd067c8-6eb2-4fd6-bd88-f6fbb72e8ede  default
UN   97.21 KiB   256          0.0%              2a986937-856b-4758-9026-146dc4620eb4  default

You should expect to see 0.0% ownership for the nodes in the new datacenter. This is because we are not yet replicating to the new datacenter.

Update replication strategy

For each keyspace that uses NetworkTopologyStrategy, update the replication strategy to include the new datacenter as follows:

ALTER KEYSPACE system_auth WITH replication = {'class': 'NetworkTopologyStrategy', 'dc1': 3, 'dc2': 3}

Run the same nodetool status command that you ran previously. You should see that the token ownership has changed for the nodes in the new datacenter.

At this point nodes in the new datacenter will start receiving writes.

Stream data to the new nodes

Next you need to stream data from the original datacenter to the new datacenter to get the latter caught up with past writes. This can be done with the nodetool rebuild command. It needs to be run on each node in the new datacenter as follows:

nodetool rebuild <old-datacenter-name>

Stop sending traffic to the old datacenter

To stop sending traffic to the old datacenter, there are two steps:

  1. Route clients to the new datacenter
  2. Update replication strategies

Route clients to the new datacenter

Update the LoadBalancingPolicy of client services to route requests to the new datacenter. Here is an example for v4.11 of the Java driver where the new datacenter is named dc2:

# application.conf

datastax-java-driver {
    basic.load-balancing-policy {
        local-datacenter = dc2

    basic.request {
        consistency = LOCAL_QUORUM

Update replication strategies

Next we need to stop replicating data to nodes in the old datacenter. For each keyspace that was previously updated, we need to update it again. This time we will remove the old datacenter.

Here is an example that specifies the system_auth keyspace:

ALTER KEYSPACE system_auth WITH replication = {'class': 'NetworkTopologyStrategy', 'dc2': 3}

Remove the old datacenter

Now we are ready to do some cleanup and remove the old datacenter and associated resources that are no longer in use. There are three steps:

  1. Decommission nodes
  2. Remove old seed nodes
  3. Delete old datacenter resources from Kubernetes

Decommission nodes

Run the following on each node in the old datacenter:

nodetool decommission

Remove the old seed nodes

Remove the seeds nodes from the additionalSeeds chart property. You can simply remove the additionaSeeds property from the chart overrides file (see again the sample file shown in the Install K8ssandra section above). Edit your version of the values file, and then run a helm upgrade for the changes to take effect. Assuming that the edited chart properties are stored in k8ssandra-values.yaml:

helm upgrade <release-name> k8ssandra/k8ssandra -f k8ssandra-values.yaml

The command above will trigger a statefulset update, which in effect is a cluster-wide (with respect to Cassandra) rolling restart.

Delete the old datacenter resources from Kubernetes

The steps necessary here will vary depending on how you installed and managed the old datacenter. You want to make sure that the StatefulSets, PersistentVolumeClaims, PersistentVolumes, and any other objects created in association with the old datacenter are deleted in the Kubernetes cluster.

Next steps

Explore other K8ssandra tasks.

See the Reference topics for information about K8ssandra Helm charts, and a glossary.