Kubernetes auto scale-up support (#29)

2024-06-24 10:14:42 +02:00 · 2022-01-07 00:41:10 +01:00 · 2022-01-07 00:41:10 +01:00 · 32fb918128
commit 32fb918128
parent d7c8b4628e
8 changed files with 233 additions and 78 deletions
--- a/README.md
+++ b/README.md
@ -12,6 +12,8 @@ Routes Minecraft client connections to backend servers based upon the requested
 ```text
  -api-binding host:port
    	The host:port bound for servicing API requests (env API_BINDING)
+  -auto-scale-up
+      Increase Kubernetes StatefulSet Replicas (only) from 0 to 1 on respective backend servers when accessed (env AUTO_SCALE_UP)
  -connection-rate-limit int
    	Max number of connections to allow per second (env CONNECTION_RATE_LIMIT) (default 1)
  -cpu-profile string
@ -19,9 +21,9 @@ Routes Minecraft client connections to backend servers based upon the requested
  -debug
    	Enable debug logs (env DEBUG)
  -in-kube-cluster
-    	Use in-cluster kubernetes config (env IN_KUBE_CLUSTER)
+    	Use in-cluster Kubernetes config (env IN_KUBE_CLUSTER)
  -kube-config string
-    	The path to a kubernetes configuration file (env KUBE_CONFIG)
+    	The path to a Kubernetes configuration file (env KUBE_CONFIG)
  -mapping string
    	Comma-separated mappings of externalHostname=host:port (env MAPPING)
  -metrics-backend string
@ -61,9 +63,9 @@ Routes Minecraft client connections to backend servers based upon the requested
    "backend": "HOST:PORT"
  }
  ```
-  
+
 * `POST /defaultRoute` (with `Content-Type: application/json`)
-  
+
  Registers a default route to the given backend. JSON body is structured as:
  ```json
  {
@ -74,7 +76,7 @@ Routes Minecraft client connections to backend servers based upon the requested
 * `DELETE /routes/{serverAddress}`

  Deletes an existing route for the given `serverAddress`
-  
+
 # Docker Multi-Architecture Image

 The [multi-architecture image published at Docker Hub](https://hub.docker.com/repository/docker/itzg/mc-router) supports amd64, arm64, and arm32v6 (i.e. RaspberryPi).
@ -86,25 +88,25 @@ configures two Minecraft server services named `vanilla` and `forge`, which also
 network aliases. _Notice those services don't need their ports exposed since the internal
 networking allows for the inter-container access._

-The `router` service is only one of the services that needs to exposed on the external 
+The `router` service is only one of the services that needs to exposed on the external
 network. The `--mapping` declares how the hostname users will enter into their Minecraft client
 will map to the internal services.

 ![](docs/compose-diagram.png)
-  
+
 To test out this example, I added these two entries to my "hosts" file:

 ```
 127.0.0.1 vanilla.example.com
 127.0.0.1 forge.example.com
 ```
-  
+
 # Kubernetes Usage

-## Using kubernetes service auto-discovery
+## Using Kubernetes Service auto-discovery

-When running `mc-router` as a kubernetes pod and you pass the `--in-kube-cluster` command-line argument, then
-it will automatically watch for any services annotated with 
+When running `mc-router` as a Kubernetes Pod and you pass the `--in-kube-cluster` command-line argument, then
+it will automatically watch for any services annotated with
 - `mc-router.itzg.me/externalServerName` : The value of the annotation will be registered as the external hostname Minecraft clients would used to connect to the
   routed service. The service's clusterIP and target port are used as the routed backend. You can use more hostnames by splitting them with comma.
 - `mc-router.itzg.me/defaultServer` : The service's clusterIP and target port are used as the default if
@ -140,13 +142,13 @@ metadata:
    "mc-router.itzg.me/externalServerName": "external.host.name,other.host.name"
 ```

