Major refactor of metric data handling

- make the  internal memory store required and default
- Rename memorystore to metricstore
- Rename metricDataDispatcher to metricdispatch
- Remove metricdata package
- Introduce metricsync package for upstream metric data pull

internal/metricstore/api.go

@@ -0,0 +1,231 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"errors"
+	"fmt"
+	"math"
+
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+	"github.com/ClusterCockpit/cc-lib/v2/util"
+)
+
+var (
+	ErrInvalidTimeRange = errors.New("[METRICSTORE]> invalid time range: 'from' must be before 'to'")
+	ErrEmptyCluster     = errors.New("[METRICSTORE]> cluster name cannot be empty")
+)
+
+type APIMetricData struct {
+	Error      *string           `json:"error,omitempty"`
+	Data       schema.FloatArray `json:"data,omitempty"`
+	From       int64             `json:"from"`
+	To         int64             `json:"to"`
+	Resolution int64             `json:"resolution"`
+	Avg        schema.Float      `json:"avg"`
+	Min        schema.Float      `json:"min"`
+	Max        schema.Float      `json:"max"`
+}
+
+type APIQueryRequest struct {
+	Cluster     string     `json:"cluster"`
+	Queries     []APIQuery `json:"queries"`
+	ForAllNodes []string   `json:"for-all-nodes"`
+	From        int64      `json:"from"`
+	To          int64      `json:"to"`
+	WithStats   bool       `json:"with-stats"`
+	WithData    bool       `json:"with-data"`
+	WithPadding bool       `json:"with-padding"`
+}
+
+type APIQueryResponse struct {
+	Queries []APIQuery        `json:"queries,omitempty"`
+	Results [][]APIMetricData `json:"results"`
+}
+
+type APIQuery struct {
+	Type        *string      `json:"type,omitempty"`
+	SubType     *string      `json:"subtype,omitempty"`
+	Metric      string       `json:"metric"`
+	Hostname    string       `json:"host"`
+	Resolution  int64        `json:"resolution"`
+	TypeIds     []string     `json:"type-ids,omitempty"`
+	SubTypeIds  []string     `json:"subtype-ids,omitempty"`
+	ScaleFactor schema.Float `json:"scale-by,omitempty"`
+	Aggregate   bool         `json:"aggreg"`
+}
+
+// TODO: Optimize this, just like the stats endpoint!
+func (data *APIMetricData) AddStats() {
+	n := 0
+	sum, min, max := 0.0, math.MaxFloat64, -math.MaxFloat64
+	for _, x := range data.Data {
+		if x.IsNaN() {
+			continue
+		}
+
+		n += 1
+		sum += float64(x)
+		min = math.Min(min, float64(x))
+		max = math.Max(max, float64(x))
+	}
+
+	if n > 0 {
+		avg := sum / float64(n)
+		data.Avg = schema.Float(avg)
+		data.Min = schema.Float(min)
+		data.Max = schema.Float(max)
+	} else {
+		data.Avg, data.Min, data.Max = schema.NaN, schema.NaN, schema.NaN
+	}
+}
+
+func (data *APIMetricData) ScaleBy(f schema.Float) {
+	if f == 0 || f == 1 {
+		return
+	}
+
+	data.Avg *= f
+	data.Min *= f
+	data.Max *= f
+	for i := 0; i < len(data.Data); i++ {
+		data.Data[i] *= f
+	}
+}
+
+func (data *APIMetricData) PadDataWithNull(ms *MemoryStore, from, to int64, metric string) {
+	minfo, ok := ms.Metrics[metric]
+	if !ok {
+		return
+	}
+
+	if (data.From / minfo.Frequency) > (from / minfo.Frequency) {
+		padfront := int((data.From / minfo.Frequency) - (from / minfo.Frequency))
+		ndata := make([]schema.Float, 0, padfront+len(data.Data))
+		for range padfront {
+			ndata = append(ndata, schema.NaN)
+		}
+		for j := 0; j < len(data.Data); j++ {
+			ndata = append(ndata, data.Data[j])
+		}
+		data.Data = ndata
+	}
+}
+
+func FetchData(req APIQueryRequest) (*APIQueryResponse, error) {
+	if req.From > req.To {
+		return nil, ErrInvalidTimeRange
+	}
+	if req.Cluster == "" && req.ForAllNodes != nil {
+		return nil, ErrEmptyCluster
+	}
+
+	req.WithData = true
+	ms := GetMemoryStore()
+	if ms == nil {
+		return nil, fmt.Errorf("memorystore not initialized")
+	}
+
+	response := APIQueryResponse{
+		Results: make([][]APIMetricData, 0, len(req.Queries)),
+	}
+	if req.ForAllNodes != nil {
+		nodes := ms.ListChildren([]string{req.Cluster})
+		for _, node := range nodes {
+			for _, metric := range req.ForAllNodes {
+				q := APIQuery{
+					Metric:   metric,
+					Hostname: node,
+				}
+				req.Queries = append(req.Queries, q)
+				response.Queries = append(response.Queries, q)
+			}
+		}
+	}
+
+	for _, query := range req.Queries {
+		sels := make([]util.Selector, 0, 1)
+		if query.Aggregate || query.Type == nil {
+			sel := util.Selector{{String: req.Cluster}, {String: query.Hostname}}
+			if query.Type != nil {
+				if len(query.TypeIds) == 1 {
+					sel = append(sel, util.SelectorElement{String: *query.Type + query.TypeIds[0]})
+				} else {
+					ids := make([]string, len(query.TypeIds))
+					for i, id := range query.TypeIds {
+						ids[i] = *query.Type + id
+					}
+					sel = append(sel, util.SelectorElement{Group: ids})
+				}
+
+				if query.SubType != nil {
+					if len(query.SubTypeIds) == 1 {
+						sel = append(sel, util.SelectorElement{String: *query.SubType + query.SubTypeIds[0]})
+					} else {
+						ids := make([]string, len(query.SubTypeIds))
+						for i, id := range query.SubTypeIds {
+							ids[i] = *query.SubType + id
+						}
+						sel = append(sel, util.SelectorElement{Group: ids})
+					}
+				}
+			}
+			sels = append(sels, sel)
+		} else {
+			for _, typeID := range query.TypeIds {
+				if query.SubType != nil {
+					for _, subTypeID := range query.SubTypeIds {
+						sels = append(sels, util.Selector{
+							{String: req.Cluster},
+							{String: query.Hostname},
+							{String: *query.Type + typeID},
+							{String: *query.SubType + subTypeID},
+						})
+					}
+				} else {
+					sels = append(sels, util.Selector{
+						{String: req.Cluster},
+						{String: query.Hostname},
+						{String: *query.Type + typeID},
+					})
+				}
+			}
+		}
+
+		// log.Printf("query: %#v\n", query)
+		// log.Printf("sels: %#v\n", sels)
+		var err error
+		res := make([]APIMetricData, 0, len(sels))
+		for _, sel := range sels {
+			data := APIMetricData{}
+
+			data.Data, data.From, data.To, data.Resolution, err = ms.Read(sel, query.Metric, req.From, req.To, query.Resolution)
+			if err != nil {
+				msg := err.Error()
+				data.Error = &msg
+				res = append(res, data)
+				continue
+			}
+
+			if req.WithStats {
+				data.AddStats()
+			}
+			if query.ScaleFactor != 0 {
+				data.ScaleBy(query.ScaleFactor)
+			}
+			if req.WithPadding {
+				data.PadDataWithNull(ms, req.From, req.To, query.Metric)
+			}
+			if !req.WithData {
+				data.Data = nil
+			}
+			res = append(res, data)
+		}
+		response.Results = append(response.Results, res)
+	}
+
+	return &response, nil
+}

internal/metricstore/archive.go

@@ -0,0 +1,196 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"archive/zip"
+	"bufio"
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"path/filepath"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+)
+
+func Archiving(wg *sync.WaitGroup, ctx context.Context) {
+	go func() {
+		defer wg.Done()
+		d, err := time.ParseDuration(Keys.Archive.Interval)
+		if err != nil {
+			cclog.Fatalf("[METRICSTORE]> error parsing archive interval duration: %v\n", err)
+		}
+		if d <= 0 {
+			return
+		}
+
+		ticker := time.NewTicker(d)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				t := time.Now().Add(-d)
+				cclog.Infof("[METRICSTORE]> start archiving checkpoints (older than %s)...", t.Format(time.RFC3339))
+				n, err := ArchiveCheckpoints(Keys.Checkpoints.RootDir,
+					Keys.Archive.RootDir, t.Unix(), Keys.Archive.DeleteInstead)
+
+				if err != nil {
+					cclog.Errorf("[METRICSTORE]> archiving failed: %s", err.Error())
+				} else {
+					cclog.Infof("[METRICSTORE]> done: %d files zipped and moved to archive", n)
+				}
+			}
+		}
+	}()
+}
+
+var ErrNoNewArchiveData error = errors.New("all data already archived")
+
+// ZIP all checkpoint files older than `from` together and write them to the `archiveDir`,
+// deleting them from the `checkpointsDir`.
+func ArchiveCheckpoints(checkpointsDir, archiveDir string, from int64, deleteInstead bool) (int, error) {
+	entries1, err := os.ReadDir(checkpointsDir)
+	if err != nil {
+		return 0, err
+	}
+
+	type workItem struct {
+		cdir, adir    string
+		cluster, host string
+	}
+
+	var wg sync.WaitGroup
+	n, errs := int32(0), int32(0)
+	work := make(chan workItem, Keys.NumWorkers)
+
+	wg.Add(Keys.NumWorkers)
+	for worker := 0; worker < Keys.NumWorkers; worker++ {
+		go func() {
+			defer wg.Done()
+			for workItem := range work {
+				m, err := archiveCheckpoints(workItem.cdir, workItem.adir, from, deleteInstead)
+				if err != nil {
+					cclog.Errorf("error while archiving %s/%s: %s", workItem.cluster, workItem.host, err.Error())
+					atomic.AddInt32(&errs, 1)
+				}
+				atomic.AddInt32(&n, int32(m))
+			}
+		}()
+	}
+
+	for _, de1 := range entries1 {
+		entries2, e := os.ReadDir(filepath.Join(checkpointsDir, de1.Name()))
+		if e != nil {
+			err = e
+		}
+
+		for _, de2 := range entries2 {
+			cdir := filepath.Join(checkpointsDir, de1.Name(), de2.Name())
+			adir := filepath.Join(archiveDir, de1.Name(), de2.Name())
+			work <- workItem{
+				adir: adir, cdir: cdir,
+				cluster: de1.Name(), host: de2.Name(),
+			}
+		}
+	}
+
+	close(work)
+	wg.Wait()
+
+	if err != nil {
+		return int(n), err
+	}
+
+	if errs > 0 {
+		return int(n), fmt.Errorf("%d errors happened while archiving (%d successes)", errs, n)
+	}
+	return int(n), nil
+}
+
+// Helper function for `ArchiveCheckpoints`.
+func archiveCheckpoints(dir string, archiveDir string, from int64, deleteInstead bool) (int, error) {
+	entries, err := os.ReadDir(dir)
+	if err != nil {
+		return 0, err
+	}
+
+	extension := Keys.Checkpoints.FileFormat
+	files, err := findFiles(entries, from, extension, false)
+	if err != nil {
+		return 0, err
+	}
+
+	if deleteInstead {
+		n := 0
+		for _, checkpoint := range files {
+			filename := filepath.Join(dir, checkpoint)
+			if err = os.Remove(filename); err != nil {
+				return n, err
+			}
+			n += 1
+		}
+		return n, nil
+	}
+
+	filename := filepath.Join(archiveDir, fmt.Sprintf("%d.zip", from))
+	f, err := os.OpenFile(filename, os.O_CREATE|os.O_WRONLY, CheckpointFilePerms)
+	if err != nil && os.IsNotExist(err) {
+		err = os.MkdirAll(archiveDir, CheckpointDirPerms)
+		if err == nil {
+			f, err = os.OpenFile(filename, os.O_CREATE|os.O_WRONLY, CheckpointFilePerms)
+		}
+	}
+	if err != nil {
+		return 0, err
+	}
+	defer f.Close()
+	bw := bufio.NewWriter(f)
+	defer bw.Flush()
+	zw := zip.NewWriter(bw)
+	defer zw.Close()
+
+	n := 0
+	for _, checkpoint := range files {
+		// Use closure to ensure file is closed immediately after use,
+		// avoiding file descriptor leak from defer in loop
+		err := func() error {
+			filename := filepath.Join(dir, checkpoint)
+			r, err := os.Open(filename)
+			if err != nil {
+				return err
+			}
+			defer r.Close()
+
+			w, err := zw.Create(checkpoint)
+			if err != nil {
+				return err
+			}
+
+			if _, err = io.Copy(w, r); err != nil {
+				return err
+			}
+
+			if err = os.Remove(filename); err != nil {
+				return err
+			}
+			return nil
+		}()
+		if err != nil {
+			return n, err
+		}
+		n += 1
+	}
+
+	return n, nil
+}

