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Connecting a Front End to a Back End Using a Service

This tutorial shows how to create a frontend and a backend microservice. The backend microservice is a hello greeter. The frontend and backend are connected using a Kubernetes Service object.

Objectives

Before you begin

Creating the backend using a Deployment

The backend is a simple hello greeter microservice. Here is the configuration file for the backend Deployment:

hello.yaml 
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: hello
spec:
  replicas: 7
  template:
    metadata:
      labels:
        app: hello
        tier: backend
        track: stable
    spec:
      containers:
        - name: hello
          image: "gcr.io/google-samples/hello-go-gke:1.0"
          ports:
            - name: http
              containerPort: 80

Create the backend Deployment:

kubectl create -f http://k8s.io/docs/tutorials/connecting-apps/hello.yaml

View information about the backend Deployment:

kubectl describe deployment hello

The output is similar to this:

Name:                           hello
Namespace:                      default
CreationTimestamp:              Mon, 24 Oct 2016 14:21:02 -0700
Labels:                         app=hello
                                tier=backend
                                track=stable
Selector:                       app=hello,tier=backend,track=stable
Replicas:                       7 updated | 7 total | 7 available | 0 unavailable
StrategyType:                   RollingUpdate
MinReadySeconds:                0
RollingUpdateStrategy:          1 max unavailable, 1 max surge
OldReplicaSets:                 <none>
NewReplicaSet:                  hello-3621623197 (7/7 replicas created)
Events:
...

Creating the backend Service object

The key to connecting a frontend to a backend is the backend Service. A Service creates a persistent IP address and DNS name entry so that the backend microservice can always be reached. A Service uses selector labels to find the Pods that it routes traffic to.

First, explore the Service configuration file:

hello-service.yaml 
kind: Service
apiVersion: v1
metadata:
  name: hello
spec:
  selector:
    app: hello
    tier: backend
  ports:
    - protocol: TCP
      port: 80
      targetPort: http

In the configuration file, you can see that the Service routes traffic to Pods that have the labels app: hello and tier: backend.

Create the hello Service:

kubectl create -f http://k8s.io/docs/tutorials/connecting-apps/hello-service.yaml

At this point, you have a backend Deployment running, and you have a Service that can route traffic to it.

Creating the frontend

Now that you have your backend, you can create a frontend that connects to the backend. The frontend connects to the backend worker Pods by using the DNS name given to the backend Service. The DNS name is “hello”, which is the value of the name field in the preceding Service configuration file.

The Pods in the frontend Deployment run an nginx image that is configured to find the hello backend Service. Here is the nginx configuration file:

frontend/frontend.conf 
upstream hello {
    server hello;
}

server {
    listen 80;

    location / {
        proxy_pass http://hello;
    }
}

Similar to the backend, the frontend has a Deployment and a Service. The configuration for the Service has type: LoadBalancer, which means that the Service uses the default load balancer of your cloud provider.

frontend.yaml 
kind: Service
apiVersion: v1
metadata:
  name: frontend
spec:
  selector:
    app: hello
    tier: frontend
  ports:
    - protocol: "TCP"
      port: 80
      targetPort: 80
  type: LoadBalancer
---
apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: frontend
spec:
  replicas: 1
  template:
    metadata:
      labels:
        app: hello
        tier: frontend
        track: stable
    spec:
      containers:
        - name: nginx
          image: "gcr.io/google-samples/hello-frontend:1.0"
          lifecycle:
            preStop:
              exec:
                command: ["/usr/sbin/nginx","-s","quit"]

Create the frontend Deployment and Service:

kubectl create -f http://k8s.io/docs/tutorials/connecting-apps/frontend.yaml

The output verifies that both resources were created:

deployment "frontend" created
service "frontend" created

Note: The nginx configuration is baked into the container image. A better way to do this would be to use a ConfigMap, so that you can change the configuration more easily.

Interact with the frontend Service

Once you’ve created a Service of type LoadBalancer, you can use this command to find the external IP:

kubectl get service frontend

The external IP field may take some time to populate. If this is the case, the external IP is listed as <pending>.

NAME       CLUSTER-IP      EXTERNAL-IP   PORT(S)  AGE
frontend   10.51.252.116   <pending>     80/TCP   10s

Repeat the same command again until it shows an external IP address:

NAME       CLUSTER-IP      EXTERNAL-IP        PORT(S)  AGE
frontend   10.51.252.116   XXX.XXX.XXX.XXX    80/TCP   1m

Send traffic through the frontend

The frontend and backends are now connected. You can hit the endpoint by using the curl command on the external IP of your frontend Service.

curl http://<EXTERNAL-IP>

The output shows the message generated by the backend:

{"message":"Hello"}

What’s next

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