Fix wording from "jitterSeed" -> "offsetSeed" for server-wide scrape offsets

In digital communication, "jitter" usually refers to how much a signal deviates
from true periodicity, see https://en.wikipedia.org/wiki/Jitter. The way we are
using the "jitterSeed" in Prometheus does not affect the true periodicity at
all, but just introduces a constant phase shift (or offset) within the period.
So it would be more correct and less confusing to call the "jitterSeed" an
"offsetSeed" instead.

Signed-off-by: Julius Volz <julius.volz@gmail.com>

scrape/manager.go

@@ -150,7 +150,7 @@ type Manager struct {
 	append    storage.Appendable
 	graceShut chan struct{}
 
-	jitterSeed    uint64     // Global jitterSeed seed is used to spread scrape workload across HA setup.
+	offsetSeed    uint64     // Global offsetSeed seed is used to spread scrape workload across HA setup.
 	mtxScrape     sync.Mutex // Guards the fields below.
 	scrapeConfigs map[string]*config.ScrapeConfig
 	scrapePools   map[string]*scrapePool
@@ -214,7 +214,7 @@ func (m *Manager) reload() {
 				level.Error(m.logger).Log("msg", "error reloading target set", "err", "invalid config id:"+setName)
 				continue
 			}
-			sp, err := newScrapePool(scrapeConfig, m.append, m.jitterSeed, log.With(m.logger, "scrape_pool", setName), m.opts)
+			sp, err := newScrapePool(scrapeConfig, m.append, m.offsetSeed, log.With(m.logger, "scrape_pool", setName), m.opts)
 			if err != nil {
 				level.Error(m.logger).Log("msg", "error creating new scrape pool", "err", err, "scrape_pool", setName)
 				continue
@@ -234,8 +234,8 @@ func (m *Manager) reload() {
 	wg.Wait()
 }
 
-// setJitterSeed calculates a global jitterSeed per server relying on extra label set.
+// setOffsetSeed calculates a global offsetSeed per server relying on extra label set.
-func (m *Manager) setJitterSeed(labels labels.Labels) error {
+func (m *Manager) setOffsetSeed(labels labels.Labels) error {
 	h := fnv.New64a()
 	hostname, err := osutil.GetFQDN()
 	if err != nil {
@@ -244,7 +244,7 @@ func (m *Manager) setJitterSeed(labels labels.Labels) error {
 	if _, err := fmt.Fprintf(h, "%s%s", hostname, labels.String()); err != nil {
 		return err
 	}
-	m.jitterSeed = h.Sum64()
+	m.offsetSeed = h.Sum64()
 	return nil
 }
 
@@ -281,7 +281,7 @@ func (m *Manager) ApplyConfig(cfg *config.Config) error {
 	}
 	m.scrapeConfigs = c
 
-	if err := m.setJitterSeed(cfg.GlobalConfig.ExternalLabels); err != nil {
+	if err := m.setOffsetSeed(cfg.GlobalConfig.ExternalLabels); err != nil {
 		return err
 	}
 

scrape/manager_test.go

@@ -596,7 +596,7 @@ func TestManagerTargetsUpdates(t *testing.T) {
 	}
 }
 
-func TestSetJitter(t *testing.T) {
+func TestSetOffsetSeed(t *testing.T) {
 	getConfig := func(prometheus string) *config.Config {
 		cfgText := `
 global:
@@ -617,24 +617,24 @@ global:
 
 	// Load the first config.
 	cfg1 := getConfig("ha1")
-	if err := scrapeManager.setJitterSeed(cfg1.GlobalConfig.ExternalLabels); err != nil {
+	if err := scrapeManager.setOffsetSeed(cfg1.GlobalConfig.ExternalLabels); err != nil {
 		t.Error(err)
 	}
-	jitter1 := scrapeManager.jitterSeed
+	offsetSeed1 := scrapeManager.offsetSeed
 
-	if jitter1 == 0 {
+	if offsetSeed1 == 0 {
-		t.Error("Jitter has to be a hash of uint64")
+		t.Error("Offset seed has to be a hash of uint64")
 	}
 
 	// Load the first config.
 	cfg2 := getConfig("ha2")
-	if err := scrapeManager.setJitterSeed(cfg2.GlobalConfig.ExternalLabels); err != nil {
+	if err := scrapeManager.setOffsetSeed(cfg2.GlobalConfig.ExternalLabels); err != nil {
 		t.Error(err)
 	}
-	jitter2 := scrapeManager.jitterSeed
+	offsetSeed2 := scrapeManager.offsetSeed
 