internal/metricstore/avroCheckpoint.go

@@ -0,0 +1,477 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"bufio"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+	"github.com/linkedin/goavro/v2"
+)
+
+var NumAvroWorkers int = DefaultAvroWorkers
+var startUp bool = true
+
+func (as *AvroStore) ToCheckpoint(dir string, dumpAll bool) (int, error) {
+	levels := make([]*AvroLevel, 0)
+	selectors := make([][]string, 0)
+	as.root.lock.RLock()
+	// Cluster
+	for sel1, l1 := range as.root.children {
+		l1.lock.RLock()
+		// Node
+		for sel2, l2 := range l1.children {
+			l2.lock.RLock()
+			// Frequency
+			for sel3, l3 := range l2.children {
+				levels = append(levels, l3)
+				selectors = append(selectors, []string{sel1, sel2, sel3})
+			}
+			l2.lock.RUnlock()
+		}
+		l1.lock.RUnlock()
+	}
+	as.root.lock.RUnlock()
+
+	type workItem struct {
+		level    *AvroLevel
+		dir      string
+		selector []string
+	}
+
+	n, errs := int32(0), int32(0)
+
+	var wg sync.WaitGroup
+	wg.Add(NumAvroWorkers)
+	work := make(chan workItem, NumAvroWorkers*2)
+	for range NumAvroWorkers {
+		go func() {
+			defer wg.Done()
+
+			for workItem := range work {
+				from := getTimestamp(workItem.dir)
+
+				if err := workItem.level.toCheckpoint(workItem.dir, from, dumpAll); err != nil {
+					if err == ErrNoNewArchiveData {
+						continue
+					}
+
+					cclog.Errorf("error while checkpointing %#v: %s", workItem.selector, err.Error())
+					atomic.AddInt32(&errs, 1)
+				} else {
+					atomic.AddInt32(&n, 1)
+				}
+			}
+		}()
+	}
+
+	for i := range len(levels) {
+		dir := path.Join(dir, path.Join(selectors[i]...))
+		work <- workItem{
+			level:    levels[i],
+			dir:      dir,
+			selector: selectors[i],
+		}
+	}
+
+	close(work)
+	wg.Wait()
+
+	if errs > 0 {
+		return int(n), fmt.Errorf("%d errors happend while creating avro checkpoints (%d successes)", errs, n)
+	}
+
+	startUp = false
+
+	return int(n), nil
+}
+
+// getTimestamp returns the timestamp from the directory name
+func getTimestamp(dir string) int64 {
+	// Extract the resolution and timestamp from the directory name
+	// The existing avro file will be in epoch timestamp format
+	// iterate over all the files in the directory and find the maximum timestamp
+	// and return it
+
+	resolution := path.Base(dir)
+	dir = path.Dir(dir)
+
+	files, err := os.ReadDir(dir)
+	if err != nil {
+		return 0
+	}
+	var maxTS int64 = 0
+
+	if len(files) == 0 {
+		return 0
+	}
+
+	for _, file := range files {
+		if file.IsDir() {
+			continue
+		}
+		name := file.Name()
+
+		if len(name) < 5 || !strings.HasSuffix(name, ".avro") || !strings.HasPrefix(name, resolution+"_") {
+			continue
+		}
+
+		ts, err := strconv.ParseInt(name[strings.Index(name, "_")+1:len(name)-5], 10, 64)
+		if err != nil {
+			fmt.Printf("error while parsing timestamp: %s\n", err.Error())
+			continue
+		}
+
+		if ts > maxTS {
+			maxTS = ts
+		}
+	}
+
+	interval, _ := time.ParseDuration(Keys.Checkpoints.Interval)
+	updateTime := time.Unix(maxTS, 0).Add(interval).Add(time.Duration(CheckpointBufferMinutes-1) * time.Minute).Unix()
+
+	if startUp {
+		return 0
+	}
+
+	if updateTime < time.Now().Unix() {
+		return 0
+	}
+
+	return maxTS
+}
+
+func (l *AvroLevel) toCheckpoint(dir string, from int64, dumpAll bool) error {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	// fmt.Printf("Checkpointing directory: %s\n", dir)
+	// filepath contains the resolution
+	intRes, _ := strconv.Atoi(path.Base(dir))
+
+	// find smallest overall timestamp in l.data map and delete it from l.data
+	minTS := int64(1<<63 - 1)
+	for ts, dat := range l.data {
+		if ts < minTS && len(dat) != 0 {
+			minTS = ts
+		}
+	}
+
+	if from == 0 && minTS != int64(1<<63-1) {
+		from = minTS
+	}
+
+	if from == 0 {
+		return ErrNoNewArchiveData
+	}
+
+	var schema string
+	var codec *goavro.Codec
+	recordList := make([]map[string]any, 0)
+
+	var f *os.File
+
+	filePath := dir + fmt.Sprintf("_%d.avro", from)
+
+	var err error
+
+	fp_, err_ := os.Stat(filePath)
+	if errors.Is(err_, os.ErrNotExist) {
+		err = os.MkdirAll(path.Dir(dir), 0o755)
+		if err != nil {
+			return fmt.Errorf("failed to create directory: %v", err)
+		}
+	} else if fp_.Size() != 0 {
+		f, err = os.Open(filePath)
+		if err != nil {
+			return fmt.Errorf("failed to open existing avro file: %v", err)
+		}
+
+		br := bufio.NewReader(f)
+
+		reader, err := goavro.NewOCFReader(br)
+		if err != nil {
+			return fmt.Errorf("failed to create OCF reader: %v", err)
+		}
+		codec = reader.Codec()
+		schema = codec.Schema()
+
+		f.Close()
+	}
+
+	timeRef := time.Now().Add(time.Duration(-CheckpointBufferMinutes+1) * time.Minute).Unix()
+
+	if dumpAll {
+		timeRef = time.Now().Unix()
+	}
+
+	// Empty values
+	if len(l.data) == 0 {
+		// we checkpoint avro files every 60 seconds
+		repeat := 60 / intRes
+
+		for range repeat {
+			recordList = append(recordList, make(map[string]any))
+		}
+	}
+
+	readFlag := true
+
+	for ts := range l.data {
+		flag := false
+		if ts < timeRef {
+			data := l.data[ts]
+
+			schemaGen, err := generateSchema(data)
+			if err != nil {
+				return err
+			}
+
+			flag, schema, err = compareSchema(schema, schemaGen)
+			if err != nil {
+				return fmt.Errorf("failed to compare read and generated schema: %v", err)
+			}
+			if flag && readFlag && !errors.Is(err_, os.ErrNotExist) {
+
+				f.Close()
+
+				f, err = os.Open(filePath)
+				if err != nil {
+					return fmt.Errorf("failed to open Avro file: %v", err)
+				}
+
+				br := bufio.NewReader(f)
+
+				ocfReader, err := goavro.NewOCFReader(br)
+				if err != nil {
+					return fmt.Errorf("failed to create OCF reader while changing schema: %v", err)
+				}
+
+				for ocfReader.Scan() {
+					record, err := ocfReader.Read()
+					if err != nil {
+						return fmt.Errorf("failed to read record: %v", err)
+					}
+
+					recordList = append(recordList, record.(map[string]any))
+				}
+
+				f.Close()
+
+				err = os.Remove(filePath)
+				if err != nil {
+					return fmt.Errorf("failed to delete file: %v", err)
+				}
+
+				readFlag = false
+			}
+			codec, err = goavro.NewCodec(schema)
+			if err != nil {
+				return fmt.Errorf("failed to create codec after merged schema: %v", err)
+			}
+
+			recordList = append(recordList, generateRecord(data))
+			delete(l.data, ts)
+		}
+	}
+
+	if len(recordList) == 0 {
+		return ErrNoNewArchiveData
+	}
+
+	f, err = os.OpenFile(filePath, os.O_CREATE|os.O_APPEND|os.O_RDWR, 0o644)
+	if err != nil {
+		return fmt.Errorf("failed to append new avro file: %v", err)
+	}
+
+	// fmt.Printf("Codec : %#v\n", codec)
+
+	writer, err := goavro.NewOCFWriter(goavro.OCFConfig{
+		W:               f,
+		Codec:           codec,
+		CompressionName: goavro.CompressionDeflateLabel,
+	})
+	if err != nil {
+		return fmt.Errorf("failed to create OCF writer: %v", err)
+	}
+
+	// Append the new record
+	if err := writer.Append(recordList); err != nil {
+		return fmt.Errorf("failed to append record: %v", err)
+	}
+
+	f.Close()
+
+	return nil
+}
+
+func compareSchema(schemaRead, schemaGen string) (bool, string, error) {
+	var genSchema, readSchema AvroSchema
+
+	if schemaRead == "" {
+		return false, schemaGen, nil
+	}
+
+	// Unmarshal the schema strings into AvroSchema structs
+	if err := json.Unmarshal([]byte(schemaGen), &genSchema); err != nil {
+		return false, "", fmt.Errorf("failed to parse generated schema: %v", err)
+	}
+	if err := json.Unmarshal([]byte(schemaRead), &readSchema); err != nil {
+		return false, "", fmt.Errorf("failed to parse read schema: %v", err)
+	}
+
+	sort.Slice(genSchema.Fields, func(i, j int) bool {
+		return genSchema.Fields[i].Name < genSchema.Fields[j].Name
+	})
+
+	sort.Slice(readSchema.Fields, func(i, j int) bool {
+		return readSchema.Fields[i].Name < readSchema.Fields[j].Name
+	})
+
+	// Check if schemas are identical
+	schemasEqual := true
+	if len(genSchema.Fields) <= len(readSchema.Fields) {
+
+		for i := range genSchema.Fields {
+			if genSchema.Fields[i].Name != readSchema.Fields[i].Name {
+				schemasEqual = false
+				break
+			}
+		}
+
+		// If schemas are identical, return the read schema
+		if schemasEqual {
+			return false, schemaRead, nil
+		}
+	}
+
+	// Create a map to hold unique fields from both schemas
+	fieldMap := make(map[string]AvroField)
+
+	// Add fields from the read schema
+	for _, field := range readSchema.Fields {
+		fieldMap[field.Name] = field
+	}
+
+	// Add or update fields from the generated schema
+	for _, field := range genSchema.Fields {
+		fieldMap[field.Name] = field
+	}
+
+	// Create a union schema by collecting fields from the map
+	var mergedFields []AvroField
+	for _, field := range fieldMap {
+		mergedFields = append(mergedFields, field)
+	}
+
+	// Sort fields by name for consistency
+	sort.Slice(mergedFields, func(i, j int) bool {
+		return mergedFields[i].Name < mergedFields[j].Name
+	})
+
+	// Create the merged schema
+	mergedSchema := AvroSchema{
+		Type:   "record",
+		Name:   genSchema.Name,
+		Fields: mergedFields,
+	}
+
+	// Check if schemas are identical
+	schemasEqual = len(mergedSchema.Fields) == len(readSchema.Fields)
+	if schemasEqual {
+		for i := range mergedSchema.Fields {
+			if mergedSchema.Fields[i].Name != readSchema.Fields[i].Name {
+				schemasEqual = false
+				break
+			}
+		}
+
+		if schemasEqual {
+			return false, schemaRead, nil
+		}
+	}
+
+	// Marshal the merged schema back to JSON
+	mergedSchemaJSON, err := json.Marshal(mergedSchema)
+	if err != nil {
+		return false, "", fmt.Errorf("failed to marshal merged schema: %v", err)
+	}
+
+	return true, string(mergedSchemaJSON), nil
+}
+
+func generateSchema(data map[string]schema.Float) (string, error) {
+	// Define the Avro schema structure
+	schema := map[string]any{
+		"type":   "record",
+		"name":   "DataRecord",
+		"fields": []map[string]any{},
+	}
+
+	fieldTracker := make(map[string]struct{})
+
+	for key := range data {
+		if _, exists := fieldTracker[key]; !exists {
+			key = correctKey(key)
+
+			field := map[string]any{
+				"name":    key,
+				"type":    "double",
+				"default": -1.0,
+			}
+			schema["fields"] = append(schema["fields"].([]map[string]any), field)
+			fieldTracker[key] = struct{}{}
+		}
+	}
+
+	schemaString, err := json.Marshal(schema)
+	if err != nil {
+		return "", fmt.Errorf("failed to marshal schema: %v", err)
+	}
+
+	return string(schemaString), nil
+}
+
+func generateRecord(data map[string]schema.Float) map[string]any {
+	record := make(map[string]any)
+
+	// Iterate through each map in data
+	for key, value := range data {
+		key = correctKey(key)
+
+		// Set the value in the record
+		// avro only accepts basic types
+		record[key] = value.Double()
+	}
+
+	return record
+}
+
+func correctKey(key string) string {
+	key = strings.ReplaceAll(key, "_", "_0x5F_")
+	key = strings.ReplaceAll(key, ":", "_0x3A_")
+	key = strings.ReplaceAll(key, ".", "_0x2E_")
+	return key
+}
+
+func ReplaceKey(key string) string {
+	key = strings.ReplaceAll(key, "_0x2E_", ".")
+	key = strings.ReplaceAll(key, "_0x3A_", ":")
+	key = strings.ReplaceAll(key, "_0x5F_", "_")
+	return key
+}

