scrape/scrape.go
func (sp *scrapePool) sync(targets []*Target) {
	sp.mtx.Lock()
	defer sp.mtx.Unlock()

	var (
		uniqueTargets   = map[uint64]struct{}{}
		interval        = time.Duration(sp.config.ScrapeInterval)
		timeout         = time.Duration(sp.config.ScrapeTimeout)
		limit           = int(sp.config.SampleLimit)
		honorLabels     = sp.config.HonorLabels
		honorTimestamps = sp.config.HonorTimestamps
		mrc             = sp.config.MetricRelabelConfigs
	)

	for _, t := range targets {
		t := t
		hash := t.hash()
		uniqueTargets[hash] = struct{}{}

		if _, ok := sp.activeTargets[hash]; !ok {
			s := &targetScraper{Target: t, client: sp.client, timeout: timeout}
			l := sp.newLoop(scrapeLoopOptions{
				target:          t,
				scraper:         s,
				limit:           limit,
				honorLabels:     honorLabels,
				honorTimestamps: honorTimestamps,
				mrc:             mrc,
			})

			sp.activeTargets[hash] = t
			sp.loops[hash] = l

			go l.run(interval, timeout, nil)
		} else {
			// Need to keep the most updated labels information
			// for displaying it in the Service Discovery web page.
			sp.activeTargets[hash].SetDiscoveredLabels(t.DiscoveredLabels())
		}
	}

	var wg sync.WaitGroup

	// Stop and remove old targets and scraper loops.
	for hash := range sp.activeTargets {
		if _, ok := uniqueTargets[hash]; !ok {
			wg.Add(1)
			go func(l loop) {

				l.stop()

				wg.Done()
			}(sp.loops[hash])

			delete(sp.loops, hash)
			delete(sp.activeTargets, hash)
		}
	}

	// Wait for all potentially stopped scrapers to terminate.
	// This covers the case of flapping targets. If the server is under high load, a new scraper
	// may be active and tries to insert. The old scraper that didn't terminate yet could still
	// be inserting a previous sample set.
	wg.Wait()
}
func (sp *scrapePool) sync(targets []*Target) {
	sp.mtx.Lock()
	defer sp.mtx.Unlock()

	var (
		uniqueTargets   = map[uint64]struct{}{}
		interval        = time.Duration(sp.config.ScrapeInterval)
		timeout         = time.Duration(sp.config.ScrapeTimeout)
		limit           = int(sp.config.SampleLimit)
		honorLabels     = sp.config.HonorLabels
		honorTimestamps = sp.config.HonorTimestamps
		mrc             = sp.config.MetricRelabelConfigs
	)

	for _, t := range targets {
		t := t
		hash := t.hash()
		uniqueTargets[hash] = struct{}{}

		if _, ok := sp.activeTargets[hash]; !ok {
			s := &targetScraper{Target: t, client: sp.client, timeout: timeout}
			l := sp.newLoop(scrapeLoopOptions{
				target:          t,
				scraper:         s,
				limit:           limit,
				honorLabels:     honorLabels,
				honorTimestamps: honorTimestamps,
				mrc:             mrc,
			})

			sp.activeTargets[hash] = t
			sp.loops[hash] = l

			go l.run(interval, timeout, nil)
		} else {
			// Need to keep the most updated labels information
			// for displaying it in the Service Discovery web page.
			sp.activeTargets[hash].SetDiscoveredLabels(t.DiscoveredLabels())
		}
	}

	var wg sync.WaitGroup

	// Stop and remove old targets and scraper loops.
	for hash := range sp.activeTargets {
		if _, ok := uniqueTargets[hash]; !ok {
			wg.Add(1)
			go func(l loop) {

				l.stop()

				wg.Done()
			}(sp.loops[hash])

			delete(sp.loops, hash)
			delete(sp.activeTargets, hash)
		}
	}

	// Wait for all potentially stopped scrapers to terminate.
	// This covers the case of flapping targets. If the server is under high load, a new scraper
	// may be active and tries to insert. The old scraper that didn't terminate yet could still
	// be inserting a previous sample set.
	wg.Wait()
}
// sync takes a list of potentially duplicated targets, deduplicates them, starts
// scrape loops for new targets, and stops scrape loops for disappeared targets.
// It returns after all stopped scrape loops terminated.