Implement the following plan:

# Fix: ReceiveNats blocks Init() during startup

## Context

The application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation — its docstring says "blocks until ctx is cancelled" and `wg.Wait()` on line 98 waits for worker goroutines that only exit when `ctx` is cancelled at shutdown. However, it is called **synchronously** from `Init()` at line 187, which prevents `Init()` from returning and the server from finishing startup.

**This is a pre-existing bug** (present since commit `ca0f9a42` that introduced `ReceiveNats`). The WAL sharding changes did not modify `ReceiveNats` or the `Init()` call to it. Previously masked because either `NumWorkers` was 1 (the `workers == 1` path in `ReceiveNats` skips `wg.Add`, so `wg.Wait()` returns immediately) or NATS subscriptions were not configured.

## Root Cause

**File:** `pkg/metricstore/metricstore.go:186-191`

```go
if Keys.Subscriptions != nil {
    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← blocks forever when workers > 1
    if err != nil {
        cclog.Fatal(err)
    }
}
```

When `workers > 1`, `ReceiveNats` calls `wg.Add(workers)` per subscription, spawns goroutines that loop on a `msgs` channel (closed only on `ctx.Done()`), and then calls `wg.Wait()` — blocking until shutdown.

The caller (`cmd/cc-backend/main.go:346`) calls `Init()` synchronously and expects it to return so the HTTP server can start.

## Fix

**File:** `pkg/metricstore/metricstore.go` — wrap `ReceiveNats` call in `wg.Go()`:

```go
if Keys.Subscriptions != nil {
    wg.Go(func() {
        if err := ReceiveNats(ms, Keys.NumWorkers, ctx); err != nil {
            cclog.Fatal(err)
        }
    })
}
```

This runs `ReceiveNats` as a background goroutine tracked by the caller's WaitGroup, allowing `Init()` to return immediately.

## Verification

1. `go build ./...`
2. `go test ./pkg/metricstore/...`
3. Verify the app starts up and serves HTTP requests when NATS subscriptions are configured

If you need specific details from before exiting plan mode (like exact code snippets, error messages, or content you generated), read the full transcript at: /Users/jan/.claude/projects/-Users-jan-prg-ClusterCockpit-cc-backend/734f1e28-b805-471f-8f2c-275878888543.jsonl


   180→	// the shutdown() function when appropriate.
   181→	// Store the shutdown function for later use by Shutdown()
   182→	shutdownFuncMu.Lock()
   183→	shutdownFunc = shutdown
   184→	shutdownFuncMu.Unlock()
   185→
   186→	if Keys.Subscriptions != nil {
   187→		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
   188→		if err != nil {
   189→			cclog.Fatal(err)
   190→		}
   191→	}
   192→}
   193→
   194→// InitMetrics initializes the singleton MemoryStore instance with the given metric configurations.
   195→//
   196→// This function must be called before GetMemoryStore() and can only be called once due to
   197→// the singleton pattern. It assigns each metric an internal offset for efficient buffer indexing.
   198→//
   199→// Parameters:

	// the shutdown() function when appropriate.
	// Store the shutdown function for later use by Shutdown()
	shutdownFuncMu.Lock()
	shutdownFunc = shutdown
	shutdownFuncMu.Unlock()

	if Keys.Subscriptions != nil {
		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
		if err != nil {
			cclog.Fatal(err)
		}
	}
}

// InitMetrics initializes the singleton MemoryStore instance with the given metric configurations.
//
// This function must be called before GetMemoryStore() and can only be called once due to
// the singleton pattern. It assigns each metric an internal offset for efficient buffer indexing.
//
// Parameters:


Now let me check if there's a `wg` variable available in this context.


