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Krishnamohan Yerrabilli 2022-08-30 14:29:18 +05:30 committed by GitHub
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<div class="col-md-8">
<h3>Overview of Kubernetes Services</h3>
<p>Kubernetes <a href="/docs/concepts/workloads/pods/">Pods</a> are mortal. Pods in fact have a <a href="/docs/concepts/workloads/pods/pod-lifecycle/">lifecycle</a>. When a worker node dies, the Pods running on the Node are also lost. A <a href="/docs/concepts/workloads/controllers/replicaset/">ReplicaSet</a> might then dynamically drive the cluster back to desired state via 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.</p>
<p>Kubernetes <a href="/docs/concepts/workloads/pods/">Pods</a> are mortal. Pods have a <a href="/docs/concepts/workloads/pods/pod-lifecycle/">lifecycle</a>. When a worker node dies, the Pods running on the Node are also lost. A <a href="/docs/concepts/workloads/controllers/replicaset/">ReplicaSet</a> 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.</p>
<p>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 <a href="/docs/concepts/configuration/overview/#general-configuration-tips">(preferred)</a> or JSON, like all Kubernetes objects. The set of Pods targeted by a Service is usually determined by a <i>LabelSelector</i> (see below for why you might want a Service without including <code>selector</code> in the spec).</p>
<p>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 <a href="/docs/concepts/configuration/overview/#general-configuration-tips">(preferred)</a> or JSON, like all Kubernetes objects. The set of Pods targeted by a Service is usually determined by a <i>LabelSelector</i> (see below for why you might want a Service without including a <code>selector</code> in the spec).</p>
<p>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 <code>type</code> in the ServiceSpec:</p>
<ul>
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<li><i>ExternalName</i> - Maps the Service to the contents of the <code>externalName</code> field (e.g. <code>foo.bar.example.com</code>), by returning a <code>CNAME</code> record with its value. No proxying of any kind is set up. This type requires v1.7 or higher of <code>kube-dns</code>, or CoreDNS version 0.0.8 or higher.</li>
</ul>
<p>More information about the different types of Services can be found in the <a href="/docs/tutorials/services/source-ip/">Using Source IP</a> tutorial. Also see <a href="/docs/concepts/services-networking/connect-applications-service">Connecting Applications with Services</a>.</p>
<p>Additionally, note that there are some use cases with Services that involve not defining <code>selector</code> in the spec. A Service created without <code>selector</code> 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 <code>type: ExternalName</code>.</p>
<p>Additionally, note that there are some use cases with Services that involve not defining the <code>selector</code> in the spec. A Service created without <code>selector</code> 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 <code>type: ExternalName</code>.</p>
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