internal/metricstore/avroHelper.go

@@ -0,0 +1,84 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"context"
+	"slices"
+	"strconv"
+	"sync"
+
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+)
+
+func DataStaging(wg *sync.WaitGroup, ctx context.Context) {
+	// AvroPool is a pool of Avro writers.
+	go func() {
+		if Keys.Checkpoints.FileFormat == "json" {
+			wg.Done() // Mark this goroutine as done
+			return    // Exit the goroutine
+		}
+
+		defer wg.Done()
+
+		var avroLevel *AvroLevel
+		oldSelector := make([]string, 0)
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case val := <-LineProtocolMessages:
+				// Fetch the frequency of the metric from the global configuration
+				freq, err := GetMetricFrequency(val.MetricName)
+				if err != nil {
+					cclog.Errorf("Error fetching metric frequency: %s\n", err)
+					continue
+				}
+
+				metricName := ""
+
+				for _, selectorName := range val.Selector {
+					metricName += selectorName + SelectorDelimiter
+				}
+
+				metricName += val.MetricName
+
+				// Create a new selector for the Avro level
+				// The selector is a slice of strings that represents the path to the
+				// Avro level. It is created by appending the cluster, node, and metric
+				// name to the selector.
+				var selector []string
+				selector = append(selector, val.Cluster, val.Node, strconv.FormatInt(freq, 10))
+
+				if !stringSlicesEqual(oldSelector, selector) {
+					// Get the Avro level for the metric
+					avroLevel = avroStore.root.findAvroLevelOrCreate(selector)
+
+					// If the Avro level is nil, create a new one
+					if avroLevel == nil {
+						cclog.Errorf("Error creating or finding the level with cluster : %s, node : %s, metric : %s\n", val.Cluster, val.Node, val.MetricName)
+					}
+					oldSelector = slices.Clone(selector)
+				}
+
+				avroLevel.addMetric(metricName, val.Value, val.Timestamp, int(freq))
+			}
+		}
+	}()
+}
+
+func stringSlicesEqual(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}

internal/metricstore/avroStruct.go

@@ -0,0 +1,167 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"sync"
+
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+)
+
+var (
+	LineProtocolMessages = make(chan *AvroStruct)
+	// SelectorDelimiter separates hierarchical selector components in metric names for Avro encoding
+	SelectorDelimiter = "_SEL_"
+)
+
+var CheckpointBufferMinutes = DefaultCheckpointBufferMin
+
+type AvroStruct struct {
+	MetricName string
+	Cluster    string
+	Node       string
+	Selector   []string
+	Value      schema.Float
+	Timestamp  int64
+}
+
+type AvroStore struct {
+	root AvroLevel
+}
+
+var avroStore AvroStore
+
+type AvroLevel struct {
+	children map[string]*AvroLevel
+	data     map[int64]map[string]schema.Float
+	lock     sync.RWMutex
+}
+
+type AvroField struct {
+	Name    string `json:"name"`
+	Type    any    `json:"type"`
+	Default any    `json:"default,omitempty"`
+}
+
+type AvroSchema struct {
+	Type   string      `json:"type"`
+	Name   string      `json:"name"`
+	Fields []AvroField `json:"fields"`
+}
+
+func (l *AvroLevel) findAvroLevelOrCreate(selector []string) *AvroLevel {
+	if len(selector) == 0 {
+		return l
+	}
+
+	// Allow concurrent reads:
+	l.lock.RLock()
+	var child *AvroLevel
+	var ok bool
+	if l.children == nil {
+		// Children map needs to be created...
+		l.lock.RUnlock()
+	} else {
+		child, ok := l.children[selector[0]]
+		l.lock.RUnlock()
+		if ok {
+			return child.findAvroLevelOrCreate(selector[1:])
+		}
+	}
+
+	// The level does not exist, take write lock for unique access:
+	l.lock.Lock()
+	// While this thread waited for the write lock, another thread
+	// could have created the child node.
+	if l.children != nil {
+		child, ok = l.children[selector[0]]
+		if ok {
+			l.lock.Unlock()
+			return child.findAvroLevelOrCreate(selector[1:])
+		}
+	}
+
+	child = &AvroLevel{
+		data:     make(map[int64]map[string]schema.Float, 0),
+		children: nil,
+	}
+
+	if l.children != nil {
+		l.children[selector[0]] = child
+	} else {
+		l.children = map[string]*AvroLevel{selector[0]: child}
+	}
+	l.lock.Unlock()
+	return child.findAvroLevelOrCreate(selector[1:])
+}
+
+func (l *AvroLevel) addMetric(metricName string, value schema.Float, timestamp int64, Freq int) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	KeyCounter := int(CheckpointBufferMinutes * 60 / Freq)
+
+	// Create keys in advance for the given amount of time
+	if len(l.data) != KeyCounter {
+		if len(l.data) == 0 {
+			for i := range KeyCounter {
+				l.data[timestamp+int64(i*Freq)] = make(map[string]schema.Float, 0)
+			}
+		} else {
+			// Get the last timestamp
+			var lastTS int64
+			for ts := range l.data {
+				if ts > lastTS {
+					lastTS = ts
+				}
+			}
+			// Create keys for the next KeyCounter timestamps
+			l.data[lastTS+int64(Freq)] = make(map[string]schema.Float, 0)
+		}
+	}
+
+	closestTS := int64(0)
+	minDiff := int64(Freq) + 1 // Start with diff just outside the valid range
+	found := false
+
+	// Iterate over timestamps and choose the one which is within range.
+	// Since its epoch time, we check if the difference is less than 60 seconds.
+	for ts, dat := range l.data {
+		// Check if timestamp is within range
+		diff := timestamp - ts
+		if diff < -int64(Freq) || diff > int64(Freq) {
+			continue
+		}
+
+		// Metric already present at this timestamp — skip
+		if _, ok := dat[metricName]; ok {
+			continue
+		}
+
+		// Check if this is the closest timestamp so far
+		if Abs(diff) < minDiff {
+			minDiff = Abs(diff)
+			closestTS = ts
+			found = true
+		}
+	}
+
+	if found {
+		l.data[closestTS][metricName] = value
+	}
+}
+
+func GetAvroStore() *AvroStore {
+	return &avroStore
+}
+
+// Abs returns the absolute value of x.
+func Abs(x int64) int64 {
+	if x < 0 {
+		return -x
+	}
+	return x
+}

internal/metricstore/buffer.go

@@ -0,0 +1,190 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"errors"
+	"sync"
+
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+)
+
+// BufferCap is the default buffer capacity.
+// buffer.data will only ever grow up to its capacity and a new link
+// in the buffer chain will be created if needed so that no copying
+// of data or reallocation needs to happen on writes.
+const BufferCap int = DefaultBufferCapacity
+
+var bufferPool sync.Pool = sync.Pool{
+	New: func() any {
+		return &buffer{
+			data: make([]schema.Float, 0, BufferCap),
+		}
+	},
+}
+
+var (
+	ErrNoData           error = errors.New("[METRICSTORE]> no data for this metric/level")
+	ErrDataDoesNotAlign error = errors.New("[METRICSTORE]> data from lower granularities does not align")
+)
+
+// Each metric on each level has it's own buffer.
+// This is where the actual values go.
+// If `cap(data)` is reached, a new buffer is created and
+// becomes the new head of a buffer list.
+type buffer struct {
+	prev      *buffer
+	next      *buffer
+	data      []schema.Float
+	frequency int64
+	start     int64
+	archived  bool
+	closed    bool
+}
+
+func newBuffer(ts, freq int64) *buffer {
+	b := bufferPool.Get().(*buffer)
+	b.frequency = freq
+	b.start = ts - (freq / 2)
+	b.prev = nil
+	b.next = nil
+	b.archived = false
+	b.closed = false
+	b.data = b.data[:0]
+	return b
+}
+
+// If a new buffer was created, the new head is returnd.
+// Otherwise, the existing buffer is returnd.
+// Normaly, only "newer" data should be written, but if the value would
+// end up in the same buffer anyways it is allowed.
+func (b *buffer) write(ts int64, value schema.Float) (*buffer, error) {
+	if ts < b.start {
+		return nil, errors.New("[METRICSTORE]> cannot write value to buffer from past")
+	}
+
+	// idx := int((ts - b.start + (b.frequency / 3)) / b.frequency)
+	idx := int((ts - b.start) / b.frequency)
+	if idx >= cap(b.data) {
+		newbuf := newBuffer(ts, b.frequency)
+		newbuf.prev = b
+		b.next = newbuf
+		b = newbuf
+		idx = 0
+	}
+
+	// Overwriting value or writing value from past
+	if idx < len(b.data) {
+		b.data[idx] = value
+		return b, nil
+	}
+
+	// Fill up unwritten slots with NaN
+	for i := len(b.data); i < idx; i++ {
+		b.data = append(b.data, schema.NaN)
+	}
+
+	b.data = append(b.data, value)
+	return b, nil
+}
+
+func (b *buffer) end() int64 {
+	return b.firstWrite() + int64(len(b.data))*b.frequency
+}
+
+func (b *buffer) firstWrite() int64 {
+	return b.start + (b.frequency / 2)
+}
+
+// Return all known values from `from` to `to`. Gaps of information are represented as NaN.
+// Simple linear interpolation is done between the two neighboring cells if possible.
+// If values at the start or end are missing, instead of NaN values, the second and thrid
+// return values contain the actual `from`/`to`.
+// This function goes back the buffer chain if `from` is older than the currents buffer start.
+// The loaded values are added to `data` and `data` is returned, possibly with a shorter length.
+// If `data` is not long enough to hold all values, this function will panic!
+func (b *buffer) read(from, to int64, data []schema.Float) ([]schema.Float, int64, int64, error) {
+	if from < b.firstWrite() {
+		if b.prev != nil {
+			return b.prev.read(from, to, data)
+		}
+		from = b.firstWrite()
+	}
+
+	i := 0
+	t := from
+	for ; t < to; t += b.frequency {
+		idx := int((t - b.start) / b.frequency)
+		if idx >= cap(b.data) {
+			if b.next == nil {
+				break
+			}
+			b = b.next
+			idx = 0
+		}
+
+		if idx >= len(b.data) {
+			if b.next == nil || to <= b.next.start {
+				break
+			}
+			data[i] += schema.NaN
+		} else if t < b.start {
+			data[i] += schema.NaN
+		} else {
+			data[i] += b.data[idx]
+		}
+		i++
+	}
+
+	return data[:i], from, t, nil
+}
+
+// Returns true if this buffer needs to be freed.
+func (b *buffer) free(t int64) (delme bool, n int) {
+	if b.prev != nil {
+		delme, m := b.prev.free(t)
+		n += m
+		if delme {
+			b.prev.next = nil
+			if cap(b.prev.data) == BufferCap {
+				bufferPool.Put(b.prev)
+			}
+			b.prev = nil
+		}
+	}
+
+	end := b.end()
+	if end < t {
+		return true, n + 1
+	}
+
+	return false, n
+}
+
+// Call `callback` on every buffer that contains data in the range from `from` to `to`.
+func (b *buffer) iterFromTo(from, to int64, callback func(b *buffer) error) error {
+	if b == nil {
+		return nil
+	}
+
+	if err := b.prev.iterFromTo(from, to, callback); err != nil {
+		return err
+	}
+
+	if from <= b.end() && b.start <= to {
+		return callback(b)
+	}
+
+	return nil
+}
+
+func (b *buffer) count() int64 {
+	res := int64(len(b.data))
+	if b.prev != nil {
+		res += b.prev.count()
+	}
+	return res
+}

