Services
Direct Access to a Pod
After a pod is created, the following problems may occur if you directly access the pod:
- The pod can be deleted and recreated at any time by a controller such as a Deployment, and the result of accessing the pod becomes unpredictable.
- The IP address of the pod is allocated only after the pod is started. Before the pod is started, the IP address of the pod is unknown.
- An application is usually composed of multiple pods that run the same image. Accessing pods one by one is not efficient.
For example, an application uses Deployments to create the frontend and backend. The frontend calls the backend for computing, as shown in Figure 1. Three pods are running in the backend, which are independent and replaceable. When a backend pod is re-created, the new pod is assigned with a new IP address, of which the frontend pod is unaware.
Using Services for Pod Access
Kubernetes Services are used to solve the preceding pod access problems. A Service has a fixed IP address. (When a CCE cluster is created, a Service CIDR block is set, which is used to allocate IP addresses to Services.) A Service forwards requests accessing the Service to pods based on labels, and at the same time, perform load balancing for these pods.
In the preceding example, a Service is added for the frontend pod to access the backend pods. In this way, the frontend pod does not need to be aware of the changes on backend pods, as shown in Figure 2.
Creating Backend Pods
Create a Deployment with three replicas, that is, three pods with label app: nginx.
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- image: nginx:latest
name: container-0
resources:
limits:
cpu: 100m
memory: 200Mi
requests:
cpu: 100m
memory: 200Mi
imagePullSecrets:
- name: default-secret
Creating a Service
In the following example, we create a Service named nginx, and use a selector to select the pod with the label app:nginx. The port of the target pod is port 80 while the exposed port of the Service is port 8080.
The Service can be accessed using Service name:Exposed port. In the example, nginx:8080 is used. In this case, other pods can access the pod associated with nginx using nginx:8080.
apiVersion: v1
kind: Service
metadata:
name: nginx #Service name
spec:
selector: # Label selector, which selects pods with the label app=nginx
app: nginx
ports:
- name: service0
targetPort: 80 # Pod port
port: 8080 # Service external port
protocol: TCP # Forwarding protocol type. The value can be TCP or UDP.
type: ClusterIP # Service type
Save the Service definition to nginx-svc.yaml and use kubectl to create the Service.
$ kubectl create -f nginx-svc.yaml service/nginx created $ kubectl get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE kubernetes ClusterIP 10.247.0.1 <none> 443/TCP 7h19m nginx ClusterIP 10.247.124.252 <none> 8080/TCP 5h48m
You can see that the Service has a ClusterIP, which is fixed unless the Service is deleted. You can use this ClusterIP to access the Service inside the cluster.
Create a pod and use the ClusterIP to access the pod. Information similar to the following is returned.
$ kubectl run -i --tty --image nginx:alpine test --rm /bin/sh If you don't see a command prompt, try pressing enter. / # curl 10.247.124.252:8080 <!DOCTYPE html> <html> <head> <title>Welcome to nginx!</title> ...
Using ServiceName to Access a Service
After the DNS resolves the domain name, you can use ServiceName:Port to access the Service, the most common practice in Kubernetes. When you are creating a CCE cluster, you are required to install the coredns add-on by default. You can view the pods of CoreDNS in the kube-system namespace.
$ kubectl get po --namespace=kube-system NAME READY STATUS RESTARTS AGE coredns-7689f8bdf-295rk 1/1 Running 0 9m11s coredns-7689f8bdf-h7n68 1/1 Running 0 11m
After coredns is installed, it becomes a DNS. After the Service is created, coredns records the Service name and IP address. In this way, the pod can obtain the Service IP address by querying the Service name from coredns.
nginx.<namespace>.svc.cluster.local is used to access the Service. nginx is the Service name, <namespace> is the namespace, and svc.cluster.local is the domain name suffix. In actual use, you can omit <namespace>.svc.cluster.local in the same namespace and use the ServiceName.
For example, if the Service named nginx is created, you can access the Service through nginx:8080 and then access backend pods.
An advantage of using ServiceName is that you can write ServiceName into the program when developing the application. In this way, you do not need to know the IP address of a specific Service.
