Help Center/ Cloud Container Engine/ Kubernetes Basics/ Containers and Kubernetes/ Kubernetes
Updated on 2024-09-24 GMT+08:00
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Kubernetes

What Is Kubernetes?

Kubernetes is a containerized application software system that can be easily deployed and managed. It facilitates container scheduling and orchestration.

For application developers, Kubernetes can be regarded as a cluster operating system. Kubernetes provides functions such as service discovery, scaling, load balancing, self-healing, and even leader election, freeing developers from infrastructure-related configurations.

When you use Kubernetes, it is like you run a large number of servers as one on which your applications run. Kubernetes enables you to deploy applications always using the same method, regardless of the number of servers in a cluster.

Figure 1 Running applications in a Kubernetes cluster

Kubernetes Cluster Architecture

A Kubernetes cluster consists of master nodes (Masters) and worker nodes (Nodes). Applications are deployed on worker nodes, and you can specify the nodes for deployment.

For CCE clusters, master nodes are hosted by CCE. You only need to create worker nodes.

The following figure shows the architecture of a Kubernetes cluster.

Figure 2 Kubernetes cluster architecture

Master node

A master node is the machine where the control plane components run, including API server, scheduler, controller manager, and etcd.

  • API server: a transit station for components to communicate with each other. It receives external requests and writes data into etcd.
  • Controller manager: carries out cluster-level functions, such as component replication, node tracing, and node fault fixing.
  • Scheduler: schedules containers to nodes based on various conditions (such as available resources and node affinity).
  • etcd: provides distributed data storage for cluster configurations.

In a production environment, multiple master nodes are deployed to ensure high cluster availability. For example, you can deploy three master nodes for your CCE cluster.

Worker node

A worker node is a compute node for running containerized applications in a cluster. A worker node consists of the following components:

  • kubelet: communicates with the container runtime, interacts with the API server, and manages containers on the node.
  • kube-proxy: an access proxy between application components.
  • Container runtime: an engine such as Docker software for downloading images and running containers.

Kubernetes Scalability

Kubernetes makes the Container Runtime Interface (CRI), Container Network Interface (CNI), and Container Storage Interface (CSI) open sourced. These interfaces maximize Kubernetes scalability and allow Kubernetes to focus on container scheduling.

  • CRI: provides computing resources for a container runtime. It shields differences between container engines and interacts with each container engine through a unified interface.
  • CNI: enables Kubernetes to support different networking implementations. For example, the custom CNI add-on of CCE allows your Kubernetes clusters to run in VPCs.
  • CSI: enables Kubernetes to support various classes of storage. For example, CCE can be interconnected with block storage (EVS), file storage (SFS), and object storage (OBS) services.

Basic Objects in Kubernetes

The following figure describes the basic objects in Kubernetes and the relationships between them.

Figure 3 Basic Kubernetes objects
  • Pod

    Pods are the smallest deployable units of computing that you can create and manage in Kubernetes. A pod is a group of one or more containers, with shared storage and network resources, and a specification for how to run the containers. Each pod has a separate IP address.

  • Deployment

    A Deployment manages a set of pods to run an application workload. It can contain one or more pods. Each pod has the same role, and the system automatically distributes requests to the pods of a Deployment.

  • StatefulSet

    A StatefulSet is used to manage stateful applications. Like Deployments, StatefulSets manage a group of pods based on an identical container spec. Where they differ is that StatefulSets maintain a fixed ID for each of their pods. These pods are created based on the same declaration but cannot replace each other. Each pod has a permanent ID regardless of how it is scheduled.

  • Job

    A job is used to control batch tasks. Jobs are different from long-term servo tasks (such as Deployments). The former can be started and terminated at specific time, while the latter runs unceasingly unless it is terminated. The pods managed by a job will be automatically removed after successfully completing tasks based on user configurations.

  • Cron job

    A cron job is a time-based job. Similar to the crontab of Linux, it runs a specified job in a specified time range.

  • DaemonSet

    A DaemonSet runs only one pod on each node in a cluster. This works well for certain system-level applications such as log collection and resource monitoring since they must run on each node and need only a few pods. A good example is kube-proxy.

  • Service

    A Service is used for pod access. With a fixed IP address, a Service forwards access traffic to pods and balances load for these pods.

  • Ingress

    Services forward requests based on Layer 4 TCP and UDP protocols. Ingresses can forward requests based on Layer 7 HTTPS and HTTPS protocols and make forwarding more targeted by domain names and paths.

  • ConfigMap

    A ConfigMap stores configurations in key-value pairs required by applications. With a ConfigMap, you can easily decouple configurations and use different configurations in different environments.

  • Secret

    A secret lets you store and manage sensitive information, such as authentication information, certificates, and private keys. Storing confidential information in a secret is safer and more flexible than putting it verbatim in a pod definition or in a container image.

  • PersistentVolume (PV)

    A PV describes a persistent data storage volume. It defines a directory for persistent storage on a host machine, for example, a mount directory of a network file system (NFS).

  • PersistentVolumeClaim (PVC)

    Kubernetes provides PVCs to apply for persistent storage. With PVCs, you only need to specify the type and capacity of storage without concerning about how to create and release underlying storage resources.

Setting Up a Kubernetes Cluster

Kubernetes introduces multiple methods for setting up a Kubernetes cluster, such as minikube and kubeadm.

If you do not want to create a Kubernetes cluster by coding, you can create one on the CCE console. The following sections use clusters created on the CCE console as examples.

Kubernetes Objects

Resources in Kubernetes can be described in YAML or JSON format. An object consists of the following parts:

  • typeMeta: metadata of the object type, specifying the API version and type of the object.
  • objectMeta: metadata of the object, including the object name and used labels.
  • spec: expected status of the object, for example, which image the object uses and how many replicas the object has.
  • status: actual status of the object, which can be viewed only after the object is created. You do not need to specify the status when creating an object.
Figure 4 YAML description file

Running Applications on Kubernetes

Delete status from the content in Figure 4 and save it as the nginx-deployment.yaml file, as shown below:

apiVersion: apps/v1
kind: Deployment
metadata:
  name:  nginx
  labels:
    app:  nginx
spec:
  selector:
    matchLabels:
      app: nginx
  replicas: 3
  template:
    metadata:
      labels:
    app:  nginx
    spec:
      containers:
      - name:  nginx
        image:  nginx:alpine
        resources:
          requests:
            cpu: 100m
            memory: 200Mi
          limits:
            cpu: 100m
            memory: 200Mi
      imagePullSecrets:
      - name: default-secret

Use kubectl to access the cluster and run the following command:

# kubectl create -f nginx-deployment.yaml 
deployment.apps/nginx created

After the command is executed, three pods are created in the Kubernetes cluster. You can run the following command to obtain the Deployment and pods:

# kubectl get deploy
NAME    READY   UP-TO-DATE   AVAILABLE   AGE
nginx   3/3     3            3           9s

# kubectl get pods
NAME                     READY   STATUS    RESTARTS   AGE
nginx-685898579b-qrt4d   1/1     Running   0          15s
nginx-685898579b-t9zd2   1/1     Running   0          15s
nginx-685898579b-w59jn   1/1     Running   0          15s

By now, we have walked you through the Kubernetes basics of containers and clusters, and provided you an example of how to use kubectl. The following sections will go deeper into Kubernetes objects, such as how they are used and related.