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Example: Deploying Cassandra with Stateful Sets

Table of Contents

The following document describes the development of a cloud native Cassandra deployment on Kubernetes. When we say cloud native, we mean an application which understands that it is running within a cluster manager, and uses this cluster management infrastructure to help implement the application. In particular, in this instance, a custom Cassandra SeedProvider is used to enable Cassandra to dynamically discover new Cassandra nodes as they join the cluster.

This example also uses some of the core components of Kubernetes:

Prerequisites

This example assumes that you have a Kubernetes version >=1.2 cluster installed and running, and that you have installed the kubectl command line tool somewhere in your path. Please see the getting started guides for installation instructions for your platform.

This example also has a few code and configuration files needed. To avoid typing these out, you can git clone the Kubernetes repository to your local computer.

Cassandra Docker

The pods use the gcr.io/google-samples/cassandra:v12 image from Google’s container registry. The docker is based on debian:jessie and includes OpenJDK 8. This image includes a standard Cassandra installation from the Apache Debian repo. Through the use of environment variables you are able to change values that are inserted into the cassandra.yaml.

ENV VAR DEFAULT VALUE
CASSANDRA_CLUSTER_NAME ‘Test Cluster’
CASSANDRA_NUM_TOKENS 32
CASSANDRA_RPC_ADDRESS 0.0.0.0

Quickstart

If you want to jump straight to the commands we will run, here are the steps:

#
# StatefulSet
#

# clone the example repository
git clone https://github.com/kubernetes/examples
cd examples

# create a service to track all cassandra statefulset nodes
kubectl create -f cassandra/cassandra-service.yaml

# create a statefulset
kubectl create -f cassandra/cassandra-statefulset.yaml

# validate the Cassandra cluster. Substitute the name of one of your pods.
kubectl exec -ti cassandra-0 -- nodetool status

# cleanup
grace=$(kubectl get po cassandra-0 -o=jsonpath='{.spec.terminationGracePeriodSeconds}') \
  && kubectl delete statefulset,po -l app=cassandra \
  && echo "Sleeping $grace" \
  && sleep $grace \
  && kubectl delete pvc -l app=cassandra

#
# Resource Controller Example
#

# create a replication controller to replicate cassandra nodes
kubectl create -f cassandra/cassandra-controller.yaml

# validate the Cassandra cluster. Substitute the name of one of your pods.
kubectl exec -ti cassandra-xxxxx -- nodetool status

# scale up the Cassandra cluster
kubectl scale rc cassandra --replicas=4

# delete the replication controller
kubectl delete rc cassandra

#
# Create a DaemonSet to place a cassandra node on each kubernetes node
#

kubectl create -f cassandra/cassandra-daemonset.yaml --validate=false

# resource cleanup
kubectl delete service -l app=cassandra
kubectl delete daemonset cassandra

Step 1: Create a Cassandra Headless Service

A Kubernetes Service describes a set of Pods that perform the same task. In Kubernetes, the atomic unit of an application is a Pod: one or more containers that must be scheduled onto the same host.

The Service is used for DNS lookups between Cassandra Pods, and Cassandra clients within the Kubernetes Cluster.

Here is the service description:

apiVersion: v1
kind: Service
metadata:
  labels:
    app: cassandra
  name: cassandra
spec:
  clusterIP: None
  ports:
    - port: 9042
  selector:
    app: cassandra

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Create the service for the StatefulSet:

$ kubectl create -f cassandra/cassandra-service.yaml

The following command shows if the service has been created.

$ kubectl get svc cassandra

The response should be like:

NAME        CLUSTER-IP   EXTERNAL-IP   PORT(S)    AGE
cassandra   None         <none>        9042/TCP   45s

If an error is returned the service create failed.

Step 2: Use a StatefulSet to create Cassandra Ring

StatefulSets (previously PetSets) are a feature that was upgraded to a Beta component in Kubernetes 1.5. Deploying stateful distributed applications, like Cassandra, within a clustered environment can be challenging. We implemented StatefulSet to greatly simplify this process. Multiple StatefulSet features are used within this example, but is out of scope of this documentation. Please refer to the Stateful Set documentation.

The StatefulSet manifest that is included below, creates a Cassandra ring that consists of three pods.

