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StatefulSet Basics and How to Debug a StatefulSet

What is Kubernetes StatefulSets

A StatefulSet is a set of pods with a unique, persistent hostname and ID. StatefulSets are designed to run stateful applications in Kubernetes with dedicated persistent storage. When pods run as part of a StatefulSet, Kubernetes keeps state data in the persistent storage volumes of the StatefulSet, even if the pods shut down.

StatefulSets are commonly used to run replicated databases with a unique persistent ID for each pod. Even if the pod is rescheduled to another machine, or moved to an entirely different data center, its identity is preserved. Persistent IDs allow you to associate specific storage volumes with pods throughout their lifecycle.

A new feature in Kubernetes 1.14 that is beneficial to StatefulSets is local persistent volumes. A local persistent volume is a local disk attached directly to a single Kubernetes node, which acts as a persistent storage resource for Kubernetes nodes. This means that you can attach and detach the same disk to multiple machines without needing remote services.

This is part of our series of articles about Kubernetes troubleshooting.

StatefulSet vs. DaemonSet vs. Deployment

StatefulSets, DaemonSets, and Deployments are different ways to deploy pods in Kubernetes. All three of these are defined via YAML configuration. When you apply this configuration in your cluster, an object is created, which is then managed by the relevant Kubernetes controller.

The key differences between these three objects can be described as follows:

  • StatefulSets run one or more pods with a persistent ID and persistent volumes, which is suitable for running stateful applications.
  • DaemonSets run one or more pods across the entire cluster or a certain set of nodes. This can be used to run administrative workloads such as logging and monitoring components.
  • Deployments run one or more pods, allowing you to define how many replicas of the pods need to run, on which types of nodes, and which deployment strategy should be used (for example, a Rolling deployment which replaces pods with a new version one by one, to prevent downtime).

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Creating StatefulSets

To create a StatefulSet, you need to define a manifest in YAML and create the StatefulSet in your cluster using kubectl apply.

After you create a StatefulSet, it continuously monitors the cluster and makes sure that the specified number of pods are running and available.

When a StatefulSet detects a pod that failed or was evicted from its node, it automatically deploys a new node with the same unique ID, connected to the same persistent storage, and with the same configuration as the original pod (for example, resource requests and limits). This ensures that clients who were previously served by the failed pod can resume their transactions.

The following example describes a manifest file for a StatefulSet. It was shared by Google Cloud. Typically, a StatefulSet is defined together with a Service object, which receives traffic and forwards it to the StatefulSet.

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: web
spec:
  selector:
    matchLabels:
      app: nginx
  serviceName: "nginx"
  replicas: 3
  template:
    metadata:
      labels:
        app: nginx
    spec:
      terminationGracePeriodSeconds: 10
      containers:
     —name: nginx
        image: k8s.gcr.io/nginx-slim:0.8
        ports:
       —containerPort: 80
          name: web
        volumeMounts:
       —name: www
          mountPath: /usr/share/nginx/html
  volumeClaimTemplates:
 —metadata:
      name: www
    spec:
      accessModes: [ "ReadWriteOnce" ]
      resources:
        requests:
          storage: 1Gi

A few important points about the StatefulSet manifest:

  • The code above creates a StatefulSet called web, containing three pods running an NGINX container image.
  • The specs.selector.matchLabels.app field must match the template.metadata.labels field (both are set to app: nginx). This ensures that the StatefulSet can correctly identify containers it is managing.
  • The pod exposes the web port defined as port 80.
  • volumeClaimTemplates provides storage using a PersistentVolumeClaim called www. This requests a storage volume that enables ReadWriteOnce access and has 1GB of storage.
  • A mount path is specified by template.spec.volumeMounts and is called www. This path indicates that storage volumes should be mounted in the /usr/share/nginx/html folder within the container.

Debugging a StatefulSet

If you are experiencing issues with a StatefulSet, take the following steps to debug it:

Step 1: List Pods in the StatefulSet

List all pods belonging to a StatefulSet using this command. Be sure to define the label specified in your StatefulSet manifest (substitute it for myapp below):

kubectl get pods -l app=myapp

The output will look like this:

NAME      READY     STATUS    RESTARTS   AGE
web-0     0/1       Pending   0          0s
web-0     0/1       Pending   0         0s
web-0     0/1       ContainerCreating   0         0s
web-0     1/1       Running   0         19s
web-1     0/1       Pending   0         0s
web-1     0/1       Pending   0         0s
web-1     0/1       ContainerCreating   0         0s
web-1     1/1       Running   0         18s

The following pod statuses indicate a problem with the pod:

Failed—all containers in the pod terminated, and at least one container exited with non-zero status or was forcibly terminated by Kubernetes.
Unknown—pods status could not be retrieved by Kubenetes, typically due to a communication error with the node.

