Add prometheus endpoint for getting metrics (#33)

pkg/metrics/active_users.go

@@ -0,0 +1,92 @@
+//  Vikunja is a todo-list application to facilitate your life.
+//  Copyright 2018 Vikunja and contributors. All rights reserved.
+//
+//  This program is free software: you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation, either version 3 of the License, or
+//  (at your option) any later version.
+//
+//  This program is distributed in the hope that it will be useful,
+//  but WITHOUT ANY WARRANTY; without even the implied warranty of
+//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//  GNU General Public License for more details.
+//
+//  You should have received a copy of the GNU General Public License
+//  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package metrics
+
+import (
+	"bytes"
+	"code.vikunja.io/api/pkg/log"
+	"encoding/gob"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"time"
+)
+
+// SecondsUntilInactive defines the seconds until a user is considered inactive
+const SecondsUntilInactive = 60
+
+// ActiveUsersKey is the key used to store active users in redis
+const ActiveUsersKey = `activeusers`
+
+// ActiveUser defines an active user
+type ActiveUser struct {
+	UserID   int64
+	LastSeen time.Time
+}
+
+func init() {
+	promauto.NewGaugeFunc(prometheus.GaugeOpts{
+		Name: "vikunja_active_users",
+		Help: "The currently active users on this node",
+	}, func() float64 {
+
+		allActiveUsers, err := GetActiveUsers()
+		if err != nil {
+			log.Log.Error(err.Error())
+		}
+		activeUsersCount := 0
+		for _, u := range allActiveUsers {
+			if time.Since(u.LastSeen) < SecondsUntilInactive*time.Second {
+				activeUsersCount++
+			}
+		}
+		return float64(activeUsersCount)
+	})
+}
+
+// GetActiveUsers returns the active users from redis
+func GetActiveUsers() (users []*ActiveUser, err error) {
+
+	activeUsersR, err := r.Get(ActiveUsersKey).Bytes()
+	if err != nil {
+		if err.Error() == "redis: nil" {
+			return users, nil
+		}
+		return
+	}
+
+	var b bytes.Buffer
+	_, err = b.Write(activeUsersR)
+	if err != nil {
+		return nil, err
+	}
+	d := gob.NewDecoder(&b)
+	if err := d.Decode(&users); err != nil {
+		return nil, err
+	}
+	return
+}
+
+// SetActiveUsers sets the active users from redis
+func SetActiveUsers(users []*ActiveUser) (err error) {
+	var b bytes.Buffer
+	e := gob.NewEncoder(&b)
+	if err := e.Encode(users); err != nil {
+		return err
+	}
+
+	return r.Set(ActiveUsersKey, b.Bytes(), 0).Err()
+}