Add Prewarming Pool Alert

that checks for every environment if the filled share of the prewarmin pool is at least the specified threshold.

internal/api/health.go

@@ -1,12 +1,42 @@
 package api
 
 import (
+	"errors"
+	"fmt"
+	"github.com/openHPI/poseidon/internal/config"
+	"github.com/openHPI/poseidon/internal/environment"
+	"github.com/openHPI/poseidon/pkg/dto"
 	"net/http"
+	"strings"
 )
 
+var ErrorPrewarmingPoolDepleting = errors.New("the prewarming pool is depleting")
+
 // Health handles the health route.
 // It responds that the server is alive.
 // If it is not, the response won't reach the client.
-func Health(writer http.ResponseWriter, _ *http.Request) {
-	writer.WriteHeader(http.StatusNoContent)
+func Health(manager environment.Manager) http.HandlerFunc {
+	return func(writer http.ResponseWriter, request *http.Request) {
+		if err := checkPrewarmingPool(manager); err != nil {
+			sendJSON(writer, &dto.InternalServerError{Message: err.Error(), ErrorCode: dto.PrewarmingPoolDepleting},
+				http.StatusServiceUnavailable, request.Context())
+			return
+		}
+
+		writer.WriteHeader(http.StatusNoContent)
+	}
+}
+
+func checkPrewarmingPool(manager environment.Manager) error {
+	var depletingEnvironments []int
+	for _, data := range manager.Statistics() {
+		if float64(data.IdleRunners)/float64(data.PrewarmingPoolSize) < config.Config.Server.PrewarmingPoolAlertThreshold {
+			depletingEnvironments = append(depletingEnvironments, data.ID)
+		}
+	}
+	if len(depletingEnvironments) > 0 {
+		arrayToString := strings.Trim(strings.Join(strings.Fields(fmt.Sprint(depletingEnvironments)), ","), "[]")
+		return fmt.Errorf("%w: environments %s", ErrorPrewarmingPoolDepleting, arrayToString)
+	}
+	return nil
 }