feat: opt-out of auto-scaling per server (k8s) (#454)

Author: moritzrinow

README.md

@@ -366,6 +366,22 @@ spec:
         - name: mc
 ```
 
+You can also opt-out of auto-scaling per server by setting the following annotations on the `Service` object:
+- `mc-router.itzg.me/autoScaleUp=false`
+- `mc-router.itzg.me/autoScaleDown=false`
+
+Example server with auto-scaling disabled explicitly:
+```yaml
+apiVersion: v1
+kind: Service
+metadata:
+  name: mc-forge
+  annotations:
+    "mc-router.itzg.me/externalServerName": "external.host.name"
+    "mc-router.itzg.me/autoScaleUp": "false"
+    "mc-router.itzg.me/autoScaleDown": "false"
+```
+
 ## REST API
 
 * `GET /routes` (with `Accept: application/json`)

go.mod

@@ -14,7 +14,7 @@ require (
 	github.com/prometheus/client_golang v1.23.2
 	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.11.1
-	golang.ngrok.com/ngrok v1.13.0
+	golang.ngrok.com/ngrok v1.12.1
 	golang.org/x/text v0.28.0
 	k8s.io/api v0.33.4
 	k8s.io/apimachinery v0.33.4

go.sum

@@ -200,8 +200,8 @@ go.yaml.in/yaml/v2 v2.4.2 h1:DzmwEr2rDGHl7lsFgAHxmNz/1NlQ7xLIrlN2h5d1eGI=
 go.yaml.in/yaml/v2 v2.4.2/go.mod h1:081UH+NErpNdqlCXm3TtEran0rJZGxAYx9hb/ELlsPU=
 golang.ngrok.com/muxado/v2 v2.0.1 h1:jM9i6Pom6GGmnPrHKNR6OJRrUoHFkSZlJ3/S0zqdVpY=
 golang.ngrok.com/muxado/v2 v2.0.1/go.mod h1:wzxJYX4xiAtmwumzL+QsukVwFRXmPNv86vB8RPpOxyM=
-golang.ngrok.com/ngrok v1.13.0 h1:6SeOS+DAeIaHlkDmNH5waFHv0xjlavOV3wml0Z59/8k=
-golang.ngrok.com/ngrok v1.13.0/go.mod h1:BKOMdoZXfD4w6o3EtE7Cu9TVbaUWBqptrZRWnVcAuI4=
+golang.ngrok.com/ngrok v1.12.1 h1:fjPyPr/R5/Et02x52iIJD2XqukwYeafsHNvM1ndJDAI=
+golang.ngrok.com/ngrok v1.12.1/go.mod h1:BKOMdoZXfD4w6o3EtE7Cu9TVbaUWBqptrZRWnVcAuI4=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20191011191535-87dc89f01550/go.mod h1:yigFU9vqHzYiE8UmvKecakEJjdnWj3jj499lnFckfCI=
 golang.org/x/crypto v0.0.0-20200622213623-75b288015ac9/go.mod h1:LzIPMQfyMNhhGPhUkYOs5KpL4U8rLKemX1yGLhDgUto=

server/k8s.go

@@ -23,6 +23,8 @@ import (
 const (
 	AnnotationExternalServerName = "mc-router.itzg.me/externalServerName"
 	AnnotationDefaultServer      = "mc-router.itzg.me/defaultServer"
+	AnnotationAutoScaleUp        = "mc-router.itzg.me/autoScaleUp"
+	AnnotationAutoScaleDown      = "mc-router.itzg.me/autoScaleDown"
 )
 
 // K8sWatcher is a RouteFinder that can find routes from kubernetes services.
@@ -279,11 +281,32 @@ func (w *K8sWatcher) buildDetails(service *core.Service, externalServiceName str
 }
 
 func (w *K8sWatcher) buildScaleFunction(service *core.Service, from int32, to int32) ScalerFunc {
-	if from <= to && !w.autoScaleUp {
-		return nil
+	// Currently, annotations can only be used to opt-out of auto-scaling.
+	// However, this logic is prepared also for opt-in, as it returns a `ScalerFunc` when flags are false but annotations are set to `enabled`.
+	if from <= to {
+		enabled, exists := service.Annotations[AnnotationAutoScaleUp]
+		if exists {
+			if enabled == "false" {
+				return nil
+			}
+		} else {
+			if !w.autoScaleUp {
+				return nil
+			}
+		}
 	}
-	if from >= to && !w.autoScaleDown {
-		return nil
+	if from >= to {
+		enabled, exists := service.Annotations[AnnotationAutoScaleDown]
+		if exists {
+			if enabled == "false" {
+				return nil
+			}
+		} else {
+			if !w.autoScaleDown {
+				return nil
+			}
+		}
+
 	}
 	return func(ctx context.Context) error {
 		serviceName := service.Name