-	if jitter1 == jitter2 {
+	if offsetSeed1 == offsetSeed2 {
-		t.Error("Jitter should not be the same on different set of external labels")
+		t.Error("Offset seed should not be the same on different set of external labels")
 	}
 }
 

scrape/scrape.go

@@ -279,7 +279,7 @@ const maxAheadTime = 10 * time.Minute
 // returning an empty label set is interpreted as "drop"
 type labelsMutator func(labels.Labels) labels.Labels
 
-func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed uint64, logger log.Logger, options *Options) (*scrapePool, error) {
+func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, offsetSeed uint64, logger log.Logger, options *Options) (*scrapePool, error) {
 	targetScrapePools.Inc()
 	if logger == nil {
 		logger = log.NewNopLogger()
@@ -325,7 +325,7 @@ func newScrapePool(cfg *config.ScrapeConfig, app storage.Appendable, jitterSeed
 			func(l labels.Labels) labels.Labels { return mutateReportSampleLabels(l, opts.target) },
 			func(ctx context.Context) storage.Appender { return app.Appender(ctx) },
 			cache,
-			jitterSeed,
+			offsetSeed,
 			opts.honorTimestamps,
 			opts.sampleLimit,
 			opts.bucketLimit,
@@ -775,7 +775,7 @@ func appender(app storage.Appender, sampleLimit, bucketLimit int) storage.Append
 type scraper interface {
 	scrape(ctx context.Context, w io.Writer) (string, error)
 	Report(start time.Time, dur time.Duration, err error)
-	offset(interval time.Duration, jitterSeed uint64) time.Duration
+	offset(interval time.Duration, offsetSeed uint64) time.Duration
 }
 
 // targetScraper implements the scraper interface for a target.
@@ -891,7 +891,7 @@ type scrapeLoop struct {
 	cache                   *scrapeCache
 	lastScrapeSize          int
 	buffers                 *pool.Pool
-	jitterSeed              uint64
+	offsetSeed              uint64
 	honorTimestamps         bool
 	forcedErr               error
 	forcedErrMtx            sync.Mutex
@@ -1175,7 +1175,7 @@ func newScrapeLoop(ctx context.Context,
 	reportSampleMutator labelsMutator,
 	appender func(ctx context.Context) storage.Appender,
 	cache *scrapeCache,
-	jitterSeed uint64,
+	offsetSeed uint64,
 	honorTimestamps bool,
 	sampleLimit int,
 	bucketLimit int,
@@ -1217,7 +1217,7 @@ func newScrapeLoop(ctx context.Context,
 		sampleMutator:           sampleMutator,
 		reportSampleMutator:     reportSampleMutator,
 		stopped:                 make(chan struct{}),
-		jitterSeed:              jitterSeed,
+		offsetSeed:              offsetSeed,
 		l:                       l,
 		parentCtx:               ctx,
 		appenderCtx:             appenderCtx,
@@ -1238,7 +1238,7 @@ func newScrapeLoop(ctx context.Context,
 
 func (sl *scrapeLoop) run(errc chan<- error) {
 	select {
-	case <-time.After(sl.scraper.offset(sl.interval, sl.jitterSeed)):
+	case <-time.After(sl.scraper.offset(sl.interval, sl.offsetSeed)):
 		// Continue after a scraping offset.
 	case <-sl.ctx.Done():
 		close(sl.stopped)

scrape/target_test.go

@@ -59,7 +59,7 @@ func TestTargetLabels(t *testing.T) {
 
 func TestTargetOffset(t *testing.T) {
 	interval := 10 * time.Second
-	jitter := uint64(0)
+	offsetSeed := uint64(0)
 
 	offsets := make([]time.Duration, 10000)
 
@@ -68,7 +68,7 @@ func TestTargetOffset(t *testing.T) {
 		target := newTestTarget("example.com:80", 0, labels.FromStrings(
 			"label", fmt.Sprintf("%d", i),
 		))
-		offsets[i] = target.offset(interval, jitter)
+		offsets[i] = target.offset(interval, offsetSeed)
 	}
 
 	// Put the offsets into buckets and validate that they are all