A new Kubernetes Deployment with 2+ replicas for high availability.
I want to be able to execute a command on the first pod only, let's say create a DB, and let the other replicas wait for the first one to complete.
To implement this, I just want to know in the pod if this is replica #1 or not.
So in the pod's entry point I can test:
if [ $REPLICA_ID -eq 1 ]; then
CreateDB
else
WaitForDB
fi
Can this be done in Kubernetes?
in Kubernetes a Deployment is considered stateless and therefore doesn't provide the feature you're looking for. You should rather look into StatefulSet and their features.
A StatefulSete.g. supports ordered creation and when combined with the generally available readinessProbe for you pods you could create the desired behaviour. Also the pod name is stable within a StatefulSet so your test could then be done with the hostname of the Pod.
Instead of the accepted answer, wouldn't an init container fit your problem description better?
Add some kind of semaphore system (if needed) to ensure it is executed correctly?
Related
Does anyone know how to delete pod from kubernetes master node? I have this one master node on bare-metal ubuntu server. When i'm trying to delete it with "kubectl delete pod .." or force deleting from there: https://kubernetes.io/docs/tasks/run-application/force-delete-stateful-set-pod/ it doesnt work. the pod is creating again and again...
The pods in a Statefulsets are managed by ReplicaSets and will be recreated again if the current and the desired replicas defined in the spec do not match.
The document you linked provides instructions as to how to kill the pods forcefully avoiding the graceful shutdown behaviour which can have unexpected behaviour depending on the application.
The link clearly states the pods will be recreated in the section:
Force deletions do not wait for confirmation from the kubelet that the Pod has been terminated. Irrespective of whether a force deletion is successful in killing a Pod, it will immediately free up the name from the apiserver. This would let the StatefulSet controller create a replacement Pod with that same identity; this can lead to the duplication of a still-running Pod, and if said Pod can still communicate with the other members of the StatefulSet, will violate the at most one semantics that StatefulSet is designed to guarantee.
If you want the pods to be stopped and new pods for the Statefulset do not get created, you need to scale down the Statefulset by changing the replicas to 0.
You can read the official docs for how to scale the Statefulset replicas.
The key to figuring out how to kill the pod will be to understand how it was created. For example, if the pod is part of a deployment with a declared replicas count as 1, Once you kill/ force kill, Kubernetes detects a mismatch between the desired state (the number of replicas defined in the deployment configuration) to the current state and will create a new pod to replace the one that was deleted - therefor in this example you will need to either scale the deployment to 0 or delete the deployment.
If we need to kill any pod we can just scale down the replica set.
kubectl scale deploy <deployment_name> --replicas=<expected_no_of_replicas>
Way of deleting pods will depends on how you created it. If you created it individually ( not part of a ReplicaSet/ReplicationController/Deployment ) then you can delete pod directly. otherwise the only option to delete is the scale option. In production setup what I believe is all are using Deployment option out of ReplicaSet/ReplicationController/Deployment( Please refer documents and understand the difference between all those three options )
I have a Kubernetes Deployment app with 3 replicas, which needs a 7GB storage for each replica, I want to be able to attach a new empty azureDisk storage to be mounted into each pod/replica created in this deployment.
Basically I have the following restrictions:
I must use Deployment, not a Statefulset
Each time a pod dies and a new pod is up, it shouldn't have a state, and it will have a new empty azureDisk attached to it.
the pods do not share their storage, each pod has its own 7GB storage.
the pods need to use azureDisk because I need a 7GB storage on demand, which means, dynamically creating azureStorage when I scale my deployment replicas.
When using azureDisk, I need to use it with Access mode type ReadWriteOnce (as says in the docs ) and it will attach the only 1 pod to this disk, that's found, but, that only works if I have 1 pod, if I have more than 1 pod, I can't use the same claim... is there any way to dynamically ask for more storages like the one in the first claim?
NOTE 1: I know there is a volumeClaimTemplates, but that's only related to a Statefulset.
NOTE 2: I don't care if a pod restarts 100 times, and this in turn creates 100 PV which only 1 is used, that is fine.