This example includes using a GCE Storage Class, please update appropriately depending on the cloud you are working with.

apiVersion: "apps/v1beta1"
kind: StatefulSet
metadata:
  name: cassandra
spec:
  serviceName: cassandra
  replicas: 3
  template:
    metadata:
      labels:
        app: cassandra
    spec:
      containers:
      - name: cassandra
        image: gcr.io/google-samples/cassandra:v12
        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", "PID=$(pidof java) && kill $PID && while ps -p $PID > /dev/null; do sleep 1; done"]
        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: CASSANDRA_AUTO_BOOTSTRAP
            value: "false"
          - 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
      annotations:
        volume.beta.kubernetes.io/storage-class: fast
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi
---
kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: fast
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-ssd

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Create the Cassandra StatefulSet as follows:

$ kubectl create -f cassandra/cassandra-statefulset.yaml

Step 3: Validate and Modify The Cassandra StatefulSet

Deploying this StatefulSet shows off two of the new features that StatefulSets provides.

  1. The pod names are known
  2. The pods deploy in incremental order

First validate that the StatefulSet has deployed, by running kubectl command below.

$ kubectl get statefulset cassandra

The command should respond like:

NAME        DESIRED   CURRENT   AGE
cassandra   3         3         13s

Next watch the Cassandra pods deploy, one after another. The StatefulSet resource deploys pods in a number fashion: 1, 2, 3, etc. If you execute the following command before the pods deploy you are able to see the ordered creation.

$ kubectl get pods -l="app=cassandra"
NAME          READY     STATUS              RESTARTS   AGE
cassandra-0   1/1       Running             0          1m
cassandra-1   0/1       ContainerCreating   0          8s

The above example shows two of the three pods in the Cassandra StatefulSet deployed. Once all of the pods are deployed the same command will respond with the full StatefulSet.

$ kubectl get pods -l="app=cassandra"
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

Running the Cassandra utility nodetool will display the status of the ring.

$ kubectl exec cassandra-0 -- nodetool status
Datacenter: DC1-K8Demo
======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address   Load       Tokens       Owns (effective)  Host ID                               Rack
UN  10.4.2.4  65.26 KiB  32           63.7%             a9d27f81-6783-461d-8583-87de2589133e  Rack1-K8Demo
UN  10.4.0.4  102.04 KiB  32           66.7%             5559a58c-8b03-47ad-bc32-c621708dc2e4  Rack1-K8Demo
UN  10.4.1.4  83.06 KiB  32           69.6%             9dce943c-581d-4c0e-9543-f519969cc805  Rack1-K8Demo

You can also run cqlsh to describe the keyspaces in the cluster.

$ kubectl exec cassandra-0 -- cqlsh -e 'desc keyspaces'

system_traces  system_schema  system_auth  system  system_distributed

In order to increase or decrease the size of the Cassandra StatefulSet, you must use kubectl edit. You can find more information about the edit command in the documentation.

Use the following command to edit the StatefulSet.

$ kubectl edit statefulset cassandra

This will create an editor in your terminal. The line you are looking to change is replicas. The example does on contain the entire contents of the terminal window, and the last line of the example below is the replicas line that you want to change.

# 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/v1beta1
kind: StatefulSet
metadata:
  creationTimestamp: 2016-08-13T18:40:58Z
  generation: 1
  labels:
    app: cassandra
  name: cassandra
  namespace: default
  resourceVersion: "323"
  selfLink: /apis/apps/v1beta1/namespaces/default/statefulsets/cassandra
  uid: 7a219483-6185-11e6-a910-42010a8a0fc0
spec:
  replicas: 3

Modify the manifest to the following, and save the manifest.

spec:
  replicas: 4

The StatefulSet will now contain four pods.

$ kubectl get statefulset cassandra

The command should respond like:

NAME        DESIRED   CURRENT   AGE
cassandra   4         4         36m

For the Kubernetes 1.5 release, the beta StatefulSet resource does not have kubectl scale functionality, like a Deployment, ReplicaSet, Replication Controller, or Job.