internal/metricstore/checkpoint.go

@@ -0,0 +1,761 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"bufio"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"sort"
+	"strconv"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+	"github.com/linkedin/goavro/v2"
+)
+
+const (
+	CheckpointFilePerms = 0o644
+	CheckpointDirPerms  = 0o755
+	GCTriggerInterval   = DefaultGCTriggerInterval
+)
+
+// Whenever changed, update MarshalJSON as well!
+type CheckpointMetrics struct {
+	Data      []schema.Float `json:"data"`
+	Frequency int64          `json:"frequency"`
+	Start     int64          `json:"start"`
+}
+
+type CheckpointFile struct {
+	Metrics  map[string]*CheckpointMetrics `json:"metrics"`
+	Children map[string]*CheckpointFile    `json:"children"`
+	From     int64                         `json:"from"`
+	To       int64                         `json:"to"`
+}
+
+var lastCheckpoint time.Time
+
+func Checkpointing(wg *sync.WaitGroup, ctx context.Context) {
+	lastCheckpoint = time.Now()
+
+	if Keys.Checkpoints.FileFormat == "json" {
+		ms := GetMemoryStore()
+
+		go func() {
+			defer wg.Done()
+			d, err := time.ParseDuration(Keys.Checkpoints.Interval)
+			if err != nil {
+				cclog.Fatal(err)
+			}
+			if d <= 0 {
+				return
+			}
+
+			ticker := time.NewTicker(d)
+			defer ticker.Stop()
+
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				case <-ticker.C:
+					cclog.Infof("[METRICSTORE]> start checkpointing (starting at %s)...", lastCheckpoint.Format(time.RFC3339))
+					now := time.Now()
+					n, err := ms.ToCheckpoint(Keys.Checkpoints.RootDir,
+						lastCheckpoint.Unix(), now.Unix())
+					if err != nil {
+						cclog.Errorf("[METRICSTORE]> checkpointing failed: %s", err.Error())
+					} else {
+						cclog.Infof("[METRICSTORE]> done: %d checkpoint files created", n)
+						lastCheckpoint = now
+					}
+				}
+			}
+		}()
+	} else {
+		go func() {
+			defer wg.Done()
+
+			select {
+			case <-ctx.Done():
+				return
+			case <-time.After(time.Duration(CheckpointBufferMinutes) * time.Minute):
+				GetAvroStore().ToCheckpoint(Keys.Checkpoints.RootDir, false)
+			}
+
+			ticker := time.NewTicker(DefaultAvroCheckpointInterval)
+			defer ticker.Stop()
+
+			for {
+				select {
+				case <-ctx.Done():
+					return
+				case <-ticker.C:
+					GetAvroStore().ToCheckpoint(Keys.Checkpoints.RootDir, false)
+				}
+			}
+		}()
+	}
+}
+
+// As `Float` implements a custom MarshalJSON() function,
+// serializing an array of such types has more overhead
+// than one would assume (because of extra allocations, interfaces and so on).
+func (cm *CheckpointMetrics) MarshalJSON() ([]byte, error) {
+	buf := make([]byte, 0, 128+len(cm.Data)*8)
+	buf = append(buf, `{"frequency":`...)
+	buf = strconv.AppendInt(buf, cm.Frequency, 10)
+	buf = append(buf, `,"start":`...)
+	buf = strconv.AppendInt(buf, cm.Start, 10)
+	buf = append(buf, `,"data":[`...)
+	for i, x := range cm.Data {
+		if i != 0 {
+			buf = append(buf, ',')
+		}
+		if x.IsNaN() {
+			buf = append(buf, `null`...)
+		} else {
+			buf = strconv.AppendFloat(buf, float64(x), 'f', 1, 32)
+		}
+	}
+	buf = append(buf, `]}`...)
+	return buf, nil
+}
+
+// Metrics stored at the lowest 2 levels are not stored away (root and cluster)!
+// On a per-host basis a new JSON file is created. I have no idea if this will scale.
+// The good thing: Only a host at a time is locked, so this function can run
+// in parallel to writes/reads.
+func (m *MemoryStore) ToCheckpoint(dir string, from, to int64) (int, error) {
+	levels := make([]*Level, 0)
+	selectors := make([][]string, 0)
+	m.root.lock.RLock()
+	for sel1, l1 := range m.root.children {
+		l1.lock.RLock()
+		for sel2, l2 := range l1.children {
+			levels = append(levels, l2)
+			selectors = append(selectors, []string{sel1, sel2})
+		}
+		l1.lock.RUnlock()
+	}
+	m.root.lock.RUnlock()
+
+	type workItem struct {
+		level    *Level
+		dir      string
+		selector []string
+	}
+
+	n, errs := int32(0), int32(0)
+
+	var wg sync.WaitGroup
+	wg.Add(Keys.NumWorkers)
+	work := make(chan workItem, Keys.NumWorkers*2)
+	for worker := 0; worker < Keys.NumWorkers; worker++ {
+		go func() {
+			defer wg.Done()
+
+			for workItem := range work {
+				if err := workItem.level.toCheckpoint(workItem.dir, from, to, m); err != nil {
+					if err == ErrNoNewArchiveData {
+						continue
+					}
+
+					cclog.Errorf("[METRICSTORE]> error while checkpointing %#v: %s", workItem.selector, err.Error())
+					atomic.AddInt32(&errs, 1)
+				} else {
+					atomic.AddInt32(&n, 1)
+				}
+			}
+		}()
+	}
+
+	for i := 0; i < len(levels); i++ {
+		dir := path.Join(dir, path.Join(selectors[i]...))
+		work <- workItem{
+			level:    levels[i],
+			dir:      dir,
+			selector: selectors[i],
+		}
+	}
+
+	close(work)
+	wg.Wait()
+
+	if errs > 0 {
+		return int(n), fmt.Errorf("[METRICSTORE]> %d errors happened while creating checkpoints (%d successes)", errs, n)
+	}
+	return int(n), nil
+}
+
+func (l *Level) toCheckpointFile(from, to int64, m *MemoryStore) (*CheckpointFile, error) {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	retval := &CheckpointFile{
+		From:     from,
+		To:       to,
+		Metrics:  make(map[string]*CheckpointMetrics),
+		Children: make(map[string]*CheckpointFile),
+	}
+
+	for metric, minfo := range m.Metrics {
+		b := l.metrics[minfo.offset]
+		if b == nil {
+			continue
+		}
+
+		allArchived := true
+		b.iterFromTo(from, to, func(b *buffer) error {
+			if !b.archived {
+				allArchived = false
+			}
+			return nil
+		})
+
+		if allArchived {
+			continue
+		}
+
+		data := make([]schema.Float, (to-from)/b.frequency+1)
+		data, start, end, err := b.read(from, to, data)
+		if err != nil {
+			return nil, err
+		}
+
+		for i := int((end - start) / b.frequency); i < len(data); i++ {
+			data[i] = schema.NaN
+		}
+
+		retval.Metrics[metric] = &CheckpointMetrics{
+			Frequency: b.frequency,
+			Start:     start,
+			Data:      data,
+		}
+	}
+
+	for name, child := range l.children {
+		val, err := child.toCheckpointFile(from, to, m)
+		if err != nil {
+			return nil, err
+		}
+
+		if val != nil {
+			retval.Children[name] = val
+		}
+	}
+
+	if len(retval.Children) == 0 && len(retval.Metrics) == 0 {
+		return nil, nil
+	}
+
+	return retval, nil
+}
+
+func (l *Level) toCheckpoint(dir string, from, to int64, m *MemoryStore) error {
+	cf, err := l.toCheckpointFile(from, to, m)
+	if err != nil {
+		return err
+	}
+
+	if cf == nil {
+		return ErrNoNewArchiveData
+	}
+
+	filepath := path.Join(dir, fmt.Sprintf("%d.json", from))
+	f, err := os.OpenFile(filepath, os.O_CREATE|os.O_WRONLY, CheckpointFilePerms)
+	if err != nil && os.IsNotExist(err) {
+		err = os.MkdirAll(dir, CheckpointDirPerms)
+		if err == nil {
+			f, err = os.OpenFile(filepath, os.O_CREATE|os.O_WRONLY, CheckpointFilePerms)
+		}
+	}
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	bw := bufio.NewWriter(f)
+	if err = json.NewEncoder(bw).Encode(cf); err != nil {
+		return err
+	}
+
+	return bw.Flush()
+}
+
+func (m *MemoryStore) FromCheckpoint(dir string, from int64, extension string) (int, error) {
+	var wg sync.WaitGroup
+	work := make(chan [2]string, Keys.NumWorkers)
+	n, errs := int32(0), int32(0)
+
+	wg.Add(Keys.NumWorkers)
+	for worker := 0; worker < Keys.NumWorkers; worker++ {
+		go func() {
+			defer wg.Done()
+			for host := range work {
+				lvl := m.root.findLevelOrCreate(host[:], len(m.Metrics))
+				nn, err := lvl.fromCheckpoint(m, filepath.Join(dir, host[0], host[1]), from, extension)
+				if err != nil {
+					cclog.Errorf("[METRICSTORE]> error while loading checkpoints for %s/%s: %s", host[0], host[1], err.Error())
+					atomic.AddInt32(&errs, 1)
+				}
+				atomic.AddInt32(&n, int32(nn))
+			}
+		}()
+	}
+
+	i := 0
+	clustersDir, err := os.ReadDir(dir)
+	for _, clusterDir := range clustersDir {
+		if !clusterDir.IsDir() {
+			err = errors.New("[METRICSTORE]> expected only directories at first level of checkpoints/ directory")
+			goto done
+		}
+
+		hostsDir, e := os.ReadDir(filepath.Join(dir, clusterDir.Name()))
+		if e != nil {
+			err = e
+			goto done
+		}
+
+		for _, hostDir := range hostsDir {
+			if !hostDir.IsDir() {
+				err = errors.New("[METRICSTORE]> expected only directories at second level of checkpoints/ directory")
+				goto done
+			}
+
+			i++
+			if i%Keys.NumWorkers == 0 && i > GCTriggerInterval {
+				// Forcing garbage collection runs here regulary during the loading of checkpoints
+				// will decrease the total heap size after loading everything back to memory is done.
+				// While loading data, the heap will grow fast, so the GC target size will double
+				// almost always. By forcing GCs here, we can keep it growing more slowly so that
+				// at the end, less memory is wasted.
+				runtime.GC()
+			}
+
+			work <- [2]string{clusterDir.Name(), hostDir.Name()}
+		}
+	}
+done:
+	close(work)
+	wg.Wait()
+
+	if err != nil {
+		return int(n), err
+	}
+
+	if errs > 0 {
+		return int(n), fmt.Errorf("[METRICSTORE]> %d errors happened while creating checkpoints (%d successes)", errs, n)
+	}
+	return int(n), nil
+}
+
+// Metrics stored at the lowest 2 levels are not loaded (root and cluster)!
+// This function can only be called once and before the very first write or read.
+// Different host's data is loaded to memory in parallel.
+func (m *MemoryStore) FromCheckpointFiles(dir string, from int64) (int, error) {
+	if _, err := os.Stat(dir); os.IsNotExist(err) {
+		// The directory does not exist, so create it using os.MkdirAll()
+		err := os.MkdirAll(dir, CheckpointDirPerms) // CheckpointDirPerms sets the permissions for the directory
+		if err != nil {
+			cclog.Fatalf("[METRICSTORE]> Error creating directory: %#v\n", err)
+		}
+		cclog.Debugf("[METRICSTORE]> %#v Directory created successfully", dir)
+	}
+
+	// Config read (replace with your actual config read)
+	fileFormat := Keys.Checkpoints.FileFormat
+	if fileFormat == "" {
+		fileFormat = "avro"
+	}
+
+	// Map to easily get the fallback format
+	oppositeFormat := map[string]string{
+		"json": "avro",
+		"avro": "json",
+	}
+
+	// First, attempt to load the specified format
+	if found, err := checkFilesWithExtension(dir, fileFormat); err != nil {
+		return 0, fmt.Errorf("[METRICSTORE]> error checking files with extension: %v", err)
+	} else if found {
+		cclog.Infof("[METRICSTORE]> Loading %s files because fileformat is %s", fileFormat, fileFormat)
+		return m.FromCheckpoint(dir, from, fileFormat)
+	}
+
+	// If not found, attempt the opposite format
+	altFormat := oppositeFormat[fileFormat]
+	if found, err := checkFilesWithExtension(dir, altFormat); err != nil {
+		return 0, fmt.Errorf("[METRICSTORE]> error checking files with extension: %v", err)
+	} else if found {
+		cclog.Infof("[METRICSTORE]> Loading %s files but fileformat is %s", altFormat, fileFormat)
+		return m.FromCheckpoint(dir, from, altFormat)
+	}
+
+	cclog.Print("[METRICSTORE]> No valid checkpoint files found in the directory")
+	return 0, nil
+}
+
+func checkFilesWithExtension(dir string, extension string) (bool, error) {
+	found := false
+
+	err := filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return fmt.Errorf("[METRICSTORE]> error accessing path %s: %v", path, err)
+		}
+		if !info.IsDir() && filepath.Ext(info.Name()) == "."+extension {
+			found = true
+			return nil
+		}
+		return nil
+	})
+	if err != nil {
+		return false, fmt.Errorf("[METRICSTORE]> error walking through directories: %s", err)
+	}
+
+	return found, nil
+}
+
+func (l *Level) loadAvroFile(m *MemoryStore, f *os.File, from int64) error {
+	br := bufio.NewReader(f)
+
+	fileName := f.Name()[strings.LastIndex(f.Name(), "/")+1:]
+	resolution, err := strconv.ParseInt(fileName[0:strings.Index(fileName, "_")], 10, 64)
+	if err != nil {
+		return fmt.Errorf("[METRICSTORE]> error while reading avro file (resolution parsing) : %s", err)
+	}
+
+	fromTimestamp, err := strconv.ParseInt(fileName[strings.Index(fileName, "_")+1:len(fileName)-5], 10, 64)
+
+	// Same logic according to lineprotocol
+	fromTimestamp -= (resolution / 2)
+
+	if err != nil {
+		return fmt.Errorf("[METRICSTORE]> error converting timestamp from the avro file : %s", err)
+	}
+
+	// fmt.Printf("File : %s with resolution : %d\n", fileName, resolution)
+
+	var recordCounter int64 = 0
+
+	// Create a new OCF reader from the buffered reader
+	ocfReader, err := goavro.NewOCFReader(br)
+	if err != nil {
+		return fmt.Errorf("[METRICSTORE]> error creating OCF reader: %w", err)
+	}
+
+	metricsData := make(map[string]schema.FloatArray)
+
+	for ocfReader.Scan() {
+		datum, err := ocfReader.Read()
+		if err != nil {
+			return fmt.Errorf("[METRICSTORE]> error while reading avro file : %s", err)
+		}
+
+		record, ok := datum.(map[string]any)
+		if !ok {
+			return fmt.Errorf("[METRICSTORE]> failed to assert datum as map[string]interface{}")
+		}
+
+		for key, value := range record {
+			metricsData[key] = append(metricsData[key], schema.ConvertToFloat(value.(float64)))
+		}
+
+		recordCounter += 1
+	}
+
+	to := (fromTimestamp + (recordCounter / (60 / resolution) * 60))
+	if to < from {
+		return nil
+	}
+
+	for key, floatArray := range metricsData {
+		metricName := ReplaceKey(key)
+
+		if strings.Contains(metricName, SelectorDelimiter) {
+			subString := strings.Split(metricName, SelectorDelimiter)
+
+			lvl := l
+
+			for i := 0; i < len(subString)-1; i++ {
+
+				sel := subString[i]
+
+				if lvl.children == nil {
+					lvl.children = make(map[string]*Level)
+				}
+
+				child, ok := lvl.children[sel]
+				if !ok {
+					child = &Level{
+						metrics:  make([]*buffer, len(m.Metrics)),
+						children: nil,
+					}
+					lvl.children[sel] = child
+				}
+				lvl = child
+			}
+
+			leafMetricName := subString[len(subString)-1]
+			err = lvl.createBuffer(m, leafMetricName, floatArray, fromTimestamp, resolution)
+			if err != nil {
+				return fmt.Errorf("[METRICSTORE]> error while creating buffers from avroReader : %s", err)
+			}
+		} else {
+			err = l.createBuffer(m, metricName, floatArray, fromTimestamp, resolution)
+			if err != nil {
+				return fmt.Errorf("[METRICSTORE]> error while creating buffers from avroReader : %s", err)
+			}
+		}
+
+	}
+
+	return nil
+}
+
+func (l *Level) createBuffer(m *MemoryStore, metricName string, floatArray schema.FloatArray, from int64, resolution int64) error {
+	n := len(floatArray)
+	b := &buffer{
+		frequency: resolution,
+		start:     from,
+		data:      floatArray[0:n:n],
+		prev:      nil,
+		next:      nil,
+		archived:  true,
+	}
+
+	minfo, ok := m.Metrics[metricName]
+	if !ok {
+		return nil
+	}
+
+	prev := l.metrics[minfo.offset]
+	if prev == nil {
+		l.metrics[minfo.offset] = b
+	} else {
+		if prev.start > b.start {
+			return fmt.Errorf("[METRICSTORE]> buffer start time %d is before previous buffer start %d", b.start, prev.start)
+		}
+
+		b.prev = prev
+		prev.next = b
+
+		missingCount := ((int(b.start) - int(prev.start)) - len(prev.data)*int(b.frequency))
+		if missingCount > 0 {
+			missingCount /= int(b.frequency)
+
+			for range missingCount {
+				prev.data = append(prev.data, schema.NaN)
+			}
+
+			prev.data = prev.data[0:len(prev.data):len(prev.data)]
+		}
+	}
+	l.metrics[minfo.offset] = b
+
+	return nil
+}
+
+func (l *Level) loadJSONFile(m *MemoryStore, f *os.File, from int64) error {
+	br := bufio.NewReader(f)
+	cf := &CheckpointFile{}
+	if err := json.NewDecoder(br).Decode(cf); err != nil {
+		return err
+	}
+
+	if cf.To != 0 && cf.To < from {
+		return nil
+	}
+
+	if err := l.loadFile(cf, m); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (l *Level) loadFile(cf *CheckpointFile, m *MemoryStore) error {
+	for name, metric := range cf.Metrics {
+		n := len(metric.Data)
+		b := &buffer{
+			frequency: metric.Frequency,
+			start:     metric.Start,
+			data:      metric.Data[0:n:n],
+			prev:      nil,
+			next:      nil,
+			archived:  true,
+		}
+
+		minfo, ok := m.Metrics[name]
+		if !ok {
+			continue
+		}
+
+		prev := l.metrics[minfo.offset]
+		if prev == nil {
+			l.metrics[minfo.offset] = b
+		} else {
+			if prev.start > b.start {
+				return fmt.Errorf("[METRICSTORE]> buffer start time %d is before previous buffer start %d", b.start, prev.start)
+			}
+
+			b.prev = prev
+			prev.next = b
+		}
+		l.metrics[minfo.offset] = b
+	}
+
+	if len(cf.Children) > 0 && l.children == nil {
+		l.children = make(map[string]*Level)
+	}
+
+	for sel, childCf := range cf.Children {
+		child, ok := l.children[sel]
+		if !ok {
+			child = &Level{
+				metrics:  make([]*buffer, len(m.Metrics)),
+				children: nil,
+			}
+			l.children[sel] = child
+		}
+
+		if err := child.loadFile(childCf, m); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (l *Level) fromCheckpoint(m *MemoryStore, dir string, from int64, extension string) (int, error) {
+	direntries, err := os.ReadDir(dir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return 0, nil
+		}
+
+		return 0, err
+	}
+
+	allFiles := make([]fs.DirEntry, 0)
+	filesLoaded := 0
+	for _, e := range direntries {
+		if e.IsDir() {
+			child := &Level{
+				metrics:  make([]*buffer, len(m.Metrics)),
+				children: make(map[string]*Level),
+			}
+
+			files, err := child.fromCheckpoint(m, path.Join(dir, e.Name()), from, extension)
+			filesLoaded += files
+			if err != nil {
+				return filesLoaded, err
+			}
+
+			l.children[e.Name()] = child
+		} else if strings.HasSuffix(e.Name(), "."+extension) {
+			allFiles = append(allFiles, e)
+		} else {
+			continue
+		}
+	}
+
+	files, err := findFiles(allFiles, from, extension, true)
+	if err != nil {
+		return filesLoaded, err
+	}
+
+	loaders := map[string]func(*MemoryStore, *os.File, int64) error{
+		"json": l.loadJSONFile,
+		"avro": l.loadAvroFile,
+	}
+
+	loader := loaders[extension]
+
+	for _, filename := range files {
+		// Use a closure to ensure file is closed immediately after use
+		err := func() error {
+			f, err := os.Open(path.Join(dir, filename))
+			if err != nil {
+				return err
+			}
+			defer f.Close()
+
+			return loader(m, f, from)
+		}()
+		if err != nil {
+			return filesLoaded, err
+		}
+
+		filesLoaded += 1
+	}
+
+	return filesLoaded, nil
+}
+
+// This will probably get very slow over time!
+// A solution could be some sort of an index file in which all other files
+// and the timespan they contain is listed.
+func findFiles(direntries []fs.DirEntry, t int64, extension string, findMoreRecentFiles bool) ([]string, error) {
+	nums := map[string]int64{}
+	for _, e := range direntries {
+		if !strings.HasSuffix(e.Name(), "."+extension) {
+			continue
+		}
+
+		ts, err := strconv.ParseInt(e.Name()[strings.Index(e.Name(), "_")+1:len(e.Name())-5], 10, 64)
+		if err != nil {
+			return nil, err
+		}
+		nums[e.Name()] = ts
+	}
+
+	sort.Slice(direntries, func(i, j int) bool {
+		a, b := direntries[i], direntries[j]
+		return nums[a.Name()] < nums[b.Name()]
+	})
+
+	filenames := make([]string, 0)
+	for i := range direntries {
+		e := direntries[i]
+		ts1 := nums[e.Name()]
+
+		if findMoreRecentFiles && t <= ts1 {
+			filenames = append(filenames, e.Name())
+		}
+		if i == len(direntries)-1 {
+			continue
+		}
+
+		enext := direntries[i+1]
+		ts2 := nums[enext.Name()]
+
+		if findMoreRecentFiles {
+			if ts1 < t && t < ts2 {
+				filenames = append(filenames, e.Name())
+			}
+		} else {
+			if ts2 < t {
+				filenames = append(filenames, e.Name())
+			}
+		}
+	}
+
+	return filenames, nil
+}

