   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)
   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:
   200→//   - metrics: Map of metric names to their configurations (frequency and aggregation strategy)
   201→//
   202→// Panics if any metric has Frequency == 0, which indicates an invalid configuration.
   203→//
   204→// After this call, the global msInstance is ready for use via GetMemoryStore().
   205→func InitMetrics(metrics map[string]MetricConfig) {
   206→	singleton.Do(func() {
   207→		offset := 0
   208→		for key, cfg := range metrics {
   209→			if cfg.Frequency == 0 {
   210→				panic("[METRICSTORE]> invalid frequency")
   211→			}
   212→
   213→			metrics[key] = MetricConfig{
   214→				Frequency:   cfg.Frequency,
   215→				Aggregation: cfg.Aggregation,
   216→				offset:      offset,
   217→			}
   218→			offset += 1
   219→		}
   220→
   221→		msInstance = &MemoryStore{
   222→			root: Level{
   223→				metrics:  make([]*buffer, len(metrics)),
   224→				children: make(map[string]*Level),
   225→			},
   226→			Metrics: metrics,
   227→		}
   228→	})
   229→}
   230→
   231→// GetMemoryStore returns the singleton MemoryStore instance.
   232→//
   233→// Returns the initialized MemoryStore singleton. Calls cclog.Fatal if InitMetrics() was not called first.
   234→//
   235→// This function is safe for concurrent use after initialization.
   236→func GetMemoryStore() *MemoryStore {
   237→	if msInstance == nil {
   238→		cclog.Warnf("[METRICSTORE]> MemoryStore not initialized!")
   239→	}
   240→
   241→	return msInstance
   242→}
   243→
   244→func (ms *MemoryStore) GetMetricFrequency(metricName string) (int64, error) {
   245→	if metric, ok := ms.Metrics[metricName]; ok {
   246→		return metric.Frequency, nil
   247→	}
   248→	return 0, fmt.Errorf("[METRICSTORE]> metric %s not found", metricName)
   249→}
   250→
   251→// SetNodeProvider sets the NodeProvider implementation for the MemoryStore.
   252→// This must be called during initialization to provide job state information
   253→// for selective buffer retention during Free operations.
   254→// If not set, the Free function will fall back to freeing all buffers.
   255→func (ms *MemoryStore) SetNodeProvider(provider NodeProvider) {
   256→	ms.nodeProvider = provider
   257→}
   258→
   259→// Shutdown performs a graceful shutdown of the metric store.
   260→//
   261→// This function cancels all background goroutines started by Init() and writes
   262→// a final checkpoint to disk before returning. It should be called when the
   263→// application receives a termination signal.
   264→//
   265→// The function will:
   266→//  1. Cancel the context to stop all background workers
   267→//  2. Close the WAL messages channel if using WAL format
   268→//  3. Write a final checkpoint to preserve in-memory data
   269→//  4. Log any errors encountered during shutdown
   270→//
   271→// Note: This function blocks until the final checkpoint is written.
   272→func Shutdown() {
   273→	shutdownFuncMu.Lock()
   274→	defer shutdownFuncMu.Unlock()
   275→	if shutdownFunc != nil {
   276→		shutdownFunc()
   277→	}
   278→
   279→	if Keys.Checkpoints.FileFormat == "wal" {
   280→		for _, ch := range walShardChs {
   281→			close(ch)
   282→		}
   283→	}
   284→
   285→	cclog.Infof("[METRICSTORE]> Writing to '%s'...\n", Keys.Checkpoints.RootDir)
   286→	var files int
   287→	var err error
   288→
   289→	ms := GetMemoryStore()
   290→
   291→	lastCheckpointMu.Lock()
   292→	from := lastCheckpoint
   293→	lastCheckpointMu.Unlock()
   294→
   295→	if Keys.Checkpoints.FileFormat == "wal" {
   296→		var hostDirs []string
   297→		files, hostDirs, err = ms.ToCheckpointWAL(Keys.Checkpoints.RootDir, from.Unix(), time.Now().Unix())
   298→		if err == nil {
   299→			RotateWALFilesAfterShutdown(hostDirs)
   300→		}
   301→	} else {
   302→		files, err = ms.ToCheckpoint(Keys.Checkpoints.RootDir, from.Unix(), time.Now().Unix())
   303→	}
   304→
   305→	if err != nil {
   306→		cclog.Errorf("[METRICSTORE]> Writing checkpoint failed: %s\n", err.Error())
   307→	}
   308→	cclog.Infof("[METRICSTORE]> Done! (%d files written)\n", files)
   309→}
   310→
   311→// Retention starts a background goroutine that periodically frees old metric data.
   312→//
   313→// This worker runs at half the retention interval and calls Free() to remove buffers
   314→// older than the configured retention time. It respects the NodeProvider to preserve
   315→// data for nodes with active jobs.
   316→//
   317→// Parameters:
   318→//   - wg: WaitGroup to signal completion when context is cancelled
   319→//   - ctx: Context for cancellation signal
   320→//
   321→// The goroutine exits when ctx is cancelled.
   322→func Retention(wg *sync.WaitGroup, ctx context.Context) {
   323→	ms := GetMemoryStore()
   324→
   325→	wg.Go(func() {
   326→		d, err := time.ParseDuration(Keys.RetentionInMemory)
   327→		if err != nil {
   328→			cclog.Fatal(err)
   329→		}
   330→		if d <= 0 {
   331→			return
   332→		}
   333→
   334→		tickInterval := d / 2
   335→		if tickInterval <= 0 {
   336→			return
   337→		}
   338→		ticker := time.NewTicker(tickInterval)
   339→		defer ticker.Stop()
   340→
   341→		for {
   342→			select {
   343→			case <-ctx.Done():
   344→				return
   345→			case <-ticker.C:
   346→				state.mu.Lock()
   347→
   348→				t := time.Now().Add(-d)
   349→


Bash

git log -p --all --reverse -- pkg/metricstore/lineprotocol.go | grep -A 30 "func ReceiveNats" | head -60


