Added cached collector.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

update.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Attempt 2

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Added blocking registry, with raw collector and transactional handler.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Added fast path to normal (empty) registry to save 8 allocs and 3K5B per Gather.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Simplified API, added tests.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Fix.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Simplified implementation.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Added benchmark.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

Optimized.

Signed-off-by: Bartlomiej Plotka <bwplotka@gmail.com>

prometheus/cache/cache.go

@@ -0,0 +1,261 @@
+// Copyright 2022 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cache
+
+import (
+	"errors"
+	"fmt"
+	"sort"
+	"sync"
+	"time"
+
+	"github.com/cespare/xxhash/v2"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/common/model"
+
+	//nolint:staticcheck // Ignore SA1019. Need to keep deprecated package for compatibility.
+	"github.com/golang/protobuf/proto"
+	"github.com/prometheus/client_golang/prometheus/internal"
+	dto "github.com/prometheus/client_model/go"
+)
+
+var _ prometheus.TransactionalGatherer = &CachedTGatherer{}
+
+var separatorByteSlice = []byte{model.SeparatorByte} // For convenient use with xxhash.
+
+// CachedTGatherer is a transactional gatherer that allows maintaining set of metrics which
+// change less frequently than scrape time, yet label values and values change over time.
+//
+// If you happen to use NewDesc, NewConstMetric or MustNewConstMetric inside Collector.Collect routine, consider
+// using CachedTGatherer instead.
+//
+// Use CachedTGatherer with classic Registry using NewMultiTRegistry and ToTransactionalGatherer helpers.
+// NOTE(bwplotka): Experimental, API and behaviour can change.
+type CachedTGatherer struct {
+	metrics            map[uint64]*dto.Metric
+	metricFamilyByName map[string]*dto.MetricFamily
+	mMu                sync.RWMutex
+}
+
+func NewCachedTGatherer() *CachedTGatherer {
+	return &CachedTGatherer{
+		metrics:            make(map[uint64]*dto.Metric),
+		metricFamilyByName: map[string]*dto.MetricFamily{},
+	}
+}
+
+// Gather implements TransactionalGatherer interface.
+func (c *CachedTGatherer) Gather() (_ []*dto.MetricFamily, done func(), err error) {
+	c.mMu.RLock()
+
+	// BenchmarkCachedTGatherer_Update shows, even for 1 million metrics with 1000 families
+	// this is efficient enough (~300µs and ~50 kB per op), no need to cache it for now.
+	return internal.NormalizeMetricFamilies(c.metricFamilyByName), c.mMu.RUnlock, nil
+}
+
+type Key struct {
+	FQName string // __name__
+
+	// Label names can be unsorted, we will be sorting them later. The only implication is cachability if
+	// consumer provide non-deterministic order of those.
+	LabelNames  []string
+	LabelValues []string
+}
+
+func (k Key) isValid() error {
+	if k.FQName == "" {
+		return errors.New("FQName cannot be empty")
+	}
+	if len(k.LabelNames) != len(k.LabelValues) {
+		return errors.New("new metric: label name has different length than values")
+	}
+
+	return nil
+}
+
+// hash returns unique hash for this key.
+func (k Key) hash() uint64 {
+	h := xxhash.New()
+	h.WriteString(k.FQName)
+	h.Write(separatorByteSlice)
+
+	for i := range k.LabelNames {
+		h.WriteString(k.LabelNames[i])
+		h.Write(separatorByteSlice)
+		h.WriteString(k.LabelValues[i])
+		h.Write(separatorByteSlice)
+	}
+	return h.Sum64()
+}
+
+// Insert represents record to set in cache.
+type Insert struct {
+	Key
+
+	Help      string
+	ValueType prometheus.ValueType
+	Value     float64
+
+	// Timestamp is optional. Pass nil for no explicit timestamp.
+	Timestamp *time.Time
+}
+
+// Update goes through inserts and deletions and updates current cache in concurrency safe manner.
+// If reset is set to true, all inserts and deletions are working on empty cache. In such case