internal/metricstore/config.go

@@ -0,0 +1,115 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"fmt"
+	"time"
+)
+
+const (
+	DefaultMaxWorkers             = 10
+	DefaultBufferCapacity         = 512
+	DefaultGCTriggerInterval      = 100
+	DefaultAvroWorkers            = 4
+	DefaultCheckpointBufferMin    = 3
+	DefaultAvroCheckpointInterval = time.Minute
+)
+
+type MetricStoreConfig struct {
+	// Number of concurrent workers for checkpoint and archive operations.
+	// If not set or 0, defaults to min(runtime.NumCPU()/2+1, 10)
+	NumWorkers  int `json:"num-workers"`
+	Checkpoints struct {
+		FileFormat string `json:"file-format"`
+		Interval   string `json:"interval"`
+		RootDir    string `json:"directory"`
+		Restore    string `json:"restore"`
+	} `json:"checkpoints"`
+	Debug struct {
+		DumpToFile string `json:"dump-to-file"`
+		EnableGops bool   `json:"gops"`
+	} `json:"debug"`
+	RetentionInMemory string `json:"retention-in-memory"`
+	Archive           struct {
+		Interval      string `json:"interval"`
+		RootDir       string `json:"directory"`
+		DeleteInstead bool   `json:"delete-instead"`
+	} `json:"archive"`
+	Subscriptions []struct {
+		// Channel name
+		SubscribeTo string `json:"subscribe-to"`
+
+		// Allow lines without a cluster tag, use this as default, optional
+		ClusterTag string `json:"cluster-tag"`
+	} `json:"subscriptions"`
+}
+
+var Keys MetricStoreConfig
+
+// AggregationStrategy for aggregation over multiple values at different cpus/sockets/..., not time!
+type AggregationStrategy int
+
+const (
+	NoAggregation AggregationStrategy = iota
+	SumAggregation
+	AvgAggregation
+)
+
+func AssignAggregationStrategy(str string) (AggregationStrategy, error) {
+	switch str {
+	case "":
+		return NoAggregation, nil
+	case "sum":
+		return SumAggregation, nil
+	case "avg":
+		return AvgAggregation, nil
+	default:
+		return NoAggregation, fmt.Errorf("[METRICSTORE]> unknown aggregation strategy: %s", str)
+	}
+}
+
+type MetricConfig struct {
+	// Interval in seconds at which measurements are stored
+	Frequency int64
+
+	// Can be 'sum', 'avg' or null. Describes how to aggregate metrics from the same timestep over the hierarchy.
+	Aggregation AggregationStrategy
+
+	// Private, used internally...
+	offset int
+}
+
+var Metrics map[string]MetricConfig
+
+func GetMetricFrequency(metricName string) (int64, error) {
+	if metric, ok := Metrics[metricName]; ok {
+		return metric.Frequency, nil
+	}
+	return 0, fmt.Errorf("[METRICSTORE]> metric %s not found", metricName)
+}
+
+// AddMetric adds logic to add metrics. Redundant metrics should be updated with max frequency.
+// use metric.Name to check if the metric already exists.
+// if not, add it to the Metrics map.
+func AddMetric(name string, metric MetricConfig) error {
+	if Metrics == nil {
+		Metrics = make(map[string]MetricConfig, 0)
+	}
+
+	if existingMetric, ok := Metrics[name]; ok {
+		if existingMetric.Frequency != metric.Frequency {
+			if existingMetric.Frequency < metric.Frequency {
+				existingMetric.Frequency = metric.Frequency
+				Metrics[name] = existingMetric
+			}
+		}
+	} else {
+		Metrics[name] = metric
+	}
+
+	return nil
+}

internal/metricstore/configSchema.go

@@ -0,0 +1,95 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+const configSchema = `{
+    "type": "object",
+    "description": "Configuration specific to built-in metric-store.",
+    "properties": {
+        "checkpoints": {
+            "description": "Configuration for checkpointing the metrics within metric-store",
+            "type": "object",
+            "properties": {
+                "file-format": {
+                    "description": "Specify the type of checkpoint file. There are 2 variants: 'avro' and 'json'. If nothing is specified, 'avro' is default.",
+                    "type": "string"
+                },
+                "interval": {
+                    "description": "Interval at which the metrics should be checkpointed.",
+                    "type": "string"
+                },
+                "directory": {
+                    "description": "Specify the parent directy in which the checkpointed files should be placed.",
+                    "type": "string"
+                },
+                "restore": {
+                    "description": "When cc-backend starts up, look for checkpointed files that are less than X hours old and load metrics from these selected checkpoint files.",
+                    "type": "string"
+                }
+            }
+        },
+        "archive": {
+            "description": "Configuration for archiving the already checkpointed files.",
+            "type": "object",
+            "properties": {
+                "interval": {
+                    "description": "Interval at which the checkpointed files should be archived.",
+                    "type": "string"
+                },
+                "directory": {
+                    "description": "Specify the parent directy in which the archived files should be placed.",
+                    "type": "string"
+                }
+            }
+        },
+        "retention-in-memory": {
+            "description": "Keep the metrics within memory for given time interval. Retention for X hours, then the metrics would be freed.",
+            "type": "string"
+        },
+        "nats": {
+            "description": "Configuration for accepting published data through NATS.",
+            "type": "array",
+            "items": {
+                "type": "object",
+                "properties": {
+                    "address": {
+                        "description": "Address of the NATS server.",
+                        "type": "string"
+                    },
+                    "username": {
+                        "description": "Optional: If configured with username/password method.",
+                        "type": "string"
+                    },
+                    "password": {
+                        "description": "Optional: If configured with username/password method.",
+                        "type": "string"
+                    },
+                    "creds-file-path": {
+                        "description": "Optional: If configured with Credential File method. Path to your NATS cred file.",
+                        "type": "string"
+                    },
+                    "subscriptions": {
+                        "description": "Array of various subscriptions. Allows to subscibe to different subjects and publishers.",
+                        "type": "array",
+                        "items": {
+                            "type": "object",
+                            "properties": {
+                                "subscribe-to": {
+                                    "description": "Channel name",
+                                    "type": "string"
+                                },
+                                "cluster-tag": {
+                                    "description": "Optional: Allow lines without a cluster tag, use this as default",
+                                    "type": "string"
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+}`