+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() {
+					defer wg.Done()
+					for m := range msgs {
+						dec := lineprotocol.NewDecoderWithBytes(m)
+						if err := DecodeLine(dec, ms, clusterTag); err != nil {
+							cclog.Errorf("error: %s", err.Error())
+						}
+					}
+				}()
+			}
+
--
 func ReceiveNats(ms *MemoryStore,

commit 4c3cd8e66a7b7367afc5bc2ae154af4d0649b35c
Merge: ca0f9a42 6ecb9349
Author: Jan Eitzinger <jan@moebiusband.org>
Date:   Fri Feb 27 09:30:32 2026 +0100

    Merge branch 'dev' into optimize-checkpoint-wal

commit 23ce1722a9fe43612e53331bb855bdaf2de55b31
Merge: 9c0104a2 6ecb9349
Author: Jan Eitzinger <jan@moebiusband.org>
Date:   Fri Feb 27 09:31:50 2026 +0100

    Merge branch 'dev' into optimize-checkpoint-loading

commit a1db8263d72b9727347ea69e0cc832ec67bd1235
Author: Jan Eitzinger <jan@moebiusband.org>
Date:   Fri Feb 27 12:30:27 2026 +0100

    Document line protocol. Optimize REST writeMetric path

diff --git a/pkg/metricstore/lineprotocol.go b/pkg/metricstore/lineprotocol.go
index f8c83e31..ecae3df1 100644
--- a/pkg/metricstore/lineprotocol.go
+++ b/pkg/metricstore/lineprotocol.go
@@ -3,9 +3,23 @@
 // Use of this source code is governed by a MIT-style


git log --all --oneline -- pkg/metricstore/lineprotocol.go | tail -5


ca0f9a42 Introduce metric store binary checkpoints with write ahead log
9c0104a2 Attempt to minimize heap allocations by using caching
9fc1836c Replace avro checkpoints with native binary format
499b4287 Switch to cclib nats client
f4130103 Move metricstore from internal to pkg


git show ca0f9a42:pkg/metricstore/lineprotocol.go 2>/dev/null | grep -A 50 "func ReceiveNats"


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() {
					defer wg.Done()
					for m := range msgs {
						dec := lineprotocol.NewDecoderWithBytes(m)
						if err := DecodeLine(dec, ms, clusterTag); err != nil {
							cclog.Errorf("error: %s", err.Error())
						}
					}
				}()
			}

			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
				select {
				case msgs <- data:
				case <-ctx.Done():
				}
			})
		} 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)
	}

	go func() {
		<-ctx.Done()
		close(msgs)


git show ca0f9a42:pkg/metricstore/lineprotocol.go 2>/dev/null | grep -A 60 "func ReceiveNats" | tail -15


	}

	go func() {
		<-ctx.Done()
		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 {


git show ca0f9a42:pkg/metricstore/metricstore.go | grep -A 30 "if Keys.Subscriptions"


	if Keys.Subscriptions != nil {
		err = ReceiveNats(ms, 1, 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,


Perfect! Now I have all the information I need. Let me provide a comprehensive analysis to the user.

## Analysis: `metricstore.Init()` Blocking via `ReceiveNats`

### Summary

I've found a **critical blocking bug** introduced by recent commits. `metricstore.Init()` now blocks indefinitely when NATS subscriptions are configured, preventing the server from fully starting up.

---

## Detailed Findings

### 1. **Startup Flow: How `Init()` is Called**

**Location**: `/Users/jan/prg/ClusterCockpit/cc-backend/cmd/cc-backend/main.go`, lines 344-346

```go
mscfg := ccconf.GetPackageConfig("metric-store")
if mscfg != nil {
    metrics := metricstore.BuildMetricList()
    metricstore.Init(mscfg, metrics, &wg)  // ← Called SYNCHRONOUSLY on main thread
```

The caller (`runServer()`) expects `Init()` to return so execution can continue. Instead:

**Location**: `/Users/jan/prg/ClusterCockpit/cc-backend/pkg/metricstore/metricstore.go`, lines 186-191

```go
if Keys.Subscriptions != nil {
    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← This BLOCKS!
    if err != nil {
        cclog.Fatal(err)
    }
}
```

`ReceiveNats()` is called **synchronously** and blocks `Init()` from returning.

---

### 2. **Why `ReceiveNats` Blocks: `wg.Wait()` at Line 98**

**Location**: `/Users/jan/prg/ClusterCockpit/cc-backend/pkg/metricstore/lineprotocol.go`, lines 45-101

```go
func ReceiveNats(ms *MemoryStore, workers int, ctx context.Context) error {
    nc := nats.GetClient()
    if nc == nil {
        return nil  // Early exit if no NATS client
    }

    var wg sync.WaitGroup    // ← Local WaitGroup
    msgs := make(chan []byte, max(workers*256, 8192))

    for _, sc := range *Keys.Subscriptions {
        clusterTag := sc.ClusterTag
        if workers > 1 {
            wg.Add(workers)  // ← Register N worker goroutines
            for range workers {
                go func() {
                    defer wg.Done()
                    for m := range msgs {      // ← Waits for messages forever
                        dec := lineprotocol.NewDecoderWithBytes(m)
                        if err := DecodeLine(dec, ms, clusterTag); err != nil {
                            cclog.Errorf("error: %s", err.Error())
                        }
                    }
                }()
            }

            nc.Subscribe(sc.SubscribeTo, ...)
        } else {
            // Single worker: decodes inline (no channel)
            nc.Subscribe(sc.SubscribeTo, ...)
        }
        cclog.Infof("NATS subscription to '%s' established", sc.SubscribeTo)
    }

    go func() {
        <-ctx.Done()      // ← Waits for context cancellation
        close(msgs)       // ← Closes channel to unblock workers
    }()

    wg.Wait()            // ← LINE 98: BLOCKS HERE until all workers finish
    return nil
}
```

**The blocking flow**:
1. `ReceiveNats()` creates `N` worker goroutines (where N = `Keys.NumWorkers`)
2. These workers loop on the `msgs` channel, waiting for messages forever
3. The loop only exits when `msgs` is closed
4. `msgs` is only closed when `ctx` is cancelled
5. `ctx` is only cancelled during shutdown
6. **Result**: `wg.Wait()` blocks forever until shutdown signal

