Add support of ntp time verification

ntp/ntp.go

@@ -0,0 +1,60 @@
+package ntp
+
+import (
+	"math/rand"
+	"time"
+
+	"github.com/beevik/ntp"
+	"github.com/juju/errors"
+	"go.uber.org/zap"
+)
+
+const autoUpdatePeriod = time.Minute
+
+var ntpEndpoints = []string{
+	"0.pool.ntp.org",
+	"1.pool.ntp.org",
+	"2.pool.ntp.org",
+	"3.pool.ntp.org",
+}
+
+// Fetch fetches the data on time drift.
+func Fetch() (time.Duration, error) {
+	url := ntpEndpoints[rand.Intn(len(ntpEndpoints))]
+	resp, err := ntp.Query(url)
+	if err != nil {
+		return 0, errors.Annotatef(err, "Cannot fetch NTP server %s", url)
+	}
+
+	offsetInt := int64(resp.ClockOffset)
+	if offsetInt < 0 {
+		offsetInt = -offsetInt
+	}
+	offset := time.Duration(offsetInt)
+
+	return offset, nil
+}
+
+// AutoUpdate runs periodic check of current time .drift state.
+func AutoUpdate() {
+	logger := zap.S().Named("ntp")
+
+	for range time.Tick(autoUpdatePeriod) {
+		diff, err := Fetch()
+		if err != nil {
+			logger.Debugw("Cannot fetch time from NTP", "error", err)
+			continue
+		}
+
+		switch {
+		case diff < 400*time.Millisecond:
+			logger.Debugw("NTP time drift", "value", diff.String())
+		case diff < 600*time.Millisecond:
+			logger.Infow("NTP time drift", "value", diff.String())
+		case diff < 800*time.Millisecond:
+			logger.Warnw("NTP time drift", "value", diff.String())
+		default:
+			logger.Errorw("NTP time drift", "value", diff.String())
+		}
+	}
+}