<p>I am currently using Deployments to manage my pods in my K8S cluster.</p>

<p>Some of my deployments require 2 pods/replicas, some require 3 pods/replicas and some of them require just 1 pod/replica. The issue Im having is the one with one pod/replica.</p>

<p>My YAML file is :</p>

<pre><code>apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: user-management-backend-deployment
spec:
  replicas: 1
  strategy:
    type: RollingUpdate
    rollingUpdate:
      maxUnavailable: 1
      maxSurge: 2
  selector:
    matchLabels:
      name: user-management-backend
  template:
    metadata:
      labels:
        name: user-management-backend
    spec:
      containers:
      - name: user-management-backend
        image: proj_csdp/user-management_backend:3.1.8
        imagePullPolicy: IfNotPresent
        ports:
          - containerPort: 8080
        livenessProbe:
          httpGet:
            port: 8080
            path: /user_management/health
          initialDelaySeconds: 300
          timeoutSeconds: 30
        readinessProbe:
          httpGet:
            port: 8080
            path: /user_management/health
          initialDelaySeconds: 10
          timeoutSeconds: 5
        volumeMounts:
          - name: nfs
            mountPath: "/vault"
      volumes:
        - name: nfs
          nfs:
            server: kube-nfs
            path: "/kubenfs/vault"
            readOnly: true
</code></pre>

<p>I have a the old version running fine.</p>

<pre><code># kubectl get po | grep  user-management-backend-deployment
user-management-backend-deployment-3264073543-mrrvl               1/1       Running        0          4d
</code></pre>

<p>Now I want to update the image:</p>

<pre><code># kubectl set image deployment  user-management-backend-deployment  user-management-backend=proj_csdp/user-management_backend:3.2.0
</code></pre>

<p>Now as per RollingUpdate design, K8S should bring up the new pod while keeping the old pod working and only once the new pod is ready to take the traffic, should the old pod get deleted. But what I see is that the old pod is immediately deleted and the new pod is created and then it takes time to start taking traffic meaning that I have to drop traffic.</p>

<pre><code># kubectl get po | grep  user-management-backend-deployment
user-management-backend-deployment-3264073543-l93m9               0/1       ContainerCreating   0          1s

# kubectl get po | grep  user-management-backend-deployment
user-management-backend-deployment-3264073543-l93m9               1/1       Running            0          33s
</code></pre>

<p>I have used <code>maxSurge: 2</code> &amp; <code>maxUnavailable: 1</code> but this does not seem to be working.</p>

<p>Any ideas why is this not working ?</p>

<p>It appears to be the <code>maxUnavailable: 1</code>; I was able to trivially reproduce your experience setting that value, and trivially achieve the correct experience by setting it to <code>maxUnavailable: 0</code></p>

<p>Here's my "pseudo-proof" of how the scheduler arrived at the behavior you are experiencing:</p>

<p>Because <code>replicas: 1</code>, the desired state for k8s is exactly one Pod in <code>Ready</code>. During a Rolling Update operation, which is the strategy you requested, it will create a new Pod, bringing the total to 2. But you granted k8s permission to leave <em>one Pod</em> in an unavailable state, and you instructed it to keep the <em>desired</em> number of Pods at 1. Thus, it fulfilled all of those constraints: 1 Pod, the desired count, in an unavailable state, permitted by the R-U strategy.</p>

<p>By setting the <code>maxUnavailable</code> to zero, you correctly direct k8s to never let any Pod be unavailable, even if that means surging Pods above the <code>replica</code> count for a short time</p>
