Cassandra

Cassandra is a popular distributed NoSQL open source database.

Using Ondat persistent volumes with Cassandra means that if a Cassandra pod fails, the cluster is only in a degraded state for as long as it takes Kubernetes to restart the pod. When the pod comes back up the pod data is immediately available. Should Kubernetes schedule the Cassandra pod on a new node, Ondat allows for the data to be available to the pod, irrespective of whether or not an Ondat master is located on the same node.

As Cassandra has features to allow it to handle replication careful consideration of whether to allow Ondat or Cassandra to handle replication is required.

Before you start, ensure you have Ondat installed and ready on a Kubernetes cluster. See our guide on how to install Ondat on Kubernetes for more information.

Deploying Cassandra on Kubernetes

1. You can find the latest files in the Ondat use cases repository

   git clone https://github.com/storageos/use-cases.git storageos-usecases
   

   StatefulSet defintion

   apiVersion: apps/v1
   kind: StatefulSet
   metadata:
    name: cassandra
   spec:
    selector:
      matchLabels:
        app: cassandra
    serviceName: cassandra
    replicas: 3
    ...
    spec:
         ...
         volumeMounts:
          - name: cassandra-data
            mountPath: /var/lib/cassandra
    ...
   volumeClaimTemplates:
    - metadata:
        name: cassandra-data
      spec:
        accessModes: ["ReadWriteOnce"]
        storageClassName: "storageos" # Ondat storageClass
        resources:
          requests:
            storage: 5Gi
   

   This excerpt is from the StatefulSet definition. This file contains the VolumeClaim template that will dynamically provision storage, using the Ondat storage class. Dynamic provisioning occurs as a volumeMount has been declared with the same name as a VolumeClaimTemplate.

2. Move into the Cassandra examples folder and create the objects

   cd storageos-usecases
   kubectl create -f ./cassandra
   

3. Confirm Cassandra is up and running.

   $ kubectl get pods -w -l app=cassandra
   NAME          READY   STATUS    RESTARTS   AGE
   cassandra-0   1/1     Running   0          8m32s
   cassandra-1   1/1     Running   0          7m51s
   cassandra-2   1/1     Running   0          6m36s
   

4. Connect to the Cassandra client pod and connect to the Cassandra server through the service

   $ kubectl exec -it cassandra-0 -- cqlsh cassandra-0.cassandra
   Connected to K8Demo at cassandra-0.cassandra:9042.
   [cqlsh 5.0.1 | Cassandra 3.11.3 | CQL spec 3.4.4 | Native protocol v4]
   Use HELP for help.
   cqlsh> SELECT cluster_name, listen_address FROM system.local;
   
    cluster_name | listen_address
   --------------+----------------
          K8Demo |   100.96.7.124
   
   (1 rows)