Avoid SPOF for webhook

At the moment, the webhook is a SPOF in certain
scenarios. Under high load or when a node failure
occurs the webhook becomes unavailable. Defining
a HPA, PDB and affinity rules solves this issue.

config/webhook-hpa.yaml

@@ -0,0 +1,65 @@
+# Copyright 2019 The Tekton Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+apiVersion: autoscaling/v2beta1
+kind: HorizontalPodAutoscaler
+metadata:
+  name: tekton-pipelines-webhook
+  namespace: tekton-pipelines
+  labels:
+    app.kubernetes.io/name: webhook
+    app.kubernetes.io/component: webhook
+    app.kubernetes.io/instance: default
+    app.kubernetes.io/version: "devel"
+    app.kubernetes.io/part-of: tekton-pipelines
+    # tekton.dev/release value replaced with inputs.params.versionTag in pipeline/tekton/publish.yaml
+    pipeline.tekton.dev/release: "devel"
+    # labels below are related to istio and should not be used for resource lookup
+    version: "devel"
+spec:
+  minReplicas: 1
+  maxReplicas: 5
+  scaleTargetRef:
+    apiVersion: apps/v1
+    kind: Deployment
+    name: tekton-pipelines-webhook
+  metrics:
+    - type: Resource
+      resource:
+        name: cpu
+        targetAverageUtilization: 100
+---
+apiVersion: policy/v1beta1
+kind: PodDisruptionBudget
+metadata:
+  name: tekton-pipelines-webhook
+  namespace: tekton-pipelines
+  labels:
+    app.kubernetes.io/name: webhook
+    app.kubernetes.io/component: webhook
+    app.kubernetes.io/instance: default
+    app.kubernetes.io/version: "devel"
+    app.kubernetes.io/part-of: tekton-pipelines
+    # tekton.dev/release value replaced with inputs.params.versionTag in pipeline/tekton/publish.yaml
+    pipeline.tekton.dev/release: "devel"
+    # labels below are related to istio and should not be used for resource lookup
+    version: "devel"
+spec:
+  minAvailable: 80%
+  selector:
+    matchLabels:
+      app.kubernetes.io/name: webhook
+      app.kubernetes.io/component: webhook
+      app.kubernetes.io/instance: default
+      app.kubernetes.io/part-of: tekton-pipelines

config/webhook.yaml

@@ -54,6 +54,18 @@ spec:
         app: tekton-pipelines-webhook
         version: "devel"
     spec:
+      affinity:
+        podAntiAffinity:
+          preferredDuringSchedulingIgnoredDuringExecution:
+          - podAffinityTerm:
+              labelSelector:
+                matchLabels:
+                  app.kubernetes.io/name: webhook
+                  app.kubernetes.io/component: webhook
+                  app.kubernetes.io/instance: default
+                  app.kubernetes.io/part-of: tekton-pipelines
+              topologyKey: kubernetes.io/hostname
+            weight: 100
       serviceAccountName: tekton-pipelines-webhook
       containers:
       - name: webhook