# Example: Deploying Cassandra with a StatefulSet

Example: Deploying Cassandra with a StatefulSetThis tutorial shows you how to run Apache Cassandra on Kubernetes. Cassandra, a database, needs persistent storage to provide data durability (application state). In this example, a custom Cassandra seed provider lets the database discover new Cassandra instances as they join the Cassandra cluster.StatefulSets make it easier to deploy stateful applications into your Kubernetes cluster. For more information on the features used in this tutorial, see StatefulSet.Note:Cassandra and Kubernetes both use the term node to mean a member of a cluster. In this tutorial, the Pods that belong to the StatefulSet are Cassandra nodes and are members of the Cassandra cluster (called a ring). When those Pods run in your Kubernetes cluster, the Kubernetes control plane schedules those Pods onto Kubernetes Nodes.When a Cassandra node starts, it uses a seed list to bootstrap discovery of other nodes in the ring. This tutorial deploys a custom Cassandra seed provider that lets the database discover new Cassandra Pods as they appear inside your Kubernetes cluster.ObjectivesCreate and validate a Cassandra headless Service.Use a StatefulSet to create a Cassandra ring.Validate the StatefulSet.Modify the StatefulSet.Delete the StatefulSet and its Pods.Before you beginYou need to have a Kubernetes cluster, and the kubectl command-line tool must be configured to communicate with your cluster. It is recommended to run this tutorial on a cluster with at least two nodes that are not acting as control plane hosts. If you do not already have a cluster, you can create one by using minikube or you can use one of these Kubernetes playgrounds:iximiuz LabsKillercodaKodeKloudPlay with KubernetesTo complete this tutorial, you should already have a basic familiarity with Pods, Services, and StatefulSets.Additional Minikube setup instructionsCaution:Minikube defaults to 2048MB of memory and 2 CPU. Running Minikube with the default resource configuration results in insufficient resource errors during this tutorial. To avoid these errors, start Minikube with the following settings:minikube start --memory 5120 --cpus=4 Creating a headless Service for CassandraIn Kubernetes, a Service describes a set of Pods that perform the same task.The following Service is used for DNS lookups between Cassandra Pods and clients within your cluster:application/cassandra/cassandra-service.yaml apiVersion: v1 kind: Service metadata: labels: app: cassandra name: cassandra spec: clusterIP: None ports: - port: 9042 selector: app: cassandra Create a Service to track all Cassandra StatefulSet members from the cassandra-service.yaml file:kubectl apply -f https://k8s.io/examples/application/cassandra/cassandra-service.yaml Validating (optional)Get the Cassandra Service.kubectl get svc cassandra The response isNAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE cassandra ClusterIP None <none> 9042/TCP 45s If you don't see a Service named cassandra, that means creation failed. Read Debug Services for help troubleshooting common issues.Using a StatefulSet to create a Cassandra ringThe StatefulSet manifest, included below, creates a Cassandra ring that consists of three Pods.Note:This example uses the default provisioner for Minikube. Please update the following StatefulSet for the cloud you are working with.application/cassandra/cassandra-statefulset.yaml apiVersion: apps/v1 kind: StatefulSet metadata: name: cassandra labels: app: cassandra spec: serviceName: cassandra replicas: 3 selector: matchLabels: app: cassandra template: metadata: labels: app: cassandra spec: terminationGracePeriodSeconds: 500 containers: - name: cassandra image: gcr.io/google-samples/cassandra:v13 imagePullPolicy: Always ports: - containerPort: 7000 name: intra-node - containerPort: 7001 name: tls-intra-node - containerPort: 7199 name: jmx - containerPort: 9042 name: cql resources: limits: cpu: "500m" memory: 1Gi requests: cpu: "500m" memory: 1Gi securityContext: capabilities: add: - IPC_LOCK lifecycle: preStop: exec: command: - /bin/sh - -c - nodetool drain env: - name: MAX_HEAP_SIZE value: 512M - name: HEAP_NEWSIZE value: 100M - name: CASSANDRA_SEEDS value: "cassandra-0.cassandra.default.svc.cluster.local" - name: CASSANDRA_CLUSTER_NAME value: "K8Demo" - name: CASSANDRA_DC value: "DC1-K8Demo" - name: CASSANDRA_RACK value: "Rack1-K8Demo" - name: POD_IP valueFrom: fieldRef: fieldPath: status.podIP readinessProbe: exec: command: - /bin/bash - -c - /ready-probe.sh initialDelaySeconds: 15 timeoutSeconds: 5 # These volume mounts are persistent. They are like inline claims, # but not exactly because the names need to match exactly one of # the stateful pod volumes. volumeMounts: - name: cassandra-data mountPath: /cassandra_data # These are converted to volume claims by the controller # and mounted