-## Example kubernetes deployment
+## Example Kubernetes deployment

-[This example deployment](docs/k8s-example-auto.yaml) 
+[This example deployment](docs/k8s-example-auto.yaml)
 * Declares an `mc-router` service that exposes a node port 25565
 * Declares a service account with access to watch and list services
 * Declares `--in-kube-cluster` in the `mc-router` container arguments
-* Two "backend" Minecraft servers are declared each with an 
+* Two "backend" Minecraft servers are declared each with an
  `"mc-router.itzg.me/externalServerName"` annotation that declares their external server name(s)

 ```bash
@ -157,8 +159,31 @@ kubectl apply -f https://raw.githubusercontent.com/itzg/mc-router/master/docs/k8

 #### Notes
 * This deployment assumes two persistent volume claims: `mc-stable` and `mc-snapshot`
-* I extended the allowed node port range by adding `--service-node-port-range=25000-32767` 
-  to `/etc/kubernetes/manifests/kube-apiserver.yaml` 
+* I extended the allowed node port range by adding `--service-node-port-range=25000-32767`
+  to `/etc/kubernetes/manifests/kube-apiserver.yaml`
+
+#### Auto Scale Up
+
+The `-auto-scale-up` flag argument makes the router "wake up" any stopped backend servers, by changing `replicas: 0` to `replicas: 1`.
+
+This requires using `kind: StatefulSet` instead of `kind: Service` for the Minecraft backend servers.
+
+It also requires the `ClusterRole` to permit `get` + `update` for `statefulsets` & `statefulsets/scale`,
+e.g. like this (or some equivalent more fine-grained one to only watch/list services+statefulsets, and only get+update scale):
+
+```yaml
+apiVersion: rbac.authorization.k8s.io/v1
+kind: ClusterRole
+metadata:
+  name: services-watcher
+rules:
+- apiGroups: [""]
+  resources: ["services"]
+  verbs: ["watch","list"]
+- apiGroups: ["apps"]
+  resources: ["statefulsets", "statefulsets/scale"]
+  verbs: ["watch","list","get","update"]
+```

 # Development

@ -168,16 +193,26 @@ kubectl apply -f https://raw.githubusercontent.com/itzg/mc-router/master/docs/k8
 docker build -t mc-router .
 ```

+## Build locally without Docker
+
+After [installing Go](https://go.dev/doc/install) and doing a `go mod download` to install all required prerequisites, just like the [Dockerfile](Dockerfile) does, you can:
+
+```bash
+make test # go test -v ./...
+go build ./cmd/mc-router/
+```
+
 ## Skaffold

 For "in-cluster development" it's convenient to use https://skaffold.dev. Any changes to Go source code
 will trigger a go build, new container image pushed to registry with a new tag, and refresh in Kubernetes
-with the image tag used in the deployment transparently updated to the new tag and thus new pod created pulling new images:
+with the image tag used in the deployment transparently updated to the new tag and thus new pod created pulling new images,
+as configured by [skaffold.yaml](skaffold.yaml):

    skaffold dev

 When using Google Cloud (GCP), first create a _Docker Artifact Registry_,
-then add the _Artifact Registry Reader_ Role to the _Compute Engine default service account_ of your GKE clusterService Account_ (to avoid error like "container mc-router is waiting to start: ...-docker.pkg.dev/... can't be pulled"),
+then add the _Artifact Registry Reader_ Role to the _Compute Engine default service account_ of your _GKE `clusterService` Account_ (to avoid error like "container mc-router is waiting to start: ...-docker.pkg.dev/... can't be pulled"),
 then use e.g. `gcloud auth configure-docker europe-docker.pkg.dev` or equivalent one time (to create a `~/.docker/config.json`),
 and then use e.g. `--default-repo=europe-docker.pkg.dev/YOUR-PROJECT/YOUR-ARTIFACT-REGISTRY` option for `skaffold dev`.

--- a/cmd/mc-router/main.go
+++ b/cmd/mc-router/main.go
@ -37,8 +37,9 @@ type Config struct {
 	CpuProfile           string   `usage:"Enables CPU profiling and writes to given path"`
 	Debug                bool     `usage:"Enable debug logs"`
 	ConnectionRateLimit  int      `default:"1" usage:"Max number of connections to allow per second"`
-	InKubeCluster        bool     `usage:"Use in-cluster kubernetes config"`
-	KubeConfig           string   `usage:"The path to a kubernetes configuration file"`
+	InKubeCluster        bool     `usage:"Use in-cluster Kubernetes config"`
+	KubeConfig           string   `usage:"The path to a Kubernetes configuration file"`
+	AutoScaleUp          bool     `usage:"Increase Kubernetes StatefulSet Replicas (only) from 0 to 1 on respective backend servers when accessed"`
 	MetricsBackend       string   `default:"discard" usage:"Backend to use for metrics exposure/publishing: discard,expvar,influxdb"`
 	UseProxyProtocol     bool     `default:"false" usage:"Send PROXY protocol to backend servers"`
 	MetricsBackendConfig MetricsBackendConfig
@ -113,14 +114,14 @@ func main() {
 	}