I'm not sure why you need to use a StatefulSet but the only I see to do this is to create your own operator for your application. The operator would have a controller that manages your pods similar to what a ReplicaSet does but with the exception that for every new pod that is instantiated a new PVC is created.
It might just be better to figure out how to run your application in a StatefulSet and use VolumeClaimTemplates
✌️
The main question is - Why? "if I have an application which doesn't have state, still I need a large volume for each pod"
Looking at this explanation you should focus on StateFull application. From my point of view it looks like you are forcing to use Deployment instead of StateFullSet for StateFull application
In your example probably you need pv which support different access modes.
The main problem you have experienced is that using pv with supported mode ReadWriteOnce you can bind at the same time only one pv by single node. So your pods in different nodes will not start due to failing volume mounting. You can use this approach only for ReadOnlyMany/ReadWriteMany scenario.
Please refer to other providers which have different capabilities for access modes like: filestore(gcp), AzureFile(azure), Glusterfs, NFS
Deployments vs. StatefulSets
I have a problem statement where in there is a Kubernetes cluster and I have some pods running on it.
Now, I want some functions/processes to run once per deployment, independent of number of replicas.
These processes use the same image like the image in deployment yaml.
I cannot use initcontainers and sidecars, because they will run along with main container on pod for each replica.
I tried to create a new image and then a pod out of it. But this pod keeps on running, which is not good for cluster resource, as it should be destroyed after it has done its job. Also, the main container depends on the completion on this process, in order to run the "command" part of K8 spec.
Looking for suggestions on how to tackle this?
Theoretically, You could write an admission controller webhook for intercepting create/update deployments and triggering your functions as you want. If your functions need to be checked, use ValidatingWebhookConfiguration for validating the process and then deny or accept commands.
i am new to kubernetes and i have some functionally that i need to implement.
i need to set an env variable for only one docker container in a service.
for example- if i have 3 users containers then 1 of them need to have env variable named master
i did it with nomad. nomad set an env variable named NOMAD_ALLOC_INDEX, that give me the index of the container, this way i checked that if the container index was 0 then it is master.
i try find if kubernetes have a similar variable but didn't find anywhere.
i also try find in google an alternative solution but ended up with nothing.
any ideas of how i can achieve it ?
If you want sequential indexes, StatefulSet is your solution. Otherwise lookup kubernetes leader election, there are ways to solve it with ie. sidecar container performing leader election and exposing status via http call so you can curl localhost:port and see if the pod is master or not.
(I am (all things considered) a Kubernetes rookie.)
I know that kubectl create -f myDeployment.yaml will send my deployment specification off to the cluster to be reified, and if it says to start three replicas of its contained pod template then Kubernetes will set about starting up three pods.
I wonder: is there a Kubernetes concept or practice of somehow uploading the deployment for reference later and then "activating" it later? Perhaps by, say, changing replicas from zero to some positive number? If this is not a meaningful question, or this isn't the Right Way To Think About Things, I'd appreciate pointers as well.
I don't think you idea would work well with Kubernetes. Firstly, there so no way of "pausing" a Deployment or any other ReplicationController or ReplicaSet, besides setting the replicas to 0, as you mentioned.
The next issue is, that the YAML you would get from the apiserver isn't the same as you created. The controller manager adds some annotations, default values and statuses. So it would be hard to verify the Deployment that way.
IMO a better way to verify Deployments is to add them to a version control system and peer-review the YAML files. Then you can create or update is on the apiserver with kubectl apply -f myDeployment.yaml. If the Deployment is wrong in term of syntax, then kubectl will complain about it and you could patch the Deployment accordingly. This also simplifies the update procedure of Deployments.
Deployment can be paused, please refer https://kubernetes.io/docs/user-guide/deployments/#pausing-and-resuming-a-deployment , or see information with kubectl rollout pause -h.
You can adjust replicas of a paused deployment, but changes on pod template will not trigger a rollout. If the deployment is paused in the middle of a rollout, then it will not continue until you resume it.