---

### 3. **This Blocking Was Always There: The Bug is Not Recent**

Commit history shows `wg.Wait()` was **part of the original implementation** when `ReceiveNats` was created:

- **Commit `ca0f9a42`** (2026-02-14, "Introduce metric store binary checkpoints with write ahead log"): First instance of `ReceiveNats` already has `wg.Wait()` at the end
- **All subsequent commits** preserve this blocking behavior
- **Commit `bf1a8a17`** (2026-03-09, "fix: Shard WAL consumer for higher throughput"): Still has `wg.Wait()`
- **Commit `50aed595`** (2026-03-11, "fix: metricstore NATS contention"): Current HEAD, still has `wg.Wait()`

The code comment even documents this blocking behavior (lines 42-44):
```go
// The function blocks until ctx is cancelled and all worker goroutines have
// finished. It returns nil when the NATS client is not configured; callers
// should treat that as a no-op rather than an error.
```

---

### 4. **Critical Bug: `ReceiveNats` Should Run in a Goroutine**

The root cause is clear: **`ReceiveNats()` is designed to block indefinitely** (until context cancellation), but **it's being called synchronously from `Init()`** instead of in a background goroutine.

**Expected behavior**:
```go
// In metricstore.go Init(), lines 186-191
if Keys.Subscriptions != nil {
    wg.Go(func() {                               // ← Should be in a goroutine!
        err := ReceiveNats(ms, Keys.NumWorkers, ctx)
        if err != nil {
            cclog.Errorf("ReceiveNats error: %s", err.Error())
        }
    })
    // Remove: err = ReceiveNats(...)
    // Remove: if err != nil { ... }
}
```

**Current (buggy) behavior**:
```go
if Keys.Subscriptions != nil {
    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← Blocks Init() here!
    if err != nil {
        cclog.Fatal(err)
    }
}
```

---

### 5. **WALStaging Interaction: Not the Root Cause**

**Location**: `/Users/jan/prg/ClusterCockpit/cc-backend/pkg/metricstore/walCheckpoint.go`, lines 147-177

The `WALStaging()` function correctly handles both cases:
- **For `"json"` format** (line 148-150): Returns immediately without calling `wg.Go()`
  ```go
  if Keys.Checkpoints.FileFormat == "json" {
      return  // ← No goroutine added to wg
  }
  ```

- **For `"wal"` format** (line 167): Calls `wg.Go()` once per shard
  ```go
  wg.Go(func() {  // ← Adds N goroutines to caller's wg
      // WAL staging loop
  })
  ```

**This is correct design** — WAL goroutines are properly tracked by the caller's WaitGroup. The issue is not with WALStaging but with ReceiveNats being called synchronously.

---

## Root Cause Summary

| Aspect | Finding |
|--------|---------|
| **Where `Init()` is called** | `/cmd/cc-backend/main.go` line 346 — **synchronously** |
| **What blocks Init** | `ReceiveNats()` at line 187 of `metricstore.go` |
| **Why ReceiveNats blocks** | Calls `wg.Wait()` (line 98 of `lineprotocol.go`) waiting for worker goroutines |
| **Why workers never finish** | They loop on `msgs` channel until `ctx.Done()`, which only happens at shutdown |
| **When this was introduced** | Commit `ca0f9a42` (2026-02-14) — the function design always included blocking |
| **Is it a WaitGroup interference?** | No — `ReceiveNats` uses its own local `wg`, not the caller's. The caller's `wg` is never incremented for NATS workers |
| **Is WALStaging involved?** | No — WALStaging correctly returns early for JSON format; for WAL format it properly calls `wg.Go()` |

---

## Fix Required

Change line 186-191 in `/Users/jan/prg/ClusterCockpit/cc-backend/pkg/metricstore/metricstore.go`:

```go
// BEFORE (buggy):
if Keys.Subscriptions != nil {
    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← Blocks!
    if err != nil {
        cclog.Fatal(err)
    }
}

// AFTER (fixed):
if Keys.Subscriptions != nil {
    wg.Go(func() {
        err := ReceiveNats(ms, Keys.NumWorkers, ctx)
        if err != nil {
            cclog.Fatalf("ReceiveNats error: %s", err.Error())
        }
    })
}
```

This way, `ReceiveNats` runs as a background goroutine and doesn't block server startup.

agentId: ac3d6ab97cbca2a87 (use SendMessage with to: 'ac3d6ab97cbca2a87' to continue this agent)
<usage>total_tokens: 55309
tool_uses: 18
duration_ms: 38966</usage>

Prompt:
I need to understand how `metricstore.Init()` is called and whether `ReceiveNats` blocking is expected behavior or a new bug.

1. Find where `metricstore.Init()` is called (likely in `cmd/cc-backend/` or similar). Read that code to understand the startup flow — does the caller expect Init to return, or is it run in a goroutine?

2. Look at the git history for `ReceiveNats` — specifically check if it was recently changed to block (with `wg.Wait()`), or if it always blocked. Run `git log --oneline -20 pkg/metricstore/lineprotocol.go` and `git log --oneline -20 pkg/metricstore/metricstore.go` and read recent diffs.

3. Check the current WALStaging implementation in walCheckpoint.go — the function now returns early for "json" format without adding anything to wg, and for "wal" format it calls wg.Go() N times. Verify whether the WaitGroup used by Init's caller could be affected by these extra goroutines.

4. Specifically look at whether `ReceiveNats`'s local `wg.Wait()` on line 98 was always there or was recently added, and whether it could interact with the Init's WaitGroup parameter somehow.


The investigation found the root cause. `ReceiveNats` blocks by design (`wg.Wait()` on line 98 waits for context cancellation), but it's called **synchronously** from `Init()` — so `Init()` never returns and the server never finishes starting up.

The fix is simple: run `ReceiveNats` in a background goroutine via `wg.Go()`.


Edit

# Fix WAL Channel Saturation (3M+ messages dropped)

## Context

After the initial slow-consumer fixes (multi-worker decode, non-blocking WAL send, 65K channel buffer, 256-batch drain), the WAL channel is still permanently full. 3M+ messages are being dropped — the single `WALStaging` goroutine fundamentally cannot keep up with `NumWorkers` concurrent decode goroutines producing WAL messages.