internal/metricstore/debug.go

@@ -0,0 +1,112 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"bufio"
+	"fmt"
+	"strconv"
+)
+
+func (b *buffer) debugDump(buf []byte) []byte {
+	if b.prev != nil {
+		buf = b.prev.debugDump(buf)
+	}
+
+	start, len, end := b.start, len(b.data), b.start+b.frequency*int64(len(b.data))
+	buf = append(buf, `{"start":`...)
+	buf = strconv.AppendInt(buf, start, 10)
+	buf = append(buf, `,"len":`...)
+	buf = strconv.AppendInt(buf, int64(len), 10)
+	buf = append(buf, `,"end":`...)
+	buf = strconv.AppendInt(buf, end, 10)
+	if b.archived {
+		buf = append(buf, `,"saved":true`...)
+	}
+	if b.next != nil {
+		buf = append(buf, `},`...)
+	} else {
+		buf = append(buf, `}`...)
+	}
+	return buf
+}
+
+func (l *Level) debugDump(m *MemoryStore, w *bufio.Writer, lvlname string, buf []byte, depth int) ([]byte, error) {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+	for i := 0; i < depth; i++ {
+		buf = append(buf, '\t')
+	}
+	buf = append(buf, '"')
+	buf = append(buf, lvlname...)
+	buf = append(buf, "\":{\n"...)
+	depth += 1
+	objitems := 0
+	for name, mc := range m.Metrics {
+		if b := l.metrics[mc.offset]; b != nil {
+			for i := 0; i < depth; i++ {
+				buf = append(buf, '\t')
+			}
+
+			buf = append(buf, '"')
+			buf = append(buf, name...)
+			buf = append(buf, `":[`...)
+			buf = b.debugDump(buf)
+			buf = append(buf, "],\n"...)
+			objitems++
+		}
+	}
+
+	for name, lvl := range l.children {
+		_, err := w.Write(buf)
+		if err != nil {
+			return nil, err
+		}
+
+		buf = buf[0:0]
+		buf, err = lvl.debugDump(m, w, name, buf, depth)
+		if err != nil {
+			return nil, err
+		}
+
+		buf = append(buf, ',', '\n')
+		objitems++
+	}
+
+	// remove final `,`:
+	if objitems > 0 {
+		buf = append(buf[0:len(buf)-1], '\n')
+	}
+
+	depth -= 1
+	for i := 0; i < depth; i++ {
+		buf = append(buf, '\t')
+	}
+	buf = append(buf, '}')
+	return buf, nil
+}
+
+func (m *MemoryStore) DebugDump(w *bufio.Writer, selector []string) error {
+	lvl := m.root.findLevel(selector)
+	if lvl == nil {
+		return fmt.Errorf("[METRICSTORE]> not found: %#v", selector)
+	}
+
+	buf := make([]byte, 0, 2048)
+	buf = append(buf, "{"...)
+
+	buf, err := lvl.debugDump(m, w, "data", buf, 0)
+	if err != nil {
+		return err
+	}
+
+	buf = append(buf, "}\n"...)
+	if _, err = w.Write(buf); err != nil {
+		return err
+	}
+
+	return w.Flush()
+}

internal/metricstore/healthcheck.go

@@ -0,0 +1,92 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"bufio"
+	"fmt"
+	"time"
+)
+
+// MaxMissingDataPoints is a threshold that allows a node to be healthy with certain number of data points missing.
+// Suppose a node does not receive last 5 data points, then healthCheck endpoint will still say a
+// node is healthy. Anything more than 5 missing points in metrics of the node will deem the node unhealthy.
+const MaxMissingDataPoints int64 = 5
+
+// MaxUnhealthyMetrics is a threshold which allows upto certain number of metrics in a node to be unhealthly.
+// Works with MaxMissingDataPoints. Say 5 metrics (including submetrics) do not receive the last
+// MaxMissingDataPoints data points, then the node will be deemed healthy. Any more metrics that does
+// not receive data for MaxMissingDataPoints data points will deem the node unhealthy.
+const MaxUnhealthyMetrics int64 = 5
+
+func (b *buffer) healthCheck() int64 {
+	// Check if the buffer is empty
+	if b.data == nil {
+		return 1
+	}
+
+	bufferEnd := b.start + b.frequency*int64(len(b.data))
+	t := time.Now().Unix()
+
+	// Check if the buffer is too old
+	if t-bufferEnd > MaxMissingDataPoints*b.frequency {
+		return 1
+	}
+
+	return 0
+}
+
+func (l *Level) healthCheck(m *MemoryStore, count int64) (int64, error) {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	for _, mc := range m.Metrics {
+		if b := l.metrics[mc.offset]; b != nil {
+			count += b.healthCheck()
+		}
+	}
+
+	for _, lvl := range l.children {
+		c, err := lvl.healthCheck(m, 0)
+		if err != nil {
+			return 0, err
+		}
+		count += c
+	}
+
+	return count, nil
+}
+
+func (m *MemoryStore) HealthCheck(w *bufio.Writer, selector []string) error {
+	lvl := m.root.findLevel(selector)
+	if lvl == nil {
+		return fmt.Errorf("[METRICSTORE]> not found: %#v", selector)
+	}
+
+	buf := make([]byte, 0, 25)
+	// buf = append(buf, "{"...)
+
+	var count int64 = 0
+
+	unhealthyMetricsCount, err := lvl.healthCheck(m, count)
+	if err != nil {
+		return err
+	}
+
+	if unhealthyMetricsCount < MaxUnhealthyMetrics {
+		buf = append(buf, "Healthy"...)
+	} else {
+		buf = append(buf, "Unhealthy"...)
+	}
+
+	// buf = append(buf, "}\n"...)
+
+	if _, err = w.Write(buf); err != nil {
+		return err
+	}
+
+	return w.Flush()
+}

internal/metricstore/level.go

@@ -0,0 +1,192 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"sync"
+	"unsafe"
+
+	"github.com/ClusterCockpit/cc-lib/v2/util"
+)
+
+// Could also be called "node" as this forms a node in a tree structure.
+// Called Level because "node" might be confusing here.
+// Can be both a leaf or a inner node. In this tree structue, inner nodes can
+// also hold data (in `metrics`).
+type Level struct {
+	children map[string]*Level
+	metrics  []*buffer
+	lock     sync.RWMutex
+}
+
+// Find the correct level for the given selector, creating it if
+// it does not exist. Example selector in the context of the
+// ClusterCockpit could be: []string{ "emmy", "host123", "cpu0" }.
+// This function would probably benefit a lot from `level.children` beeing a `sync.Map`?
+func (l *Level) findLevelOrCreate(selector []string, nMetrics int) *Level {
+	if len(selector) == 0 {
+		return l
+	}
+
+	// Allow concurrent reads:
+	l.lock.RLock()
+	var child *Level
+	var ok bool
+	if l.children == nil {
+		// Children map needs to be created...
+		l.lock.RUnlock()
+	} else {
+		child, ok = l.children[selector[0]]
+		l.lock.RUnlock()
+		if ok {
+			return child.findLevelOrCreate(selector[1:], nMetrics)
+		}
+	}
+
+	// The level does not exist, take write lock for unique access:
+	l.lock.Lock()
+	// While this thread waited for the write lock, another thread
+	// could have created the child node.
+	if l.children != nil {
+		child, ok = l.children[selector[0]]
+		if ok {
+			l.lock.Unlock()
+			return child.findLevelOrCreate(selector[1:], nMetrics)
+		}
+	}
+
+	child = &Level{
+		metrics:  make([]*buffer, nMetrics),
+		children: nil,
+	}
+
+	if l.children != nil {
+		l.children[selector[0]] = child
+	} else {
+		l.children = map[string]*Level{selector[0]: child}
+	}
+	l.lock.Unlock()
+	return child.findLevelOrCreate(selector[1:], nMetrics)
+}
+
+func (l *Level) free(t int64) (int, error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	n := 0
+	for i, b := range l.metrics {
+		if b != nil {
+			delme, m := b.free(t)
+			n += m
+			if delme {
+				if cap(b.data) == BufferCap {
+					bufferPool.Put(b)
+				}
+				l.metrics[i] = nil
+			}
+		}
+	}
+
+	for _, l := range l.children {
+		m, err := l.free(t)
+		n += m
+		if err != nil {
+			return n, err
+		}
+	}
+
+	return n, nil
+}
+
+func (l *Level) sizeInBytes() int64 {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+	size := int64(0)
+
+	for _, b := range l.metrics {
+		if b != nil {
+			size += b.count() * int64(unsafe.Sizeof(util.Float(0)))
+		}
+	}
+
+	for _, child := range l.children {
+		size += child.sizeInBytes()
+	}
+
+	return size
+}
+
+func (l *Level) findLevel(selector []string) *Level {
+	if len(selector) == 0 {
+		return l
+	}
+
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	lvl := l.children[selector[0]]
+	if lvl == nil {
+		return nil
+	}
+
+	return lvl.findLevel(selector[1:])
+}
+
+func (l *Level) findBuffers(selector util.Selector, offset int, f func(b *buffer) error) error {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	if len(selector) == 0 {
+		b := l.metrics[offset]
+		if b != nil {
+			return f(b)
+		}
+
+		for _, lvl := range l.children {
+			err := lvl.findBuffers(nil, offset, f)
+			if err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	sel := selector[0]
+	if len(sel.String) != 0 && l.children != nil {
+		lvl, ok := l.children[sel.String]
+		if ok {
+			err := lvl.findBuffers(selector[1:], offset, f)
+			if err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	if sel.Group != nil && l.children != nil {
+		for _, key := range sel.Group {
+			lvl, ok := l.children[key]
+			if ok {
+				err := lvl.findBuffers(selector[1:], offset, f)
+				if err != nil {
+					return err
+				}
+			}
+		}
+		return nil
+	}
+
+	if sel.Any && l.children != nil {
+		for _, lvl := range l.children {
+			if err := lvl.findBuffers(selector[1:], offset, f); err != nil {
+				return err
+			}
+		}
+		return nil
+	}
+
+	return nil
+}

