feat: Add metrics

Fixes #7.

Signed-off-by: Julien Riou <julien@riou.xyz>

src/server/config.go

@@ -7,18 +7,22 @@ import (
 )
 
 type Config struct {
-	Title              string `json:"title"`
-	DatabaseType       string `json:"database_type"`
-	DatabaseDsn        string `json:"database_dsn"`
-	IDLength           int    `json:"id_length"`
-	PasswordLength     int    `json:"password_length"`
-	ExpirationInterval int    `json:"expiration_interval"`
-	ListenAddress      string `json:"listen_address"`
-	ListenPort         int    `json:"listen_port"`
-	Expirations        []int  `json:"expirations"`
-	Expiration         int    `json:"expiration"`
-	MaxUploadSize      int64  `json:"max_upload_size"`
-	ShowVersion        bool   `json:"show_version"`
+	Title                 string `json:"title"`
+	DatabaseType          string `json:"database_type"`
+	DatabaseDsn           string `json:"database_dsn"`
+	IDLength              int    `json:"id_length"`
+	PasswordLength        int    `json:"password_length"`
+	ExpirationInterval    int    `json:"expiration_interval"`
+	ListenAddress         string `json:"listen_address"`
+	ListenPort            int    `json:"listen_port"`
+	Expirations           []int  `json:"expirations"`
+	Expiration            int    `json:"expiration"`
+	MaxUploadSize         int64  `json:"max_upload_size"`
+	ShowVersion           bool   `json:"show_version"`
+	EnableMetrics         bool   `json:"enable_metrics"`
+	PrometheusRoute       string `json:"prometheus_route"`
+	PrometheusNotesMetric string `json:"prometheus_notes_metric"`
+	ObservationInterval   int    `json:"observation_internal"`
 }
 
 func NewConfig() *Config {
@@ -38,9 +42,13 @@ func NewConfig() *Config {
 			604800,   // 7 days
 			18144000, // 30 days
 		},
-		Expiration:    604800,   // 7 days
-		MaxUploadSize: 10485760, // 10MiB (encoded)
-		ShowVersion:   false,
+		Expiration:            604800,   // 7 days
+		MaxUploadSize:         10485760, // 10MiB (encoded)
+		ShowVersion:           false,
+		EnableMetrics:         false,
+		PrometheusRoute:       "/metrics",
+		PrometheusNotesMetric: "collerd_notes",
+		ObservationInterval:   60,
 	}
 }
 

src/server/metrics.go

@@ -0,0 +1,55 @@
+package server
+
+import (
+	"fmt"
+	"log/slog"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type Metrics struct {
+	logger   *slog.Logger
+	notes    prometheus.Gauge
+	db       *Database
+	reg      *prometheus.Registry
+	interval int
+}
+
+func NewMetrics(logger *slog.Logger, reg *prometheus.Registry, config *Config, db *Database) (*Metrics, error) {
+	l := logger.With("module", "metrics")
+	m := &Metrics{
+		logger: l,
+		db:     db,
+		notes: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: config.PrometheusNotesMetric,
+		}),
+		reg:      reg,
+		interval: config.ObservationInterval,
+	}
+	if err := reg.Register(m.notes); err != nil {
+		return nil, err
+	}
+
+	go m.ObserveThread()
+
+	return m, nil
+}
+
+func (m *Metrics) ObserveThread() {
+	m.logger.Debug("starting observations")
+	for {
+		m.logger.Debug("counting notes")
+		var count int64
+		m.db.db.Model(&Note{}).Count(&count)
+		m.notes.Set(float64(count))
+		m.logger.Debug("counted notes", slog.Any("count", count))
+
+		wording := "second"
+		if m.interval > 1 {
+			wording += "s"
+		}
+		m.logger.Debug(fmt.Sprintf("waiting for %d %s before next observation", m.interval, wording))
+		time.Sleep(time.Duration(m.interval) * time.Second)
+	}
+}

src/server/server.go

@@ -14,6 +14,7 @@ import (
 
 	"git.riou.xyz/jriou/coller/internal"
 	"github.com/gorilla/mux"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 var passwordLength = internal.MIN_PASSWORD_LENGTH
@@ -23,6 +24,7 @@ type Server struct {
 	db      *Database
 	config  *Config
 	version string
+	metrics *Metrics
 }
 
 func NewServer(logger *slog.Logger, db *Database, config *Config, version string) (*Server, error) {
@@ -44,6 +46,10 @@ func (s *Server) SetPasswordLength(length int) {
 	passwordLength = length
 }
 
+func (s *Server) SetMetrics(metrics *Metrics) {
+	s.metrics = metrics
+}
+
 type ErrorResponse struct {
 	Message string `json:"message"`
 	Error   string `json:"error"`
@@ -312,8 +318,15 @@ var templatesFS embed.FS
 func (s *Server) Start() error {
 	r := mux.NewRouter().StrictSlash(true)
 
-	// API
+	// Healthchecks
 	r.HandleFunc("/health", HealthHandler)
+
+	// Metrics
+	if s.metrics != nil && s.metrics.reg != nil {
+		r.Path(s.config.PrometheusRoute).Handler(promhttp.HandlerFor(s.metrics.reg, promhttp.HandlerOpts{Registry: s.metrics.reg})).Methods("GET")
+	}
+
+	// API
 	r.Path("/api/note").Handler(&CreateNoteHandler{logger: s.logger, db: s.db, maxUploadSize: s.config.MaxUploadSize}).Methods("POST")
 	r.Path("/{id:[a-zA-Z0-9]+}/{password:[a-zA-Z0-9]+}").Handler(&GetProtectedNoteHandler{logger: s.logger, db: s.db}).Methods("GET")
 	r.Path("/{id:[a-zA-Z0-9]+}").Handler(&GetNoteHandler{logger: s.logger, db: s.db}).Methods("GET")