<p>Not sure if such if there was such a question, so pardon me if I couldn't find such.</p>
<p>I have a cluster based on 3 nodes, my application consists of a frontend and a backend with each running 2 replicas:</p>
<ul>
<li>front1 - running on <code>node1</code></li>
<li>front2 - running on <code>node2</code></li>
<li>be1 - <code>node1</code></li>
<li>be2 - <code>node2</code></li>
<li>Both <code>FE</code> pods are served behind <code>frontend-service</code></li>
<li>Both <code>BE</code> pods are service behind <code>be-service</code></li>
</ul>
<p>When I shutdown <code>node-2</code>, the application stopped and in my UI I could see application errors.</p>
<p>I've checked the logs and found out that my application attempted to reach the service type of the backend pods and it failed to respond since <code>be2</code> wasn't running, the scheduler is yet to terminate the existing one.</p>
<p>Only when the node was terminated and removed from the cluster, the pods were rescheduled to the 3rd node and the application was back online.</p>
<p>I know a service mesh can help by removing the pods that aren't responding from the traffic, however, I don't want to implement it yet, and trying to understand what is the best solution to route the traffic to the healthy pods in a fast and easy way, 5 minutes of downtime is a lot of time.</p>
<p>Here's my <code>be</code> deployment spec:</p>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    app: backend
  name: backend
  namespace: default
spec:
  progressDeadlineSeconds: 600
  replicas: 2
  revisionHistoryLimit: 10
  selector:
    matchLabels:
      app: backend
  strategy:
    rollingUpdate:
      maxSurge: 0
      maxUnavailable: 1
    type: RollingUpdate
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: backend
    spec:
      affinity:
        nodeAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
            nodeSelectorTerms:
            - matchExpressions:
              - key: node-Application
                operator: In
                values:
                - &quot;true&quot;
        podAntiAffinity:
          requiredDuringSchedulingIgnoredDuringExecution:
          - labelSelector:
              matchExpressions:
              - key: app
                operator: In
                values:
                - backend
            topologyKey: kubernetes.io/hostname
      containers:
      - env:
        - name: SSL_ENABLED
          value: &quot;false&quot;
        image: quay.io/something:latest
        imagePullPolicy: IfNotPresent
        livenessProbe:
          failureThreshold: 3
          httpGet:
            path: /liveness
            port: 16006
            scheme: HTTP
          initialDelaySeconds: 60
          periodSeconds: 20
          successThreshold: 1
          timeoutSeconds: 10
        name: backend
        ports:
        - containerPort: 16006
          protocol: TCP
        - containerPort: 8457
          protocol: TCP
        readinessProbe:
          failureThreshold: 3
          httpGet:
            path: /readiness
            port: 16006
            scheme: HTTP
          initialDelaySeconds: 10
          periodSeconds: 10
          successThreshold: 1
          timeoutSeconds: 5
        resources:
          limits:
            cpu: 1500m
            memory: 8500Mi
          requests:
            cpu: 6m
            memory: 120Mi
      dnsPolicy: ClusterFirst
</code></pre>
<p>Here's my backend service:</p>
<pre class="lang-yaml prettyprint-override"><code>apiVersion: v1
kind: Service
metadata:
  labels:
    app: identity
  name: backend
  namespace: default
spec:
  clusterIP: 10.233.34.115
  ports:
  - name: tcp
    port: 16006
    protocol: TCP
    targetPort: 16006
  - name: internal-http-rpc
    port: 8457
    protocol: TCP
    targetPort: 8457
  selector:
    app: backend
  sessionAffinity: None
  type: ClusterIP
</code></pre>

<p>This is a community wiki answer. Feel free to expand it.</p>
<p>As already mentioned by @TomerLeibovich the main issue here was due to the <a href="https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#configure-probes" rel="nofollow noreferrer">Probes Configuration</a>:</p>
<blockquote>
<p><a href="https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.20/#probe-v1-core" rel="nofollow noreferrer">Probes</a> have a number of fields that you can use to more precisely
control the behavior of liveness and readiness checks:</p>
<ul>
<li><p><code>initialDelaySeconds</code>: Number of seconds after the container has started before liveness or readiness probes are initiated. Defaults to
0 seconds. Minimum value is 0.</p>
</li>
<li><p><code>periodSeconds</code>: How often (in seconds) to perform the probe. Default to 10 seconds. Minimum value is 1.</p>
</li>
<li><p><code>timeoutSeconds</code>: Number of seconds after which the probe times out. Defaults to 1 second. Minimum value is 1.</p>
</li>
<li><p><code>successThreshold</code>: Minimum consecutive successes for the probe to be considered successful after having failed. Defaults to 1. Must be 1
for liveness and startup Probes. Minimum value is 1.</p>
</li>
<li><p><code>failureThreshold</code>: When a probe fails, Kubernetes will try failureThreshold times before giving up. Giving up in case of liveness
probe means restarting the container. In case of readiness probe the
Pod will be marked Unready. Defaults to 3. Minimum value is 1.</p>
</li>
</ul>
</blockquote>
<p>Plus the proper <a href="https://kubernetes.io/docs/tasks/administer-cluster/out-of-resource/" rel="nofollow noreferrer">Pod eviction configuration</a>:</p>
<blockquote>
<p>The <code>kubelet</code> needs to preserve node stability when available compute
resources are low. This is especially important when dealing with
incompressible compute resources, such as memory or disk space. If
such resources are exhausted, nodes become unstable.</p>
</blockquote>
<p>Changing the threshold to 1 instead of 3 and reducing the pod-eviction solved the issue as the Pod is now being evicted sooner.</p>
<p><strong>EDIT:</strong></p>
<p>The other possible solution in this scenario is to label other nodes with the app backend to make sure that each backend/pod was deployed on different nodes. In your current situation one pod deployed on the node was removed from the endpoint and the application became unresponsive.</p>
<p>Also, the workaround for triggering pod eviction from the unhealthy node is to add tolerations to</p>
<pre><code>deployment.spec. template.spec: tolerations: - key: &quot;node.kubernetes.io/unreachable&quot; operator: &quot;Exists&quot; effect: &quot;NoExecute&quot; tolerationSeconds: 60 
</code></pre>
<p>instead of using the default value: <code>tolerationSeconds: 300</code>.</p>
<p>You can find more information in <a href="https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/#taint-based-evictions" rel="nofollow noreferrer">this documentation</a>.</p>