Now, create a pod and access the pod. Query the IP address of the nginx Service domain name, which is 10.247.124.252. Access the domain name of the pod and information similar to the following is returned.
$ kubectl run -i --tty --image tutum/dnsutils dnsutils --restart=Never --rm /bin/sh If you don't see a command prompt, try pressing enter. / # nslookup nginx Server: 10.247.3.10 Address: 10.247.3.10#53 Name: nginx.default.svc.cluster.local Address: 10.247.124.252 / # curl nginx:8080 <!DOCTYPE html> <html> <head> <title>Welcome to nginx!</title> ...
Using Services for Service Discovery
After a Service is deployed, it can discover the pod no matter how the pod changes.
If you run the kubectl describe command to query the Service, information similar to the following is displayed:
$ kubectl describe svc nginx Name: nginx ...... Endpoints: 172.16.2.132:80,172.16.3.6:80,172.16.3.7:80 ......
One Endpoints record is displayed. An endpoint is also a resource object in Kubernetes. Kubernetes monitors the pod IP addresses through endpoints so that a Service can discover pods.
$ kubectl get endpoints NAME ENDPOINTS AGE nginx 172.16.2.132:80,172.16.3.6:80,172.16.3.7:80 5h48m
In this example, 172.16.2.132:80 is the IP:port of the pod. You can run the following command to view the IP address of the pod, which is the same as the preceding IP address.
$ kubectl get po -o wide NAME READY STATUS RESTARTS AGE IP NODE nginx-869759589d-dnknn 1/1 Running 0 5h40m 172.16.3.7 192.168.0.212 nginx-869759589d-fcxhh 1/1 Running 0 5h40m 172.16.3.6 192.168.0.212 nginx-869759589d-r69kh 1/1 Running 0 5h40m 172.16.2.132 192.168.0.94
If a pod is deleted, the Deployment re-creates the pod and the IP address of the new pod changes.
$ kubectl delete po nginx-869759589d-dnknn pod "nginx-869759589d-dnknn" deleted $ kubectl get po -o wide NAME READY STATUS RESTARTS AGE IP NODE nginx-869759589d-fcxhh 1/1 Running 0 5h41m 172.16.3.6 192.168.0.212 nginx-869759589d-r69kh 1/1 Running 0 5h41m 172.16.2.132 192.168.0.94 nginx-869759589d-w98wg 1/1 Running 0 7s 172.16.3.10 192.168.0.212
Check the endpoints again. You can see that the content under ENDPOINTS changes with the pod.
$ kubectl get endpoints NAME ENDPOINTS AGE kubernetes 192.168.0.127:5444 7h20m nginx 172.16.2.132:80,172.16.3.10:80,172.16.3.6:80 5h49m
Let's take a closer look at how this happens.
We have introduced kube-proxy on worker nodes in Kubernetes Cluster Architecture. Actually, all Service-related operations are performed by kube-proxy. When a Service is created, Kubernetes allocates an IP address to the Service and notifies kube-proxy on all nodes of the Service creation through the API server. After receiving the notification, each kube-proxy records the IP address and port number of the Service through iptables. In this way, the Service can be obtained on each node.
The following figure shows how a Service is accessed. Pod X accesses the Service (10.247.124.252:8080). When pod X sends data packets, the destination IP:Port is replaced with the IP:Port of pod 1 based on the iptables rule. In this way, the real backend pod can be accessed through the Service.
In addition to recording the IP address and port number of a Service, kube-proxy monitors the changes of the Service and their endpoints to ensure that pods can still be accessed through the Service after the pods are rebuilt.
Service Types and Application Scenarios
Services of the ClusterIP, NodePort, LoadBalancer, and Headless Service types offer different functions.
- ClusterIP: used to make the Service only reachable within a cluster.
- NodePort: used for access from outside a cluster. A NodePort Service is accessed through the port on the node. For details, see NodePort Services.
- LoadBalancer: used for access from outside a cluster. It is an extension of NodePort, to which a load balancer routes, and external systems only need to access the load balancer. For details, see LoadBalancer Services.
- Headless Service: used by pods to discover each other. No separate cluster IP address will be allocated to this type of Service, and the cluster will not balance loads or perform routing for it. You can create a headless Service by setting the spec.clusterIP value to None. For details, see Headless Services.