internal/metricstore/lineprotocol.go

@@ -0,0 +1,258 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"time"
+
+	"github.com/ClusterCockpit/cc-backend/pkg/nats"
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+	"github.com/influxdata/line-protocol/v2/lineprotocol"
+)
+
+func ReceiveNats(ms *MemoryStore,
+	workers int,
+	ctx context.Context,
+) error {
+	nc := nats.GetClient()
+
+	if nc == nil {
+		cclog.Warn("NATS client not initialized")
+		return nil
+	}
+
+	var wg sync.WaitGroup
+
+	msgs := make(chan []byte, workers*2)
+
+	for _, sc := range Keys.Subscriptions {
+		clusterTag := sc.ClusterTag
+		if workers > 1 {
+			wg.Add(workers)
+
+			for range workers {
+				go func() {
+					for m := range msgs {
+						dec := lineprotocol.NewDecoderWithBytes(m)
+						if err := DecodeLine(dec, ms, clusterTag); err != nil {
+							cclog.Errorf("error: %s", err.Error())
+						}
+					}
+
+					wg.Done()
+				}()
+			}
+
+			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
+				msgs <- data
+			})
+		} else {
+			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
+				dec := lineprotocol.NewDecoderWithBytes(data)
+				if err := DecodeLine(dec, ms, clusterTag); err != nil {
+					cclog.Errorf("error: %s", err.Error())
+				}
+			})
+		}
+		cclog.Infof("NATS subscription to '%s' established", sc.SubscribeTo)
+	}
+
+	close(msgs)
+	wg.Wait()
+
+	return nil
+}
+
+// Place `prefix` in front of `buf` but if possible,
+// do that inplace in `buf`.
+func reorder(buf, prefix []byte) []byte {
+	n := len(prefix)
+	m := len(buf)
+	if cap(buf) < m+n {
+		return append(prefix[:n:n], buf...)
+	} else {
+		buf = buf[:n+m]
+		for i := m - 1; i >= 0; i-- {
+			buf[i+n] = buf[i]
+		}
+		for i := range n {
+			buf[i] = prefix[i]
+		}
+		return buf
+	}
+}
+
+// Decode lines using dec and make write calls to the MemoryStore.
+// If a line is missing its cluster tag, use clusterDefault as default.
+func DecodeLine(dec *lineprotocol.Decoder,
+	ms *MemoryStore,
+	clusterDefault string,
+) error {
+	// Reduce allocations in loop:
+	t := time.Now()
+	metric, metricBuf := Metric{}, make([]byte, 0, 16)
+	selector := make([]string, 0, 4)
+	typeBuf, subTypeBuf := make([]byte, 0, 16), make([]byte, 0)
+
+	// Optimize for the case where all lines in a "batch" are about the same
+	// cluster and host. By using `WriteToLevel` (level = host), we do not need
+	// to take the root- and cluster-level lock as often.
+	var lvl *Level = nil
+	prevCluster, prevHost := "", ""
+
+	var ok bool
+	for dec.Next() {
+		rawmeasurement, err := dec.Measurement()
+		if err != nil {
+			return err
+		}
+
+		// Needs to be copied because another call to dec.* would
+		// invalidate the returned slice.
+		metricBuf = append(metricBuf[:0], rawmeasurement...)
+
+		// The go compiler optimizes map[string(byteslice)] lookups:
+		metric.MetricConfig, ok = ms.Metrics[string(rawmeasurement)]
+		if !ok {
+			continue
+		}
+
+		typeBuf, subTypeBuf := typeBuf[:0], subTypeBuf[:0]
+		cluster, host := clusterDefault, ""
+		for {
+			key, val, err := dec.NextTag()
+			if err != nil {
+				return err
+			}
+			if key == nil {
+				break
+			}
+
+			// The go compiler optimizes string([]byte{...}) == "...":
+			switch string(key) {
+			case "cluster":
+				if string(val) == prevCluster {
+					cluster = prevCluster
+				} else {
+					cluster = string(val)
+					lvl = nil
+				}
+			case "hostname", "host":
+				if string(val) == prevHost {
+					host = prevHost
+				} else {
+					host = string(val)
+					lvl = nil
+				}
+			case "type":
+				if string(val) == "node" {
+					break
+				}
+
+				// We cannot be sure that the "type" tag comes before the "type-id" tag:
+				if len(typeBuf) == 0 {
+					typeBuf = append(typeBuf, val...)
+				} else {
+					typeBuf = reorder(typeBuf, val)
+				}
+			case "type-id":
+				typeBuf = append(typeBuf, val...)
+			case "subtype":
+				// We cannot be sure that the "subtype" tag comes before the "stype-id" tag:
+				if len(subTypeBuf) == 0 {
+					subTypeBuf = append(subTypeBuf, val...)
+				} else {
+					subTypeBuf = reorder(subTypeBuf, val)
+					// subTypeBuf = reorder(typeBuf, val)
+				}
+			case "stype-id":
+				subTypeBuf = append(subTypeBuf, val...)
+			default:
+			}
+		}
+
+		// If the cluster or host changed, the lvl was set to nil
+		if lvl == nil {
+			selector = selector[:2]
+			selector[0], selector[1] = cluster, host
+			lvl = ms.GetLevel(selector)
+			prevCluster, prevHost = cluster, host
+		}
+
+		// subtypes:
+		selector = selector[:0]
+		if len(typeBuf) > 0 {
+			selector = append(selector, string(typeBuf)) // <- Allocation :(
+			if len(subTypeBuf) > 0 {
+				selector = append(selector, string(subTypeBuf))
+			}
+		}
+
+		for {
+			key, val, err := dec.NextField()
+			if err != nil {
+				return err
+			}
+
+			if key == nil {
+				break
+			}
+
+			if string(key) != "value" {
+				return fmt.Errorf("host %s: unknown field: '%s' (value: %#v)", host, string(key), val)
+			}
+
+			if val.Kind() == lineprotocol.Float {
+				metric.Value = schema.Float(val.FloatV())
+			} else if val.Kind() == lineprotocol.Int {
+				metric.Value = schema.Float(val.IntV())
+			} else if val.Kind() == lineprotocol.Uint {
+				metric.Value = schema.Float(val.UintV())
+			} else {
+				return fmt.Errorf("host %s: unsupported value type in message: %s", host, val.Kind().String())
+			}
+		}
+
+		if t, err = dec.Time(lineprotocol.Second, t); err != nil {
+			t = time.Now()
+			if t, err = dec.Time(lineprotocol.Millisecond, t); err != nil {
+				t = time.Now()
+				if t, err = dec.Time(lineprotocol.Microsecond, t); err != nil {
+					t = time.Now()
+					if t, err = dec.Time(lineprotocol.Nanosecond, t); err != nil {
+						return fmt.Errorf("host %s: timestamp : %#v with error : %#v", host, t, err.Error())
+					}
+				}
+			}
+		}
+
+		if err != nil {
+			return fmt.Errorf("host %s: timestamp : %#v with error : %#v", host, t, err.Error())
+		}
+
+		time := t.Unix()
+
+		if Keys.Checkpoints.FileFormat != "json" {
+			LineProtocolMessages <- &AvroStruct{
+				MetricName: string(metricBuf),
+				Cluster:    cluster,
+				Node:       host,
+				Selector:   append([]string{}, selector...),
+				Value:      metric.Value,
+				Timestamp:  time,
+			}
+		}
+
+		if err := ms.WriteToLevel(lvl, selector, time, []Metric{metric}); err != nil {
+			return err
+		}
+	}
+	return nil
+}

internal/metricstore/memorystore.go

@@ -0,0 +1,423 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+// Package metricstore provides an efficient in-memory time-series metric storage system
+// with support for hierarchical data organization, checkpointing, and archiving.
+//
+// The package organizes metrics in a tree structure (cluster → host → component) and
+// provides concurrent read/write access to metric data with configurable aggregation strategies.
+// Background goroutines handle periodic checkpointing (JSON or Avro format), archiving old data,
+// and enforcing retention policies.
+//
+// Key features:
+//   - In-memory metric storage with configurable retention
+//   - Hierarchical data organization (selectors)
+//   - Concurrent checkpoint/archive workers
+//   - Support for sum and average aggregation
+//   - NATS integration for metric ingestion
+package metricstore
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"runtime"
+	"sync"
+	"time"
+
+	"github.com/ClusterCockpit/cc-backend/internal/config"
+	"github.com/ClusterCockpit/cc-backend/pkg/archive"
+	cclog "github.com/ClusterCockpit/cc-lib/v2/ccLogger"
+	"github.com/ClusterCockpit/cc-lib/v2/resampler"
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+	"github.com/ClusterCockpit/cc-lib/v2/util"
+)
+
+var (
+	singleton  sync.Once
+	msInstance *MemoryStore
+	// shutdownFunc stores the context cancellation function created in Init
+	// and is called during Shutdown to cancel all background goroutines
+	shutdownFunc context.CancelFunc
+)
+
+type Metric struct {
+	Name         string
+	Value        schema.Float
+	MetricConfig MetricConfig
+}
+
+type MemoryStore struct {
+	Metrics map[string]MetricConfig
+	root    Level
+}
+
+func Init(rawConfig json.RawMessage, wg *sync.WaitGroup) {
+	startupTime := time.Now()
+
+	if rawConfig != nil {
+		config.Validate(configSchema, rawConfig)
+		dec := json.NewDecoder(bytes.NewReader(rawConfig))
+		// dec.DisallowUnknownFields()
+		if err := dec.Decode(&Keys); err != nil {
+			cclog.Abortf("[METRICSTORE]> Metric Store Config Init: Could not decode config file '%s'.\nError: %s\n", rawConfig, err.Error())
+		}
+	}
+
+	// Set NumWorkers from config or use default
+	if Keys.NumWorkers <= 0 {
+		Keys.NumWorkers = min(runtime.NumCPU()/2+1, DefaultMaxWorkers)
+	}
+	cclog.Debugf("[METRICSTORE]> Using %d workers for checkpoint/archive operations\n", Keys.NumWorkers)
+
+	// Helper function to add metric configuration
+	addMetricConfig := func(mc schema.MetricConfig) {
+		agg, err := AssignAggregationStrategy(mc.Aggregation)
+		if err != nil {
+			cclog.Warnf("Could not find aggregation strategy for metric config '%s': %s", mc.Name, err.Error())
+		}
+
+		AddMetric(mc.Name, MetricConfig{
+			Frequency:   int64(mc.Timestep),
+			Aggregation: agg,
+		})
+	}
+
+	for _, c := range archive.Clusters {
+		for _, mc := range c.MetricConfig {
+			addMetricConfig(*mc)
+		}
+
+		for _, sc := range c.SubClusters {
+			for _, mc := range sc.MetricConfig {
+				addMetricConfig(mc)
+			}
+		}
+	}
+
+	// Pass the config.MetricStoreKeys
+	InitMetrics(Metrics)
+
+	ms := GetMemoryStore()
+
+	d, err := time.ParseDuration(Keys.Checkpoints.Restore)
+	if err != nil {
+		cclog.Fatal(err)
+	}
+
+	restoreFrom := startupTime.Add(-d)
+	cclog.Infof("[METRICSTORE]> Loading checkpoints newer than %s\n", restoreFrom.Format(time.RFC3339))
+	files, err := ms.FromCheckpointFiles(Keys.Checkpoints.RootDir, restoreFrom.Unix())
+	loadedData := ms.SizeInBytes() / 1024 / 1024 // In MB
+	if err != nil {
+		cclog.Fatalf("[METRICSTORE]> Loading checkpoints failed: %s\n", err.Error())
+	} else {
+		cclog.Infof("[METRICSTORE]> Checkpoints loaded (%d files, %d MB, that took %fs)\n", files, loadedData, time.Since(startupTime).Seconds())
+	}
+
+	// Try to use less memory by forcing a GC run here and then
+	// lowering the target percentage. The default of 100 means
+	// that only once the ratio of new allocations execeds the
+	// previously active heap, a GC is triggered.
+	// Forcing a GC here will set the "previously active heap"
+	// to a minumum.
+	runtime.GC()
+
+	ctx, shutdown := context.WithCancel(context.Background())
+
+	wg.Add(4)
+
+	Retention(wg, ctx)
+	Checkpointing(wg, ctx)
+	Archiving(wg, ctx)
+	DataStaging(wg, ctx)
+
+	// Note: Signal handling has been removed from this function.
+	// The caller is responsible for handling shutdown signals and calling
+	// the shutdown() function when appropriate.
+	// Store the shutdown function for later use by Shutdown()
+	shutdownFunc = shutdown
+
+	err = ReceiveNats(ms, 1, ctx)
+	if err != nil {
+		cclog.Fatal(err)
+	}
+}
+
+// InitMetrics creates a new, initialized instance of a MemoryStore.
+// Will panic if values in the metric configurations are invalid.
+func InitMetrics(metrics map[string]MetricConfig) {
+	singleton.Do(func() {
+		offset := 0
+		for key, cfg := range metrics {
+			if cfg.Frequency == 0 {
+				panic("[METRICSTORE]> invalid frequency")
+			}
+
+			metrics[key] = MetricConfig{
+				Frequency:   cfg.Frequency,
+				Aggregation: cfg.Aggregation,
+				offset:      offset,
+			}
+			offset += 1
+		}
+
+		msInstance = &MemoryStore{
+			root: Level{
+				metrics:  make([]*buffer, len(metrics)),
+				children: make(map[string]*Level),
+			},
+			Metrics: metrics,
+		}
+	})
+}
+
+func GetMemoryStore() *MemoryStore {
+	if msInstance == nil {
+		cclog.Fatalf("[METRICSTORE]> MemoryStore not initialized!")
+	}
+
+	return msInstance
+}
+
+func Shutdown() {
+	// Cancel the context to signal all background goroutines to stop
+	if shutdownFunc != nil {
+		shutdownFunc()
+	}
+
+	cclog.Infof("[METRICSTORE]> Writing to '%s'...\n", Keys.Checkpoints.RootDir)
+	var files int
+	var err error
+
+	ms := GetMemoryStore()
+
+	if Keys.Checkpoints.FileFormat == "json" {
+		files, err = ms.ToCheckpoint(Keys.Checkpoints.RootDir, lastCheckpoint.Unix(), time.Now().Unix())
+	} else {
+		files, err = GetAvroStore().ToCheckpoint(Keys.Checkpoints.RootDir, true)
+		close(LineProtocolMessages)
+	}
+
+	if err != nil {
+		cclog.Errorf("[METRICSTORE]> Writing checkpoint failed: %s\n", err.Error())
+	}
+	cclog.Infof("[METRICSTORE]> Done! (%d files written)\n", files)
+}
+
+func getName(m *MemoryStore, i int) string {
+	for key, val := range m.Metrics {
+		if val.offset == i {
+			return key
+		}
+	}
+	return ""
+}
+
+func Retention(wg *sync.WaitGroup, ctx context.Context) {
+	ms := GetMemoryStore()
+
+	go func() {
+		defer wg.Done()
+		d, err := time.ParseDuration(Keys.RetentionInMemory)
+		if err != nil {
+			cclog.Fatal(err)
+		}
+		if d <= 0 {
+			return
+		}
+
+		tickInterval := d / 2
+		if tickInterval <= 0 {
+			return
+		}
+		ticker := time.NewTicker(tickInterval)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ctx.Done():
+				return
+			case <-ticker.C:
+				t := time.Now().Add(-d)
+				cclog.Infof("[METRICSTORE]> start freeing buffers (older than %s)...\n", t.Format(time.RFC3339))
+				freed, err := ms.Free(nil, t.Unix())
+				if err != nil {
+					cclog.Errorf("[METRICSTORE]> freeing up buffers failed: %s\n", err.Error())
+				} else {
+					cclog.Infof("[METRICSTORE]> done: %d buffers freed\n", freed)
+				}
+			}
+		}
+	}()
+}
+
+// Write all values in `metrics` to the level specified by `selector` for time `ts`.
+// Look at `findLevelOrCreate` for how selectors work.
+func (m *MemoryStore) Write(selector []string, ts int64, metrics []Metric) error {
+	var ok bool
+	for i, metric := range metrics {
+		if metric.MetricConfig.Frequency == 0 {
+			metric.MetricConfig, ok = m.Metrics[metric.Name]
+			if !ok {
+				metric.MetricConfig.Frequency = 0
+			}
+			metrics[i] = metric
+		}
+	}
+
+	return m.WriteToLevel(&m.root, selector, ts, metrics)
+}
+
+func (m *MemoryStore) GetLevel(selector []string) *Level {
+	return m.root.findLevelOrCreate(selector, len(m.Metrics))
+}
+
+// WriteToLevel assumes that `minfo` in `metrics` is filled in
+func (m *MemoryStore) WriteToLevel(l *Level, selector []string, ts int64, metrics []Metric) error {
+	l = l.findLevelOrCreate(selector, len(m.Metrics))
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	for _, metric := range metrics {
+		if metric.MetricConfig.Frequency == 0 {
+			continue
+		}
+
+		b := l.metrics[metric.MetricConfig.offset]
+		if b == nil {
+			// First write to this metric and level
+			b = newBuffer(ts, metric.MetricConfig.Frequency)
+			l.metrics[metric.MetricConfig.offset] = b
+		}
+
+		nb, err := b.write(ts, metric.Value)
+		if err != nil {
+			return err
+		}
+
+		// Last write created a new buffer...
+		if b != nb {
+			l.metrics[metric.MetricConfig.offset] = nb
+		}
+	}
+	return nil
+}
+
+// Read returns all values for metric `metric` from `from` to `to` for the selected level(s).
+// If the level does not hold the metric itself, the data will be aggregated recursively from the children.
+// The second and third return value are the actual from/to for the data. Those can be different from
+// the range asked for if no data was available.
+func (m *MemoryStore) Read(selector util.Selector, metric string, from, to, resolution int64) ([]schema.Float, int64, int64, int64, error) {
+	if from > to {
+		return nil, 0, 0, 0, errors.New("[METRICSTORE]> invalid time range")
+	}
+
+	minfo, ok := m.Metrics[metric]
+	if !ok {
+		return nil, 0, 0, 0, errors.New("[METRICSTORE]> unknown metric: " + metric)
+	}
+
+	n, data := 0, make([]schema.Float, (to-from)/minfo.Frequency+1)
+
+	err := m.root.findBuffers(selector, minfo.offset, func(b *buffer) error {
+		cdata, cfrom, cto, err := b.read(from, to, data)
+		if err != nil {
+			return err
+		}
+
+		if n == 0 {
+			from, to = cfrom, cto
+		} else if from != cfrom || to != cto || len(data) != len(cdata) {
+			missingfront, missingback := int((from-cfrom)/minfo.Frequency), int((to-cto)/minfo.Frequency)
+			if missingfront != 0 {
+				return ErrDataDoesNotAlign
+			}
+
+			newlen := len(cdata) - missingback
+			if newlen < 1 {
+				return ErrDataDoesNotAlign
+			}
+			cdata = cdata[0:newlen]
+			if len(cdata) != len(data) {
+				return ErrDataDoesNotAlign
+			}
+
+			from, to = cfrom, cto
+		}
+
+		data = cdata
+		n += 1
+		return nil
+	})
+
+	if err != nil {
+		return nil, 0, 0, 0, err
+	} else if n == 0 {
+		return nil, 0, 0, 0, errors.New("[METRICSTORE]> metric or host not found")
+	} else if n > 1 {
+		if minfo.Aggregation == AvgAggregation {
+			normalize := 1. / schema.Float(n)
+			for i := 0; i < len(data); i++ {
+				data[i] *= normalize
+			}
+		} else if minfo.Aggregation != SumAggregation {
+			return nil, 0, 0, 0, errors.New("[METRICSTORE]> invalid aggregation")
+		}
+	}
+
+	data, resolution, err = resampler.LargestTriangleThreeBucket(data, minfo.Frequency, resolution)
+	if err != nil {
+		return nil, 0, 0, 0, err
+	}
+
+	return data, from, to, resolution, nil
+}
+
+// Free releases all buffers for the selected level and all its children that
+// contain only values older than `t`.
+func (m *MemoryStore) Free(selector []string, t int64) (int, error) {
+	return m.GetLevel(selector).free(t)
+}
+
+func (m *MemoryStore) FreeAll() error {
+	for k := range m.root.children {
+		delete(m.root.children, k)
+	}
+
+	return nil
+}
+
+func (m *MemoryStore) SizeInBytes() int64 {
+	return m.root.sizeInBytes()
+}
+
+// ListChildren , given a selector, returns a list of all children of the level
+// selected.
+func (m *MemoryStore) ListChildren(selector []string) []string {
+	lvl := &m.root
+	for lvl != nil && len(selector) != 0 {
+		lvl.lock.RLock()
+		next := lvl.children[selector[0]]
+		lvl.lock.RUnlock()
+		lvl = next
+		selector = selector[1:]
+	}
+
+	if lvl == nil {
+		return nil
+	}
+
+	lvl.lock.RLock()
+	defer lvl.lock.RUnlock()
+
+	children := make([]string, 0, len(lvl.children))
+	for child := range lvl.children {
+		children = append(children, child)
+	}
+
+	return children
+}