   100→	root         Level
   101→	nodeProvider NodeProvider
   102→}
   103→
   104→// Init initializes the metric store from configuration and starts background workers.
   105→//
   106→// This function must be called exactly once before any other metricstore operations.
   107→// It performs the following initialization steps:
   108→//  1. Validates and decodes the metric store configuration
   109→//  2. Configures worker pool size (defaults to NumCPU/2+1, max 10)
   110→//  3. Loads metric configurations from all registered clusters
   111→//  4. Restores checkpoints within the retention window
   112→//  5. Starts background workers for retention, checkpointing, archiving, and monitoring
   113→//  6. Optionally subscribes to NATS for real-time metric ingestion
   114→//
   115→// Parameters:
   116→//   - rawConfig: JSON configuration for the metric store (see MetricStoreConfig)
   117→//   - wg: WaitGroup that will be incremented for each background goroutine started
   118→//
   119→// The function will call cclog.Fatal on critical errors during initialization.
   120→// Use Shutdown() to cleanly stop all background workers started by Init().
   121→//
   122→// Note: Signal handling must be implemented by the caller. Call Shutdown() when
   123→// receiving termination signals to ensure checkpoint data is persisted.
   124→func Init(rawConfig json.RawMessage, metrics map[string]MetricConfig, wg *sync.WaitGroup) {
   125→	startupTime := time.Now()
   126→
   127→	if rawConfig != nil {
   128→		config.Validate(configSchema, rawConfig)
   129→		dec := json.NewDecoder(bytes.NewReader(rawConfig))
   130→		dec.DisallowUnknownFields()
   131→		if err := dec.Decode(&Keys); err != nil {
   132→			cclog.Abortf("[METRICSTORE]> Metric Store Config Init: Could not decode config file '%s'.\nError: %s\n", rawConfig, err.Error())
   133→		}
   134→	}
   135→
   136→	// Set NumWorkers from config or use default
   137→	if Keys.NumWorkers <= 0 {
   138→		Keys.NumWorkers = min(runtime.NumCPU()/2+1, DefaultMaxWorkers)
   139→	}
   140→	cclog.Debugf("[METRICSTORE]> Using %d workers for checkpoint/archive operations\n", Keys.NumWorkers)
   141→
   142→	// Pass the config.MetricStoreKeys
   143→	InitMetrics(metrics)
   144→
   145→	ms := GetMemoryStore()
   146→
   147→	d, err := time.ParseDuration(Keys.RetentionInMemory)
   148→	if err != nil {
   149→		cclog.Fatal(err)
   150→	}
   151→
   152→	restoreFrom := startupTime.Add(-d)
   153→	cclog.Infof("[METRICSTORE]> Loading checkpoints newer than %s\n", restoreFrom.Format(time.RFC3339))
   154→	files, err := ms.FromCheckpointFiles(Keys.Checkpoints.RootDir, restoreFrom.Unix())
   155→	loadedData := ms.SizeInBytes() / 1024 / 1024 // In MB
   156→	if err != nil {
   157→		cclog.Fatalf("[METRICSTORE]> Loading checkpoints failed: %s\n", err.Error())
   158→	} else {
   159→		cclog.Infof("[METRICSTORE]> Checkpoints loaded (%d files, %d MB, that took %fs)\n", files, loadedData, time.Since(startupTime).Seconds())
   160→	}
   161→
   162→	// Try to use less memory by forcing a GC run here and then
   163→	// lowering the target percentage. The default of 100 means
   164→	// that only once the ratio of new allocations execeds the
   165→	// previously active heap, a GC is triggered.
   166→	// Forcing a GC here will set the "previously active heap"
   167→	// to a minumum.
   168→	// runtime.GC()
   169→
   170→	ctx, shutdown := context.WithCancel(context.Background())
   171→
   172→	Retention(wg, ctx)
   173→	Checkpointing(wg, ctx)
   174→	CleanUp(wg, ctx)
   175→	WALStaging(wg, ctx)
   176→	MemoryUsageTracker(wg, ctx)
   177→
   178→	// Note: Signal handling has been removed from this function.
   179→	// The caller is responsible for handling shutdown signals and calling
   180→	// the shutdown() function when appropriate.
   181→	// Store the shutdown function for later use by Shutdown()
   182→	shutdownFuncMu.Lock()
   183→	shutdownFunc = shutdown
   184→	shutdownFuncMu.Unlock()
   185→
   186→	if Keys.Subscriptions != nil {
   187→		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
   188→		if err != nil {
   189→			cclog.Fatal(err)

	root         Level
	nodeProvider NodeProvider
}

// Init initializes the metric store from configuration and starts background workers.
//
// This function must be called exactly once before any other metricstore operations.
// It performs the following initialization steps:
//  1. Validates and decodes the metric store configuration
//  2. Configures worker pool size (defaults to NumCPU/2+1, max 10)
//  3. Loads metric configurations from all registered clusters
//  4. Restores checkpoints within the retention window
//  5. Starts background workers for retention, checkpointing, archiving, and monitoring
//  6. Optionally subscribes to NATS for real-time metric ingestion
//
// Parameters:
//   - rawConfig: JSON configuration for the metric store (see MetricStoreConfig)
//   - wg: WaitGroup that will be incremented for each background goroutine started
//
// The function will call cclog.Fatal on critical errors during initialization.
// Use Shutdown() to cleanly stop all background workers started by Init().
//
// Note: Signal handling must be implemented by the caller. Call Shutdown() when
// receiving termination signals to ensure checkpoint data is persisted.
func Init(rawConfig json.RawMessage, metrics map[string]MetricConfig, 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)

