Use struct for request JSON, assume that there's always an error message if...

Use struct for request JSON, assume that there's always an error message if the result is null (according to docs)

main.go

@@ -33,10 +33,20 @@ var (
 	showVersion     = flag.Bool("version", false, "Print version information.")
 )
 
+type ApiRequest struct {
+	Object  string   `json:"object"`
+	Method  string   `json:"method"`
+	Params  []string `json:"params"`
+}
+
 type ApiResponse struct {
 	TGFlash string      `json:"tg_flash"`
 	Result  interface{} `json:"result"`
-	Error   interface{} `json:"error"`
+	Error   ApiError    `json:"error"`
+}
+
+type ApiError struct {
+	Message string `json:"message"`
 }
 
 type ResultObject struct {
@@ -61,13 +71,13 @@ func NewExporter(uri string) *Exporter {
 		URI: uri,
 		up: prometheus.NewDesc(
 			prometheus.BuildFQName(namespace, "", "up"),
-			"Could the Nexenta API be reached",
+			"Is the Nexenta API reachable",
 			nil,
 			nil),
 		scrapeFailures: prometheus.NewCounter(prometheus.CounterOpts{
 			Namespace: namespace,
 			Name:      "exporter_scrape_failures_total",
-			Help:      "Number of errors while scraping Nexenta API.",
+			Help:      "Number of errors while scraping the Nexenta API",
 		}),
 		volumeOnline: prometheus.NewDesc(
 			prometheus.BuildFQName(namespace, "volume", "online"),
@@ -92,7 +102,6 @@ func (e *Exporter) Describe(ch chan<- *prometheus.Desc) {
 func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
 	e.mutex.Lock() // To protect metrics from concurrent collects.
 	defer e.mutex.Unlock()
-	e.apiReachable = 0
 	err := e.collect(ch)
 	if err != nil {
 		log.Errorf("%v", err)
@@ -124,9 +133,7 @@ func (e *Exporter) collect(ch chan<- prometheus.Metric) error {
 
 func (e *Exporter) getVolumes(ch chan<- prometheus.Metric) ([]string, error) {
 
-	var reqJson = []byte(`{"object": "volume", "params": [""], "method": "get_names"}`)
-
-	apiResponse, err := e.queryApi(ch, reqJson); if err != nil { return nil, err }
+	apiResponse, err := e.queryApi(ch, "volume", "get_names", []string{""}); if err != nil { return nil, err }
 	apiResult, err := json.Marshal(apiResponse.Result); if err != nil { return nil, err }
 
 	var volumes []string
@@ -137,8 +144,7 @@ func (e *Exporter) getVolumes(ch chan<- prometheus.Metric) ([]string, error) {
 
 func (e *Exporter) getVolumeStatus(ch chan<- prometheus.Metric, volume string) error {
 
-	var reqJson = []byte(`{"object": "volume", "params": ["` + volume + `"], "method": "get_status"}`)
-	apiResponse, err := e.queryApi(ch, reqJson); if err != nil { return err }
+	apiResponse, err := e.queryApi(ch, "volume", "get_status", []string{volume}); if err != nil { return err }
 	apiResult, err := json.Marshal(apiResponse.Result); if err != nil { return err }
 
 	var result = new(ResultObject)
@@ -154,13 +160,20 @@ func (e *Exporter) getVolumeStatus(ch chan<- prometheus.Metric, volume string) e
 	return err
 }
 
-func (e *Exporter) queryApi(ch chan<- prometheus.Metric, reqJson []byte) (*ApiResponse, error) {
+func (e *Exporter) queryApi(ch chan<- prometheus.Metric, object string, method string, params []string) (*ApiResponse, error) {
+
+	reqObject := &ApiRequest{
+		Object: object,
+		Method: method,
+		Params: params,
+	}
 
+	e.apiReachable = 0
+	reqJson, _ := json.Marshal(reqObject)
 	resp, err := e.client.Post(e.URI, "application/json", bytes.NewBuffer(reqJson))
 	if err != nil {
 		return nil, fmt.Errorf("Error scraping Nexenta API: %v", err)
 	}
-
 	e.apiReachable = 1
 
 	data, err := ioutil.ReadAll(resp.Body)
@@ -171,7 +184,7 @@ func (e *Exporter) queryApi(ch chan<- prometheus.Metric, reqJson []byte) (*ApiRe
 		}
 		e.scrapeFailures.Inc()
 		e.scrapeFailures.Collect(ch)
-		return nil, fmt.Errorf("Status %s (%d) %s", resp.Status, resp.StatusCode, data)
+		return nil, fmt.Errorf("Request Error: Status %s %s", resp.Status, data)
 	}
 
 	//log.Infof("response: %s", data)
@@ -182,13 +195,7 @@ func (e *Exporter) queryApi(ch chan<- prometheus.Metric, reqJson []byte) (*ApiRe
 	if apiResponse.Result == nil {
 		e.scrapeFailures.Inc()
 		e.scrapeFailures.Collect(ch)
-		return nil, fmt.Errorf("Field 'result' in API response is null, API Error: %v", apiResponse.Error)
-	}
-
-	if apiResponse.Error != nil {
-		e.scrapeFailures.Inc()
-		e.scrapeFailures.Collect(ch)
-		return nil, fmt.Errorf("API Error: %v", apiResponse.Error)
+		return nil, fmt.Errorf("API Error: %v", apiResponse.Error.Message)
 	}
 
 	return apiResponse, nil