NodePort Services
A NodePort Service enables each node in a Kubernetes cluster to reserve the same port. External systems first access the Node IP:Port and then the NodePort Service forwards the requests to the pod backing the Service.
apiVersion: v1
kind: Service
metadata:
name: nodeport-service
spec:
type: NodePort
ports:
- port: 8080
targetPort: 80
nodePort: 30120
selector:
app: nginx
Create and view the Service. The value of PORT for the NodePort Service is 8080:30120/TCP, indicating that port 8080 of the Service is mapped to port 30120 of the node.
$ kubectl create -f nodeport.yaml service/nodeport-service created $ kubectl get svc -o wide NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE SELECTOR kubernetes ClusterIP 10.247.0.1 <none> 443/TCP 107m <none> nginx ClusterIP 10.247.124.252 <none> 8080/TCP 16m app=nginx nodeport-service NodePort 10.247.210.174 <none> 8080:30120/TCP 17s app=nginx
Access the Service by using Node IP:Port number to access the pod.
$ kubectl run -i --tty --image nginx:alpine test --rm /bin/sh If you don't see a command prompt, try pressing enter. / # curl 192.168.0.212:30120 <!DOCTYPE html> <html> <head> <title>Welcome to nginx!</title> ......
LoadBalancer Services
A Service is exposed externally using a load balancer that forwards requests to the NodePort of the node.
Load balancers are not a Kubernetes component. Different cloud service providers have different implementations. For example, CCE interconnects with Elastic Load Balance (ELB). As a result, there are different implementation methods of creating a LoadBalancer Service.
apiVersion: v1
kind: Service
metadata:
annotations:
kubernetes.io/elb.id: 3c7caa5a-a641-4bff-801a-feace27424b6
labels:
app: nginx
name: nginx
spec:
loadBalancerIP: 10.78.42.242 # IP address of the ELB instance
ports:
- name: service0
port: 80
protocol: TCP
targetPort: 80
nodePort: 30120
selector:
app: nginx
type: LoadBalancer # Service type (LoadBalancer)
The parameters in annotations under metadata are required for CCE LoadBalancer Services. They specify the ELB instance to which the Service is bound. CCE also allows you to create an ELB instance when creating a LoadBalancer Service. For details, see LoadBalancer.
Headless Services
A Service allows a client to access a pod associated with the Service for both internal and external network communication. However, the following problems persist:
- Accessing all pods at the same time
- Allowing pods in a Service to access each other
Kubernetes provides headless Services to solve these problems. When a client accesses a non-headless Service, only the cluster IP address of the Service is returned for a DNS query. The pod to be accessed is determined based on the cluster forwarding rule (IPVS or iptables). A headless Service is not allocated with a separate cluster IP address. During a DNS query, the DNS records of all pods will be returned. In this way, the IP address of each pod can be obtained. StatefulSets in StatefulSets use headless Services for mutual access between pods.
apiVersion: v1
kind: Service # Object type (Service)
metadata:
name: nginx-headless
labels:
app: nginx
spec:
ports:
- name: nginx # Name of the port for communication between pods
port: 80 # Port number for communication between pods
selector:
app: nginx # Select the pod labeled with app:nginx.
clusterIP: None # Set this parameter to None, indicating a headless Service.
Run the following command to create a headless Service:
# kubectl create -f headless.yaml service/nginx-headless created
After the Service is created, you can query the Service.
# kubectl get svc NAME TYPE CLUSTER-IP EXTERNAL-IP PORT(S) AGE nginx-headless ClusterIP None <none> 80/TCP 5s
Create a pod to query the DNS. You can view the records of all pods. In this way, all pods can be accessed.
$ kubectl run -i --tty --image tutum/dnsutils dnsutils --restart=Never --rm /bin/sh If you don't see a command prompt, try pressing enter. / # nslookup nginx-headless Server: 10.247.3.10 Address: 10.247.3.10#53 Name: nginx-headless.default.svc.cluster.local Address: 172.16.0.31 Name: nginx-headless.default.svc.cluster.local Address: 172.16.0.18 Name: nginx-headless.default.svc.cluster.local Address: 172.16.0.19
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