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Example: Deploying Cassandra with a StatefulSet
ahmetb
tutorial
30

This 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 {{< glossary_tooltip text="Nodes" term_id="node" >}}.

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. {{< /note >}}

{{% heading "objectives" %}}

  • Create and validate a Cassandra headless {{< glossary_tooltip text="Service" term_id="service" >}}.
  • Use a {{< glossary_tooltip term_id="StatefulSet" >}} to create a Cassandra ring.
  • Validate the StatefulSet.
  • Modify the StatefulSet.
  • Delete the StatefulSet and its {{< glossary_tooltip text="Pods" term_id="pod" >}}.

{{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}}

To complete this tutorial, you should already have a basic familiarity with {{< glossary_tooltip text="Pods" term_id="pod" >}}, {{< glossary_tooltip text="Services" term_id="service" >}}, and {{< glossary_tooltip text="StatefulSets" term_id="StatefulSet" >}}.

Additional Minikube setup instructions

{{< caution >}} 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

{{< /caution >}}

Creating a headless Service for Cassandra {#creating-a-cassandra-headless-service}

In Kubernetes, a {{< glossary_tooltip text="Service" term_id="service" >}} describes a set of {{< glossary_tooltip text="Pods" term_id="pod" >}} that perform the same task.

The following Service is used for DNS lookups between Cassandra Pods and clients within your cluster:

{{% code_sample file="application/cassandra/cassandra-service.yaml" %}}

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) {#validating}

Get the Cassandra Service.

kubectl get svc cassandra

The response is

NAME        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 ring

The 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. {{< /note >}}

{{% code_sample file="application/cassandra/cassandra-statefulset.yaml" %}}

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 StatefulSet

  1. Get 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.

  2. 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

  3. 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 StatefulSet

Use kubectl edit to modify the size of a Cassandra StatefulSet.

  1. 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

  2. Change the number of replicas to 4, and then save the manifest.

    The StatefulSet now scales to run with 4 Pods.

  3. 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

{{% heading "cleanup" %}}

Deleting 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. {{< /warning >}}

  1. 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

  2. Run the following command to delete the Service you set up for Cassandra:

    kubectl delete service -l app=cassandra

Cassandra container environment variables

The 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 variable Default value
CASSANDRA_CLUSTER_NAME 'Test Cluster'
CASSANDRA_NUM_TOKENS 32
CASSANDRA_RPC_ADDRESS 0.0.0.0

{{% heading "whatsnext" %}}