## Root Cause

The single `WALStaging` goroutine is the bottleneck. For every message it must:
1. `path.Join()` to build hostDir string — **allocates**
2. Map lookup in `hostFiles`
3. `buildWALPayload()` — **allocates a new `[]byte`** per message
4. `writeWALRecord()` — **allocates another `[]byte`** (record = magic+len+payload+crc)
5. `bufio.Write()` — copies into buffer, occasionally flushes to disk

With `NumWorkers` producers (e.g., 5-10 goroutines) each decoding batches of metrics, the single consumer cannot match. The 65K channel buffer fills in seconds, then every message is dropped.

## Plan: Sharded WAL Writers

Replace the single `WALStaging` goroutine with **N sharded writer goroutines**, each owning a subset of hosts. Messages are routed to shards by hashing `cluster+node`. This:
- Parallelizes serialization and I/O across goroutines
- Keeps per-host writes serial (required for correct WAL files)
- Eliminates the single-goroutine bottleneck entirely

Additionally, eliminate per-message heap allocations in `buildWALPayload`/`writeWALRecord` by encoding directly into the `bufio.Writer`.

### Change 1: Sharded WAL writer goroutines
**File:** `pkg/metricstore/walCheckpoint.go`

Replace the single `WALMessages` channel + `WALStaging()` goroutine with:

```go
const walShards = 16  // or NumWorkers

// Per-shard channel + goroutine
var walShardChs []chan *WALMessage

func walShard(msg *WALMessage) int {
    // FNV-1a hash of cluster+node, mod shard count
    h := fnv.New32a()
    h.Write([]byte(msg.Cluster))
    h.Write([]byte{0})
    h.Write([]byte(msg.Node))
    return int(h.Sum32()) % len(walShardChs)
}
```

Each shard goroutine:
- Owns its own `hostFiles` map (no contention)
- Drains its channel with the existing batch-drain pattern
- Handles rotate requests for its hosts

**Shard count:** Use `NumWorkers` (already configured, sensible default).

**Channel buffer per shard:** `65536 / numShards` — same total memory, but each shard only buffers its hosts.

### Change 2: Eliminate per-message allocations
**File:** `pkg/metricstore/walCheckpoint.go`

Replace `buildWALPayload()` → `writeWALRecord()` (2 allocations per message) with `writeWALRecordDirect(w *bufio.Writer, msg *WALMessage)` that encodes directly into the `bufio.Writer` using a small stack-allocated scratch buffer:

```go
func writeWALRecordDirect(w *bufio.Writer, msg *WALMessage) error {
    // Build payload into a pooled or stack buffer, compute CRC inline,
    // write magic+len, then payload, then CRC — all directly to w.
    // Zero heap allocations in the hot path.
    var scratch [128]byte  // stack-allocated, covers most records
    // ... encode payload into scratch or a pooled []byte if >128 ...
}
```

Keep the old `writeWALRecord`/`buildWALPayload` for use in tests or non-hot paths, but the shard goroutines use the direct version.

### Change 3: Route WAL sends to shards from DecodeLine
**File:** `pkg/metricstore/lineprotocol.go:355-372`

Replace:
```go
select {
case WALMessages <- msg:
default:
    // dropped
}
```
With:
```go
shard := walShard(msg)
select {
case walShardChs[shard] <- msg:
default:
    // dropped
}
```

### Change 4: Shard-aware rotation
**File:** `pkg/metricstore/walCheckpoint.go`

`RotateWALFiles` must send rotate requests to the correct shard. Each shard has its own `walRotateCh`. The checkpoint goroutine hashes the hostDir to find the right shard and sends the rotate request there.

### Change 5: Shutdown
**File:** `pkg/metricstore/metricstore.go` (Shutdown function)

Close all shard channels instead of the single `WALMessages` channel. Each shard goroutine drains and flushes on close.

## Files to Modify

- `pkg/metricstore/walCheckpoint.go` — sharded goroutines, direct record writing, shard-aware rotation
- `pkg/metricstore/lineprotocol.go` — route WAL sends to shard channels
- `pkg/metricstore/metricstore.go` — `WALStaging()` call updated, `Shutdown()` closes shard channels

## Verification

1. `go build ./...`
2. `go test ./pkg/metricstore/...`
3. Deploy and verify:
   - WAL drop count stays at 0 (or near-zero during extreme bursts)
   - WAL files are correct (load a checkpoint, verify data)
   - No "WAL channel full" log messages
   - Memory usage roughly same (total channel buffer size unchanged)

# Fix: ReceiveNats blocks Init() during startup

## Context

After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

## Root Cause

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

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

`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

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


The file /Users/jan/.claude/plans/bright-wandering-flurry.md has been updated successfully.

# Fix WAL Channel Saturation (3M+ messages dropped)

## Context

After the initial slow-consumer fixes (multi-worker decode, non-blocking WAL send, 65K channel buffer, 256-batch drain), the WAL channel is still permanently full. 3M+ messages are being dropped — the single `WALStaging` goroutine fundamentally cannot keep up with `NumWorkers` concurrent decode goroutines producing WAL messages.

## Root Cause

The single `WALStaging` goroutine is the bottleneck. For every message it must:
1. `path.Join()` to build hostDir string — **allocates**
2. Map lookup in `hostFiles`
3. `buildWALPayload()` — **allocates a new `[]byte`** per message
4. `writeWALRecord()` — **allocates another `[]byte`** (record = magic+len+payload+crc)
5. `bufio.Write()` — copies into buffer, occasionally flushes to disk

With `NumWorkers` producers (e.g., 5-10 goroutines) each decoding batches of metrics, the single consumer cannot match. The 65K channel buffer fills in seconds, then every message is dropped.

## Plan: Sharded WAL Writers

Replace the single `WALStaging` goroutine with **N sharded writer goroutines**, each owning a subset of hosts. Messages are routed to shards by hashing `cluster+node`. This:
- Parallelizes serialization and I/O across goroutines
- Keeps per-host writes serial (required for correct WAL files)
- Eliminates the single-goroutine bottleneck entirely

Additionally, eliminate per-message heap allocations in `buildWALPayload`/`writeWALRecord` by encoding directly into the `bufio.Writer`.