at the paths mentioned above. # do not use these in production until ssd GCEPersistentDisk or other ssd pd volumeClaimTemplates: - metadata: name: cassandra-data spec: accessModes: [ "ReadWriteOnce" ] storageClassName: fast resources: requests: storage: 1Gi --- kind: StorageClass apiVersion: storage.k8s.io/v1 metadata: name: fast provisioner: k8s.io/minikube-hostpath parameters: type: pd-ssd Create the Cassandra StatefulSet from the cassandra-statefulset.yaml file:# Use this if you are able to apply cassandra-statefulset.yaml unmodified kubectl apply -f https://k8s.io/examples/application/cassandra/cassandra-statefulset.yaml If you need to modify cassandra-statefulset.yaml to suit your cluster, download https://k8s.io/examples/application/cassandra/cassandra-statefulset.yaml and then apply that manifest, from the folder you saved the modified version into:# Use this if you needed to modify cassandra-statefulset.yaml locally kubectl apply -f cassandra-statefulset.yaml Validating the Cassandra StatefulSetGet the Cassandra StatefulSet:kubectl get statefulset cassandra The response should be similar to:NAME DESIRED CURRENT AGE cassandra 3 0 13s The StatefulSet resource deploys Pods sequentially.Get the Pods to see the ordered creation status:kubectl get pods -l="app=cassandra" The response should be similar to:NAME READY STATUS RESTARTS AGE cassandra-0 1/1 Running 0 1m cassandra-1 0/1 ContainerCreating 0 8s It can take several minutes for all three Pods to deploy. Once they are deployed, the same command returns output similar to:NAME READY STATUS RESTARTS AGE cassandra-0 1/1 Running 0 10m cassandra-1 1/1 Running 0 9m cassandra-2 1/1 Running 0 8m Run the Cassandra nodetool inside the first Pod, to display the status of the ring.kubectl exec -it cassandra-0 -- nodetool status The response should look something like:Datacenter: DC1-K8Demo ====================== Status=Up/Down |/ State=Normal/Leaving/Joining/Moving -- Address Load Tokens Owns (effective) Host ID Rack UN 172.17.0.5 83.57 KiB 32 74.0% e2dd09e6-d9d3-477e-96c5-45094c08db0f Rack1-K8Demo UN 172.17.0.4 101.04 KiB 32 58.8% f89d6835-3a42-4419-92b3-0e62cae1479c Rack1-K8Demo UN 172.17.0.6 84.74 KiB 32 67.1% a6a1e8c2-3dc5-4417-b1a0-26507af2aaad Rack1-K8Demo Modifying the Cassandra StatefulSetUse kubectl edit to modify the size of a Cassandra StatefulSet.Run the following command:kubectl edit statefulset cassandra This command opens an editor in your terminal. The line you need to change is the replicas field. The following sample is an excerpt of the StatefulSet file:# Please edit the object below. Lines beginning with a '#' will be ignored, # and an empty file will abort the edit. If an error occurs while saving this file will be # reopened with the relevant failures. # apiVersion: apps/v1 kind: StatefulSet metadata: creationTimestamp: 2016-08-13T18:40:58Z generation: 1 labels: app: cassandra name: cassandra namespace: default resourceVersion: "323" uid: 7a219483-6185-11e6-a910-42010a8a0fc0 spec: replicas: 3 Change the number of replicas to 4, and then save the manifest.The StatefulSet now scales to run with 4 Pods.Get the Cassandra StatefulSet to verify your change:kubectl get statefulset cassandra The response should be similar to:NAME DESIRED CURRENT AGE cassandra 4 4 36m Cleaning upDeleting or scaling a StatefulSet down does not delete the volumes associated with the StatefulSet. This setting is for your safety because your data is more valuable than automatically purging all related StatefulSet resources.Warning:Depending on the storage class and reclaim policy, deleting the PersistentVolumeClaims may cause the associated volumes to also be deleted. Never assume you'll be able to access data if its volume claims are deleted.Run the following commands (chained together into a single command) to delete everything in the Cassandra StatefulSet:grace=$(kubectl get pod cassandra-0 -o=jsonpath='{.spec.terminationGracePeriodSeconds}') \ && kubectl delete statefulset -l app=cassandra \ && echo "Sleeping ${grace} seconds" 1>&2 \ && sleep $grace \ && kubectl delete persistentvolumeclaim -l app=cassandra Run the following command to delete the Service you set up for Cassandra:kubectl delete service -l app=cassandra Cassandra container environment variablesThe Pods in this tutorial use the gcr.io/google-samples/cassandra:v13 image from Google's container registry. The Docker image above is based on debian-base and includes OpenJDK 8.This image includes a standard Cassandra installation from the Apache Debian repo. By using environment variables you can change values that are inserted into cassandra.yaml.Environment variableDefault valueCASSANDRA_CLUSTER_NAME'Test Cluster'CASSANDRA_NUM_TOKENS32CASSANDRA_RPC_ADDRESS0.0.0.0What's nextLearn how to Scale a StatefulSet.Learn more about the KubernetesSeedProviderSee more custom Seed Provider Configurations