 	if config.InKubeCluster {
-		err = server.K8sWatcher.StartInCluster()
+		err = server.K8sWatcher.StartInCluster(config.AutoScaleUp)
 		if err != nil {
 			logrus.WithError(err).Fatal("Unable to start k8s integration")
 		} else {
 			defer server.K8sWatcher.Stop()
 		}
 	} else if config.KubeConfig != "" {
-		err := server.K8sWatcher.StartWithConfig(config.KubeConfig)
+		err := server.K8sWatcher.StartWithConfig(config.KubeConfig, config.AutoScaleUp)
 		if err != nil {
 			logrus.WithError(err).Fatal("Unable to start k8s integration")
 		} else {
--- a/docs/k8s-deployment.yaml
+++ b/docs/k8s-deployment.yaml
@ -20,6 +20,7 @@ spec:
      containers:
      - image: itzg/mc-router:latest
        name: mc-router
+        # Add "--auto-scale-up" here for https://github.com/itzg/mc-router/#auto-scale-up
        args: ["--api-binding", ":8080", "--in-kube-cluster"]
        ports:
        - name: proxy
--- a/server/connector.go
+++ b/server/connector.go
@ -167,7 +167,15 @@ func (c *connectorImpl) HandleConnection(ctx context.Context, frontendConn net.C
 func (c *connectorImpl) findAndConnectBackend(ctx context.Context, frontendConn net.Conn,
 	clientAddr net.Addr, preReadContent io.Reader, serverAddress string) {

-	backendHostPort, resolvedHost := Routes.FindBackendForServerAddress(serverAddress)
+	backendHostPort, resolvedHost, waker := Routes.FindBackendForServerAddress(ctx, serverAddress)
+	if waker != nil {
+		if err := waker(ctx); err != nil {
+			logrus.WithFields(logrus.Fields{"serverAddress": serverAddress}).WithError(err).Error("failed to wake up backend")
+			c.metrics.Errors.With("type", "wakeup_failed").Add(1)
+			return
+		}
+	}
+
 	if backendHostPort == "" {
 		logrus.WithField("serverAddress", serverAddress).Warn("Unable to find registered backend")
 		c.metrics.Errors.With("type", "missing_backend").Add(1)
--- a/server/k8s.go
+++ b/server/k8s.go
@ -1,13 +1,19 @@
 package server

 import (
+	"context"
+	"fmt"
 	"net"
 	"strconv"
 	"strings"
+	"sync"

 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
-	v1 "k8s.io/api/core/v1"
+	apps "k8s.io/api/apps/v1"
+	autoscaling "k8s.io/api/autoscaling/v1"
+	core "k8s.io/api/core/v1"
+	meta "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/fields"
 	"k8s.io/client-go/kubernetes"
 	"k8s.io/client-go/rest"
@ -21,51 +27,57 @@ const (
 )

 type IK8sWatcher interface {
-	StartWithConfig(kubeConfigFile string) error
-	StartInCluster() error
+	StartWithConfig(kubeConfigFile string, autoScaleUp bool) error
+	StartInCluster(autoScaleUp bool) error
 	Stop()
 }

 var K8sWatcher IK8sWatcher = &k8sWatcherImpl{}

 type k8sWatcherImpl struct {
-	stop chan struct{}
+	sync.RWMutex
+	// The key in mappings is a Service, and the value the StatefulSet name
+	mappings map[string]string
+
+	clientset *kubernetes.Clientset
+	stop      chan struct{}
 }

-func (w *k8sWatcherImpl) StartInCluster() error {
+func (w *k8sWatcherImpl) StartInCluster(autoScaleUp bool) error {
 	config, err := rest.InClusterConfig()
 	if err != nil {
 		return errors.Wrap(err, "Unable to load in-cluster config")
 	}

-	return w.startWithLoadedConfig(config)
+	return w.startWithLoadedConfig(config, autoScaleUp)
 }

-func (w *k8sWatcherImpl) StartWithConfig(kubeConfigFile string) error {
+func (w *k8sWatcherImpl) StartWithConfig(kubeConfigFile string, autoScaleUp bool) error {
 	config, err := clientcmd.BuildConfigFromFlags("", kubeConfigFile)
 	if err != nil {
 		return errors.Wrap(err, "Could not load kube config file")
 	}