Step 4: Delete Cassandra StatefulSet

Deleting and/or scaling a StatefulSet down will not delete the volumes associated with the StatefulSet. This is done to ensure safety first, your data is more valuable than an auto purge of all related StatefulSet resources. Deleting the Persistent Volume Claims may result in a deletion of the associated volumes, depending on the storage class and reclaim policy. You should never assume ability to access a volume after claim deletion.

Use the following commands to delete the StatefulSet.

$ grace=$(kubectl get po cassandra-0 -o=jsonpath='{.spec.terminationGracePeriodSeconds}') \
  && kubectl delete statefulset -l app=cassandra \
  && echo "Sleeping $grace" \
  && sleep $grace \
  && kubectl delete pvc -l app=cassandra

Step 5: Use a Replication Controller to create Cassandra node pods

A Kubernetes Replication Controller is responsible for replicating sets of identical pods. Like a Service, it has a selector query which identifies the members of its set. Unlike a Service, it also has a desired number of replicas, and it will create or delete Pods to ensure that the number of Pods matches up with its desired state.

The Replication Controller, in conjunction with the Service we just defined, will let us easily build a replicated, scalable Cassandra cluster.

Let’s create a replication controller with two initial replicas.

apiVersion: v1
kind: ReplicationController
metadata:
  name: cassandra
  # The labels will be applied automatically
  # from the labels in the pod template, if not set
  # labels:
    # app: cassandra
spec:
  replicas: 2
  # The selector will be applied automatically
  # from the labels in the pod template, if not set.
  # selector:
      # app: cassandra
  template:
    metadata:
      labels:
        app: cassandra
    spec:
      containers:
        - command:
            - /run.sh
          resources:
            limits:
              cpu: 0.5
          env:
            - name: MAX_HEAP_SIZE
              value: 512M
            - name: HEAP_NEWSIZE
              value: 100M
            - name: CASSANDRA_SEED_PROVIDER
              value: "io.k8s.cassandra.KubernetesSeedProvider"
            - name: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: POD_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.podIP
          image: gcr.io/google-samples/cassandra:v12
          name: cassandra
          ports:
            - containerPort: 7000
              name: intra-node
            - containerPort: 7001
              name: tls-intra-node
            - containerPort: 7199
              name: jmx
            - containerPort: 9042
              name: cql
          volumeMounts:
            - mountPath: /cassandra_data
              name: data
      volumes:
        - name: data
          emptyDir: {}

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There are a few things to note in this description.

The selector attribute contains the controller’s selector query. It can be explicitly specified, or applied automatically from the labels in the pod template if not set, as is done here.

The pod template’s label, app:cassandra, matches the Service selector from Step 1. This is how pods created by this replication controller are picked up by the Service.”

The replicas attribute specifies the desired number of replicas, in this case 2 initially. We’ll scale up to more shortly.

Create the Replication Controller:


$ kubectl create -f cassandra/cassandra-controller.yaml

You can list the new controller:


$ kubectl get rc -o wide
NAME        DESIRED   CURRENT   AGE       CONTAINER(S)   IMAGE(S)                             SELECTOR
cassandra   2         2         11s       cassandra      gcr.io/google-samples/cassandra:v12   app=cassandra

Now if you list the pods in your cluster, and filter to the label app=cassandra, you should see two Cassandra pods. (The wide argument lets you see which Kubernetes nodes the pods were scheduled onto.)


$ kubectl get pods -l="app=cassandra" -o wide
NAME              READY     STATUS    RESTARTS   AGE       NODE
cassandra-21qyy   1/1       Running   0          1m        kubernetes-minion-b286
cassandra-q6sz7   1/1       Running   0          1m        kubernetes-minion-9ye5

Because these pods have the label app=cassandra, they map to the service we defined in Step 1.

You can check that the Pods are visible to the Service using the following service endpoints query:


$ kubectl get endpoints cassandra -o yaml
apiVersion: v1
kind: Endpoints
metadata:
  creationTimestamp: 2015-06-21T22:34:12Z
  labels:
    app: cassandra
  name: cassandra
  namespace: default
  resourceVersion: "944373"
  selfLink: /api/v1/namespaces/default/endpoints/cassandra
  uid: a3d6c25f-1865-11e5-a34e-42010af01bcc
subsets:
- addresses:
  - ip: 10.244.3.15
    targetRef:
      kind: Pod
      name: cassandra
      namespace: default
      resourceVersion: "944372"
      uid: 9ef9895d-1865-11e5-a34e-42010af01bcc
  ports:
  - port: 9042
    protocol: TCP

To show that the SeedProvider logic is working as intended, you can use the nodetool command to examine the status of the Cassandra cluster. To do this, use the kubectl exec command, which lets you run nodetool in one of your Cassandra pods. Again, substitute cassandra-xxxxx with the actual name of one of your pods.


