<p>From what I understand, with the above configuration, it is possible to have 2 pods that exist in the cluster associated with the deployment. However, the old Pod is guranteed to be in the 'Terminated' state. An example scenario is updating the image version associated with the deployment.</p>
<p>There should not be any scenario where there are 2 Pods that are associated with the deployment and both are in the 'Running' phase. Is this correct?</p>
<p>In the scenarios I tried, for example, Pod eviction or updating the Pod spec. The existing Pod enters 'Terminating' state and a new Pod is deployed.</p>
<p>This is what I expected. Just wanted to make sure that all possible scenarios around updating Pod spec or Pod eviction cannot end up with two Pods in the 'Running' state as it would violate the replica count = 1 config.</p>

user2819943
<p>It depends on your <a href="https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#strategy" rel="nofollow noreferrer">update strategy</a>. Many times it's desired to have the new pod running and healthy before you shut down the old pod, otherwise you have downtime which may not be acceptable as per business requirements. By default, it's doing rolling updates.</p>
<p>The defaults look like the below, so if you don't specify anything, that's what will be used.</p>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: apps/v1
kind: Deployment
spec:
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxSurge: 25%
      maxUnavailable: 25%
</code></pre>
<p>So usually, you would have a moment where both pods are <code>running</code>. But Kubernetes will terminate the old pod as soon as the new pod becomes <code>ready</code>, so it will be hard, if not impossible, to literally see both in the state <code>ready</code>.</p>
<p>You can read about it in the docs: <a href="https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#updating-a-deployment" rel="nofollow noreferrer">https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#updating-a-deployment</a></p>
<blockquote>
<p>Deployment ensures that only a certain number of Pods are down while they are being updated. By default, it ensures that at least 75% of the desired number of Pods are up (25% max unavailable).</p>
<p>Deployment also ensures that only a certain number of Pods are created above the desired number of Pods. By default, it ensures that at most 125% of the desired number of Pods are up (25% max surge).</p>
<p>For example, if you look at the above Deployment closely, you will see that it first creates a new Pod, then deletes an old Pod, and creates another new one. It does not kill old Pods until a sufficient number of new Pods have come up, and does not create new Pods until a sufficient number of old Pods have been killed. It makes sure that at least 3 Pods are available and that at max 4 Pods in total are available. In case of a Deployment with 4 replicas, the number of Pods would be between 3 and 5.</p>
</blockquote>
<p>This is also explained here: <a href="https://kubernetes.io/docs/tutorials/kubernetes-basics/update/update-intro/" rel="nofollow noreferrer">https://kubernetes.io/docs/tutorials/kubernetes-basics/update/update-intro/</a></p>
<blockquote>
<p>Users expect applications to be available all the time and developers are expected to deploy new versions of them several times a day. In Kubernetes this is done with rolling updates. Rolling updates allow Deployments' update to take place with zero downtime by incrementally updating Pods instances with new ones. The new Pods will be scheduled on Nodes with available resources.</p>
</blockquote>
<p>To get the behaviour, described by you, you would set <code>spec.strategy.type</code> to <code>Recreate</code>.</p>
<blockquote>
<p>All existing Pods are killed before new ones are created when .spec.strategy.type==Recreate.</p>
</blockquote>
