website/content/en/docs/tutorials/stateful-application/cassandra.md

---
title: "Example: Deploying Cassandra with a StatefulSet"
reviewers:
- ahmetb
content_template: templates/tutorial
weight: 30
---

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This tutorial shows you how to run [Apache Cassandra](http://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 >}}
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* 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" >}}.
{{% /capture %}}

{{% capture 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](/docs/getting-started-guides/minikube/) defaults to 1024MiB of memory and 1 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 >}}

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## 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-application-cluster/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
    ```

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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/kubernetes/tree/master/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`        |


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{{% capture 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)

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