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.
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.
You 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:
To complete this tutorial, you should already have a basic familiarity with Pods, Services, and StatefulSets.
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
In 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
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.
The StatefulSet manifest, included below, creates a Cassandra ring that consists of three Pods.
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: 1800
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
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.
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
Use 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
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.
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
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 |
© 2022 The Kubernetes Authors
Documentation Distributed under CC BY 4.0.
https://kubernetes.io/docs/tutorials/stateful-application/cassandra/