	// Pass the config.MetricStoreKeys
	InitMetrics(metrics)

	ms := GetMemoryStore()

	d, err := time.ParseDuration(Keys.RetentionInMemory)
	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())

	Retention(wg, ctx)
	Checkpointing(wg, ctx)
	CleanUp(wg, ctx)
	WALStaging(wg, ctx)
	MemoryUsageTracker(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()
	shutdownFuncMu.Lock()
	shutdownFunc = shutdown
	shutdownFuncMu.Unlock()

	if Keys.Subscriptions != nil {
		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
		if err != nil {
			cclog.Fatal(err)


The `wg` parameter is available. Let me also quickly check `ReceiveNats` signature to confirm it takes `ctx`.


pkg/metricstore/lineprotocol.go:45:func ReceiveNats(ms *MemoryStore,


Good. Now apply the fix:


Edit

	if Keys.Subscriptions != nil {
		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
		if err != nil {
			cclog.Fatal(err)
		}
	}

	if Keys.Subscriptions != nil {
		wg.Go(func() {
			if err := ReceiveNats(ms, Keys.NumWorkers, ctx); err != nil {
				cclog.Fatal(err)
			}
		})
	}


The file /Users/jan/prg/ClusterCockpit/cc-backend/pkg/metricstore/metricstore.go has been updated successfully.

	if Keys.Subscriptions != nil {
		wg.Go(func() {
			if err := ReceiveNats(ms, Keys.NumWorkers, ctx); err != nil {
				cclog.Fatal(err)
			}
		})
	}

// 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"
	"fmt"
	"runtime"
	"runtime/debug"
	"slices"
	"sync"
	"time"

	"github.com/ClusterCockpit/cc-backend/internal/config"
	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"
)

// GlobalState holds the global state for the metric store with thread-safe access.
type GlobalState struct {
	mu                sync.RWMutex
	lastRetentionTime int64
	selectorsExcluded bool
}

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
	shutdownFuncMu sync.Mutex // Protects shutdownFunc from concurrent access
	// Create a global instance
	state = &GlobalState{}
)

// NodeProvider provides information about nodes currently in use by running jobs.
//
// This interface allows metricstore to query job information without directly
// depending on the repository package, breaking the import cycle.
//
// Implementations should return nodes that are actively processing jobs started
// before the given timestamp. These nodes will be excluded from retention-based
// garbage collection to prevent data loss for jobs that are still running or
// recently completed.
type NodeProvider interface {
	// GetUsedNodes returns a map of cluster names to sorted lists of unique hostnames
	// that are currently in use by jobs that started before the given timestamp.
	//
	// Parameters:
	//   - ts: Unix timestamp threshold - returns nodes with jobs started before this time
	//
	// Returns:
	//   - Map of cluster names to lists of node hostnames that should be excluded from garbage collection
	//   - Error if the query fails
	GetUsedNodes(ts int64) (map[string][]string, error)
}

// Metric represents a single metric data point to be written to the store.
type Metric struct {
	Name  string
	Value schema.Float
	// MetricConfig contains frequency and aggregation settings for this metric.
	// If Frequency is 0, configuration will be looked up from MemoryStore.Metrics during Write().
	MetricConfig MetricConfig
}

// MemoryStore is the main in-memory time-series metric storage implementation.
//
// It organizes metrics in a hierarchical tree structure where each level represents
// a component of the system hierarchy (e.g., cluster → host → CPU). Each level can
// store multiple metrics as time-series buffers.
//
// The store is initialized as a singleton via InitMetrics() and accessed via GetMemoryStore().
// All public methods are safe for concurrent use.
type MemoryStore struct {
	Metrics      map[string]MetricConfig
	root         Level
	nodeProvider NodeProvider
}

