---
title: "Example: Deploying Cassandra with a StatefulSet"
reviewers:
- ahmetb
content_type: tutorial
weight: 30
---
<!-- overview -->
This tutorial shows you how to run [Apache Cassandra](https://cassandra.apache.org/) 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](/docs/concepts/workloads/controllers/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](https://minikube.sigs.k8s.io/docs/) 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:
```shell
minikube start --memory 5120 --cpus=4
```
{{< /caution >}}
<!-- lessoncontent -->
## 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:
{{< codenew file="application/cassandra/cassandra-service.yaml" >}}
Create a Service to track all Cassandra StatefulSet members from the `cassandra-service.yaml` file:
```shell
kubectl apply -f https://k8s.io/examples/application/cassandra/cassandra-service.yaml
```
### Validating (optional) {#validating}
Get the Cassandra Service.
```shell
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](/docs/tasks/debug/debug-application/debug-service/)
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 >}}
{{< codenew file="application/cassandra/cassandra-statefulset.yaml" >}}
Create the Cassandra StatefulSet from the `cassandra-statefulset.yaml` file:
```shell
# 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:
```shell
# 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:
```shell
kubectl get statefulset cassandra
```
The response should be similar to:
```
NAME DESIRED CURRENT AGE
cassandra 3 0 13s
```
The `StatefulSet` resource deploys Pods sequentially.
1. Get the Pods to see the ordered creation status:
```shell
kubectl get pods -l="app=cassandra"
```
The response should be similar to:
```shell
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](https://cwiki.apache.org/confluence/display/CASSANDRA2/NodeTool) inside the first Pod, to
display the status of the ring.
```shell
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:
```shell
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:
```yaml
# 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
```
1. Change the number of replicas to 4, and then save the manifest.
The StatefulSet now scales to run with 4 Pods.
1. Get the Cassandra StatefulSet to verify your change:
```shell
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:
```shell
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
```
1. Run the following command to delete the Service you set up for Cassandra:
```shell
kubectl delete service -l app=cassandra
```
## Cassandra container environment variables
The Pods in this tutorial use the [`gcr.io/google-samples/cassandra:v13`](https://github.com/kubernetes/examples/blob/master/cassandra/image/Dockerfile)
image from Google's [container registry](https://cloud.google.com/container-registry/docs/).
The Docker image above is based on [debian-base](https://github.com/kubernetes/release/tree/master/images/build/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" %}}
* Learn how to [Scale a StatefulSet](/docs/tasks/run-application/scale-stateful-set/).
* Learn more about the [*KubernetesSeedProvider*](https://github.com/kubernetes/examples/blob/master/cassandra/java/src/main/java/io/k8s/cassandra/KubernetesSeedProvider.java)
* See more custom [Seed Provider Configurations](https://git.k8s.io/examples/cassandra/java/README.md)