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A Statefulset is a Kubernetes controller that is used to manage and maintain one or more Pods. However, so do other controllers like ReplicaSets and, the more robust, Deployments. So what does Kubernetes use StatefulSets for? To answer this question, we need to discuss stateless versus stateful applications.
A stateless application is one that does not care which network it is using, and it does not need permanent storage. Examples of stateless apps may include web servers (Apache, Nginx, or Tomcat).
On the other hand, we have stateful apps. Let’s say you have a Solr database cluster that is managed by several Zookeeper instances. For such an application to function correctly, each Solr instance must be aware of the Zookeeper instances that are controlling it. Similarly, the Zookeeper instances themselves establish connections between each other to elect a master node. Due to such a design, Solr clusters are an example of stateful applications. If you deploy Zookeeper on Kubernetes, you’ll need to ensure that pods can reach each other through a unique identity that does not change (hostnames, IPs...etc.). Other examples of stateful applications include MySQL clusters, Redis, Kafka, MongoDB, and others.
Given this difference, Deployment is more suited to work with stateless applications. As far as a Deployment is concerned, Pods are interchangeable. While a StatefulSet keeps a unique identity for each Pod it manages. It uses the same identity whenever it needs to reschedule those Pods.
A Kubernetes Service acts as an abstraction layer. In a stateless application like an Nginx web server, the client does not (and should not) care which pod receives a response to the request. The connection reaches the Service, and it routes it to any backend pod. This is not the case in stateful apps. In the above diagram, a Solr pod may need to reach the Zookeeper master, not any pod. For this reason, part of the Statefulset definition entails a Headless Service. A Headless Service does not contain a ClusterIP. Instead, it creates several Endpoints that are used to produce DNS records. Each DNS record is bound to a pod. All of this is done internally by Kubernetes, but it’s good to have an idea about how it does it.
In below code snippet we are deploying a stateful application. For simplicity, are using Apache as the pod image. The deployment is made up of three Apache web servers; all of them are connected to a persistent volume. Create a new file and name it apache_stateful.yaml. Add the following to it:
kind: StorageClass apiVersion: storage.k8s.io/v1 metadata: name: www-disk provisioner: kubernetes.io/gce-pd parameters: type: pd-ssd zone: us-central1-a --- apiVersion: apps/v1 kind: StatefulSet metadata: name: webapp spec: selector: matchLabels: app: web serviceName: web-svc replicas: 3 template: metadata: labels: app: web spec: terminationGracePeriodSeconds: 10 containers: - name: web image: httpd:2.4 ports: - containerPort: 80 name: http volumeMounts: - name: www mountPath: /var/www/html volumeClaimTemplates: - metadata: name: www annotations: volume.beta.kubernetes.io/storage-class: www-disk spec: accessModes: [ "ReadWriteOnce" ] resources: requests: storage: 1Gi
Before we start discussing the details of this definition, notice that the file actually contains two definitions: the storage class that the StatefulSet is using and the StatefulSet itself.
Storage classes are Kubernetes objects that let the users specify which type of storage they need from the cloud provider. Different storage classes represent various service quality, such as disk latency and throughput, and are selected depending on the scenario they are used for and the cloud provider’s support. Persistent Volumes and Persistent Volume Claims use Storage Classes.
Persistent volumes act as an abstraction layer to save the user from going into the details of how storage is managed and provisioned by each cloud provider (in this example, we are using Google GCE). By definition, StatefulSets are the most frequent users of Persistent Volumes since they need permanent storage for their pods.
A Persistent Volume Claim is a request to use a Persistent Volume. If we are to use the Pods and Nodes analogy, then consider Persistent Volumes as the “nodes” and Persistent Volume Claims as the “pods” that use the node resources. The resources we are talking about here are storage properties, such as storage size, latency, throughput, etc.
apiVersion: apps/v1 kind: StatefulSet metadata: name: webapp
The definition starts with the API version that recognizes StatefulSets, the kind of the resource, and the metadata. In this example, we only added the resource name to the metadata, but you can also add labels to identify it further.
spec: selector: matchLabels: app: web serviceName: web-svc
The next thing we have is the spec, and the spec contains the pod selector. Like other Kubernetes controllers (for example, Deployments and ReplicaSets), StatefulSets use matchLabels to choose and acquire the pods that it should manage.
We also have a reference to a service that we are yet to create, the Headless Service that provides access to our pods.
The replicas object defines how many pods will this StatefulSet create.
The pod template defines how the pods get created. It contains the labels that the pods have (they must match the pod selector).
template: metadata: labels: app: web spec: terminationGracePeriodSeconds: 10 containers: - name: web image: httpd:2.4 ports: - containerPort: 80 name: http volumeMounts: - name: www mountPath: /var/www/html
The pod template spec part contains the terminationGracePeriodSeconds. This setting defines how long should Kubernetes wait after sending the shutdown signal to the pod before forcing deleting it. Kubernetes strongly recommends that you do not set this value to 0 (disabled) as pods in a StatefulSet are usually parts of clusters and should be allowed enough period to do a clean shutdown.