+// this implementation tries to reuse memory from existing cached item when possible.
+//
+// Update reuses insert struct memory, so after use, Insert slice and its elements cannot be reused
+// outside of this method.
+// TODO(bwplotka): Lack of copying can pose memory safety problems if insert variables are reused. Consider copying if value
+// is different. Yet it gives significant allocation gains.
+func (c *CachedTGatherer) Update(reset bool, inserts []Insert, deletions []Key) error {
+	c.mMu.Lock()
+	defer c.mMu.Unlock()
+
+	currMetrics := c.metrics
+	currMetricFamilies := c.metricFamilyByName
+	if reset {
+		currMetrics = make(map[uint64]*dto.Metric, len(c.metrics))
+		currMetricFamilies = make(map[string]*dto.MetricFamily, len(c.metricFamilyByName))
+	}
+
+	errs := prometheus.MultiError{}
+	for i := range inserts {
+		// TODO(bwplotka): Validate more about this insert?
+		if err := inserts[i].isValid(); err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		// Update metric family.
+		mf, ok := c.metricFamilyByName[inserts[i].FQName]
+		if !ok {
+			mf = &dto.MetricFamily{}
+			mf.Name = &inserts[i].FQName
+		} else if reset {
+			// Reset metric slice, since we want to start from scratch.
+			mf.Metric = mf.Metric[:0]
+		}
+		mf.Type = inserts[i].ValueType.ToDTO()
+		mf.Help = &inserts[i].Help
+
+		currMetricFamilies[inserts[i].FQName] = mf
+
+		// Update metric pointer.
+		hSum := inserts[i].hash()
+		m, ok := c.metrics[hSum]
+		if !ok {
+			m = &dto.Metric{Label: make([]*dto.LabelPair, 0, len(inserts[i].LabelNames))}
+			for j := range inserts[i].LabelNames {
+				m.Label = append(m.Label, &dto.LabelPair{
+					Name:  &inserts[i].LabelNames[j],
+					Value: &inserts[i].LabelValues[j],
+				})
+			}
+			sort.Sort(internal.LabelPairSorter(m.Label))
+		}
+
+		switch inserts[i].ValueType {
+		case prometheus.CounterValue:
+			v := m.Counter
+			if v == nil {
+				v = &dto.Counter{}
+			}
+			v.Value = &inserts[i].Value
+			m.Counter = v
+			m.Gauge = nil
+			m.Untyped = nil
+		case prometheus.GaugeValue:
+			v := m.Gauge
+			if v == nil {
+				v = &dto.Gauge{}
+			}
+			v.Value = &inserts[i].Value
+			m.Counter = nil
+			m.Gauge = v
+			m.Untyped = nil
+		case prometheus.UntypedValue:
+			v := m.Untyped
+			if v == nil {
+				v = &dto.Untyped{}
+			}
+			v.Value = &inserts[i].Value
+			m.Counter = nil
+			m.Gauge = nil
+			m.Untyped = v
+		default:
+			return fmt.Errorf("unsupported value type %v", inserts[i].ValueType)
+		}
+
+		m.TimestampMs = nil
+		if inserts[i].Timestamp != nil {
+			m.TimestampMs = proto.Int64(inserts[i].Timestamp.Unix()*1000 + int64(inserts[i].Timestamp.Nanosecond()/1000000))
+		}
+		currMetrics[hSum] = m
+
+		if !reset && ok {
+			// If we did update without reset and we found metric in previous
+			// map, we know metric pointer exists in metric family map, so just continue.
+			continue
+		}
+
+		// Will be sorted later anyway, so just append.
+		mf.Metric = append(mf.Metric, m)
+	}
+
+	for _, del := range deletions {
+		if err := del.isValid(); err != nil {
+			errs.Append(err)
+			continue
+		}
+
+		hSum := del.hash()
+		m, ok := currMetrics[hSum]
+		if !ok {
+			continue
+		}
+		delete(currMetrics, hSum)
+
+		mf, ok := currMetricFamilies[del.FQName]
+		if !ok {
+			// Impossible, but well...
+			errs.Append(fmt.Errorf("could not remove metric %s(%s) from metric family, metric family does not exists", del.FQName, del.LabelValues))
+			continue
+		}
+
+		toDel := -1
+		for i := range mf.Metric {
+			if mf.Metric[i] == m {
+				toDel = i
+				break
+			}
+		}
+
+		if toDel == -1 {
+			errs.Append(fmt.Errorf("could not remove metric %s(%s) from metric family, metric family does not have such metric", del.FQName, del.LabelValues))
+			continue
+		}
+
+		if len(mf.Metric) == 1 {
+			delete(currMetricFamilies, del.FQName)
+			continue
+		}
+
+		mf.Metric = append(mf.Metric[:toDel], mf.Metric[toDel+1:]...)
+	}
+
+	c.metrics = currMetrics
+	c.metricFamilyByName = currMetricFamilies
+	return errs.MaybeUnwrap()
+}