# Using a Service to Expose Your App

Using a Service to Expose Your AppObjectivesLearn about a Service in Kubernetes.Understand how labels and selectors relate to a Service.Expose an application outside a Kubernetes cluster.Overview of Kubernetes ServicesKubernetes Pods are mortal. Pods have a lifecycle. When a worker node dies, the Pods running on the Node are also lost. A Replicaset might then dynamically drive the cluster back to the desired state via the creation of new Pods to keep your application running. As another example, consider an image-processing backend with 3 replicas. Those replicas are exchangeable; the front-end system should not care about backend replicas or even if a Pod is lost and recreated. That said, each Pod in a Kubernetes cluster has a unique IP address, even Pods on the same Node, so there needs to be a way of automatically reconciling changes among Pods so that your applications continue to function.A Kubernetes Service is an abstraction layer which defines a logical set of Pods and enables external traffic exposure, load balancing and service discovery for those Pods.A Service in Kubernetes is an abstraction which defines a logical set of Pods and a policy by which to access them. Services enable a loose coupling between dependent Pods. A Service is defined using YAML or JSON, like all Kubernetes object manifests. The set of Pods targeted by a Service is usually determined by a label selector (see below for why you might want a Service without including a selector in the spec).Although each Pod has a unique IP address, those IPs are not exposed outside the cluster without a Service. Services allow your applications to receive traffic. Services can be exposed in different ways by specifying a type in the spec of the Service:ClusterIP (default) - Exposes the Service on an internal IP in the cluster. This type makes the Service only reachable from within the cluster.NodePort - Exposes the Service on the same port of each selected Node in the cluster using NAT. Makes a Service accessible from outside the cluster using NodeIP:NodePort. Superset of ClusterIP.LoadBalancer - Creates an external load balancer in the current cloud (if supported) and assigns a fixed, external IP to the Service. Superset of NodePort.ExternalName - Maps the Service to the contents of the externalName field (e.g. foo.bar.example.com), by returning a CNAME record with its value. No proxying of any kind is set up. This type requires v1.7 or higher of kube-dns, or CoreDNS version 0.0.8 or higher.More information about the different types of Services can be found in the Using Source IP tutorial. Also see Connecting Applications with Services.Additionally, note that there are some use cases with Services that involve not defining a selector in the spec. A Service created without selector will also not create the corresponding Endpoints object. This allows users to manually map a Service to specific endpoints. Another possibility why there may be no selector is you are strictly using type: ExternalName.Services and LabelsA Service routes traffic across a set of Pods. Services are the abstraction that allows pods to die and replicate in Kubernetes without impacting your application. Discovery and routing among dependent Pods (such as the frontend and backend components in an application) are handled by Kubernetes Services.Services match a set of Pods using labels and selectors, a grouping primitive that allows logical operation on objects in Kubernetes. Labels are key/value pairs attached to objects and can be used in any number of ways:Designate objects for development, test, and productionEmbed version tagsClassify an object using tagsLabels can be attached to objects at creation time or later on. They can be modified at any time. Let's expose our application now using a Service and apply some labels.Step 1: Creating a new ServiceLet’s verify that our application is running. We’ll use the kubectl get command and look for existing Pods:kubectl get pods If no Pods are running then it means the objects from the previous tutorials were cleaned up. In this case, go back and recreate the deployment from the Using kubectl to create a Deployment tutorial. Please wait a couple of seconds and list the Pods again. You can continue once you see the one Pod running.