diff --git a/PLAN_AGGREGATOR.md b/PLAN_AGGREGATOR.md
new file mode 100644
index 0000000..0ae85db
--- /dev/null
+++ b/PLAN_AGGREGATOR.md
@@ -0,0 +1,250 @@
+# Aggregator v0 — Implementation Plan
+
+Module path: `git.ipng.ch/ipng/nginx-logtail`
+
+**Scope:** A working aggregator that subscribes to `StreamSnapshots` from all configured
+collectors, maintains a merged in-memory cache, and serves the same `LogtailService` gRPC
+interface as the collector. Tolerates partial collector failures.
+
+---
+
+## Step 1 — Extract shared logic to `internal/store`
+
+The aggregator's cache is structurally identical to the collector's store: same `Entry` and
+`snapshot` types, same tiered ring buffers, same heap-based top-K, same label encoding
+(`encodeTuple`, `labelTuple`), same `matchesFilter` and `dimensionLabel` functions.
+
+Rather than duplicating ~200 lines of load-bearing code, extract these to a shared internal
+package before writing any aggregator code. Then refactor the collector to import it.
+
+**New package: `internal/store`**
+
+Move from `cmd/collector/store.go` into `internal/store/store.go`:
+- `Tuple4` struct
+- `Entry` struct
+- `snapshot` struct (unexported → exported: `Snapshot`)
+- `entryHeap` + heap interface methods
+- `encodeTuple`, `labelTuple`, `splitN`, `indexOf`
+- `matchesFilter`, `dimensionLabel`
+- `topKFromMap`, `topK`
+- `trendPoint`
+- `ringView`, `bucketsForWindow`
+- All ring-buffer constants (`fineRingSize`, `coarseRingSize`, `fineTopK`, `coarseTopK`,
+  `coarseEvery`)
+
+Keep in `cmd/collector/store.go` (collector-specific):
+- `liveMapCap`
+- `Store` struct (live map + ring buffers + subscriber fan-out + `Run` goroutine)
+- `ingest`, `rotate`, `mergeFineBuckets`
+- `QueryTopN`, `QueryTrend`, `Subscribe`, `Unsubscribe`, `broadcast`
+- The `Store` embeds the ring buffers using the types from `internal/store`
+
+Collector tests must continue to pass unchanged after the refactor.
+
+---
+
+## Step 2 — subscriber.go
+
+One goroutine per collector. Dials the collector, calls `StreamSnapshots`, and forwards each
+received `pb.Snapshot` to the merger. Reconnects with exponential backoff on any stream error.
+
+```
+CollectorSub struct:
+  addr    string
+  merger  *Merger
+  source  string          // filled from first snapshot received
+  fails   int             // consecutive failures
+```
+
+Lifecycle:
+1. `Dial(addr)` → `client.StreamSnapshots(ctx, &pb.SnapshotRequest{})`
+2. Loop: `stream.Recv()` → `merger.Apply(snap)`; on error: close, `fails++`
+3. If `fails >= 3`: call `merger.Zero(addr)`, log degraded warning
+4. Backoff sleep (100 ms → doubles → cap 30 s), then go to step 1
+5. On successful `Recv()` after degraded: `fails = 0`, log recovery
+
+Context cancellation exits the goroutine cleanly.
+
+---
+
+## Step 3 — merger.go
+
+Maintains the per-collector maps and a single running merged map. Uses a delta strategy:
+when a new snapshot arrives from collector X, subtract X's previous entries from `merged`,
+add the new entries, and replace X's stored map. This is O(snapshot_size) rather than
+O(N_collectors × snapshot_size).
+
+```
+Merger struct:
+  mu           sync.Mutex
+  perCollector map[string]map[string]int64  // addr → (label → count)
+  merged       map[string]int64             // label → total count across all collectors
+```
+
+Methods:
+- `Apply(snap *pb.Snapshot)` — lock, subtract old, add new, store new, unlock
+- `Zero(addr string)` — lock, subtract perCollector[addr] from merged, delete entry, unlock
+- `TopK(k int) []store.Entry` — lock, call `store.TopKFromMap(merged, k)`, unlock
+
+`Apply` is called from multiple subscriber goroutines concurrently — the mutex is the only
+synchronisation point. No channels needed here.