-	return w.startWithLoadedConfig(config)
+	return w.startWithLoadedConfig(config, autoScaleUp)
 }

-func (w *k8sWatcherImpl) startWithLoadedConfig(config *rest.Config) error {
+func (w *k8sWatcherImpl) startWithLoadedConfig(config *rest.Config, autoScaleUp bool) error {
+	w.stop = make(chan struct{}, 1)
+
 	clientset, err := kubernetes.NewForConfig(config)
 	if err != nil {
 		return errors.Wrap(err, "Could not create kube clientset")
 	}
+	w.clientset = clientset

-	watchlist := cache.NewListWatchFromClient(
-		clientset.CoreV1().RESTClient(),
-		string(v1.ResourceServices),
-		v1.NamespaceAll,
-		fields.Everything(),
-	)
-
-	_, controller := cache.NewInformer(
-		watchlist,
-		&v1.Service{},
+	_, serviceController := cache.NewInformer(
+		cache.NewListWatchFromClient(
+			clientset.CoreV1().RESTClient(),
+			string(core.ResourceServices),
+			core.NamespaceAll,
+			fields.Everything(),
+		),
+		&core.Service{},
 		0,
 		cache.ResourceEventHandlerFuncs{
 			AddFunc:    w.handleAdd,
@ -73,17 +85,64 @@ func (w *k8sWatcherImpl) startWithLoadedConfig(config *rest.Config) error {
 			UpdateFunc: w.handleUpdate,
 		},
 	)
+	go serviceController.Run(w.stop)

-	w.stop = make(chan struct{}, 1)
-	logrus.Info("Monitoring kubernetes for minecraft services")
-	go controller.Run(w.stop)
+	w.mappings = make(map[string]string)
+	if autoScaleUp {
+		_, statefulSetController := cache.NewInformer(
+			cache.NewListWatchFromClient(
+				clientset.AppsV1().RESTClient(),
+				"statefulSets",
+				core.NamespaceAll,
+				fields.Everything(),
+			),
+			&apps.StatefulSet{},
+			0,
+			cache.ResourceEventHandlerFuncs{
+				AddFunc: func(obj interface{}) {
+					statefulSet, ok := obj.(*apps.StatefulSet)
+					if !ok {
+						return
+					}
+					w.RLock()
+					defer w.RUnlock()
+					w.mappings[statefulSet.Spec.ServiceName] = statefulSet.Name
+				},
+				DeleteFunc: func(obj interface{}) {
+					statefulSet, ok := obj.(*apps.StatefulSet)
+					if !ok {
+						return
+					}
+					w.RLock()
+					defer w.RUnlock()
+					delete(w.mappings, statefulSet.Spec.ServiceName)
+				},
+				UpdateFunc: func(oldObj, newObj interface{}) {
+					oldStatefulSet, ok := oldObj.(*apps.StatefulSet)
+					if !ok {
+						return
+					}
+					newStatefulSet, ok := newObj.(*apps.StatefulSet)
+					if !ok {
+						return
+					}
+					w.RLock()
+					defer w.RUnlock()
+					delete(w.mappings, oldStatefulSet.Spec.ServiceName)
+					w.mappings[newStatefulSet.Spec.ServiceName] = newStatefulSet.Name
+				},
+			},
+		)
+		go statefulSetController.Run(w.stop)
+	}

+	logrus.Info("Monitoring Kubernetes for Minecraft services")
 	return nil
 }

 // oldObj and newObj are expected to be *v1.Service
 func (w *k8sWatcherImpl) handleUpdate(oldObj interface{}, newObj interface{}) {
-	for _, oldRoutableService := range extractRoutableServices(oldObj) {
+	for _, oldRoutableService := range w.extractRoutableServices(oldObj) {
 		logrus.WithFields(logrus.Fields{
 			"old": oldRoutableService,
 		}).Debug("UPDATE")
@ -92,12 +151,12 @@ func (w *k8sWatcherImpl) handleUpdate(oldObj interface{}, newObj interface{}) {
 		}
 	}

