Introduce app state for the monitoring module

refs #2835

modules/monitoring/library/Monitoring/ProvidedHook/ApplicationState.php

@@ -0,0 +1,32 @@
+<?php
+/* Icinga Web 2 | (c) 2018 Icinga Development Team | GPLv2+ */
+
+namespace Icinga\Module\Monitoring\ProvidedHook;
+
+use Icinga\Application\Hook\ApplicationStateHook;
+use Icinga\Module\Monitoring\Backend\MonitoringBackend;
+
+class ApplicationState extends ApplicationStateHook
+{
+    public function collectMessages()
+    {
+        $backend = MonitoringBackend::instance();
+
+        $programStatus = $backend
+            ->select()
+            ->from(
+                'programstatus',
+                ['is_currently_running', 'status_update_time']
+            )
+            ->fetchRow();
+
+        if ($programStatus === false || ! (bool) $programStatus->is_currently_running) {
+            $message = sprintf(
+                mt('monitoring', "Monitoring backend '%s' is not running."),
+                $backend->getName()
+            );
+
+            $this->addError('monitoring/backend-down', $programStatus->status_update_time, $message);
+        }
+    }
+}