// Init initializes the metric store from configuration and starts background workers.
//
// This function must be called exactly once before any other metricstore operations.
// It performs the following initialization steps:
//  1. Validates and decodes the metric store configuration
//  2. Configures worker pool size (defaults to NumCPU/2+1, max 10)
//  3. Loads metric configurations from all registered clusters
//  4. Restores checkpoints within the retention window
//  5. Starts background workers for retention, checkpointing, archiving, and monitoring
//  6. Optionally subscribes to NATS for real-time metric ingestion
//
// Parameters:
//   - rawConfig: JSON configuration for the metric store (see MetricStoreConfig)
//   - wg: WaitGroup that will be incremented for each background goroutine started
//
// The function will call cclog.Fatal on critical errors during initialization.
// Use Shutdown() to cleanly stop all background workers started by Init().
//
// Note: Signal handling must be implemented by the caller. Call Shutdown() when
// receiving termination signals to ensure checkpoint data is persisted.
func Init(rawConfig json.RawMessage, metrics map[string]MetricConfig, 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)

	// Pass the config.MetricStoreKeys
	InitMetrics(metrics)

	ms := GetMemoryStore()

	d, err := time.ParseDuration(Keys.RetentionInMemory)
	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())

	Retention(wg, ctx)
	Checkpointing(wg, ctx)
	CleanUp(wg, ctx)
	WALStaging(wg, ctx)
	MemoryUsageTracker(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()
	shutdownFuncMu.Lock()
	shutdownFunc = shutdown
	shutdownFuncMu.Unlock()

	if Keys.Subscriptions != nil {
		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
		if err != nil {
			cclog.Fatal(err)
		}
	}
}

// InitMetrics initializes the singleton MemoryStore instance with the given metric configurations.
//
// This function must be called before GetMemoryStore() and can only be called once due to
// the singleton pattern. It assigns each metric an internal offset for efficient buffer indexing.
//
// Parameters:
//   - metrics: Map of metric names to their configurations (frequency and aggregation strategy)
//
// Panics if any metric has Frequency == 0, which indicates an invalid configuration.
//
// After this call, the global msInstance is ready for use via GetMemoryStore().
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,
		}
	})
}

// GetMemoryStore returns the singleton MemoryStore instance.
//
// Returns the initialized MemoryStore singleton. Calls cclog.Fatal if InitMetrics() was not called first.
//
// This function is safe for concurrent use after initialization.
func GetMemoryStore() *MemoryStore {
	if msInstance == nil {
		cclog.Warnf("[METRICSTORE]> MemoryStore not initialized!")
	}

	return msInstance
}

func (ms *MemoryStore) GetMetricFrequency(metricName string) (int64, error) {
	if metric, ok := ms.Metrics[metricName]; ok {
		return metric.Frequency, nil
	}
	return 0, fmt.Errorf("[METRICSTORE]> metric %s not found", metricName)
}

// SetNodeProvider sets the NodeProvider implementation for the MemoryStore.
// This must be called during initialization to provide job state information
// for selective buffer retention during Free operations.
// If not set, the Free function will fall back to freeing all buffers.
func (ms *MemoryStore) SetNodeProvider(provider NodeProvider) {
	ms.nodeProvider = provider
}

// Shutdown performs a graceful shutdown of the metric store.
//
// This function cancels all background goroutines started by Init() and writes
// a final checkpoint to disk before returning. It should be called when the
// application receives a termination signal.
//
// The function will:
//  1. Cancel the context to stop all background workers
//  2. Close the WAL messages channel if using WAL format
//  3. Write a final checkpoint to preserve in-memory data
//  4. Log any errors encountered during shutdown
//
// Note: This function blocks until the final checkpoint is written.
func Shutdown() {
	shutdownFuncMu.Lock()
	defer shutdownFuncMu.Unlock()
	if shutdownFunc != nil {
		shutdownFunc()
	}

	if Keys.Checkpoints.FileFormat == "wal" {
		for _, ch := range walShardChs {
			close(ch)
		}
	}

	cclog.Infof("[METRICSTORE]> Writing to '%s'...\n", Keys.Checkpoints.RootDir)
	var files int
	var err error

	ms := GetMemoryStore()

	lastCheckpointMu.Lock()
	from := lastCheckpoint
	lastCheckpointMu.Unlock()

	if Keys.Checkpoints.FileFormat == "wal" {
		var hostDirs []string
		files, hostDirs, err = ms.ToCheckpointWAL(Keys.Checkpoints.RootDir, from.Unix(), time.Now().Unix())
		if err == nil {
			RotateWALFilesAfterShutdown(hostDirs)
		}
	} else {
		files, err = ms.ToCheckpoint(Keys.Checkpoints.RootDir, from.Unix(), time.Now().Unix())
	}

	if err != nil {
		cclog.Errorf("[METRICSTORE]> Writing checkpoint failed: %s\n", err.Error())
	}
	cclog.Infof("[METRICSTORE]> Done! (%d files written)\n", files)
}

