Make input registers GaugeFuncs

main.go

@@ -25,6 +25,7 @@ import (
 	"log"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 
 	"gitea.drjosh.dev/josh/sungrow/modbus"
@@ -33,12 +34,16 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+const maxScrapeAge = 3 * time.Second
+
 var (
 	httpAddr       = flag.String("http-addr", ":9455", "Address to listen on")
 	inverterAddrs  = flag.String("inverter-addrs", "rakmodule_00DBC1:502,192.168.86.6:502", "Comma-separated list of inverter addresses (modbus-tcp with 'encryption')")
 	scrapeInterval = flag.Duration("scrape-interval", 15*time.Second, "Period of modbus scraping loop")
 
-	registerGauges = make(map[uint16]prometheus.Gauge)
+	scrapeMu   sync.Mutex
+	lastScrape time.Time
+	lastValues = make(map[uint16]float64)
 
 	scrapeCounter = promauto.NewCounter(prometheus.CounterOpts{
 		Namespace: "sungrow",
@@ -100,21 +105,13 @@ func init() {
 		},
 		func() float64 { return solarFeedInTariff.pricePerKWh(time.Now()) },
 	)
-
-	for addr, reg := range sungrowInputRegs {
-		registerGauges[addr] = promauto.NewGauge(prometheus.GaugeOpts{
-			Namespace: "sungrow",
-			Subsystem: "inverter",
-			Name:      reg.name,
-			Help:      fmt.Sprintf("addr: %d, unit: %s", addr, reg.unit),
-		})
-	}
 }
 
 func statusHandler(w http.ResponseWriter, r *http.Request) {
 	fmt.Fprintf(w, "current time: %v\n", time.Now())
 }
 
+// Called under scrapeMu.
 func readRegs(client modbus.Client, start, qty uint16) {
 	data, err := client.ReadInputRegisters(start, qty)
 	if err != nil {
@@ -136,16 +133,18 @@ func readRegs(client modbus.Client, start, qty uint16) {
 			log.Fatalf("Couldn't parse input register data at %d: %v", addr, err)
 		}
 		//fmt.Printf("%s: %v %s\n", reg.name, val, reg.unit)
-		registerGauges[addr].Set(val)
+		lastValues[addr] = val
 	}
 }
 
+// Called under scrapeMu.
 func scrape(client modbus.Client) {
 	start := time.Now()
 	scrapeStart.SetToCurrentTime()
 	readRegs(client, 5000, 100)
 	readRegs(client, 5100, 50)
 	scrapeEnd.SetToCurrentTime()
+	lastScrape = time.Now()
 	scrapeDuration.Set(time.Since(start).Seconds())
 	scrapeCounter.Inc()
 }
@@ -173,18 +172,34 @@ func main() {
 	http.Handle("/metrics", promhttp.Handler())
 	http.HandleFunc("/", statusHandler)
 
-	// Modbus scrape loop
 	handler := modbus.TCPHandlerFromConnection(sgc)
 	handler.SlaveId = 0x01
 	//handler.Connect()
 	defer handler.Close()
 	client := modbus.NewClient(handler)
+
+	// Finally, create all the register gauges.
+	// These are GaugeFuncs to more closely align Prometheus scrape time with
+	// the modbus scrape time.
+	for addr, reg := range sungrowInputRegs {
+		promauto.NewGaugeFunc(
+			prometheus.GaugeOpts{
+				Namespace: "sungrow",
+				Subsystem: "inverter",
+				Name:      reg.name,
+				Help:      fmt.Sprintf("addr: %d, unit: %s", addr, reg.unit),
+			},
+			func() float64 {
+				scrapeMu.Lock()
+				defer scrapeMu.Unlock()
+				if time.Since(lastScrape) <= maxScrapeAge {
+					return lastValues[addr]
+				}
 				scrape(client)
-	// Start http interface only after first successful scrape
-	go func() {
+				return lastValues[addr]
+			},
+		)
+	}
+
 	log.Fatalf("http.ListenAndServe: %v", http.ListenAndServe(*httpAddr, nil))
-	}()
-	for range time.Tick(*scrapeInterval) {
-		scrape(client)
-	}
 }