How does Kubernetes create Pods?
I.e. what are the sequential steps involved in creating a Pod, is it implemented in Kubernetes?
Any code reference in Kubernetes repo would also be helpful.
A Pod is described in a definition file, and ran as a set of Docker containers on a given host which is part of the Kubernetes cluster, much like docker-compose does, but with several differences.
Precisely, a Pod always contains multiple Docker containers, even though, only the containers defined by the user are usually visible through the API: A Pod has one container that is a placeholder generated by the Kubernetes API, that will hold the IP for the Pod (so that when a Pod is restarted, it's actually the client containers that are restarted, but the placeholder container remains and keeps the same IP, unlike in straight Docker or docker-compose, where recreating a composition or container changes the IP.)
How Pods are scheduled, created, started, restarted if needed, re-scheduled etc... it a much longer story and very broad question.
Related
I have a k8s cluster that runs the main workload and has a lot of nodes.
I also have a node (I call it the special node) that some of special container are running on that that is NOT part of the cluster. The node has access to some resources that are required for those special containers.
I want to be able to manage containers on the special node along with the cluster, and make it possible to access them inside the cluster, so the idea is to add the node to the cluster as a worker node and taint it to prevent normal workloads to be scheduled on it, and add tolerations on the pods running special containers.
The idea looks fine, but there may be a problem. There will be some other containers and non-container daemons and services running on the special node that are not managed by the cluster (they belong to other activities that have to be separated from the cluster). I'm not sure that will be a problem, but I have not seen running non-cluster containers along with pod containers on a worker node before, and I could not find a similar question on the web about that.
So please enlighten me, is it ok to have non-cluster containers and other daemon services on a worker node? Does is require some cautions, or I'm just worrying too much?
Ahmad from the above description, I could understand that you are trying to deploy a kubernetes cluster using kudeadm or minikube or any other similar kind of solution. In this you have some servers and in those servers one is having some special functionality like GPU etc., for deploying your special pods you can use node selector and I hope you are already doing this.
Coming to running separate container runtime on one of these nodes you need to consider two points mainly
This can be done and if you didn’t integrated the container runtime with
kubernetes it will be one more software that is running on your server
let’s say you used kubeadm on all the nodes and you want to run docker
containers this will be separate provided you have drafted a proper
architecture and configured separate isolated virtual network
accordingly.
Now comes the storage part, you need to create separate storage volumes
for kubernetes and container runtime separately because if any one
software gets failed or corrupted it should not affect the second one and
also for providing the isolation.
If you maintain proper isolation starting from storage to network then you can run both kubernetes and container runtime separately however it is not a suggested way of implementation for production environments.
Is it possible to get a shell inside a running POD instead of container running inside the POD ?
kubectl exec or rsh open a shell inside a running container not the POD.
No you can not. A pod is kubernetes concept and does not provide such ability. A pod can consist of one or more containers, and the pod describes what to be run and how in the containers.
Nope. what you are trying to do is not possible. Pod is just to manage containers. and Pod must contain at least one container.
My predecessors are right, you cannot shell inside a running POD. I suggest getting familiar with the official documentation regarding Pods in order to get a better understanding of this particular concept:
Pods are the smallest deployable units of computing that you can
create and manage in Kubernetes.
A Pod (as in a pod of whales or pea pod) is a group of one or more
containers, with shared storage/network resources, and a specification
for how to run the containers.
You can find more info regarding using or working with them as well as other details like resource sharing and communication.
Is it possible to plug and play storage to an active pod without restarting the pod? I want to bind a new storage to a running pod without restarting the pod. Does Kubernetes support this?
Most things in a Pod are immutable. In particular if you look at the API definition of a PodSpec it says in part (emphasis mine)
container: List of containers belonging to the pod. Containers cannot currently be added or removed. There must be at least one container in a Pod. Cannot be updated.
Typically you don't directly work with Pods; you work with a higher-level controller like a Deployment. There you can edit these things, and it reacts by creating new Pods with the new pod spec and then deleting the old Pods.
Also remember that sometimes the cluster itself will delete or restart a Pod (if its Node is over capacity or fails, for example) and you don't have any control over this. It's better to plan for your Pods to be periodically restarted than to try to prevent it.
I have my Kubernetes cluster and I need to know how long it takes to create a pod? Is there any Kubernetes command show me that ?
Thanks in advance
What you are asking for is not existing.
I think you should first understand the Pod Overview.
A Pod is the basic building block of Kubernetes–the smallest and simplest unit in the Kubernetes object model that you create or deploy. A Pod represents a running process on your cluster.
A Pod encapsulates an application container (or, in some cases, multiple containers), storage resources, a unique network IP, and options that govern how the container(s) should run. A Pod represents a unit of deployment: a single instance of an application in Kubernetes, which might consist of either a single container or a small number of containers that are tightly coupled and that share resources.
While you are deploying a POD it's going through phases
Pending
The Pod has been accepted by the Kubernetes system, but one or more of the Container images has not been created. This includes time before being scheduled as well as time spent downloading images over the network, which could take a while.
Running
The Pod has been bound to a node, and all of the Containers have been created. At least one Container is still running, or is in the process of starting or restarting.
Succeeded
All Containers in the Pod have terminated in success, and will not be restarted.
Failed
All Containers in the Pod have terminated, and at least one Container has terminated in failure. That is, the Container either exited with non-zero status or was terminated by the system.
Unknown
For some reason the state of the Pod could not be obtained, typically due to an error in communicating with the
host of the Pod.
As for Pod Conditions it have a type which can have following values:
PodScheduled: the Pod has been scheduled to a node;
Ready: the Pod is able to serve requests and should be added to the load balancing pools of all matching Services;
Initialized: all init containers have started successfully;
Unschedulable: the scheduler cannot schedule the Pod right now, for example due to lacking of resources or other constraints;
ContainersReady: all containers in the Pod are ready.
Please refer to the documentation regarding Pod Lifecycle for more information.
When you are deploying your POD, you have to consider how many containers will be running in it.
The image will have to be downloaded, depending on the size it might take longer. Also default pull policy is IfNotPresent, which means that Kubernetes will skip the image pull if it already exists.
You can find more about Updating Images can be found here.
You also need to consider how much resources your Master and Node has.
Kubernetes is an orchestration tool for the management of containers.
Kubernetes creates pods which are containing containers, instead of managing containers directly.
I read this about pods
I'm working with OpenShift V3 which is using pods. But in my apps, all demo's and all examples I see:
One pod contains one containers (it's possible to contain more and that could be an advantage of using pods). But in an OpenShift environment I don't see the advantage of this pods.
Can some explain me why OpenShift V3 is using kubernetes with pods and containers instead of an orchestration tool which is working with containers immediately (without pods).
There are many cases where our users want to run pods with multiple containers within OpenShift. A common use-case for running multiple containers is where a pod has a 'primary' container that does some job, and a 'side-car' container that does something like write logs to a logging agent.
The motivation for pods is twofold -- to make it easier to share resources between containers, and to enable deploying and replicating groups of containers that share resources. You can read more about them in the user-guide.
The reason we still use a Pod when only a single container is that containers do not have all the notions that are attached to pods. For example, pods have IP addresses. Containers do not -- they share the IP address associated with the pod's network namespace.
Hope that helps. Let me know if you'd like more clarification, or we can discuss on slack.