feat: json logging (#17)

README.md

@@ -75,6 +75,7 @@ Metrics are refreshed every `POLL_INTERVAL` (default 60 seconds).
 | `OCTOPUS_SOLAR_DEVICE_ID` | No | Use a specific solar export smart device ID directly |
 | `PORT` | No | Port to expose metrics on (default: `9359`) |
 | `POLL_INTERVAL` | No | How often to poll Octopus APIs (Go duration, default: `60s`) |
+| `LOG_LEVEL` | No | Minimum log level: `debug`, `info`, `warn`, `error` (default: `info`). Logs are emitted as JSON to stderr |
 
 If no filter variables are set, the exporter auto-discovers the first smart meter of each type found on the account. Use `OCTOPUS_MPAN` / `OCTOPUS_MPRN` to pin to a specific meter on accounts with multiple meters.
 

cmd/octopus_exporter/client.go

@@ -6,7 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"log"
+	"log/slog"
 	"net/http"
 	"strconv"
 	"strings"
@@ -76,7 +76,7 @@ func executeWithRetry(makeReq func() (*http.Request, error)) ([]byte, error) {
 					wait = time.Duration(secs) * time.Second
 				}
 			}
-			log.Printf("rate limited; retrying in %v (attempt %d/%d)", wait, attempt+1, maxRetries)
+			slog.Warn("rate limited; retrying", "wait", wait, "attempt", attempt+1, "max_retries", maxRetries)
 			time.Sleep(wait)
 			backoff *= 2
 			continue

cmd/octopus_exporter/consumption.go

@@ -7,6 +7,8 @@ import (
 	"time"
 )
 
+var errNoConsumptionData = errors.New("no consumption data in last 7 days")
+
 type consumptionReading struct {
 	KWh           float64
 	IntervalStart time.Time
@@ -34,7 +36,7 @@ func getLatestConsumption(kind meterKind, id, serial, key string) (*consumptionR
 
 	results := toSlice(result["results"])
 	if len(results) == 0 {
-		return nil, errors.New("no consumption data in last 7 days")
+		return nil, errNoConsumptionData
 	}
 
 	latest, ok := results[len(results)-1].(map[string]any)

cmd/octopus_exporter/main.go

@@ -3,7 +3,7 @@ package main
 import (
 	"context"
 	"errors"
-	"log"
+	"log/slog"
 	"net/http"
 	"os"
 	"os/signal"
@@ -23,10 +23,15 @@ var (
 	commit  = "none"
 )
 
+func fatal(msg string, args ...any) {
+	slog.Error(msg, args...)
+	os.Exit(1)
+}
+
 func mustEnv(key string) string {
 	v := os.Getenv(key)
 	if v == "" {
-		log.Fatalf("required environment variable %s is not set", key)
+		fatal("required environment variable not set", "var", key)
 	}
 	return v
 }
@@ -53,51 +58,63 @@ func parseInterval(key string, def time.Duration) time.Duration {
 	}
 	d, err := time.ParseDuration(v)
 	if err != nil {
-		log.Printf("invalid %s=%q, falling back to %s: %v", key, v, def, err)
+		slog.Warn("invalid duration, falling back to default", "key", key, "value", v, "default", def, "err", err)
 		return def
 	}
 	return d
 }
 
 func main() {
+	logLevel := slog.LevelInfo
+	badLevel := os.Getenv("LOG_LEVEL")
+	if badLevel != "" {
+		if err := logLevel.UnmarshalText([]byte(badLevel)); err == nil {
+			badLevel = ""
+		}
+	}
+	slog.SetDefault(slog.New(slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{Level: logLevel})))
+	if badLevel != "" {
+		slog.Warn("invalid LOG_LEVEL, falling back to info", "value", badLevel)
+	}
+
 	apiKey = mustEnv("OCTOPUS_API_KEY")
 	port = envOrDefault("PORT", "9359")
 	pollInterval := parseInterval("POLL_INTERVAL", 60*time.Second)
-	log.Printf("octopus_exporter %s (%s), poll interval %s", version, commit, pollInterval)
+	slog.Info("starting octopus_exporter", "version", version, "commit", commit, "poll_interval", pollInterval)
 
 	token, err := getKrakenToken(apiKey)
 	if err != nil {
-		log.Fatalf("failed to get initial token: %v", err)
+		fatal("failed to get initial token", "err", err)
 	}
 
-	log.Println("discovering meters from account...")
+	slog.Info("discovering meters from account")
 	candidates, err := getMeters(token)
 	if err != nil {
-		log.Fatalf("failed to discover meters: %v", err)
+		fatal("failed to discover meters", "err", err)
 	}
 
 	elecMeter, err := resolveMeter(candidates, electricity)
 	if err != nil {
-		log.Fatalf("failed to resolve electricity meter: %v", err)
+		fatal("failed to resolve electricity meter", "err", err)
 	}
 	if elecMeter == nil {
-		log.Fatal("no electricity smart meter found on account")
+		fatal("no electricity smart meter found on account")
 	}
 
 	gasMeter, err := resolveMeter(candidates, gas)
 	if err != nil {
-		log.Fatalf("failed to resolve gas meter: %v", err)
+		fatal("failed to resolve gas meter", "err", err)
 	}
 	if gasMeter == nil {
-		log.Println("no gas smart meter found — gas metrics disabled")
+		slog.Info("no gas smart meter found — gas metrics disabled")
 	}
 
 	solarMeter, err := resolveMeter(candidates, solar)
 	if err != nil {
-		log.Fatalf("failed to resolve solar export meter: %v", err)
+		fatal("failed to resolve solar export meter", "err", err)
 	}
 	if solarMeter == nil {
-		log.Println("no solar export meter found — solar metrics disabled")
+		slog.Info("no solar export meter found — solar metrics disabled")
 	}
 