$ kubectl exec -ti cassandra-xxxxx -- nodetool status
Datacenter: datacenter1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load       Tokens  Owns (effective)  Host ID                               Rack
UN  10.244.0.5  74.09 KB   256     100.0%            86feda0f-f070-4a5b-bda1-2eeb0ad08b77  rack1
UN  10.244.3.3  51.28 KB   256     100.0%            dafe3154-1d67-42e1-ac1d-78e7e80dce2b  rack1

Step 6: Scale up the Cassandra cluster

Now let’s scale our Cassandra cluster to 4 pods. We do this by telling the Replication Controller that we now want 4 replicas.


$ kubectl scale rc cassandra --replicas=4

You can see the new pods listed:


$ kubectl get pods -l="app=cassandra" -o wide
NAME              READY     STATUS    RESTARTS   AGE       NODE
cassandra-21qyy   1/1       Running   0          6m        kubernetes-minion-b286
cassandra-81m2l   1/1       Running   0          47s       kubernetes-minion-b286
cassandra-8qoyp   1/1       Running   0          47s       kubernetes-minion-9ye5
cassandra-q6sz7   1/1       Running   0          6m        kubernetes-minion-9ye5

In a few moments, you can examine the Cassandra cluster status again, and see that the new pods have been detected by the custom SeedProvider:


$ kubectl exec -ti cassandra-xxxxx -- nodetool status
Datacenter: datacenter1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load       Tokens  Owns (effective)  Host ID                               Rack
UN  10.244.0.6  51.67 KB   256     48.9%             d07b23a5-56a1-4b0b-952d-68ab95869163  rack1
UN  10.244.1.5  84.71 KB   256     50.7%             e060df1f-faa2-470c-923d-ca049b0f3f38  rack1
UN  10.244.1.6  84.71 KB   256     47.0%             83ca1580-4f3c-4ec5-9b38-75036b7a297f  rack1
UN  10.244.0.5  68.2 KB    256     53.4%             72ca27e2-c72c-402a-9313-1e4b61c2f839  rack1

Step 7: Delete the Replication Controller

Before you start Step 5, delete the replication controller you created above:


$ kubectl delete rc cassandra

Step 8: Use a DaemonSet instead of a Replication Controller

In Kubernetes, a Daemon Set can distribute pods onto Kubernetes nodes, one-to-one. Like a ReplicationController, it has a selector query which identifies the members of its set. Unlike a ReplicationController, it has a node selector to limit which nodes are scheduled with the templated pods, and replicates not based on a set target number of pods, but rather assigns a single pod to each targeted node.

An example use case: when deploying to the cloud, the expectation is that instances are ephemeral and might die at any time. Cassandra is built to replicate data across the cluster to facilitate data redundancy, so that in the case that an instance dies, the data stored on the instance does not, and the cluster can react by re-replicating the data to other running nodes.

DaemonSet is designed to place a single pod on each node in the Kubernetes cluster. That will give us data redundancy. Let’s create a DaemonSet to start our storage cluster:

apiVersion: extensions/v1beta1
kind: DaemonSet
metadata:
  labels:
    name: cassandra
  name: cassandra
spec:
  template:
    metadata:
      labels:
        app: cassandra
    spec:
      # Filter to specific nodes:
      # nodeSelector:
      #  app: cassandra
      containers:
        - command:
            - /run.sh
          env:
            - name: MAX_HEAP_SIZE
              value: 512M
            - name: HEAP_NEWSIZE
              value: 100M
            - name: CASSANDRA_SEED_PROVIDER
              value: "io.k8s.cassandra.KubernetesSeedProvider"
            - name: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: POD_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.podIP
          image: gcr.io/google-samples/cassandra:v12
          name: cassandra
          ports:
            - containerPort: 7000
              name: intra-node
            - containerPort: 7001
              name: tls-intra-node
            - containerPort: 7199
              name: jmx
            - containerPort: 9042
              name: cql
              # If you need it it is going away in C* 4.0
              #- containerPort: 9160
              #  name: thrift
          resources:
            requests:
              cpu: 0.5
          volumeMounts:
            - mountPath: /cassandra_data
              name: data
      volumes:
        - name: data
          emptyDir: {}

