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 using Kubernetes, it's like you run a large number of servers as one on which your applications run. Regardless of the number of servers in a Kubernetes cluster, the method for deploying applications in Kubernetes is always the same.

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 nodes, and you can deploy applications on specific nodes as configured.

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: functions as a transit station for components to communicate with each other, receives external requests, and writes information to etcd.
• Controller manager: performs 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: serves as a distributed data storage component that stores cluster configuration information.

In the production environment, multiple master nodes are deployed to ensure cluster high availability. For example, three master nodes will be deployed to create a CCE cluster in HA mode.

Worker node

A worker node is a compute node in a cluster, that is, a node running containerized applications. A worker node has the following components:

• kubelet: communicates with the container runtime, interacts with the API server, and manages containers on the node.
• kube-proxy: serves as an access proxy between application components.
• Container runtime: functions as the software for running containers. You can download images to build your container runtime, such as Docker.

Kubernetes Scalability

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

• Container Runtime Interface (CRI): provides computing resources when a container is running. It shields differences between container engines and interacts with each container engine through a unified interface.
• Container Network Interface (CNI): enables Kubernetes to support different networking implementations. For example, CCE has developed customized CNI plug-ins that allow your Kubernetes clusters to run in VPCs.
• Container Storage Interface (CSI): enables Kubernetes to support various classes of storage. For example, CCE is 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

  A pod is the smallest and simplest unit that you create or deploy in Kubernetes. A pod encapsulates one or more containers, storage resources, a unique network IP address, and options that govern how the containers should run.

• Deployment

  A Deployment can be viewed as an application encapsulating pods. 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. 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 the Linux system, it runs a specified job in a specified time range.

• DaemonSet

  A DaemonSet runs a pod on each node in a cluster and ensures that there is only one pod. 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 performs load balancing 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 configuration information 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 password, 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.

kubectl

kubectl is a command line tool for Kubernetes clusters. You can install kubectl on any machine and run kubectl commands to operate your Kubernetes cluster.

For details about how to install kubectl, see Connecting to a Cluster Through kubectl. After connection, you can run the kubectl cluster-info command to view the cluster information, as shown below.

# kubectl cluster-info
Kubernetes master is running at https://*.*.*.*:5443
CoreDNS is running at https://*.*.*.*:5443/api/v1/namespaces/kube-system/services/coredns:dns/proxy

To further debug and diagnose cluster problems, use 'kubectl cluster-info dump'.

Run the kubectl get nodes command to view information about nodes in the cluster.

 
NAME            STATUS    ROLES     AGE       VERSION
192.168.0.153   Ready     <none>    7m        v1.15.6-r1-20.3.0.2.B001-15.30.2
192.168.0.207   Ready     <none>    7m        v1.15.6-r1-20.3.0.2.B001-15.30.2
192.168.0.221   Ready     <none>    7m        v1.15.6-r1-20.3.0.2.B001-15.30.2

Description of Kubernetes Objects

Resources in Kubernetes can be described using a YAML or JSON file. An object description can be divided into the following four parts:

• typeMeta: metadata of the object type, specifying the API version and type of the object.
• objectMeta: metadata about 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 connect to 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 query 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.