You can run kubectl describe pod [pod-name] to get more information about pods that appear to be malfunctioning.

Step 2: Debug Individual Pods

Once you identify a problem with a pod, you’ll find it difficult to debug it, because the StatefulSet automatically terminates malfunctioning pods. To enable debugging, StatefulSets provide a special annotation you can use to suspend all controller actions in a pod, in particular scaling operations, allowing you to debug it.

Use this command to set the initialize=”false” annotation and prevent the StatefulSet from scaling the problematic pod:

kubectl annotate pods [pod-name]
pod.alpha.kubernetes.io/initialized="false" --overwrite

This will pause all operations of the StatefulSet on the pod and will prevent the StatefulSet from scaling down (deleting) the pod. You can then set a debug hook and execute commands within the pod’s containers, without interference from scaling operations.

Note that when initialized is set to "false", the entire StatefulSet will become unresponsive if the pod is unhealthy or unavailable.

When you are done debugging, run the same command and set the annotation to "true".

Step 3: Step-wise Initialization

If you didn’t succeed in debugging the pod using the above technique, this could mean there are race conditions when the StatefulSet is bootstrapped by Kubernetes. To overcome this, you can set initialized="false" in the StatefulSet manifest, and then create it in the cluster with this annotation.

Here is how to add the annotation directly to the manifest:

apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
  name: my-app
spec:
  serviceName: "my-app"
  replicas: 3
  template:
    metadata:
      labels:
        app: my-app
      annotations:
        pod.alpha.kubernetes.io/initialized: "false"
...

Now, when you use kubectl apply to create the StatefulSet, the following process will take place:

  • The StatefulSet loads the first pod and waits.
  • You can debug the pod to make sure it initializes correctly.
  • When you are done examining the pod, set the annotation to initialize="true" on the latest pod that was created:
    kubectl annotate pods [pod-name] 
    pod.alpha.kubernetes.io/initialized="true" --overwrite
    
  • The StatefulSet will now load the next pod and wait to allow you to debug it.
  • Repeat steps 3-4 until you examine all pods in the StatefulSet.

Deleting a StatefulSet as Part of Troubleshooting

Normally, when using a StatefulSet you do not need to manually remove StatefulSet pods. The StatefulSet controller is responsible for creating, resizing, and removing members of the StatefulSet, to make sure that the specified number of pods are ready to receive requests. A StatefulSet guarantees that, at most, one pod with a particular ID is running in the cluster at any given time (this is called the “at most one” semantic).

When debugging a StatefulSet, you might need to manually force a delete of pods. But be very careful when doing this, because it can violate the “at most one” semantic. StatefulSets are used to run distributed applications that require reliable network identity and storage. Having multiple members with the same ID can cause system failure and data loss (for example it can create a split brain scenario).

To delete a StatefulSet and all its pods, run this command:

kubectl delete statefulsets 

After removing the StatefulSet itself you may need to remove the associated Service object:

kubectl delete service 

If you need to delete the StatefulSet objects but keep the pods, run this command instead:

kubectl delete -f  --cascade=orphan

Later on, to delete the individual pods, use this command (substituting myapp with the label used by your pods):

kubectl delete pods -l app=myapp

These steps will help you identify basic issues with StatefulSets and resolve them. However, in many real-life scenarios, troubleshooting will be more complex. You will need to consider multiple aspects of the Kubernetes environment and diagnose issues in multiple moving parts. This can be extremely time-consuming without specialized tools – which is where Komodor comes in.

StatefulSet Troubleshooting Made Simple with Komodor

Kubernetes troubleshooting relies on the ability to quickly contextualize the problem with what’s happening in the rest of the cluster. More often than not, you will be conducting your investigation during fires in production. StatefulSet issues can involve issues related to pods, nodes, persistent volumes, applications, the underlying infrastructure, or a combination of these.

This is the reason why we created Komodor, a tool that helps dev and ops teams stop wasting their precious time looking for needles in (hay)stacks every time things go wrong.

Acting as a single source of truth (SSOT) for all of your k8s troubleshooting needs, Komodor offers:

  • Change intelligence: Every issue is a result of a change. Within seconds we can help you understand exactly who did what and when.
  • In-depth visibility: A complete activity timeline, showing all code and config changes, deployments, alerts, code diffs, pod logs and etc. All within one pane of glass with easy drill-down options.
  • Insights into service dependencies: An easy way to understand cross-service changes and visualize their ripple effects across your entire system.
  • Seamless notifications: Direct integration with your existing communication channels (e.g., Slack) so you’ll have all the information you need, when you need it.

If you are interested in checking out Komodor, use this link to sign up for a Free Trial.