EVEREST-468 Restrict scaling down to single node cluster

api/database_cluster.go

@@ -148,17 +148,9 @@ func (e *EverestServer) UpdateDatabaseCluster(ctx echo.Context, kubernetesID str
 	if err != nil {
 		return errors.Join(err, errors.New("could not get old Database Cluster"))
 	}
-	if dbc.Spec.Engine.Version != nil {
-		// XXX: Right now we do not support upgrading of versions
-		// because it varies across different engines. Also, we should
-		// prohibit downgrades. Hence, if versions are not equal we just return an error
-		if oldDB.Spec.Engine.Version != *dbc.Spec.Engine.Version {
-			return ctx.JSON(http.StatusBadRequest, Error{
-				Message: pointer.ToString("Changing version is not allowed"),
-			})
-		}
+	if err := validateDatabaseClusterOnUpdate(dbc, oldDB); err != nil {
+		return ctx.JSON(http.StatusBadRequest, Error{Message: pointer.ToString(err.Error())})
 	}
-
 	newMonitoringName := monitoringNameFrom(dbc)
 	newBackupNames := backupStorageNamesFrom(dbc)
 	err = e.createResources(ctx.Request().Context(), oldDB, kubeClient, newMonitoringName, newBackupNames)

api/validation.go

@@ -556,3 +556,24 @@ func validateStorageSize(cluster *DatabaseCluster) error {
 	}
 	return nil
 }
+
+func validateDatabaseClusterOnUpdate(dbc *DatabaseCluster, oldDB *everestv1alpha1.DatabaseCluster) error {
+	if dbc.Spec.Engine.Version != nil {
+		// XXX: Right now we do not support upgrading of versions
+		// because it varies across different engines. Also, we should
+		// prohibit downgrades. Hence, if versions are not equal we just return an error
+		if oldDB.Spec.Engine.Version != *dbc.Spec.Engine.Version {
+			return errors.New("changing version is not allowed")
+		}
+	}
+	if *dbc.Spec.Engine.Replicas < oldDB.Spec.Engine.Replicas && *dbc.Spec.Engine.Replicas == 1 {
+		// XXX: We can scale down multiple node clusters to a single node but we need to set
+		// `allowUnsafeConfigurations` to `true`. Having this configuration is not recommended
+		// and makes a database cluster unsafe. Once allowUnsafeConfigurations set to true you
+		// can't set it to false for all operators and psmdb operator does not support it.
+		//
+		// Once it is supported by all operators we can revert this.
+		return fmt.Errorf("cannot scale down %d node cluster to 1. The operation is not supported", oldDB.Spec.Engine.Replicas)
+	}
+	return nil
+}