1, Service discovery

1. Concept

In short, service discovery is the process of mutual positioning between services (Applications).
Service discovery is not unique in the era of cloud computing. It will also be used in the era of traditional single architecture. Service discovery is more needed in the following application scenarios
Services (Applications) are highly dynamic
Service (application) updates and releases frequently
The service (application) supports automatic scaling
In k8s cluster, the IP of POD is constantly changing. How to "respond to changes with invariance"
The service resource is abstracted and associated with a group of pods through the tag selector
The cluster network is abstracted, and the service access point is fixed through the relatively fixed "cluster IP"
So how to automatically associate the "name" and "cluster network IP" of Service resources to achieve the purpose that services are automatically discovered by the cluster?
Consider the traditional DNS model: hdss7-21 host. com->10.4.7.21
Can we build such a model in k8s: nginx DS - > 192.168.0.5
2. k8s service discovery method - DNS
3. Plug in (software) to realize DNS function in k8s
　　　　kube-dns-kubernetes-v1.2 to kubernetes-v1 ten
　　　　Coredns-kubernetes-v1.11 to date
Note: DNS in k8s is not omnipotent! It should only be responsible for automatically maintaining the relationship between "service name" - > "cluster network IP"

2, Core components deployed k8s (CoreDNS)

1. Write configuration file

vim /etc/nginx/conf.d/k8s-yaml.od.com.conf
server {
      listen        80;
      server_name   k8s-yaml.od.com;

     location / {
          autoindex on;
          default_type text/plain;
          root /data/k8s-yaml;
     }
}
mkdir /data/k8s-yaml/coredns -p

2. Create yaml file

vim /data/k8s-yaml/coredns/rbac.yaml
apiVersion: v1
kind: ServiceAccount
metadata:
  name: coredns
  namespace: kube-system
  labels:
      kubernetes.io/cluster-service: "true"
      addonmanager.kubernetes.io/mode: Reconcile
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
    addonmanager.kubernetes.io/mode: Reconcile
  name: system:coredns
rules:
- apiGroups:
  - ""
  resources:
  - endpoints
  - services
  - pods
  - namespaces
  verbs:
  - list
  - watch
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  annotations:
    rbac.authorization.kubernetes.io/autoupdate: "true"
  labels:
    kubernetes.io/bootstrapping: rbac-defaults
    addonmanager.kubernetes.io/mode: EnsureExists
  name: system:coredns
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: system:coredns
subjects:
- kind: ServiceAccount
  name: coredns
  namespace: kube-system
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        log
        health
        kubernetes cluster.local 182.168.0.0/16      #Receive this information according to kubelet's configuration file
        forward . 192.168.112.60
        cache 30
        loop
        reload
        loadbalance
    }

vim /data/k8s-yaml/coredns/cm.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: coredns
  namespace: kube-system
data:
  Corefile: |
    .:53 {
        errors
        log
        health
        kubernetes cluster.local 192.168.0.0/16      #Receive this information according to kubelet's configuration file
        forward  . 192.168.112.60
        cache 30
    }

vim /data/k8s-yaml/coredns/dp.yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: coredns
    kubernetes.io/name: "CoreDNS"
spec:
  replicas: 1
  selector:
    matchLabels:
      k8s-app: coredns
  template:
    metadata:
      labels:
        k8s-app: coredns
    spec:
      priorityClassName: system-cluster-critical
      serviceAccountName: coredns
      containers:
      - name: coredns
        image: harbor.od.com/public/coredns:v1.6.1
        args: [ "-conf", "/etc/coredns/Corefile" ]
        volumeMounts:
        - name: config-volume
          mountPath: /etc/coredns
        ports:
        - containerPort: 53
          name: dns
          protocol: UDP
        - containerPort: 53
          name: dns-tcp
          protocol: TCP
        - containerPort: 9153
          name: metrics
          protocol: TCP
        livenessProbe:
          httpGet:
            path: /health
            port: 8080
            scheme: HTTP
          initialDelaySeconds: 60
          timeoutSeconds: 5
          successThreshold: 1
          failureThreshold: 5
      dnsPolicy: Default
      volumes:
        - name: config-volume
          configMap:
            name: coredns
            items:
            - key: Corefile
              path: Corefile

vim /data/k8s-yaml/coredns/svc.yaml
apiVersion: v1
kind: Service
metadata:
  name: coredns
  namespace: kube-system
  labels:
    k8s-app: coredns
    kubernetes.io/cluster-service: "true"
    kubernetes.io/name: "CoreDNS"
spec:
  selector:
    k8s-app: coredns
  clusterIP: 192.168.0.2
  ports:
  - name: dns
    port: 53
    protocol: UDP
  - name: dns-tcp
    port: 53
  - name: metrics
    port: 9153
    protocol: TCP

3. Modify dns

[root@slave1 ~]# vim /var/named/od.com.zone
k8s-yaml A 192.168.112.24

systemctl restart named
dig -t A k8s-yaml.od.com @192.168.112.20 +short

4. Download the image from docker

docker pull docker.io/coredns/coredns:1.6.1
docker tag c0f6e815079e harbor.od.com/public/coredns:v1.6.1
docker push harbor.od.com/public/coredns:v1.6.1

5. Use statement

kubectl apply -f http://k8s-yaml.od.com/coredns/rbac.yaml
kubectl apply -f http://k8s-yaml.od.com/coredns/cm.yaml
kubectl apply -f http://k8s-yaml.od.com/coredns/dp.yaml
kubectl apply -f http://k8s-yaml.od.com/coredns/svc.yaml

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