The containers part defines the image that this container uses (Apache httpd:2.4 in our case) and the port that the container will listen on (80).
The volumeMounts part is where you define the Persistent Volume Claim. The name of the volume claim is essential here, and it must match the VolumeClaimTemplate. It also represents the mount point that will be used inside the pod to access the persistent disk (/var/www/html).
volumeClaimTemplates: - metadata: name: www annotations: volume.beta.kubernetes.io/storage-class: www-disk spec: accessModes: [ "ReadWriteOnce" ] resources: requests: storage: 1Gi
Now that we have the definition file in place, we can use kubectl to apply it as follows:
kubectl apply -f apache-stateful.yaml
Since the definition file contains a StorageClass and a StatefulSet resource, the following output is displayed:
storageclass.storage.k8s.io/www-disk created statefulset.apps/webapp created
Our resources are available. Let’s see whether or not we have pods:
kubectl get pods NAME READY STATUS RESTARTS AGE webapp-0 0/1 ContainerCreating 0 8s
You may notice two things here: (1) there is only one pod created while we asked for three, and (2) the pod name contains the StatefulSet name.
This is the expected behavior. The StatefulSet will not create all the pods at once, like a Deployment, for example. It maintains order when starting and stopping the pods. Since StatefulSets maintain the pod identity, the pod name is the StatefulSet name followed by an incremental number.
Wait a few seconds and issue kubectl get pods again, you should see an output similar to the following:
NAME READY STATUS RESTARTS AGE webapp-0 1/1 Running 0 43s webapp-1 0/1 ContainerCreating 0 11s
Later on, the output becomes:
NAME READY STATUS RESTARTS AGE webapp-0 1/1 Running 0 112m webapp-1 1/1 Running 0 111m webapp-2 1/1 Running 0 111m
All our pods are now started.
Right now, the pods are running. But how can a web server access another one? This is done through the Service, so we need to create one. Open a new YAML file called apache_statefulset_service.yaml and add the following to it:
apiVersion: v1 kind: Service metadata: name: web-svc labels: app: web spec: ports: - port: 80 name: web clusterIP: None selector: app: web
Don’t forget to specify the service name and the pod selector labels to match the ones that you defined in the StatefulSet definition file.
Create the service by using kubectl:
kubectl apply -f apache_statefulset_service.yamlservice/web-access-svc created
Let’s have a look at the created components:
$ kubectl get statefulset NAME READY AGE webapp 3/3 21h
$ kubectl get pv NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE pvc-077a1891-a25b-11e9-9ecf-42010a800184 1Gi RWO Delete Bound default/www-webapp-2 www-disk 21h pvc-e79d8843-a25a-11e9-9ecf-42010a800184 1Gi RWO Delete Bound default/www-webapp-0 www-disk 21h pvc-fa398e2a-a25a-11e9-9ecf-42010a800184 1Gi RWO Delete Bound default/www-webapp-1 www-disk 21h
We have the Persistent Volumes
$ kubectl get pvc NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE www-webapp-0 Bound pvc-e79d8843-a25a-11e9-9ecf-42010a800184 1Gi RWO www-disk 21h www-webapp-1 Bound pvc-fa398e2a-a25a-11e9-9ecf-42010a800184 1Gi RWO www-disk 21h www-webapp-2 Bound pvc-077a1891-a25b-11e9-9ecf-42010a800184 1Gi RWO www-disk 21h
And the Persistent Volume Claims. Let’s see how we can connect and use our pods.
We need to test our setup. Let’s open a bash shell to one of the pods:
kubectl exec -it webapp-0 bash
The httpd image isn’t shipped with curl by default, so we need to install it:
apt update && apt install curl
Once it is installed, we can try connecting to the Service:
root@webapp-0:/usr/local/apache2# curl web-svc<html><body><h1>It works!</h1></body></html>>
This is the default page that Apache displays. The Service is routing the request to the backend pods.
The StatefulSet is all about uniquely identifying pods. So, let’s try connecting to a specific pod:
root@webapp-0:/usr/local/apache2# curl webapp-1.web-svc<html><body><h1>It works!</h1></body></html>
By prefixing the service name to the pod name, you can connect to that specific pod.
We start by deleting the Headless Service:
kubectl delete -f apache_stateful_service.yamlservice "web-svc" deleted
We could equally achieve the same result by running kubectl delete service web-svc
To delete the StatefulSet with the Persistent Volume and the Persistent Volume Claims, we use the definition file:
kubectl delete -f apache_stateful.yamlstorageclass.storage.k8s.io "www-disk" deletedstatefulset.apps "webapp" deleted
The controller honors the ten seconds grace time and gives the pods time to clean up. In our example, Apache should not take more than a few milliseconds to shut down. But, if it were serving thousands of requests, it would take more time to terminate.