tsdb: Support gauge float histogram with recoding of chunk

Signed-off-by: Ganesh Vernekar <ganeshvern@gmail.com>

tsdb/chunkenc/float_histogram.go

@@ -174,6 +174,7 @@ func newFloatHistogramIterator(b []byte) *floatHistogramIterator {
 	// The first 3 bytes contain chunk headers.
 	// We skip that for actual samples.
 	_, _ = it.br.readBits(24)
+	it.counterResetHeader = CounterResetHeader(b[2] & 0b11000000)
 	return it
 }
 
@@ -196,6 +197,14 @@ type FloatHistogramAppender struct {
 	pBuckets, nBuckets []xorValue
 }
 
+func (a *FloatHistogramAppender) GetCounterResetHeader() CounterResetHeader {
+	return CounterResetHeader(a.b.bytes()[2] & 0b11000000)
+}
+
+func (a *FloatHistogramAppender) NumSamples() int {
+	return int(binary.BigEndian.Uint16(a.b.bytes()))
+}
+
 // Append implements Appender. This implementation panics because normal float
 // samples must never be appended to a histogram chunk.
 func (a *FloatHistogramAppender) Append(int64, float64) {
@@ -211,6 +220,7 @@ func (a *FloatHistogramAppender) AppendHistogram(int64, *histogram.Histogram) {
 // Appendable returns whether the chunk can be appended to, and if so
 // whether any recoding needs to happen using the provided interjections
 // (in case of any new buckets, positive or negative range, respectively).
+// If the sample is a gauge histogram, AppendableGauge must be used instead.
 //
 // The chunk is not appendable in the following cases:
 //
@@ -228,6 +238,9 @@ func (a *FloatHistogramAppender) Appendable(h *histogram.FloatHistogram) (
 	positiveInterjections, negativeInterjections []Interjection,
 	okToAppend, counterReset bool,
 ) {
+	if a.NumSamples() > 0 && a.GetCounterResetHeader() == GaugeType {
+		return
+	}
 	if value.IsStaleNaN(h.Sum) {
 		// This is a stale sample whose buckets and spans don't matter.
 		okToAppend = true
@@ -284,17 +297,21 @@ func (a *FloatHistogramAppender) Appendable(h *histogram.FloatHistogram) (
 //  2. Any recoding needs to happen for the histogram being appended, using the backward interjections
 //     (in case of any missing buckets, positive or negative range, respectively).
 //
-// The chunk is not appendable in the following cases:
+// This method must be only used for gauge histograms.
 //
-// • The schema has changed.
-// • The threshold for the zero bucket has changed.
-// • The last sample in the chunk was stale while the current sample is not stale.
+// The chunk is not appendable in the following cases:
+//  - The schema has changed.
+//  - The threshold for the zero bucket has changed.
+//  - The last sample in the chunk was stale while the current sample is not stale.
 func (a *FloatHistogramAppender) AppendableGauge(h *histogram.FloatHistogram) (
 	positiveInterjections, negativeInterjections []Interjection,
 	backwardPositiveInterjections, backwardNegativeInterjections []Interjection,
 	positiveSpans, negativeSpans []histogram.Span,
 	okToAppend bool,
 ) {
+	if a.NumSamples() > 0 && a.GetCounterResetHeader() != GaugeType {
+		return
+	}
 	if value.IsStaleNaN(h.Sum) {
 		// This is a stale sample whose buckets and spans don't matter.
 		okToAppend = true
@@ -537,11 +554,29 @@ func (a *FloatHistogramAppender) Recode(
 	return hc, app
 }
 
+// RecodeHistogramm converts the current histogram (in-place) to accommodate an expansion of the set of
+// (positive and/or negative) buckets used.
+func (a *FloatHistogramAppender) RecodeHistogramm(
+	fh *histogram.FloatHistogram,
+	pBackwardInter, nBackwardInter []Interjection,
+) {
+	if len(pBackwardInter) > 0 {
+		numPositiveBuckets := countSpans(fh.PositiveSpans)
+		fh.PositiveBuckets = interject(fh.PositiveBuckets, make([]float64, numPositiveBuckets), pBackwardInter, false)
+	}
+	if len(nBackwardInter) > 0 {
+		numNegativeBuckets := countSpans(fh.NegativeSpans)
+		fh.NegativeBuckets = interject(fh.NegativeBuckets, make([]float64, numNegativeBuckets), nBackwardInter, false)
+	}
+}
+
 type floatHistogramIterator struct {
 	br       bstreamReader
 	numTotal uint16
 	numRead  uint16
 