-	for _, newRoutableService := range extractRoutableServices(newObj) {
+	for _, newRoutableService := range w.extractRoutableServices(newObj) {
 		logrus.WithFields(logrus.Fields{
 			"new": newRoutableService,
 		}).Debug("UPDATE")
 		if newRoutableService.externalServiceName != "" {
-			Routes.CreateMapping(newRoutableService.externalServiceName, newRoutableService.containerEndpoint)
+			Routes.CreateMapping(newRoutableService.externalServiceName, newRoutableService.containerEndpoint, newRoutableService.autoScaleUp)
 		} else {
 			Routes.SetDefaultRoute(newRoutableService.containerEndpoint)
 		}
@ -106,7 +165,7 @@ func (w *k8sWatcherImpl) handleUpdate(oldObj interface{}, newObj interface{}) {

 // obj is expected to be a *v1.Service
 func (w *k8sWatcherImpl) handleDelete(obj interface{}) {
-	routableServices := extractRoutableServices(obj)
+	routableServices := w.extractRoutableServices(obj)
 	for _, routableService := range routableServices {
 		if routableService != nil {
 			logrus.WithField("routableService", routableService).Debug("DELETE")
@ -122,13 +181,13 @@ func (w *k8sWatcherImpl) handleDelete(obj interface{}) {

 // obj is expected to be a *v1.Service
 func (w *k8sWatcherImpl) handleAdd(obj interface{}) {
-	routableServices := extractRoutableServices(obj)
+	routableServices := w.extractRoutableServices(obj)
 	for _, routableService := range routableServices {
 		if routableService != nil {
 			logrus.WithField("routableService", routableService).Debug("ADD")

 			if routableService.externalServiceName != "" {
-				Routes.CreateMapping(routableService.externalServiceName, routableService.containerEndpoint)
+				Routes.CreateMapping(routableService.externalServiceName, routableService.containerEndpoint, routableService.autoScaleUp)
 			} else {
 				Routes.SetDefaultRoute(routableService.containerEndpoint)
 			}
@ -138,18 +197,19 @@ func (w *k8sWatcherImpl) handleAdd(obj interface{}) {

 func (w *k8sWatcherImpl) Stop() {
 	if w.stop != nil {
-		w.stop <- struct{}{}
+		close(w.stop)
 	}
 }

 type routableService struct {
 	externalServiceName string
 	containerEndpoint   string
+	autoScaleUp         func(ctx context.Context) error
 }

 // obj is expected to be a *v1.Service
-func extractRoutableServices(obj interface{}) []*routableService {
-	service, ok := obj.(*v1.Service)
+func (w *k8sWatcherImpl) extractRoutableServices(obj interface{}) []*routableService {
+	service, ok := obj.(*core.Service)
 	if !ok {
 		return nil
 	}
@ -158,17 +218,17 @@ func extractRoutableServices(obj interface{}) []*routableService {
 	if externalServiceName, exists := service.Annotations[AnnotationExternalServerName]; exists {
 		serviceNames := strings.Split(externalServiceName, ",")
 		for _, serviceName := range serviceNames {
-			routableServices = append(routableServices, buildDetails(service, serviceName))
+			routableServices = append(routableServices, w.buildDetails(service, serviceName))
 		}
 		return routableServices
 	} else if _, exists := service.Annotations[AnnotationDefaultServer]; exists {
-		return []*routableService{buildDetails(service, "")}
+		return []*routableService{w.buildDetails(service, "")}
 	}

