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PLEASE NOTE: This document applies to the HEAD of the source tree

If you are using a released version of Kubernetes, you should refer to the docs that go with that version.

Documentation for other releases can be found at releases.k8s.io.

Cloud Native Deployments of Cassandra using Kubernetes

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:v9 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

Custom Seed Provider

A custom SeedProvider is included for running Cassandra on top of Kubernetes. 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 vis 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).

tl;dr Quickstart

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

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

# create a replication controller to replicate cassandra nodes
kubectl create -f examples/storage/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

# then, create a daemonset to place a cassandra node on each kubernetes node
kubectl create -f examples/storage/cassandra/cassandra-daemonset.yaml --validate=false

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

Step 1: Create a Cassandra 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.

An important use for a Service is to create a load balancer which distributes traffic across members of the set of Pods. But a Service can also be used as a standing query which makes a dynamically changing set of Pods available via the Kubernetes API. We'll show that in this example.

Here is the service description:

apiVersion: v1
kind: Service
metadata:
  labels:
    app: cassandra
  name: cassandra
spec:
  ports:
    - port: 9042
  selector:
    app: cassandra

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An important thing to note here is the selector. It is a query over labels, that identifies the set of Pods contained by this Service. In this case the selector is app=cassandra. If there are any pods with that label, they will be selected for membership in this service. We'll see that in action shortly.

Create the Cassandra service as follows:

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

Step 2: Use a Replication Controller to create Cassandra node pods

As we noted above, in Kubernetes, the atomic unit of an application is a Pod. A Pod is one or more containers that must be scheduled onto the same host. All containers in a pod share a network namespace, and may optionally share mounted volumes.

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: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: POD_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.podIP
          image: gcr.io/google-samples/cassandra:v9
          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
          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 examples/storage/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:v9   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 3: 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 4: Delete the Replication Controller

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

$ kubectl delete rc cassandra

Step 5: 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: POD_NAMESPACE
              valueFrom:
                fieldRef:
                  fieldPath: metadata.namespace
            - name: POD_IP
              valueFrom:
                fieldRef:
                  fieldPath: status.podIP
          image: gcr.io/google-samples/cassandra:v9
          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:
            request:
              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 examples/storage/cassandra/cassandra-daemonset.yaml

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

$ kubectl create -f examples/storage/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 6: Resource Cleanup

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

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

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