feat: Provide option to specify readiness condition (#464)

* feat: Allow to specify readiness condition in health handler

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

* added unit tests

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

* add health check path option

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

* adapted unit test

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

* added comments to new options

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

* added comment

Signed-off-by: Florian Bacher <florian.bacher@dynatrace.com>

pkg/api/utils/healthCheck.go

@@ -7,6 +7,10 @@ import (
 	"net/http"
 )
 
+const defaultHealthEndpointPath = "/health"
+
+type ReadinessConditionFunc func() bool
+
 type StatusBody struct {
 	Status string `json:"status"`
 }
@@ -16,7 +20,48 @@ func (s *StatusBody) ToJSON() ([]byte, error) {
 	return json.Marshal(s)
 }
 
-func healthHandler(w http.ResponseWriter, r *http.Request) {
+type HealthHandlerOption func(h *healthHandler)
+
+// WithReadinessConditionFunc allows to specify a function that should determine if the endpoint should return an HTTP 200 (OK), or
+// a 412 (Precondition failed) response
+func WithReadinessConditionFunc(rc ReadinessConditionFunc) HealthHandlerOption {
+	return func(h *healthHandler) {
+		h.readinessConditionFunc = rc
+	}
+}
+
+// WithPath allows to specify the path under which the endpoint should be reachable
+func WithPath(path string) HealthHandlerOption {
+	return func(h *healthHandler) {
+		h.path = path
+	}
+}
+
+type healthHandler struct {
+	readinessConditionFunc ReadinessConditionFunc
+	path                   string
+}
+
+func newHealthHandler(opts ...HealthHandlerOption) *healthHandler {
+	h := &healthHandler{
+		path: defaultHealthEndpointPath,
+	}
+	for _, o := range opts {
+		o(h)
+	}
+	return h
+}
+
+func (h *healthHandler) healthCheck(w http.ResponseWriter, r *http.Request) {
+	ready := true
+	if h.readinessConditionFunc != nil {
+		ready = h.readinessConditionFunc()
+	}
+
+	if !ready {
+		w.WriteHeader(http.StatusPreconditionFailed)
+		return
+	}
 	status := StatusBody{Status: "OK"}
 
 	body, err := status.ToJSON()
@@ -32,9 +77,11 @@ func healthHandler(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-func RunHealthEndpoint(port string) {
-
-	http.HandleFunc("/health", healthHandler)
+// RunHealthEndpoint starts an http server on the specified port and provides a simple HTTP Get endpoint that can be used for health checks
+// per default, the endpoint will be reachable under the path '/health'
+func RunHealthEndpoint(port string, opts ...HealthHandlerOption) {
+	h := newHealthHandler(opts...)
+	http.HandleFunc(h.path, h.healthCheck)
 	err := http.ListenAndServe(fmt.Sprintf(":%s", port), nil)
 	if err != nil {
 		log.Println(err)

pkg/api/utils/healthCheck_test.go

@@ -0,0 +1,69 @@
+package api
+
+import (
+	"github.com/stretchr/testify/require"
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestRunHealthEndpoint(t *testing.T) {
+	go RunHealthEndpoint("8080")
+
+	require.Eventually(t, func() bool {
+		get, err := http.Get("http://localhost:8080/health")
+		if err != nil {
+			return false
+		}
+		if get.StatusCode != http.StatusOK {
+			return false
+		}
+		return true
+	}, 2*time.Second, 50*time.Millisecond)
+}
+
+func TestRunHealthEndpoint_WithReadinessCondition(t *testing.T) {
+	ready := false
+	go RunHealthEndpoint("8080", WithPath("/ready"), WithReadinessConditionFunc(func() bool {
+		return ready
+	}))
+
+	require.Eventually(t, func() bool {
+		get, err := http.Get("http://localhost:8080/ready")
+		if err != nil {
+			return false
+		}
+		if get.StatusCode != http.StatusPreconditionFailed {
+			return false
+		}
+		return true
+	}, 2*time.Second, 50*time.Millisecond)
+
+	ready = true
+
+	require.Eventually(t, func() bool {
+		get, err := http.Get("http://localhost:8080/ready")
+		if err != nil {
+			return false
+		}
+		if get.StatusCode != http.StatusOK {
+			return false
+		}
+		return true
+	}, 2*time.Second, 50*time.Millisecond)
+}
+
+func TestRunHealthEndpointCustomPath(t *testing.T) {
+	go RunHealthEndpoint("8080", WithPath("/readiness"))
+
+	require.Eventually(t, func() bool {
+		get, err := http.Get("http://localhost:8080/readiness")
+		if err != nil {
+			return false
+		}
+		if get.StatusCode != http.StatusOK {
+			return false
+		}
+		return true
+	}, 2*time.Second, 50*time.Millisecond)
+}