In Kubernetes deployment with 4 static pods and no autoscaling, what happens by default if one pod crashes? Will it be re-created automatically with the same ID/different ID or will the application continue running on 3 pods?
When a pod crashes, it will automatically be restarted. You will see this by the incrementing value of the pod's "Restarts" value when you do kubectl get pods
From the documentation: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#pod-template
Only a .spec.template.spec.restartPolicy equal to Always is allowed, which is the default if not specified.
In other words, a deployment will ALWAYS restart your pod, regardless, and you cannot change that behaviour.
A restart will not change the name of the pod (or ID has you have called it)
The only time the pod name will change is if the pod gets deleted. This can happen during autoscaling processes or if the pod gets evicted from a node.
You've specified no autoscaling in your deployment, but if you have specified a value of 4 replicas, as I suspect you have, then the eviction will cause that one pod to change names, as it gets recreated by another node, in order to meet your request for 4 replica.
By "changing names" I just mean the hash at the end of the pod name will change. So your pod named my-test-g4gsv may be renamed to my-test-4dsv4 after it goes to a new node.
There is a backoff policy for restarts. So if Kubernetes detects a pod has been restarted repeatedly, it will start delaying its restart attempts. You will notice this as a CrashLoopBackoff value under the pod status (instead of Running). While in this state, the pod is not started, so during this time, your deployment is essentially running with reduced replicas until Kubernetes starts it.
Related
I have a pod that had a Pod Disruption Budget that says at least one has to be running. While it generally works very well it leads to a peculiar problem.
I have this pod sometimes in a failed state (due to some development) so I have two pods, scheduled for two different nodes, both in a CrashLoopBackOff state.
Now if I want to run a drain or k8s version upgrade, what happens is that pod cannot ever be evicted since it knows that there should be at least one running, which will never happen.
So k8s does not evict a pod due to Pod Disruption Budget even if the pod is not running. Is there a way to do something with this? I think ideally k8s should treat failed pods as candidates for eviction regardless of the budget (as deleting a failing pod cannot "break" anything anyway)
...if I want to run a drain or k8s version upgrade, what happens is that pod cannot ever be evicted since it knows that there should be at least one running...
kubectl drain --disable-eviction <node> will delete pod that is protected by PDB. Since you are fully aware of the downtime, you can first delete the PDB in question before draining the node.
I hit this issue too during k8s upgrade. Fyi, as mentioned in the other answer, kubectl drain --disable-eviction <node> may cause service downtime, and deleting pods might not work always when deleted pods are immediately recreated by the deployment managing the pods. Also, even if the pods are deleted successfully, it may cause service downtime depending on PodDisruptionBudget.
Instead, I increased the number of replicas of the pods in the deployment to honor PodDisruptionBudget.minAvailable or PodDisruptionBudget.maxUnavailable and was able to successfully upgrade k8s while honoring PodDisruptionBudget.
I have a testing scenario to check if the API requests are being handled by another pod if one goes down. I know this is the default behaviour, but I want to stimulate the following scenario.
Pod replicas - 2 (pod A and B)
During my API requests, I want to kill/stop only pod A.
During downtime of A, requests should be handled by B.
I am aware that we can restart the deployment and also scale replicas to 0 and again to 2, but this won't work for me.
Is there any way to kill/stop/crash only pod A?
Any help will be appreciated.
If you want to simulate what happens if one of the pods just gets lost, you can scale down the deployment
kubectl scale deployment the-deployment-name --replicas=1
and Kubernetes will terminate all but one of the pods; you should almost immediately see all of the traffic going to the surviving pod.
But if instead you want to simulate what happens if one of the pods crashes and restarts, you can delete the pod
# kubectl scale deployment the-deployment-name --replicas=2
kubectl get pods
kubectl delete pod the-deployment-name-12345-f7h9j
Once the pod starts getting deleted, the Kubernetes Service should route all of the traffic to the surviving pod(s) (those with Running status). However, the pod is managed by a ReplicaSet that wants there to be 2 replicas, so if one of the pods is deleted, the ReplicaSet will immediately create a new one. This is similar to what would happen if the pod crashes and restarts (in this scenario you'd get the same pod and the same node, if you delete the pod it might come back in a different place).
As you mentioned you can manually kill or restart the pod that is the only solution to test the case or else you can try crashing the one single POD but in the end, it will create the same scenario POD will auto restart.
Or else may you can increase the Graceful shutdown period for deployment so this way POD might take time and stay in terminating state for a good amount of time and you can perform the test.