### Change 1: Sharded WAL writer goroutines
**File:** `pkg/metricstore/walCheckpoint.go`

Replace the single `WALMessages` channel + `WALStaging()` goroutine with:

```go
const walShards = 16  // or NumWorkers

// Per-shard channel + goroutine
var walShardChs []chan *WALMessage

func walShard(msg *WALMessage) int {
    // FNV-1a hash of cluster+node, mod shard count
    h := fnv.New32a()
    h.Write([]byte(msg.Cluster))
    h.Write([]byte{0})
    h.Write([]byte(msg.Node))
    return int(h.Sum32()) % len(walShardChs)
}
```

Each shard goroutine:
- Owns its own `hostFiles` map (no contention)
- Drains its channel with the existing batch-drain pattern
- Handles rotate requests for its hosts

**Shard count:** Use `NumWorkers` (already configured, sensible default).

**Channel buffer per shard:** `65536 / numShards` — same total memory, but each shard only buffers its hosts.

### Change 2: Eliminate per-message allocations
**File:** `pkg/metricstore/walCheckpoint.go`

Replace `buildWALPayload()` → `writeWALRecord()` (2 allocations per message) with `writeWALRecordDirect(w *bufio.Writer, msg *WALMessage)` that encodes directly into the `bufio.Writer` using a small stack-allocated scratch buffer:

```go
func writeWALRecordDirect(w *bufio.Writer, msg *WALMessage) error {
    // Build payload into a pooled or stack buffer, compute CRC inline,
    // write magic+len, then payload, then CRC — all directly to w.
    // Zero heap allocations in the hot path.
    var scratch [128]byte  // stack-allocated, covers most records
    // ... encode payload into scratch or a pooled []byte if >128 ...
}
```

Keep the old `writeWALRecord`/`buildWALPayload` for use in tests or non-hot paths, but the shard goroutines use the direct version.

### Change 3: Route WAL sends to shards from DecodeLine
**File:** `pkg/metricstore/lineprotocol.go:355-372`

Replace:
```go
select {
case WALMessages <- msg:
default:
    // dropped
}
```
With:
```go
shard := walShard(msg)
select {
case walShardChs[shard] <- msg:
default:
    // dropped
}
```

### Change 4: Shard-aware rotation
**File:** `pkg/metricstore/walCheckpoint.go`

`RotateWALFiles` must send rotate requests to the correct shard. Each shard has its own `walRotateCh`. The checkpoint goroutine hashes the hostDir to find the right shard and sends the rotate request there.

### Change 5: Shutdown
**File:** `pkg/metricstore/metricstore.go` (Shutdown function)

Close all shard channels instead of the single `WALMessages` channel. Each shard goroutine drains and flushes on close.

## Files to Modify

- `pkg/metricstore/walCheckpoint.go` — sharded goroutines, direct record writing, shard-aware rotation
- `pkg/metricstore/lineprotocol.go` — route WAL sends to shard channels
- `pkg/metricstore/metricstore.go` — `WALStaging()` call updated, `Shutdown()` closes shard channels

## Verification

1. `go build ./...`
2. `go test ./pkg/metricstore/...`
3. Deploy and verify:
   - WAL drop count stays at 0 (or near-zero during extreme bursts)
   - WAL files are correct (load a checkpoint, verify data)
   - No "WAL channel full" log messages
   - Memory usage roughly same (total channel buffer size unchanged)

# Fix: ReceiveNats blocks Init() during startup

## Context

After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

## Root Cause

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

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

`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

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

-# Fix WAL Channel Saturation (3M+ messages dropped)

+# Fix: ReceiveNats blocks Init() during startup

-After the initial slow-consumer fixes (multi-worker decode, non-blocking WAL send, 65K channel buffer, 256-batch drain), the WAL channel is still permanently full. 3M+ messages are being dropped — the single `WALStaging` goroutine fundamentally cannot keep up with `NumWorkers` concurrent decode goroutines producing WAL messages.

+After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

-The single `WALStaging` goroutine is the bottleneck. For every message it must:

-1. `path.Join()` to build hostDir string — **allocates**

-3. `buildWALPayload()` — **allocates a new `[]byte`** per message

-4. `writeWALRecord()` — **allocates another `[]byte`** (record = magic+len+payload+crc)

-5. `bufio.Write()` — copies into buffer, occasionally flushes to disk

-With `NumWorkers` producers (e.g., 5-10 goroutines) each decoding batches of metrics, the single consumer cannot match. The 65K channel buffer fills in seconds, then every message is dropped.

-Replace the single `WALStaging` goroutine with **N sharded writer goroutines**, each owning a subset of hosts. Messages are routed to shards by hashing `cluster+node`. This:

-- Parallelizes serialization and I/O across goroutines

-- Keeps per-host writes serial (required for correct WAL files)

-- Eliminates the single-goroutine bottleneck entirely

-Additionally, eliminate per-message heap allocations in `buildWALPayload`/`writeWALRecord` by encoding directly into the `bufio.Writer`.

-### Change 1: Sharded WAL writer goroutines

-Replace the single `WALMessages` channel + `WALStaging()` goroutine with:

-    // FNV-1a hash of cluster+node, mod shard count

-    return int(h.Sum32()) % len(walShardChs)

+    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← blocks forever

-- Owns its own `hostFiles` map (no contention)

-- Drains its channel with the existing batch-drain pattern

-- Handles rotate requests for its hosts

+`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

-**Shard count:** Use `NumWorkers` (already configured, sensible default).

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

-**Channel buffer per shard:** `65536 / numShards` — same total memory, but each shard only buffers its hosts.

-### Change 2: Eliminate per-message allocations

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

-Replace `buildWALPayload()` → `writeWALRecord()` (2 allocations per message) with `writeWALRecordDirect(w *bufio.Writer, msg *WALMessage)` that encodes directly into the `bufio.Writer` using a small stack-allocated scratch buffer:

-func writeWALRecordDirect(w *bufio.Writer, msg *WALMessage) error {

-    // Build payload into a pooled or stack buffer, compute CRC inline,

-    // write magic+len, then payload, then CRC — all directly to w.

-    // Zero heap allocations in the hot path.