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Most of this DaemonSet definition is identical to the ReplicationController definition above; it simply gives the daemon set a recipe to use when it creates new Cassandra pods, and targets all Cassandra nodes in the cluster.

Differentiating aspects are the nodeSelector attribute, which allows the DaemonSet to target a specific subset of nodes (you can label nodes just like other resources), and the lack of a replicas attribute due to the 1-to-1 node- pod relationship.

Create this DaemonSet:


$ kubectl create -f cassandra/cassandra-daemonset.yaml

You may need to disable config file validation, like so:


$ kubectl create -f cassandra/cassandra-daemonset.yaml --validate=false

You can see the DaemonSet running:


$ kubectl get daemonset
NAME        DESIRED   CURRENT   NODE-SELECTOR
cassandra   3         3         <none>

Now, if you list the pods in your cluster, and filter to the label app=cassandra, you should see one (and only one) new cassandra pod for each node in your network.


$ kubectl get pods -l="app=cassandra" -o wide
NAME              READY     STATUS    RESTARTS   AGE       NODE
cassandra-ico4r   1/1       Running   0          4s        kubernetes-minion-rpo1
cassandra-kitfh   1/1       Running   0          1s        kubernetes-minion-9ye5
cassandra-tzw89   1/1       Running   0          2s        kubernetes-minion-b286

To prove that this all worked as intended, you can again use the nodetool command to examine the status of the cluster. To do this, use the kubectl exec command to run nodetool in one of your newly-launched cassandra pods.


$ kubectl exec -ti cassandra-xxxxx -- nodetool status
Datacenter: datacenter1
=======================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address     Load       Tokens  Owns (effective)  Host ID                               Rack
UN  10.244.0.5  74.09 KB   256     100.0%            86feda0f-f070-4a5b-bda1-2eeb0ad08b77  rack1
UN  10.244.4.2  32.45 KB   256     100.0%            0b1be71a-6ffb-4895-ac3e-b9791299c141  rack1
UN  10.244.3.3  51.28 KB   256     100.0%            dafe3154-1d67-42e1-ac1d-78e7e80dce2b  rack1

Note: This example had you delete the cassandra Replication Controller before you created the DaemonSet. This is because – to keep this example simple – the RC and the DaemonSet are using the same app=cassandra label (so that their pods map to the service we created, and so that the SeedProvider can identify them).

If we didn’t delete the RC first, the two resources would conflict with respect to how many pods they wanted to have running. If we wanted, we could support running both together by using additional labels and selectors.

Step 9: Resource Cleanup

When you are ready to take down your resources, do the following:


$ kubectl delete service -l app=cassandra
$ kubectl delete daemonset cassandra

Custom Seed Provider

A custom SeedProvider is included for running Cassandra on top of Kubernetes. Only when you deploy Cassandra via a replication control or a daemonset, you will need to use the custom seed provider. In Cassandra, a SeedProvider bootstraps the gossip protocol that Cassandra uses to find other Cassandra nodes. Seed addresses are hosts deemed as contact points. Cassandra instances use the seed list to find each other and learn the topology of the ring. The KubernetesSeedProvider discovers Cassandra seeds IP addresses via the Kubernetes API, those Cassandra instances are defined within the Cassandra Service.

Refer to the custom seed provider README for further KubernetesSeedProvider configurations. For this example you should not need to customize the Seed Provider configurations.

See the image directory of this example for specifics on how the container docker image was built and what it contains.

You may also note that we are setting some Cassandra parameters (MAX_HEAP_SIZE and HEAP_NEWSIZE), and adding information about the namespace. We also tell Kubernetes that the container exposes both the CQL and Thrift API ports. Finally, we tell the cluster manager that we need 0.1 cpu (0.1 core).

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