Move lastSamplePair method up to memorySeries

This implies a slight change of behavior as only samples added to the
respective instance of a memorySeries are returned. However, this is
most likely anyway what we want.

Following cases:

- Server has been restarted: Given the time it takes to cleanly
  shutdown and start up a server, the series are now stale anyway. An
  improved staleness handling (still to be implemented) will be based
  on tracking if a given target is continuing to expose samples for a
  given time series. In that case, we need a full scrape cycle to
  decide about staleness. So again, it makes sense to consider
  everything stale directly after a server restart.

- Series unarchived due to a read request: The series is definitely
  stale so we don't want to return anything anyway.

- Freshly created time series or series unarchived because of a sample
  append: That happens because appending a sample is imminent. Before
  the fingerprint lock is released, the series will have received a
  sample, and lastSamplePair will always returned the expected value.

storage/local/chunk.go

@@ -81,13 +81,6 @@ const (
 // is populated upon creation of a chunkDesc, so it is alway safe to call
 // firstTime. The firstTime method is arguably not needed and only there for
 // consistency with lastTime.
-//
-// Yet another (deprecated) case is lastSamplePair. It's used in federation and
-// must be callable without pinning. Locking the fingerprint of the series is
-// still required (to avoid concurrent appends to the chunk). The call is
-// relatively expensive because of the required acquisition of the evict
-// mutex. It will go away, though, once tracking the lastSamplePair has been
-// moved into the series object.
 type chunkDesc struct {
 	sync.Mutex           // Protects pinning.
 	c              chunk // nil if chunk is evicted.
@@ -194,24 +187,6 @@ func (cd *chunkDesc) maybePopulateLastTime() {
 	}
 }
 
-// lastSamplePair returns the last sample pair of the underlying chunk, or
-// ZeroSampleValue if the chunk is evicted. For safe concurrent access, this
-// method requires the fingerprint of the time series to be locked.
-// TODO(beorn7): Move up into memorySeries.
-func (cd *chunkDesc) lastSamplePair() model.SamplePair {
-	cd.Lock()
-	defer cd.Unlock()
-
-	if cd.c == nil {
-		return ZeroSamplePair
-	}
-	it := cd.c.newIterator()
-	return model.SamplePair{
-		Timestamp: it.lastTimestamp(),
-		Value:     it.lastSampleValue(),
-	}
-}
-
 // isEvicted returns whether the chunk is evicted. For safe concurrent access,
 // the caller must have locked the fingerprint of the series.
 func (cd *chunkDesc) isEvicted() bool {

storage/local/series.go

@@ -162,9 +162,15 @@ type memorySeries struct {
 	// first chunk before its chunk desc is evicted. In doubt, this field is
 	// just set to the oldest possible timestamp.
 	savedFirstTime model.Time
-	// The timestamp of the last sample in this series. Needed for fast access to
+	// The timestamp of the last sample in this series. Needed for fast
-	// ensure timestamp monotonicity during ingestion.
+	// access for federation and to ensure timestamp monotonicity during
+	// ingestion.
 	lastTime model.Time
+	// The last ingested sample value. Needed for fast access for
+	// federation.
+	lastSampleValue model.SampleValue
+	// Whether lastSampleValue has been set already.
+	lastSampleValueSet bool
 	// Whether the current head chunk has already been finished.  If true,
 	// the current head chunk must not be modified anymore.
 	headChunkClosed bool
@@ -242,6 +248,8 @@ func (s *memorySeries) add(v *model.SamplePair) int {
 	}
 
 	s.lastTime = v.Timestamp
+	s.lastSampleValue = v.Value
+	s.lastSampleValueSet = true
 	return len(chunks) - 1
 }
 
@@ -435,8 +443,9 @@ func (s *memorySeries) head() *chunkDesc {
 	return s.chunkDescs[len(s.chunkDescs)-1]
 }
 
-// firstTime returns the timestamp of the first sample in the series. The caller
+// firstTime returns the timestamp of the first sample in the series.
-// must have locked the fingerprint of the memorySeries.
+//
+// The caller must have locked the fingerprint of the memorySeries.
 func (s *memorySeries) firstTime() model.Time {
 	if s.chunkDescsOffset == 0 && len(s.chunkDescs) > 0 {
 		return s.chunkDescs[0].firstTime()
@@ -444,6 +453,23 @@ func (s *memorySeries) firstTime() model.Time {
 	return s.savedFirstTime
 }
 
+// lastSamplePair returns the last ingested SamplePair. It returns
+// ZeroSamplePair if this memorySeries has never received a sample (via the add
+// method), which is the case for freshly unarchived series or newly created
+// ones and also for all series after a server restart. However, in that case,
+// series will most likely be considered stale anyway.
+//
+// The caller must have locked the fingerprint of the memorySeries.
+func (s *memorySeries) lastSamplePair() model.SamplePair {
+	if !s.lastSampleValueSet {
+		return ZeroSamplePair
+	}
+	return model.SamplePair{
+		Timestamp: s.lastTime,
+		Value:     s.lastSampleValue,
+	}
+}
+
 // chunksToPersist returns a slice of chunkDescs eligible for persistence. It's
 // the caller's responsibility to actually persist the returned chunks
 // afterwards. The method sets the persistWatermark and the dirty flag

storage/local/storage.go

@@ -354,7 +354,7 @@ func (s *memorySeriesStorage) LastSamplePairForFingerprint(fp model.Fingerprint)
 	if !ok {
 		return ZeroSamplePair
 	}
-	return series.head().lastSamplePair()
+	return series.lastSamplePair()
 }
 
 // boundedIterator wraps a SeriesIterator and does not allow fetching
@@ -576,16 +576,18 @@ func (s *memorySeriesStorage) Append(sample *model.Sample) error {
 
 	if sample.Timestamp <= series.lastTime {
 		s.fpLocker.Unlock(fp)
-		// Don't log and track equal timestamps, as they are a common occurrence
+		// Don't report "no-op appends", i.e. where timestamp and sample
-		// when using client-side timestamps (e.g. Pushgateway or federation).
+		// value are the same as for the last append, as they are a
-		// It would be even better to also compare the sample values here, but
+		// common occurrence when using client-side timestamps
-		// we don't have efficient access to a series's last value.
+		// (e.g. Pushgateway or federation).
-		if sample.Timestamp != series.lastTime {
+		if sample.Timestamp == series.lastTime &&
+			series.lastSampleValueSet &&
+			sample.Value == series.lastSampleValue {
+			return nil
+		}
 		s.outOfOrderSamplesCount.Inc()
 		return ErrOutOfOrderSample
 	}
-		return nil
-	}
 	completedChunksCount := series.add(&model.SamplePair{
 		Value:     sample.Value,
 		Timestamp: sample.Timestamp,