-    var scratch [128]byte  // stack-allocated, covers most records

-    // ... encode payload into scratch or a pooled []byte if >128 ...

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

-Keep the old `writeWALRecord`/`buildWALPayload` for use in tests or non-hot paths, but the shard goroutines use the direct version.

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

-### Change 3: Route WAL sends to shards from DecodeLine

-`RotateWALFiles` must send rotate requests to the correct shard. Each shard has its own `walRotateCh`. The checkpoint goroutine hashes the hostDir to find the right shard and sends the rotate request there.

-**File:** `pkg/metricstore/metricstore.go` (Shutdown function)

-Close all shard channels instead of the single `WALMessages` channel. Each shard goroutine drains and flushes on close.

-- `pkg/metricstore/walCheckpoint.go` — sharded goroutines, direct record writing, shard-aware rotation

-- `pkg/metricstore/lineprotocol.go` — route WAL sends to shard channels

-- `pkg/metricstore/metricstore.go` — `WALStaging()` call updated, `Shutdown()` closes shard channels

-   - WAL drop count stays at 0 (or near-zero during extreme bursts)

-   - WAL files are correct (load a checkpoint, verify data)

-   - No "WAL channel full" log messages

-   - Memory usage roughly same (total channel buffer size unchanged)

+3. Verify the app starts up and serves HTTP requests when NATS subscriptions are configured


ToolSearch

Question: select:ExitPlanMode


Question: select:ExitPlanMode


The fix is a one-line change: wrap the `ReceiveNats` call in `wg.Go()` so it runs as a background goroutine instead of blocking `Init()` from returning.


ExitPlanMode

# Fix: ReceiveNats blocks Init() during startup

## Context

After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

## Root Cause

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

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

`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

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


Error: The user doesn't want to proceed with this tool use. The tool use was rejected (eg. if it was a file edit, the new_string was NOT written to the file). To tell you how to proceed, the user said:
Why wasn't this a problem before the changes?


Good question. Let me look more carefully at why this would be newly exposed.


pkg/metricstore/metricstore.go:136:	// Set NumWorkers from config or use default
pkg/metricstore/metricstore.go:137:	if Keys.NumWorkers <= 0 {
pkg/metricstore/metricstore.go:138:		Keys.NumWorkers = min(runtime.NumCPU()/2+1, DefaultMaxWorkers)
pkg/metricstore/metricstore.go:140:	cclog.Debugf("[METRICSTORE]> Using %d workers for checkpoint/archive operations\n", Keys.NumWorkers)
pkg/metricstore/metricstore.go:187:		err = ReceiveNats(ms, Keys.NumWorkers, ctx)
pkg/metricstore/config.go:13://	├─ NumWorkers: Parallel checkpoint/archive workers
pkg/metricstore/config.go:53:	DefaultMaxWorkers                 = 10
pkg/metricstore/config.go:114://   - NumWorkers:        Parallel workers for checkpoint/archive (0 = auto: min(NumCPU/2+1, 10))
pkg/metricstore/config.go:124:	NumWorkers         int            `json:"num-workers"`
pkg/metricstore/archive.go:108:	work := make(chan workItem, Keys.NumWorkers)
pkg/metricstore/archive.go:110:	wg.Add(Keys.NumWorkers)
pkg/metricstore/archive.go:111:	for worker := 0; worker < Keys.NumWorkers; worker++ {
pkg/metricstore/archive.go:205:		}, Keys.NumWorkers)
pkg/metricstore/archive.go:210:		wg.Add(Keys.NumWorkers)
pkg/metricstore/archive.go:211:		for w := 0; w < Keys.NumWorkers; w++ {
pkg/metricstore/checkpoint.go:184:// Uses worker pool (Keys.NumWorkers) for parallel processing. Only locks one host
pkg/metricstore/checkpoint.go:222:	wg.Add(Keys.NumWorkers)
pkg/metricstore/checkpoint.go:223:	work := make(chan workItem, Keys.NumWorkers*2)
pkg/metricstore/checkpoint.go:224:	for worker := 0; worker < Keys.NumWorkers; worker++ {
pkg/metricstore/checkpoint.go:397:	work := make(chan [2]string, Keys.NumWorkers*4)
pkg/metricstore/checkpoint.go:400:	wg.Add(Keys.NumWorkers)
pkg/metricstore/checkpoint.go:401:	for worker := 0; worker < Keys.NumWorkers; worker++ {
pkg/metricstore/walCheckpoint.go:152:	walNumShards = max(Keys.NumWorkers, 1)
pkg/metricstore/walCheckpoint.go:705:	wg.Add(Keys.NumWorkers)
pkg/metricstore/walCheckpoint.go:706:	work := make(chan workItem, Keys.NumWorkers*2)
pkg/metricstore/walCheckpoint.go:708:	for range Keys.NumWorkers {


Now let me check the `ReceiveNats` behavior difference when `workers == 1` vs `workers > 1`:


    45→func ReceiveNats(ms *MemoryStore,
    46→	workers int,
    47→	ctx context.Context,
    48→) error {
    49→	nc := nats.GetClient()
    50→
    51→	if nc == nil {
    52→		cclog.Warn("NATS client not initialized")
    53→		return nil
    54→	}
    55→
    56→	var wg sync.WaitGroup
    57→	msgs := make(chan []byte, max(workers*256, 8192))
    58→
    59→	for _, sc := range *Keys.Subscriptions {
    60→		clusterTag := sc.ClusterTag
    61→		if workers > 1 {
    62→			wg.Add(workers)
    63→
    64→			for range workers {
    65→				go func() {
    66→					defer wg.Done()
    67→					for m := range msgs {
    68→						dec := lineprotocol.NewDecoderWithBytes(m)
    69→						if err := DecodeLine(dec, ms, clusterTag); err != nil {
    70→							cclog.Errorf("error: %s", err.Error())
    71→						}
    72→					}
    73→				}()
    74→			}
    75→
    76→			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
    77→				select {
    78→				case msgs <- data:
    79→				case <-ctx.Done():
    80→				}
    81→			})
    82→		} else {
    83→			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
    84→				dec := lineprotocol.NewDecoderWithBytes(data)
    85→				if err := DecodeLine(dec, ms, clusterTag); err != nil {
    86→					cclog.Errorf("error: %s", err.Error())
    87→				}
    88→			})
    89→		}
    90→		cclog.Infof("NATS subscription to '%s' established", sc.SubscribeTo)
    91→	}
    92→
    93→	go func() {
    94→		<-ctx.Done()
    95→		close(msgs)
    96→	}()
    97→
    98→	wg.Wait()
    99→
   100→	return nil
   101→}
   102→
   103→// reorder prepends prefix to buf in-place when buf has enough spare capacity,
   104→// avoiding an allocation. Falls back to a regular append otherwise.
   105→//
   106→// It is used to assemble the "type<type-id>" and "subtype<stype-id>" selector
   107→// strings when the type tag arrives before the type-id tag in the line, so the
   108→// two byte slices need to be concatenated in tag-declaration order regardless
   109→// of wire order.
   110→func reorder(buf, prefix []byte) []byte {
   111→	n := len(prefix)
   112→	m := len(buf)
   113→	if cap(buf) < m+n {
   114→		return append(prefix[:n:n], buf...)
   115→	} else {
   116→		buf = buf[:n+m]
   117→		for i := m - 1; i >= 0; i-- {
   118→			buf[i+n] = buf[i]
   119→		}
   120→		for i := range n {
   121→			buf[i] = prefix[i]
   122→		}
   123→		return buf
   124→	}
   125→}
   126→
   127→// walDropped counts WAL messages dropped due to a full channel.
   128→// Logged periodically; not critical since binary snapshots capture the data.
   129→var walDropped atomic.Int64
   130→
   131→// decodeState holds the per-call scratch buffers used by DecodeLine.
   132→// Instances are recycled via decodeStatePool to avoid repeated allocations
   133→// during high-throughput ingestion.
   134→type decodeState struct {
   135→	// metricBuf holds a copy of the current measurement name (line-protocol
   136→	// measurement field). Copied because dec.Measurement() returns a slice
   137→	// that is invalidated by the next decoder call.
   138→	metricBuf []byte
   139→
   140→	// selector is the sub-device path passed to WriteToLevel and WALMessage
   141→	// (e.g. ["socket0"] or ["socket0", "memctrl1"]). Reused across lines.
   142→	selector []string
   143→
   144→	// typeBuf accumulates the concatenated "type"+"type-id" tag value for the
   145→	// current line. Reset at the start of each line's tag-decode loop.

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, max(workers*256, 8192))

	for _, sc := range *Keys.Subscriptions {
		clusterTag := sc.ClusterTag
		if workers > 1 {
			wg.Add(workers)

			for range workers {
				go func() {
					defer wg.Done()
					for m := range msgs {
						dec := lineprotocol.NewDecoderWithBytes(m)
						if err := DecodeLine(dec, ms, clusterTag); err != nil {
							cclog.Errorf("error: %s", err.Error())
						}
					}
				}()
			}

			nc.Subscribe(sc.SubscribeTo, func(subject string, data []byte) {
				select {
				case msgs <- data:
				case <-ctx.Done():
				}
			})
		} 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)
	}

	go func() {
		<-ctx.Done()
		close(msgs)
	}()

	wg.Wait()

	return nil
}

// reorder prepends prefix to buf in-place when buf has enough spare capacity,
// avoiding an allocation. Falls back to a regular append otherwise.
//
// It is used to assemble the "type<type-id>" and "subtype<stype-id>" selector
// strings when the type tag arrives before the type-id tag in the line, so the
// two byte slices need to be concatenated in tag-declaration order regardless
// of wire order.
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
	}
}

// walDropped counts WAL messages dropped due to a full channel.
// Logged periodically; not critical since binary snapshots capture the data.
var walDropped atomic.Int64

// decodeState holds the per-call scratch buffers used by DecodeLine.
// Instances are recycled via decodeStatePool to avoid repeated allocations
// during high-throughput ingestion.
type decodeState struct {
	// metricBuf holds a copy of the current measurement name (line-protocol
	// measurement field). Copied because dec.Measurement() returns a slice
	// that is invalidated by the next decoder call.
	metricBuf []byte

	// selector is the sub-device path passed to WriteToLevel and WALMessage
	// (e.g. ["socket0"] or ["socket0", "memctrl1"]). Reused across lines.
	selector []string

	// typeBuf accumulates the concatenated "type"+"type-id" tag value for the
	// current line. Reset at the start of each line's tag-decode loop.


Now let me check the git log to see what `ReceiveNats` looked like before the WAL sharding changes:


Bash

git log --oneline -10 pkg/metricstore/lineprotocol.go

Show recent commits for lineprotocol.go


bf1a8a17 fix: Shard WAL consumer for higher throughput
50aed595 fix: metricstore NATS contention
8234ad31 fix: Fix metricstore memory explosion from broken emergency free and batch aborts
a1db8263 Document line protocol. Optimize REST writeMetric path
4c3cd8e6 Merge branch 'dev' into optimize-checkpoint-wal
6ecb9349 Switch to CC line-protocol package. Update cc-lib.
ca0f9a42 Introduce metric store binary checkpoints with write ahead log
499b4287 Switch to cclib nats client
f4130103 Move metricstore from internal to pkg


git diff 50aed595..bf1a8a17 -- pkg/metricstore/lineprotocol.go pkg/metricstore/metricstore.go

Show WAL sharding diff for these two files


diff --git a/pkg/metricstore/lineprotocol.go b/pkg/metricstore/lineprotocol.go
index d7b96b91..caec82e9 100644
--- a/pkg/metricstore/lineprotocol.go
+++ b/pkg/metricstore/lineprotocol.go
@@ -361,13 +361,11 @@ func DecodeLine(dec *lineprotocol.Decoder,
 				Value:      metric.Value,
 				Timestamp:  time,
 			}
-			select {
-			case WALMessages <- msg:
-			default:
-				// WAL channel full — metric is written to memory store but not WAL.
+			if !SendWALMessage(msg) {
+				// WAL shard channel full — metric is written to memory store but not WAL.
 				// Next binary snapshot will capture it.
 				if dropped := walDropped.Add(1); dropped%10000 == 1 {
-					cclog.Warnf("[METRICSTORE]> WAL channel full, dropped %d messages (data safe in memory, next snapshot will capture)", dropped)
+					cclog.Warnf("[METRICSTORE]> WAL shard channel full, dropped %d messages (data safe in memory, next snapshot will capture)", dropped)
 				}
 			}
 		}
diff --git a/pkg/metricstore/metricstore.go b/pkg/metricstore/metricstore.go
index cd74f481..83c78ab2 100644
--- a/pkg/metricstore/metricstore.go
+++ b/pkg/metricstore/metricstore.go
@@ -277,7 +277,9 @@ func Shutdown() {
 	}