In kubernetes where pods are controlled by the replicaSet, if you kill a pod it will again be recreated. So the only way to do this is to scale down the number of replicas.
Let's say if your deployment had 4 replicas. You can scale down to 3 by running the command below
kubectl scale deployment <deployment-name> --replicas=3
My example is as show below
kubectl scale deployment hello-world --replicas=3
deployment.apps/hello-world scaled
I have a deployment that deploys a single pod with a persistent volume claim. If I switch off the node it is running on, after a while k8s terminates the pod and tries to spin it up elsewhere. However the new pod cannot attach the volume (Multi-Attach error for volume "pvc-...").
I can manually delete the old 'Terminating' pod with kubectl delete pod <PODNAME> --grace-period=0 --force and then things recover.
Is there a way to get Kubernetes to force delete the 'Terminating' pods after a timeout or something? Tx.
According to the docs:
A Pod is not deleted automatically when a node is unreachable. The
Pods running on an unreachable Node enter the 'Terminating' or
'Unknown' state after a timeout. Pods may also enter these states when
the user attempts graceful deletion of a Pod on an unreachable Node.
The only ways in which a Pod in such a state can be removed from the
apiserver are as follows:
The Node object is deleted (either by you, or by the Node Controller).
The kubelet on the unresponsive Node starts responding, kills the Pod and removes the entry from the apiserver.
Force deletion of the Pod by the user.
So I assume you are not deleting nor draining the node that is being shut down.
In general I'd advice to ensure any broken nodes are deleted from the node list and that should make Terminating pods to be deleted by controller manager.
Node deletion normally happens automatically, at least on kubernetes clusters running on the main cloud providers, but if that's not happening for you than you need a way to remove nodes that are not healthy.
Use Recreate in .spec.strategy.type of your Deployment. This tell Kubernetes to delete the old pods before creating new ones.
Ref: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#strategy
If I create a POD manifest (pod-definition.yaml) and set the restartPolicy: Always does that Pod also need to be associated with any controller (i.e., a Replicaset or Deployment)? The end goal here it to auto-start the container in the Pod should it die. Without a Pod being associated with a controller will that container automatically restart? What happens if the Pod has only one container?
The documentation is not clear here but it lead me to believe that the Pod must be under a controller for this to work, i.e., if you implicitly create a 8Ks object and specify a restart policy of Never you'll get a pod. If you specify always (the default) you'll get a deployment.
Pod without a controller(deployment, replication controller etc) and only with restartPolicy will not be restarted/rescheduled if the node(to be exact the kubelet on that node) where its running dies or drained or rebooted or for some other reason pod is evicted from the node. If the node is in good state and for some reason pod crashes it will be restarted on the same node without the need of a controller.
The reason is pod restartPolicy is handled by kubelet i.e pod is restarted by kubelet of the node.Now if the node dies kubelet is also dead and can not restart the pod. Hence you need to have a controller which will restart it in another node.
From the docs
restartPolicy only refers to restarts of the Containers by the kubelet
on the same node
In short if you want pods to survive a node failure or a kubelet failure of a node you should have a higher level controller.
I have Pod definition file only. Kubernetes will bring up the pod. What happens if it goes down? Would Kubernetes bring it up automatically? Or if we want certain numbers of pods up at all time, we MUST take the help of ReplicationController( or ReplicaSet in new versions)?
Although your question is not clear , but yes , if you have deployed the pod through deployment or replicaSet , then kubernetes will create another one if you or someone else deletes that pod.
If you have just the pod without any controller like ReplicaSet , then it goes forever as there is no one to take care of it.
In case , the app crashes inside pod then:
A CrashloopBackOff means that you have a pod starting, crashing, starting again, and then crashing again.
A PodSpec has a restartPolicy field with possible values Always, OnFailure, and Never which applies to all containers in a pod. The default value is Always and the restartPolicy only refers to restarts of the containers by the kubelet on the same node (so the restart count will reset if the pod is rescheduled in a different node). Failed containers that are restarted by the kubelet are restarted with an exponential back-off delay (10s, 20s, 40s …) capped at five minutes, and is reset after ten minutes of successful execution.
https://sysdig.com/blog/debug-kubernetes-crashloopbackoff/
restartPolicy pod only refers to restarts of the Containers by the kubelet on the same node.If there is no replication controller or deployment then if a node goes down kubernetes will not reschedule or restart the pods of that node into any other nodes.This is the reason pods are not recommended to be used directly in production.