+	counterResetHeader CounterResetHeader
+
 	// Layout:
 	schema         int32
 	zThreshold     float64
@@ -594,7 +629,12 @@ func (it *floatHistogramIterator) AtFloatHistogram() (int64, *histogram.FloatHis
 		return it.t, &histogram.FloatHistogram{Sum: it.sum.value}
 	}
 	it.atFloatHistogramCalled = true
+	crHint := histogram.UnknownCounterReset
+	if it.counterResetHeader == GaugeType {
+		crHint = histogram.GaugeType
+	}
 	return it.t, &histogram.FloatHistogram{
+		CounterResetHint: crHint,
 		Count:            it.cnt.value,
 		ZeroCount:        it.zCnt.value,
 		Sum:              it.sum.value,
@@ -622,6 +662,8 @@ func (it *floatHistogramIterator) Reset(b []byte) {
 	it.numTotal = binary.BigEndian.Uint16(b)
 	it.numRead = 0
 
+	it.counterResetHeader = CounterResetHeader(b[2] & 0b11000000)
+
 	it.t, it.tDelta = 0, 0
 	it.cnt, it.zCnt, it.sum = xorValue{}, xorValue{}, xorValue{}
 

tsdb/head_append.go

@@ -1138,9 +1138,10 @@ func (s *memSeries) append(t int64, v float64, appendID uint64, chunkDiskMapper
 
 // appendHistogram adds the histogram.
 // It is unsafe to call this concurrently with s.iterator(...) without holding the series lock.
+// TODO(codesome): Support gauge histograms here.
 func (s *memSeries) appendHistogram(t int64, h *histogram.Histogram, appendID uint64, chunkDiskMapper *chunks.ChunkDiskMapper, chunkRange int64) (sampleInOrder, chunkCreated bool) {
 	// Head controls the execution of recoding, so that we own the proper
-	// chunk reference afterwards.  We check for Appendable before
+	// chunk reference afterwards. We check for Appendable from appender before
 	// appendPreprocessor because in case it ends up creating a new chunk,
 	// we need to know if there was also a counter reset or not to set the
 	// meta properly.
@@ -1209,24 +1210,37 @@ func (s *memSeries) appendHistogram(t int64, h *histogram.Histogram, appendID ui
 // It is unsafe to call this concurrently with s.iterator(...) without holding the series lock.
 func (s *memSeries) appendFloatHistogram(t int64, fh *histogram.FloatHistogram, appendID uint64, chunkDiskMapper *chunks.ChunkDiskMapper, chunkRange int64) (sampleInOrder, chunkCreated bool) {
 	// Head controls the execution of recoding, so that we own the proper
-	// chunk reference afterwards.  We check for Appendable before
+	// chunk reference afterwards.  We check for Appendable from appender before
 	// appendPreprocessor because in case it ends up creating a new chunk,
 	// we need to know if there was also a counter reset or not to set the
 	// meta properly.
 	app, _ := s.app.(*chunkenc.FloatHistogramAppender)
 	var (
 		positiveInterjections, negativeInterjections []chunkenc.Interjection
+		pBackwardInter, nBackwardInter               []chunkenc.Interjection
+		pMergedSpans, nMergedSpans                   []histogram.Span
 		okToAppend, counterReset                     bool
 	)
 	c, sampleInOrder, chunkCreated := s.appendPreprocessor(t, chunkenc.EncFloatHistogram, chunkDiskMapper, chunkRange)
 	if !sampleInOrder {
 		return sampleInOrder, chunkCreated
 	}
+	gauge := fh.CounterResetHint == histogram.GaugeType
 	if app != nil {
+		if gauge {
+			positiveInterjections, negativeInterjections, pBackwardInter, nBackwardInter,
+				pMergedSpans, nMergedSpans, okToAppend = app.AppendableGauge(fh)
+		} else {
 			positiveInterjections, negativeInterjections, okToAppend, counterReset = app.Appendable(fh)
 		}
+	}
 
 	if !chunkCreated {
+		if len(pBackwardInter)+len(nBackwardInter) > 0 {
+			fh.PositiveSpans = pMergedSpans
+			fh.NegativeSpans = nMergedSpans
+			app.RecodeHistogramm(fh, pBackwardInter, nBackwardInter)
+		}
 		// We have 3 cases here
 		// - !okToAppend -> We need to cut a new chunk.
 		// - okToAppend but we have interjections → Existing chunk needs
@@ -1251,7 +1265,9 @@ func (s *memSeries) appendFloatHistogram(t int64, fh *histogram.FloatHistogram,
 	if chunkCreated {
 		hc := s.headChunk.chunk.(*chunkenc.FloatHistogramChunk)
 		header := chunkenc.UnknownCounterReset
-		if counterReset {
+		if gauge {
+			header = chunkenc.GaugeType
+		} else if counterReset {
 			header = chunkenc.CounterReset
 		} else if okToAppend {
 			header = chunkenc.NotCounterReset