 	return nil
 }

-func buildDetails(service *v1.Service, externalServiceName string) *routableService {
+func (w *k8sWatcherImpl) buildDetails(service *core.Service, externalServiceName string) *routableService {
 	clusterIp := service.Spec.ClusterIP
 	port := "25565"
 	for _, p := range service.Spec.Ports {
@ -179,6 +239,45 @@ func buildDetails(service *v1.Service, externalServiceName string) *routableServ
 	rs := &routableService{
 		externalServiceName: externalServiceName,
 		containerEndpoint:   net.JoinHostPort(clusterIp, port),
+		autoScaleUp:         w.buildScaleUpFunction(service),
 	}
 	return rs
 }
+
+func (w *k8sWatcherImpl) buildScaleUpFunction(service *core.Service) func(ctx context.Context) error {
+	return func(ctx context.Context) error {
+		serviceName := service.Name
+		if statefulSetName, exists := w.mappings[serviceName]; exists {
+			if scale, err := w.clientset.AppsV1().StatefulSets(service.Namespace).GetScale(ctx, statefulSetName, meta.GetOptions{}); err == nil {
+				replicas := scale.Status.Replicas
+				logrus.WithFields(logrus.Fields{
+					"service":     serviceName,
+					"statefulSet": statefulSetName,
+					"replicas":    replicas,
+				}).Debug("StatefulSet of Service Replicas")
+				if replicas == 0 {
+					if _, err := w.clientset.AppsV1().StatefulSets(service.Namespace).UpdateScale(ctx, statefulSetName, &autoscaling.Scale{
+						ObjectMeta: meta.ObjectMeta{
+							Name:            scale.Name,
+							Namespace:       scale.Namespace,
+							UID:             scale.UID,
+							ResourceVersion: scale.ResourceVersion,
+						},
+						Spec: autoscaling.ScaleSpec{Replicas: 1}}, meta.UpdateOptions{},
+					); err == nil {
+						logrus.WithFields(logrus.Fields{
+							"service":     serviceName,
+							"statefulSet": statefulSetName,
+							"replicas":    replicas,
+						}).Info("StatefulSet Replicas Autoscaled from 0 to 1 (wake up)")
+					} else {
+						return errors.Wrap(err, "UpdateScale for Replicas=1 failed for StatefulSet: "+statefulSetName)
+					}
+				}
+			} else {
+				return fmt.Errorf("GetScale failed for StatefulSet %s: %w", statefulSetName, err)
+			}
+		}
+		return nil
+	}
+}
--- a/server/k8s_test.go
+++ b/server/k8s_test.go
@ -1,6 +1,7 @@
 package server

 import (
+	"context"
 	"encoding/json"
 	"testing"

@ -87,7 +88,7 @@ func TestK8sWatcherImpl_handleAddThenUpdate(t *testing.T) {

 			watcher.handleAdd(&initialSvc)
 			for _, s := range test.initial.scenarios {
-				backend, _ := Routes.FindBackendForServerAddress(s.given)
+				backend, _, _ := Routes.FindBackendForServerAddress(context.Background(), s.given)
 				assert.Equal(t, s.expect, backend, "initial: given=%s", s.given)
 			}

@ -97,7 +98,7 @@ func TestK8sWatcherImpl_handleAddThenUpdate(t *testing.T) {

 			watcher.handleUpdate(&initialSvc, &updatedSvc)
 			for _, s := range test.update.scenarios {
-				backend, _ := Routes.FindBackendForServerAddress(s.given)
+				backend, _, _ := Routes.FindBackendForServerAddress(context.Background(), s.given)
 				assert.Equal(t, s.expect, backend, "update: given=%s", s.given)
 			}
 		})
@ -159,13 +160,13 @@ func TestK8sWatcherImpl_handleAddThenDelete(t *testing.T) {

 			watcher.handleAdd(&initialSvc)
 			for _, s := range test.initial.scenarios {
-				backend, _ := Routes.FindBackendForServerAddress(s.given)
+				backend, _, _ := Routes.FindBackendForServerAddress(context.Background(), s.given)
 				assert.Equal(t, s.expect, backend, "initial: given=%s", s.given)
 			}

 			watcher.handleDelete(&initialSvc)
 			for _, s := range test.delete {
-				backend, _ := Routes.FindBackendForServerAddress(s.given)
+				backend, _, _ := Routes.FindBackendForServerAddress(context.Background(), s.given)
 				assert.Equal(t, s.expect, backend, "update: given=%s", s.given)
 			}
 		})
--- a/server/routes.go
+++ b/server/routes.go
@ -1,6 +1,7 @@
 package server

 import (
+	"context"
 	"encoding/json"
 	"net/http"
 	"strings"
@ -61,7 +62,7 @@ func routesCreateHandler(writer http.ResponseWriter, request *http.Request) {
 		return
 	}

-	Routes.CreateMapping(definition.ServerAddress, definition.Backend)
+	Routes.CreateMapping(definition.ServerAddress, definition.Backend, func(ctx context.Context) error { return nil })
 	writer.WriteHeader(http.StatusCreated)
 }

@ -87,12 +88,12 @@ func routesSetDefault(writer http.ResponseWriter, request *http.Request) {
 type IRoutes interface {
 	RegisterAll(mappings map[string]string)
 	// FindBackendForServerAddress returns the host:port for the external server address, if registered.
-	// Otherwise, an empty string is returned
-	// Also returns the normalized version of the given serverAddress
-	FindBackendForServerAddress(serverAddress string) (string, string)
+	// Otherwise, an empty string is returned. Also returns the normalized version of the given serverAddress.
+	// The 3rd value returned is an (optional) "waker" function which a caller must invoke to wake up serverAddress.
+	FindBackendForServerAddress(ctx context.Context, serverAddress string) (string, string, func(ctx context.Context) error)
 	GetMappings() map[string]string
 	DeleteMapping(serverAddress string) bool
-	CreateMapping(serverAddress string, backend string)
+	CreateMapping(serverAddress string, backend string, waker func(ctx context.Context) error)
 	SetDefaultRoute(backend string)
 }