// Retention starts a background goroutine that periodically frees old metric data.
//
// This worker runs at half the retention interval and calls Free() to remove buffers
// older than the configured retention time. It respects the NodeProvider to preserve
// data for nodes with active jobs.
//
// Parameters:
//   - wg: WaitGroup to signal completion when context is cancelled
//   - ctx: Context for cancellation signal
//
// The goroutine exits when ctx is cancelled.
func Retention(wg *sync.WaitGroup, ctx context.Context) {
	ms := GetMemoryStore()

	wg.Go(func() {
		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:
				state.mu.Lock()

				t := time.Now().Add(-d)

				state.lastRetentionTime = t.Unix()

				cclog.Infof("[METRICSTORE]> start freeing buffers (older than %s)...\n", t.Format(time.RFC3339))

				freed, err := Free(ms, t)
				if err != nil {
					cclog.Errorf("[METRICSTORE]> freeing up buffers failed: %s\n", err.Error())
				} else {
					cclog.Infof("[METRICSTORE]> done: %d buffers freed\n", freed)
				}

				state.mu.Unlock()

				// Clean up the buffer pool
				bufferPool.Clean(state.lastRetentionTime)
			}
		}
	})
}

// MemoryUsageTracker starts a background goroutine that monitors memory usage.
//
// This worker checks actual process memory usage (via runtime.MemStats) periodically
// and force-frees buffers if memory exceeds the configured cap. It uses FreeOSMemory()
// to return memory to the OS after freeing buffers, avoiding aggressive GC that causes
// performance issues.
//
// The tracker logs both actual memory usage (heap allocated) and metric data size for
// visibility into memory overhead from Go runtime structures and allocations.
//
// Parameters:
//   - wg: WaitGroup to signal completion when context is cancelled
//   - ctx: Context for cancellation signal
//
// The goroutine exits when ctx is cancelled.
func MemoryUsageTracker(wg *sync.WaitGroup, ctx context.Context) {
	ms := GetMemoryStore()

	wg.Go(func() {
		normalInterval := DefaultMemoryUsageTrackerInterval
		fastInterval := 30 * time.Second

		if normalInterval <= 0 {
			return
		}

		ticker := time.NewTicker(normalInterval)
		defer ticker.Stop()
		currentInterval := normalInterval

		for {
			select {
			case <-ctx.Done():
				return
			case <-ticker.C:
				var mem runtime.MemStats
				runtime.ReadMemStats(&mem)
				actualMemoryGB := float64(mem.Alloc) / 1e9
				metricDataGB := ms.SizeInGB()
				cclog.Infof("[METRICSTORE]> memory usage: %.2f GB actual (%.2f GB metric data)", actualMemoryGB, metricDataGB)

				freedExcluded := 0
				freedEmergency := 0
				var err error

				state.mu.RLock()
				lastRetention := state.lastRetentionTime
				selectorsExcluded := state.selectorsExcluded
				state.mu.RUnlock()

				if lastRetention != 0 && selectorsExcluded {
					freedExcluded, err = ms.Free(nil, lastRetention)
					if err != nil {
						cclog.Errorf("[METRICSTORE]> error while force-freeing the excluded buffers: %s", err)
					}

					if freedExcluded > 0 {
						debug.FreeOSMemory()
						cclog.Infof("[METRICSTORE]> done: %d excluded buffers force-freed", freedExcluded)
					}
				}

				runtime.ReadMemStats(&mem)
				actualMemoryGB = float64(mem.Alloc) / 1e9

				if actualMemoryGB > float64(Keys.MemoryCap) {
					cclog.Warnf("[METRICSTORE]> memory usage %.2f GB exceeds cap %d GB, starting emergency buffer freeing", actualMemoryGB, Keys.MemoryCap)

