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Components of Kubernetes Cluster

April 23, 2021

Components of Kubernetes Cluster

学内勉強会で使用した,Kubernetes clusterの構成要素について解説するslide.


April 23, 2021

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  1. Not Following... • What are these? ◦ Container ◦ Docker

    ◦ Kubernetes • How to ... ◦ construct k8s cluster ◦ use kubectl ◦ use built-in resources(e.g. Deployment) properly
  2. Components of k8s cluster • A cluster is a set

    of worker nodes. • Every worker node runs the Pods • Kubernetes cluster consists of ◦ C-plane components ▪ may include an interface providing connection to Cloud provider API ◦ Node components
  3. Components of k8s cluster#C-plane • C-plane has a responsibility to

    ◦ manage worker node(s) ◦ detect several events in a cluster ◦ serve API to interconnect with cloud provider (optional) ▪ AWS/GCE/OpenStack/etc • esp, kube-apiserver is the core-system of it. • In general, these components are deployed in a Node. ◦ the node is known as "master node" ◦ in prod, you should deploy C-plane comps to multiple machines(using Kubeadm or stuff)
  4. Components of k8s cluster#kube-apiserver • exposes the Kubernetes API to

    cluster's outside ◦ so it plays an important role as the front-end of C-plane comps. • Note that kube-apiserver scales "horizontally" (not vertically) ◦ this feature enables us to balance traffics between those instances • kube-apiserver is the only component is connected with etcd. ◦ other all components need to communicate with etcd through apiserver. ▪ even if it is a C-plane component!
  5. Components of k8s cluster#etcd • etcd is well known as

    "distributed key-value store" • You can construct a "etcd cluster" ◦ a consensus algorithm called "Raft" works in it ◦ actually the number of nodes in cluster should be odd
  6. Components of k8s cluster#kube-scheduler • kube-scheduler assigns a Pod to

    Node • When a Pod is created newly, It's not determined where It deploys to yet. • kube-scheduler detects some Pods they're not assigned any node yet • And apply a scheduling algorithm, then a Node is selected.
  7. Components of k8s cluster#k-c-m(stripped) • A controller is a control

    loop that watches the state of clusters, nodes, and resources. ◦ If a current state isn't desirable, a controller makes changes by requesting to kube-apiserver. • k-c-m is a set of built-in controllers. ◦ includes replicaset/deployment/service/etc
  8. Components of k8s cluster#kubelet • An agent that is in

    each Node. • Start some Pods scheduled by kube-scheduler, by communicating with container-runtime. ◦ You can deploy pods to a specified node by using a mechanism called "Static Pod".
  9. Components of k8s cluster#container-runtime • A software that is responsible

    for running containers • Kubernetes support any implementation of CRI ◦ Docker ◦ containerd ◦ CRI-O • If you're operating a cluster in a multi-tenant network ◦ preferred to use secure OCI runtime(e.g. kata-runtime)
  10. Components of k8s cluster#kube-proxy • A network-proxy runs on each

    Node. • You may need to know about Kubernetes Service before studying it.
  11. Appendix#Service • In k8s cluster, each Pod has its IP

    address. • A container will communicate to others with "localhost" in a pod. • There is a few issue if a pod wants to be connected with pods they're created dynamically(e.g. using deployment). ◦ How to get their IP addresses? ◦ Is there a way to balance traffics to them smart?
  12. Appendix#Service • A Service is a way to expose an

    application runs on a cluster. ◦ can also load-balance L4 traffics to several Pods. ◦ create an endpoint with given ServiceType. ▪ ClusterIP … provide a VIP it's only used in a cluster ▪ NodePort … allocating a port that is listened to by every Node. ▪ LoadBalancer … using an external LB. • A Service marks pods by label-selector ◦ marked Pods are "targeted" by a Service. • we're going back to kube-proxy.
  13. Components of k8s cluster#kube-proxy • kube-proxy receives some traffics to

    ClusterIP/NodePort. • kube-proxy can be configured with proxy-mode ◦ userspace … running transporter in user space ◦ iptables … running transporter in kernel space ▪ more efficiently than userspace mode ▪ iptables isn't designed for load-balancing ◦ IPVS … opmizing workloads using IP Virtual Server ▪ can use more optimized LB algorithms. • least-connection • source-hashing