@ -100,7 +101,7 @@ var Routes IRoutes = &routesImpl{}

 func NewRoutes() IRoutes {
 	r := &routesImpl{
-		mappings: make(map[string]string),
+		mappings: make(map[string]mapping),
 	}

 	return r
@ -110,12 +111,20 @@ func (r *routesImpl) RegisterAll(mappings map[string]string) {
 	r.Lock()
 	defer r.Unlock()

-	r.mappings = mappings
+	r.mappings = make(map[string]mapping)
+	for k, v := range mappings {
+		r.mappings[k] = mapping{backend: v, waker: func(ctx context.Context) error { return nil }}
+	}
+}
+
+type mapping struct {
+	backend string
+	waker   func(ctx context.Context) error
 }

 type routesImpl struct {
 	sync.RWMutex
-	mappings     map[string]string
+	mappings     map[string]mapping
 	defaultRoute string
 }

@ -127,7 +136,7 @@ func (r *routesImpl) SetDefaultRoute(backend string) {
 	}).Info("Using default route")
 }

-func (r *routesImpl) FindBackendForServerAddress(serverAddress string) (string, string) {
+func (r *routesImpl) FindBackendForServerAddress(ctx context.Context, serverAddress string) (string, string, func(ctx context.Context) error) {
 	r.RLock()
 	defer r.RUnlock()

@ -136,11 +145,11 @@ func (r *routesImpl) FindBackendForServerAddress(serverAddress string) (string,
 	address := strings.ToLower(addressParts[0])

 	if r.mappings != nil {
-		if route, exists := r.mappings[address]; exists {
-			return route, address
+		if mapping, exists := r.mappings[address]; exists {
+			return mapping.backend, address, mapping.waker
 		}
 	}
-	return r.defaultRoute, address
+	return r.defaultRoute, address, nil
 }

 func (r *routesImpl) GetMappings() map[string]string {
@ -149,7 +158,7 @@ func (r *routesImpl) GetMappings() map[string]string {

 	result := make(map[string]string, len(r.mappings))
 	for k, v := range r.mappings {
-		result[k] = v
+		result[k] = v.backend
 	}
 	return result
 }
@ -167,7 +176,7 @@ func (r *routesImpl) DeleteMapping(serverAddress string) bool {
 	}
 }

-func (r *routesImpl) CreateMapping(serverAddress string, backend string) {
+func (r *routesImpl) CreateMapping(serverAddress string, backend string, waker func(ctx context.Context) error) {
 	r.Lock()
 	defer r.Unlock()

@ -177,5 +186,5 @@ func (r *routesImpl) CreateMapping(serverAddress string, backend string) {
 		"serverAddress": serverAddress,
 		"backend":       backend,
 	}).Info("Creating route")
-	r.mappings[serverAddress] = backend
+	r.mappings[serverAddress] = mapping{backend: backend, waker: waker}
 }
--- a/server/routes_test.go
+++ b/server/routes_test.go
@ -1,6 +1,7 @@
 package server

 import (
+	"context"
 	"testing"

 	"github.com/stretchr/testify/assert"
@ -45,9 +46,9 @@ func Test_routesImpl_FindBackendForServerAddress(t *testing.T) {
 		t.Run(tt.name, func(t *testing.T) {
 			r := NewRoutes()

-			r.CreateMapping(tt.mapping.serverAddress, tt.mapping.backend)
+			r.CreateMapping(tt.mapping.serverAddress, tt.mapping.backend, func(ctx context.Context) error { return nil })

-			if got, server := r.FindBackendForServerAddress(tt.args.serverAddress); got != tt.want {
+			if got, server, _ := r.FindBackendForServerAddress(context.Background(), tt.args.serverAddress); got != tt.want {
 				t.Errorf("routesImpl.FindBackendForServerAddress() = %v, want %v", got, tt.want)
 			} else {
 				assert.Equal(t, tt.mapping.serverAddress, server)