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connecting-frontend-backend.md

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Connect a Front End to a Back End Using a Service
tutorial
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This task 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 {{< glossary_tooltip term_id="service" >}} object.

{{% heading "objectives" %}}

  • Create and run a microservice using a {{< glossary_tooltip term_id="deployment" >}} object.
  • Route traffic to the backend using a frontend.
  • Use a Service object to connect the frontend application to the backend application.

{{% heading "prerequisites" %}}

{{< include "task-tutorial-prereqs.md" >}} {{< version-check >}}

This task uses Services with external load balancers, which require a supported environment. If your environment does not support this, you can use a Service of type NodePort instead.

Creating the backend using a Deployment

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

{{< codenew file="service/access/hello.yaml" >}}

Create the backend Deployment:

kubectl apply -f https://k8s.io/examples/service/access/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
Annotations:                    deployment.kubernetes.io/revision=1
Selector:                       app=hello,tier=backend,track=stable
Replicas:                       7 desired | 7 updated | 7 total | 7 available | 0 unavailable
StrategyType:                   RollingUpdate
MinReadySeconds:                0
RollingUpdateStrategy:          1 max unavailable, 1 max surge
Pod Template:
  Labels:       app=hello
                tier=backend
                track=stable
  Containers:
   hello:
    Image:              "gcr.io/google-samples/hello-go-gke:1.0"
    Port:               80/TCP
    Environment:        <none>
    Mounts:             <none>
  Volumes:              <none>
Conditions:
  Type          Status  Reason
  ----          ------  ------
  Available     True    MinimumReplicasAvailable
  Progressing   True    NewReplicaSetAvailable
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 {{< glossary_tooltip text="selectors" term_id="selector" >}} to find the Pods that it routes traffic to.

First, explore the Service configuration file:

{{< codenew file="service/access/hello-service.yaml" >}}

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 apply -f https://k8s.io/examples/service/access/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:

{{< codenew file="service/access/frontend.conf" >}}

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.

{{< codenew file="service/access/frontend.yaml" >}}

Create the frontend Deployment and Service:

kubectl apply -f https://k8s.io/examples/service/access/frontend.yaml

The output verifies that both resources were created:

deployment.apps/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. {{< /note >}}

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 --watch

This displays the configuration for the frontend Service and watches for changes. Initially, the external IP is listed as <pending>:

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

As soon as an external IP is provisioned, however, the configuration updates to include the new IP under the EXTERNAL-IP heading:

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

That IP can now be used to interact with the frontend service from outside the cluster.

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} # replace this with the EXTERNAL-IP you saw earlier

The output shows the message generated by the backend:

{"message":"Hello"}

{{% heading "cleanup" %}}

To delete the Services, enter this command:

kubectl delete services frontend hello

To delete the Deployments, the ReplicaSets and the Pods that are running the backend and frontend applications, enter this command:

kubectl delete deployment frontend hello

{{% heading "whatsnext" %}}