Add --log-format flag

Author: alexandre-daubois

docs/cli-reference.md

@@ -20,6 +20,7 @@ ember [flags]
 | `--expose` | string | _(none)_ | Start Prometheus metrics endpoint on this address (e.g. `:9191`). See [Prometheus Export](prometheus-export.md). |
 | `--daemon` | bool | `false` | Headless mode: no TUI. Requires `--expose`. See [Prometheus Export](prometheus-export.md). |
 | `--metrics-prefix` | string | _(none)_ | Prefix for exported Prometheus metric names. See [Prometheus Export](prometheus-export.md). |
+| `--log-format` | string | `text` | Log format for daemon/json modes (`text` or `json`). JSON format is suitable for log aggregation systems. |
 | `--no-color` | bool | `false` | Disable colors |
 | `--version` | | | Print version and exit |
 

internal/app/daemon.go

@@ -3,9 +3,7 @@ package app
 import (
 	"context"
 	"errors"
-	"fmt"
 	"net/http"
-	"os"
 	"time"
 
 	"github.com/alexandre-daubois/ember/internal/exporter"
@@ -34,16 +32,17 @@ func runDaemon(ctx context.Context, f fetcher.Fetcher, cfg *config) error {
 
 	go func() {
 		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
-			cancel(fmt.Errorf("metrics server: %w", err))
+			cancel(err)
 		}
 	}()
 
-	fmt.Fprintf(os.Stderr, "ember daemon: exposing metrics on %s\n", metricsURL(cfg.expose))
+	log := cfg.logger
+	log.Info("daemon started", "metrics_url", metricsURL(cfg.expose))
 
 	poll := func() {
 		snap, err := f.Fetch(ctx)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "error: %v\n", err)
+			log.Error("fetch failed", "err", err)
 			return
 		}
 		state.Update(snap)

internal/app/json.go

@@ -3,7 +3,7 @@ package app
 import (
 	"context"
 	"encoding/json"
-	"fmt"
+	"log/slog"
 	"os"
 	"time"
 
@@ -49,14 +49,14 @@ type jsonDerived struct {
 	P99       *float64 `json:"p99,omitempty"`
 }
 
-func runJSON(ctx context.Context, f fetcher.Fetcher, interval time.Duration, once bool) {
+func runJSON(ctx context.Context, f fetcher.Fetcher, interval time.Duration, once bool, log *slog.Logger) {
 	enc := json.NewEncoder(os.Stdout)
 	var state model.State
 
 	poll := func() {
 		snap, err := f.Fetch(ctx)
 		if err != nil {
-			fmt.Fprintf(os.Stderr, "error: %v\n", err)
+			log.Error("fetch failed", "err", err)
 			return
 		}
 		state.Update(snap)

internal/app/json_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"log/slog"
 	"net/http"
 	"net/http/httptest"
 	"os"
@@ -244,7 +245,7 @@ caddy_http_requests_total{host="test.com",code="200"} 100
 	os.Stdout = w
 
 	ctx := context.Background()
-	runJSON(ctx, f, time.Second, true)
+	runJSON(ctx, f, time.Second, true, slog.New(slog.NewTextHandler(os.Stderr, nil)))
 
 	w.Close()
 	os.Stdout = origStdout

internal/app/run.go

@@ -3,6 +3,7 @@ package app
 import (
 	"context"
 	"fmt"
+	"log/slog"
 	"os"
 	"os/signal"
 	"syscall"
@@ -24,6 +25,8 @@ type config struct {
 	expose        string
 	daemon        bool
 	metricsPrefix string
+	logFormat     string
+	logger        *slog.Logger
 }
 
 func newRootCmd(version string) *cobra.Command {
@@ -61,6 +64,7 @@ Keybindings:
 		SilenceUsage:  true,
 		SilenceErrors: true,
 		PreRunE: func(cmd *cobra.Command, args []string) error {
+			initLogger(&cfg)
 			return validate(&cfg)
 		},
 		RunE: func(cmd *cobra.Command, args []string) error {
@@ -89,7 +93,7 @@ Keybindings:
 
 			switch {
 			case cfg.jsonMode:
-				runJSON(ctx, f, cfg.interval, cfg.once)
+				runJSON(ctx, f, cfg.interval, cfg.once, cfg.logger)
 			case cfg.daemon:
 				return runDaemon(ctx, f, &cfg)
 			default:
@@ -113,6 +117,7 @@ Keybindings:
 	f.StringVar(&cfg.expose, "expose", "", "Expose Prometheus metrics (e.g. :9191)")
 	f.BoolVar(&cfg.daemon, "daemon", false, "Headless mode (requires --expose)")
 	f.StringVar(&cfg.metricsPrefix, "metrics-prefix", "", "Prefix for exported Prometheus metric names")
+	f.StringVar(&cfg.logFormat, "log-format", "text", "Log format for daemon/json modes (text or json)")
 
 	cmd.AddCommand(newStatusCmd(&cfg))
 	cmd.AddCommand(newWaitCmd(&cfg))
@@ -135,6 +140,15 @@ func contextWithTimeout(parent context.Context, timeout time.Duration) (context.
 	return parent, func() {}
 }
 
+func initLogger(cfg *config) {
+	switch cfg.logFormat {
+	case "json":
+		cfg.logger = slog.New(slog.NewJSONHandler(os.Stderr, nil))
+	default:
+		cfg.logger = slog.New(slog.NewTextHandler(os.Stderr, nil))
+	}
+}
+
 func validate(cfg *config) error {
 	if cfg.daemon && cfg.expose == "" {
 		return fmt.Errorf("--daemon requires --expose")

internal/app/run_test.go

@@ -168,6 +168,36 @@ func TestRun_TimeoutInheritedBySubcommands(t *testing.T) {
 	assert.Contains(t, buf.String(), "--timeout")
 }
 
+func TestInitLogger_Text(t *testing.T) {
+	cfg := &config{logFormat: "text"}
+	initLogger(cfg)
+	assert.NotNil(t, cfg.logger)
+}
+
+func TestInitLogger_JSON(t *testing.T) {
+	cfg := &config{logFormat: "json"}
+	initLogger(cfg)
+	assert.NotNil(t, cfg.logger)
+}
+
+func TestInitLogger_DefaultIsText(t *testing.T) {
+	cfg := &config{}
+	initLogger(cfg)
+	assert.NotNil(t, cfg.logger)
+}
+
+func TestRun_LogFormatFlagAvailable(t *testing.T) {
+	cmd := newRootCmd("1.0.0")
+	cmd.SetArgs([]string{"--help"})
+	var buf bytes.Buffer
+	cmd.SetOut(&buf)
+
+	err := cmd.Execute()
+
+	assert.NoError(t, err)
+	assert.Contains(t, buf.String(), "--log-format")
+}
+
 func TestRun_HelpContainsKeybindings(t *testing.T) {
 	cmd := newRootCmd("1.0.0")
 	cmd.SetArgs([]string{"--help"})