					// Use progressive time-based Free with increasing threshold
					// instead of ForceFree loop — fewer tree traversals, more effective
					d, parseErr := time.ParseDuration(Keys.RetentionInMemory)
					if parseErr != nil {
						cclog.Errorf("[METRICSTORE]> cannot parse retention duration: %s", parseErr)
					} else {
						thresholds := []float64{0.75, 0.5, 0.25}
						for _, fraction := range thresholds {
							threshold := time.Now().Add(-time.Duration(float64(d) * fraction))
							freed, freeErr := ms.Free(nil, threshold.Unix())
							if freeErr != nil {
								cclog.Errorf("[METRICSTORE]> error while freeing buffers at %.0f%% retention: %s", fraction*100, freeErr)
							}
							freedEmergency += freed

							bufferPool.Clear()
							runtime.GC()
							runtime.ReadMemStats(&mem)
							actualMemoryGB = float64(mem.Alloc) / 1e9

							if actualMemoryGB < float64(Keys.MemoryCap) {
								break
							}
						}
					}

					bufferPool.Clear()
					debug.FreeOSMemory()

					runtime.ReadMemStats(&mem)
					actualMemoryGB = float64(mem.Alloc) / 1e9

					if actualMemoryGB >= float64(Keys.MemoryCap) {
						cclog.Errorf("[METRICSTORE]> after emergency frees (%d buffers), memory usage %.2f GB still at/above cap %d GB", freedEmergency, actualMemoryGB, Keys.MemoryCap)
					} else {
						cclog.Infof("[METRICSTORE]> emergency freeing complete: %d buffers freed, memory now %.2f GB", freedEmergency, actualMemoryGB)
					}
				}

				// Adaptive ticker: check more frequently when memory is high
				memoryRatio := actualMemoryGB / float64(Keys.MemoryCap)
				if memoryRatio > 0.8 && currentInterval != fastInterval {
					ticker.Reset(fastInterval)
					currentInterval = fastInterval
					cclog.Infof("[METRICSTORE]> memory at %.0f%% of cap, switching to fast check interval (30s)", memoryRatio*100)
				} else if memoryRatio <= 0.8 && currentInterval != normalInterval {
					ticker.Reset(normalInterval)
					currentInterval = normalInterval
					cclog.Infof("[METRICSTORE]> memory at %.0f%% of cap, switching to normal check interval", memoryRatio*100)
				}
			}
		}
	})
}

// Free removes metric data older than the given time while preserving data for active nodes.
//
// This function implements intelligent retention by consulting the NodeProvider (if configured)
// to determine which nodes are currently in use by running jobs. Data for these nodes is
// preserved even if older than the retention time.
//
// Parameters:
//   - ms: The MemoryStore instance
//   - t: Time threshold - buffers with data older than this will be freed
//
// Returns:
//   - Number of buffers freed
//   - Error if NodeProvider query fails
//
// Behavior:
//   - If no NodeProvider is set: frees all buffers older than t
//   - If NodeProvider returns empty map: frees all buffers older than t
//   - Otherwise: preserves buffers for nodes returned by GetUsedNodes(), frees others
func Free(ms *MemoryStore, t time.Time) (int, error) {
	// If no NodeProvider is configured, free all buffers older than t
	if ms.nodeProvider == nil {
		return ms.Free(nil, t.Unix())
	}

	excludeSelectors, err := ms.nodeProvider.GetUsedNodes(t.Unix())
	if err != nil {
		return 0, err
	}

	switch lenMap := len(excludeSelectors); lenMap {

	// If the length of the map returned by GetUsedNodes() is 0,
	// then use default Free method with nil selector
	case 0:
		state.selectorsExcluded = false
		return ms.Free(nil, t.Unix())

	// Else formulate selectors, exclude those from the map
	// and free the rest of the selectors
	default:
		state.selectorsExcluded = true
		selectors := GetSelectors(ms, excludeSelectors)
		return FreeSelected(ms, selectors, t)
	}
}

// FreeSelected frees buffers for specific selectors while preserving others.
//
// This function is used when we want to retain some specific nodes beyond the retention time.
// It iterates through the provided selectors and frees their associated buffers.
//
// Parameters:
//   - ms: The MemoryStore instance
//   - selectors: List of selector paths to free (e.g., [["cluster1", "node1"], ["cluster2", "node2"]])
//   - t: Time threshold for freeing buffers
//
// Returns the total number of buffers freed and any error encountered.
func FreeSelected(ms *MemoryStore, selectors [][]string, t time.Time) (int, error) {
	freed := 0

	for _, selector := range selectors {

		freedBuffers, err := ms.Free(selector, t.Unix())
		if err != nil {
			cclog.Errorf("error while freeing selected buffers: %#v", err)
		}
		freed += freedBuffers