internal/metricstore/memorystore_test.go

@@ -0,0 +1,156 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"testing"
+
+	"github.com/ClusterCockpit/cc-lib/v2/schema"
+)
+
+func TestAssignAggregationStrategy(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected AggregationStrategy
+		wantErr  bool
+	}{
+		{"empty string", "", NoAggregation, false},
+		{"sum", "sum", SumAggregation, false},
+		{"avg", "avg", AvgAggregation, false},
+		{"invalid", "invalid", NoAggregation, true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result, err := AssignAggregationStrategy(tt.input)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("AssignAggregationStrategy(%q) error = %v, wantErr %v", tt.input, err, tt.wantErr)
+				return
+			}
+			if result != tt.expected {
+				t.Errorf("AssignAggregationStrategy(%q) = %v, want %v", tt.input, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestAddMetric(t *testing.T) {
+	// Reset Metrics before test
+	Metrics = make(map[string]MetricConfig)
+
+	err := AddMetric("test_metric", MetricConfig{
+		Frequency:   60,
+		Aggregation: SumAggregation,
+	})
+	if err != nil {
+		t.Errorf("AddMetric() error = %v", err)
+	}
+
+	if _, ok := Metrics["test_metric"]; !ok {
+		t.Error("AddMetric() did not add metric to Metrics map")
+	}
+
+	// Test updating with higher frequency
+	err = AddMetric("test_metric", MetricConfig{
+		Frequency:   120,
+		Aggregation: SumAggregation,
+	})
+	if err != nil {
+		t.Errorf("AddMetric() error = %v", err)
+	}
+
+	if Metrics["test_metric"].Frequency != 120 {
+		t.Errorf("AddMetric() frequency = %d, want 120", Metrics["test_metric"].Frequency)
+	}
+
+	// Test updating with lower frequency (should not update)
+	err = AddMetric("test_metric", MetricConfig{
+		Frequency:   30,
+		Aggregation: SumAggregation,
+	})
+	if err != nil {
+		t.Errorf("AddMetric() error = %v", err)
+	}
+
+	if Metrics["test_metric"].Frequency != 120 {
+		t.Errorf("AddMetric() frequency = %d, want 120 (should not downgrade)", Metrics["test_metric"].Frequency)
+	}
+}
+
+func TestGetMetricFrequency(t *testing.T) {
+	// Reset Metrics before test
+	Metrics = map[string]MetricConfig{
+		"test_metric": {
+			Frequency:   60,
+			Aggregation: SumAggregation,
+		},
+	}
+
+	freq, err := GetMetricFrequency("test_metric")
+	if err != nil {
+		t.Errorf("GetMetricFrequency() error = %v", err)
+	}
+	if freq != 60 {
+		t.Errorf("GetMetricFrequency() = %d, want 60", freq)
+	}
+
+	_, err = GetMetricFrequency("nonexistent")
+	if err == nil {
+		t.Error("GetMetricFrequency() expected error for nonexistent metric")
+	}
+}
+
+func TestBufferWrite(t *testing.T) {
+	b := newBuffer(100, 10)
+
+	// Test writing value
+	nb, err := b.write(100, schema.Float(42.0))
+	if err != nil {
+		t.Errorf("buffer.write() error = %v", err)
+	}
+	if nb != b {
+		t.Error("buffer.write() created new buffer unexpectedly")
+	}
+	if len(b.data) != 1 {
+		t.Errorf("buffer.write() len(data) = %d, want 1", len(b.data))
+	}
+	if b.data[0] != schema.Float(42.0) {
+		t.Errorf("buffer.write() data[0] = %v, want 42.0", b.data[0])
+	}
+
+	// Test writing value from past (should error)
+	_, err = b.write(50, schema.Float(10.0))
+	if err == nil {
+		t.Error("buffer.write() expected error for past timestamp")
+	}
+}
+
+func TestBufferRead(t *testing.T) {
+	b := newBuffer(100, 10)
+	
+	// Write some test data
+	b.write(100, schema.Float(1.0))
+	b.write(110, schema.Float(2.0))
+	b.write(120, schema.Float(3.0))
+
+	// Read data
+	data := make([]schema.Float, 3)
+	result, from, to, err := b.read(100, 130, data)
+	if err != nil {
+		t.Errorf("buffer.read() error = %v", err)
+	}
+	// Buffer read should return from as firstWrite (start + freq/2)
+	if from != 100 {
+		t.Errorf("buffer.read() from = %d, want 100", from)
+	}
+	if to != 130 {
+		t.Errorf("buffer.read() to = %d, want 130", to)
+	}
+	if len(result) != 3 {
+		t.Errorf("buffer.read() len(result) = %d, want 3", len(result))
+	}
+}

internal/metricstore/stats.go

@@ -0,0 +1,124 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"errors"
+	"math"
+
+	"github.com/ClusterCockpit/cc-lib/v2/util"
+)
+
+type Stats struct {
+	Samples int
+	Avg     util.Float
+	Min     util.Float
+	Max     util.Float
+}
+
+func (b *buffer) stats(from, to int64) (Stats, int64, int64, error) {
+	if from < b.start {
+		if b.prev != nil {
+			return b.prev.stats(from, to)
+		}
+		from = b.start
+	}
+
+	// TODO: Check if b.closed and if so and the full buffer is queried,
+	// use b.statistics instead of iterating over the buffer.
+
+	samples := 0
+	sum, min, max := 0.0, math.MaxFloat32, -math.MaxFloat32
+
+	var t int64
+	for t = from; t < to; t += b.frequency {
+		idx := int((t - b.start) / b.frequency)
+		if idx >= cap(b.data) {
+			b = b.next
+			if b == nil {
+				break
+			}
+			idx = 0
+		}
+
+		if t < b.start || idx >= len(b.data) {
+			continue
+		}
+
+		xf := float64(b.data[idx])
+		if math.IsNaN(xf) {
+			continue
+		}
+
+		samples += 1
+		sum += xf
+		min = math.Min(min, xf)
+		max = math.Max(max, xf)
+	}
+
+	return Stats{
+		Samples: samples,
+		Avg:     util.Float(sum) / util.Float(samples),
+		Min:     util.Float(min),
+		Max:     util.Float(max),
+	}, from, t, nil
+}
+
+// Returns statistics for the requested metric on the selected node/level.
+// Data is aggregated to the selected level the same way as in `MemoryStore.Read`.
+// If `Stats.Samples` is zero, the statistics should not be considered as valid.
+func (m *MemoryStore) Stats(selector util.Selector, metric string, from, to int64) (*Stats, int64, int64, error) {
+	if from > to {
+		return nil, 0, 0, errors.New("invalid time range")
+	}
+
+	minfo, ok := m.Metrics[metric]
+	if !ok {
+		return nil, 0, 0, errors.New("unknown metric: " + metric)
+	}
+
+	n, samples := 0, 0
+	avg, min, max := util.Float(0), math.MaxFloat32, -math.MaxFloat32
+	err := m.root.findBuffers(selector, minfo.offset, func(b *buffer) error {
+		stats, cfrom, cto, err := b.stats(from, to)
+		if err != nil {
+			return err
+		}
+
+		if n == 0 {
+			from, to = cfrom, cto
+		} else if from != cfrom || to != cto {
+			return ErrDataDoesNotAlign
+		}
+
+		samples += stats.Samples
+		avg += stats.Avg
+		min = math.Min(min, float64(stats.Min))
+		max = math.Max(max, float64(stats.Max))
+		n += 1
+		return nil
+	})
+	if err != nil {
+		return nil, 0, 0, err
+	}
+
+	if n == 0 {
+		return nil, 0, 0, ErrNoData
+	}
+
+	if minfo.Aggregation == AvgAggregation {
+		avg /= util.Float(n)
+	} else if n > 1 && minfo.Aggregation != SumAggregation {
+		return nil, 0, 0, errors.New("invalid aggregation")
+	}
+
+	return &Stats{
+		Samples: samples,
+		Avg:     avg,
+		Min:     util.Float(min),
+		Max:     util.Float(max),
+	}, from, to, nil
+}