 	// --- Metrics ---
@@ -190,9 +207,9 @@ func main() {
 		ReadHeaderTimeout: 5 * time.Second,
 	}
 	go func() {
-		log.Printf("serving metrics on :%s/metrics", port)
+		slog.Info("serving metrics", "addr", ":"+port+"/metrics")
 		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
-			log.Fatal(err)
+			fatal("HTTP server failed", "err", err)
 		}
 	}()
 
@@ -216,7 +233,7 @@ func main() {
 			newT, e := getKrakenToken(apiKey)
 			if e != nil {
 				tokenMu.Unlock()
-				log.Printf("token refresh failed: %v", e)
+				slog.Error("token refresh failed", "err", e)
 				return err
 			}
 			token = newT
@@ -237,8 +254,8 @@ func main() {
 			failedAny atomic.Bool
 		)
 
-		fail := func(format string, args ...any) {
-			log.Printf(format, args...)
+		fail := func(msg string, args ...any) {
+			slog.Error(msg, args...)
 			pollErrors.Inc()
 			failedAny.Store(true)
 		}
@@ -247,8 +264,15 @@ func main() {
 			wg.Add(1)
 			go func() {
 				defer wg.Done()
-				if err := fn(); err != nil {
-					fail("%s error: %v", name, err)
+				err := fn()
+				switch {
+				case err == nil:
+				case errors.Is(err, errNoTelemetryData):
+					slog.Warn("no telemetry data", "collector", name)
+				case errors.Is(err, errNoConsumptionData):
+					slog.Info("no consumption data", "collector", name)
+				default:
+					fail(name+" error", "err", err)
 				}
 			}()
 		}
@@ -387,7 +411,7 @@ func main() {
 			if collectedRates.ElectricityIsAgile && collectedRates.ElectricityProductCode != "" && collectedRates.ElectricityTariffCode != "" {
 				agileRate, err := getCurrentAgileRate(collectedRates.ElectricityProductCode, collectedRates.ElectricityTariffCode, apiKey)
 				if err != nil {
-					fail("agile rate error: %v", err)
+					fail("agile rate error", "err", err)
 				} else {
 					unitRate = agileRate
 				}
@@ -415,11 +439,11 @@ func main() {
 	for {
 		select {
 		case <-ctx.Done():
-			log.Println("shutdown signal received, draining HTTP server")
+			slog.Info("shutdown signal received, draining HTTP server")
 			shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 			defer cancel()
 			if err := srv.Shutdown(shutdownCtx); err != nil {
-				log.Printf("HTTP shutdown error: %v", err)
+				slog.Error("HTTP shutdown error", "err", err)
 			}
 			return
 		case <-ticker.C:

cmd/octopus_exporter/meters.go

@@ -2,7 +2,7 @@ package main
 
 import (
 	"fmt"
-	"log"
+	"log/slog"
 	"os"
 )
 
@@ -179,11 +179,11 @@ func resolveMeter(candidates []meterCandidate, kind meterKind) (*resolvedMeter,
 		m := &resolvedMeter{deviceID: c.deviceID, mpan: c.mpan, mprn: c.mprn, serial: c.serial}
 		switch kind {
 		case electricity:
-			log.Printf("using electricity meter: MPAN=%s serial=%s deviceID=%s", m.mpan, m.serial, m.deviceID)
+			slog.Info("using electricity meter", "mpan", m.mpan, "serial", m.serial, "device_id", m.deviceID)
 		case gas:
-			log.Printf("using gas meter: MPRN=%s serial=%s deviceID=%s", m.mprn, m.serial, m.deviceID)
+			slog.Info("using gas meter", "mprn", m.mprn, "serial", m.serial, "device_id", m.deviceID)
 		case solar:
-			log.Printf("using solar export meter: MPAN=%s serial=%s deviceID=%s", m.mpan, m.serial, m.deviceID)
+			slog.Info("using solar export meter", "mpan", m.mpan, "serial", m.serial, "device_id", m.deviceID)
 		}
 		return m, nil
 	}

cmd/octopus_exporter/telemetry.go

@@ -6,6 +6,8 @@ import (
 	"fmt"
 )
 
+var errNoTelemetryData = errors.New("no telemetry data returned")
+
 type telemetryReading struct {
 	ReadAt      string    `json:"readAt"`
 	Consumption jsonFloat `json:"consumption"`
@@ -29,7 +31,7 @@ func getLiveConsumption(token, deviceID string) (*telemetryReading, error) {
 	}
 	telemetry := toSlice(data["smartMeterTelemetry"])
 	if len(telemetry) == 0 {
-		return nil, errors.New("no telemetry data returned")
+		return nil, errNoTelemetryData
 	}
 
 	raw, err := json.Marshal(telemetry[0])