	}

	return freed, nil
}

// GetSelectors returns all selectors at depth 2 (cluster/node level) that are NOT in the exclusion map.
//
// This function generates a list of selectors whose buffers should be freed by excluding
// selectors that correspond to nodes currently in use by running jobs.
//
// Parameters:
//   - ms: The MemoryStore instance
//   - excludeSelectors: Map of cluster names to node hostnames that should NOT be freed
//
// Returns a list of selectors ([]string paths) that can be safely freed.
//
// Example:
//
//	If the tree has paths ["emmy", "node001"] and ["emmy", "node002"],
//	and excludeSelectors contains {"emmy": ["node001"]},
//	then only [["emmy", "node002"]] is returned.
func GetSelectors(ms *MemoryStore, excludeSelectors map[string][]string) [][]string {
	allSelectors := ms.GetPaths(2)

	filteredSelectors := make([][]string, 0, len(allSelectors))

	for _, path := range allSelectors {
		if len(path) < 2 {
			continue
		}

		key := path[0]   // The "Key" (Level 1)
		value := path[1] // The "Value" (Level 2)

		exclude := false

		// Check if the key exists in our exclusion map
		if excludedValues, exists := excludeSelectors[key]; exists {
			// The key exists, now check if the specific value is in the exclusion list
			if slices.Contains(excludedValues, value) {
				exclude = true
			}
		}

		if !exclude {
			filteredSelectors = append(filteredSelectors, path)
		}
	}

	return filteredSelectors
}

// GetPaths returns a list of lists (paths) to the specified depth.
func (ms *MemoryStore) GetPaths(targetDepth int) [][]string {
	var results [][]string

	// Start recursion. Initial path is empty.
	// We treat Root as depth 0.
	ms.root.collectPaths(0, targetDepth, []string{}, &results)

	return results
}

// 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 {
				cclog.Debugf("[METRICSTORE]> Unknown metric '%s' in Write() - skipping", metric.Name)
				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, ErrNoHostOrMetric
	} 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)
}

// ForceFree unconditionally removes the oldest buffer from each metric chain.
func (m *MemoryStore) ForceFree() (int, error) {
	return m.GetLevel(nil).forceFree()
}

func (m *MemoryStore) FreeAll() error {
	m.root.lock.Lock()
	defer m.root.lock.Unlock()

	for k := range m.root.children {
		delete(m.root.children, k)
	}

	return nil
}

func (m *MemoryStore) SizeInBytes() int64 {
	return m.root.sizeInBytes()
}

func (m *MemoryStore) SizeInGB() float64 {
	return float64(m.root.sizeInBytes()) / 1e9
}

// 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
}

-    err = ReceiveNats(ms, Keys.NumWorkers, ctx)

+      if err := ReceiveNats(ms, Keys.NumWorkers, ctx); err != nil {


Wait — `wg` is a `*sync.WaitGroup`, which doesn't have a `.Go()` method. Let me check if this is a standard `sync.WaitGroup` or a custom one.


pkg/metricstore/metricstore.go:187:		wg.Go(func() {
pkg/metricstore/metricstore.go:326:	wg.Go(func() {
pkg/metricstore/metricstore.go:389:	wg.Go(func() {
pkg/metricstore/walCheckpoint.go:167:		wg.Go(func() {
pkg/metricstore/checkpoint.go:101:	wg.Go(func() {
pkg/metricstore/archive.go:45:	wg.Go(func() {


`wg.Go()` is already used throughout the codebase — it's not a standard `sync.WaitGroup` but a custom or extended one. The fix is correct. Let me verify it builds and tests pass.


Bash

Build all packages


(Bash completed with no output)


Run metricstore tests


ok  	github.com/ClusterCockpit/cc-backend/pkg/metricstore	(cached)


Build and tests pass. The fix wraps the blocking `ReceiveNats` call in `wg.Go()`, which runs it as a background goroutine tracked by the caller's WaitGroup. This allows `Init()` to return immediately so the HTTP server can start, while NATS subscriptions are processed in the background.