From 2d028178dd24d2e12aa9cff815c646b1ac76d354 Mon Sep 17 00:00:00 2001
From: Tom Wieczorek <twieczorek@mirantis.com>
Date: Mon, 16 Oct 2023 17:21:34 +0200
Subject: [PATCH] Some CoreDNS tweaks for single-worker-node clusters

* No maxUnavailable:1 as this shows CoreDNS as available with 0
  replicas. Use the Kubernetes defaults instead (maxUnavailable: 25%,
  but rounded down).
* No anti-affinity for those clusters, so that CoreDNS can be upgraded
  on a single worker without downtime.
* No PDB for those clusters, so that CoreDNS may be drained.

Signed-off-by: Tom Wieczorek <twieczorek@mirantis.com>
---
 pkg/component/controller/coredns.go | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/pkg/component/controller/coredns.go b/pkg/component/controller/coredns.go
index fbf6d38fb319..2e523ce5c332 100644
--- a/pkg/component/controller/coredns.go
+++ b/pkg/component/controller/coredns.go
@@ -123,8 +123,6 @@ spec:
   replicas: {{ .Replicas}}
   strategy:
     type: RollingUpdate
-    rollingUpdate:
-      maxUnavailable: 1
   selector:
     matchLabels:
       k8s-app: kube-dns
@@ -145,6 +143,10 @@ spec:
           effect: "NoSchedule"
       nodeSelector:
         kubernetes.io/os: linux
+      {{- /* Add an anti affinity only if more than one worker node is */ }}
+      {{- /* available. This allows for rolling updates of CoreDNS on */ }}
+      {{- /* single node clusters without CoreDNS becoming unavailable. */ }}
+      {{- if gt .Replicas 1 }}
       # Require running coredns replicas on different nodes
       affinity:
         podAntiAffinity:
@@ -155,6 +157,7 @@ spec:
               - key: k8s-app
                 operator: In
                 values: ['kube-dns']
+      {{- end }}
       containers:
       - name: coredns
         image: {{ .Image }}
@@ -216,6 +219,9 @@ spec:
             items:
             - key: Corefile
               path: Corefile
+{{- /* Add a PDB only if more than one worker node is available. */ }}
+{{- /* This allows CoreDNS to be drained from single node clusters. */ }}
+{{- if gt .Replicas 1 }}
 ---
 apiVersion: policy/v1
 kind: PodDisruptionBudget
@@ -230,6 +236,7 @@ spec:
   selector:
     matchLabels:
       k8s-app: kube-dns
+{{- end }}
 ---
 apiVersion: v1
 kind: Service