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

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


git show 50aed595:pkg/metricstore/metricstore.go | grep -n -A 10 'ReceiveNats'

Show ReceiveNats call before WAL sharding


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.


git show 50aed595:pkg/metricstore/lineprotocol.go | grep -n 'wg.Wait\|wg.Add\|workers'

Check ReceiveNats blocking in pre-sharding code


38:// When workers > 1 a pool of goroutines drains a shared channel so that
39:// multiple messages can be decoded in parallel. With workers == 1 the NATS
46:	workers int,
57:	msgs := make(chan []byte, max(workers*256, 8192))
61:		if workers > 1 {
62:			wg.Add(workers)
64:			for range workers {
98:	wg.Wait()


So `ReceiveNats` was **identically blocking** before the WAL sharding changes — the `wg.Wait()` on line 98 and the `workers > 1` path with `wg.Add(workers)` existed in commit `50aed595` (the commit before the sharding). The WAL sharding didn't change `ReceiveNats` or the `Init()` call to it at all.

Let me check when this blocking behavior was introduced vs when `ReceiveNats` was first added:


git log --oneline --all -- pkg/metricstore/lineprotocol.go | head -20

Full history of lineprotocol.go


6af2a920 Completed 'Explore' agent: Investigate ReceiveNats blocking (toolu_01ECFd)
bf1a8a17 fix: Shard WAL consumer for higher throughput
50aed595 fix: metricstore NATS contention
818a05f2 carry forward: uncommitted session files
48a6fbb0 carry forward: uncommitted session files
8234ad31 fix: Fix metricstore memory explosion from broken emergency free and batch aborts
ecd29b28 carry forward: uncommitted session files
21b6c1b5 carry forward: uncommitted session files
a1db8263 Document line protocol. Optimize REST writeMetric path
23ce1722 Merge branch 'dev' into optimize-checkpoint-loading
4c3cd8e6 Merge branch 'dev' into optimize-checkpoint-wal
6ecb9349 Switch to CC line-protocol package. Update cc-lib.
ca0f9a42 Introduce metric store binary checkpoints with write ahead log
9c0104a2 Attempt to minimize heap allocations by using caching
9fc1836c Replace avro checkpoints with native binary format
499b4287 Switch to cclib nats client
f4130103 Move metricstore from internal to pkg


git show 50aed595:pkg/metricstore/lineprotocol.go | head -5; echo "---"; git show 50aed595:pkg/metricstore/lineprotocol.go | tail -10

Check the NATS contention fix commit for ReceiveNats


// 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.

---
			}
		}

		if err := ms.WriteToLevel(lvl, st.selector, time, []Metric{metric}); err != nil {
			cclog.Warnf("write error for host %s metric %s at ts %d: %s", host, string(st.metricBuf), time, err.Error())
			continue
		}
	}
	return nil
}


git log --oneline 50aed595 -1; echo "---"; git show 8234ad31:pkg/metricstore/lineprotocol.go | grep -n 'wg.Wait\|func ReceiveNats'

Check if wg.Wait existed in earlier commits


50aed595 fix: metricstore NATS contention
---
44:func ReceiveNats(ms *MemoryStore,
97:	wg.Wait()


git show ca0f9a42:pkg/metricstore/lineprotocol.go | grep -n 'wg.Wait\|func ReceiveNats\|wg.Add'

Check original ReceiveNats implementation


20:func ReceiveNats(ms *MemoryStore,
37:			wg.Add(workers)
73:	wg.Wait()


The `wg.Wait()` has been there since the very first commit (`ca0f9a42`) that introduced `ReceiveNats`. It was **always a blocking call**. The WAL sharding changes (`bf1a8a17`) did not modify `ReceiveNats` or the `Init()` call to it at all — the diff confirms only the WAL send path and shutdown changed.

This is a **pre-existing bug** — `ReceiveNats` was always designed to block (its own docstring says so), but `Init()` was always calling it synchronously. The reason it wasn't a problem before is likely one of:

1. **`NumWorkers` was 1** — when `workers == 1`, `ReceiveNats` takes the `else` branch (inline decode, no `wg.Add`), so `wg.Wait()` returns immediately
2. **NATS wasn't configured** — `Keys.Subscriptions` was nil, or `nats.GetClient()` returned nil

But the fix is the same regardless — `ReceiveNats` needs to run in a background goroutine. Let me update the plan with this context.


Edit

# Fix: ReceiveNats blocks Init() during startup

## Context

After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

## Root Cause

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

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

`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

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

# 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


# Fix: ReceiveNats blocks Init() during startup

## Context

After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

## Root Cause

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

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

`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

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

# 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

-After the sharded WAL changes, the application blocks during startup in `ReceiveNats`. `ReceiveNats` is designed to block until context cancellation (its `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 of `metricstore.go`, which prevents `Init()` from returning and the server from finishing startup.

+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.

-    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← blocks forever

+    err = ReceiveNats(ms, Keys.NumWorkers, ctx)  // ← blocks forever when workers > 1

-`ReceiveNats` (in `lineprotocol.go`) spawns `NumWorkers` goroutines that loop on a `msgs` channel. The channel is only closed when `ctx` is cancelled (at shutdown). So `wg.Wait()` at line 98 blocks until shutdown, and `Init()` never returns.

+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.


ExitPlanMode

# 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


The user doesn't want to proceed with this tool use. The tool use was rejected (eg. if it was a file edit, the new_string was NOT written to the file). STOP what you are doing and wait for the user to tell you how to proceed.


[Request interrupted by user for tool use]