+
+---
+
+## Step 4 — cache.go
+
+The aggregator's equivalent of the collector's `Store`. Holds the tiered ring buffers and
+answers `TopN`/`Trend`/`StreamSnapshots` queries. Populated by a 1-minute ticker that snapshots
+the current merged view from the merger.
+
+```
+Cache struct:
+  source  string
+  merger  *Merger
+
+  mu            sync.RWMutex
+  fineRing      [fineRingSize]store.Snapshot
+  fineHead      int
+  fineFilled    int
+  coarseRing    [coarseRingSize]store.Snapshot
+  coarseHead    int
+  coarseFilled  int
+  fineTick      int
+
+  subMu  sync.Mutex
+  subs   map[chan store.Snapshot]struct{}
+```
+
+`Run(ctx context.Context)`:
+- 1-minute ticker → `rotate(time.Now())`
+- `rotate`: `merger.TopK(fineTopK)` → fine ring slot; every 5 ticks → merge last 5 fine slots
+  into coarse ring slot (identical logic to collector `Store.rotate`)
+- After writing: broadcast fine snapshot to subscribers
+
+`QueryTopN`, `QueryTrend`, `Subscribe`, `Unsubscribe`, `broadcast`: identical to collector
+`Store`, backed by `internal/store` helpers.
+
+**Why tick-based and not snapshot-triggered?**
+Collectors send snapshots roughly once per minute but not in sync. Triggering a ring write on
+every incoming snapshot would produce N writes per minute (one per collector), inflating the ring
+and misaligning time windows. A single ticker keeps the aggregator ring aligned with the same
+1-minute cadence as the collectors.
+
+---
+
+## Step 5 — server.go
+
+Identical structure to `cmd/collector/server.go`. Implements `pb.LogtailServiceServer` backed by
+the `Cache` instead of the collector's `Store`. No new logic; just a different backing type.
+
+`StreamSnapshots` sends merged fine snapshots (from `cache.Subscribe`) to downstream consumers
+(frontend, CLI, or a second-tier aggregator).
+
+---
+
+## Step 6 — main.go
+
+Flags:
+
+| Flag           | Default      | Description                                            |
+|----------------|--------------|--------------------------------------------------------|
+| `--listen`     | `:9091`      | gRPC listen address                                    |
+| `--collectors` | —            | Comma-separated `host:port` addresses of collectors    |
+| `--source`     | hostname     | Name for this aggregator in query responses            |
+
+Wire-up:
+1. Parse collector addresses
+2. Create `Merger`
+3. Create `Cache(merger, source)`
+4. Start `cache.Run(ctx)` goroutine (ticker + ring rotation)
+5. Start one `CollectorSub.Run(ctx)` goroutine per collector address
+6. Start gRPC server
+7. `signal.NotifyContext` for clean shutdown on SIGINT/SIGTERM
+
+---
+
+## Step 7 — Tests
+
+| Test | What it covers |
+|------|----------------|
+| `TestMergerApply` | Two collectors send snapshots; merged map sums correctly |
+| `TestMergerReplacement` | Second snapshot from same collector replaces first, not adds |
+| `TestMergerZero` | Marking a collector degraded removes its contribution from merged |
+| `TestMergerConcurrent` | `Apply` and `Zero` from concurrent goroutines; no race (run with `-race`) |
+| `TestCacheRotation` | After one ticker fire, fine ring has 1 entry with correct counts |
+| `TestCacheCoarseRing` | After 5 ticker fires, coarse ring has 1 entry |
+| `TestCacheQueryTopN` | TopN returns correct merged rankings |
+| `TestCacheQueryTrend` | Trend returns per-bucket sums oldest-first |
+| `TestCacheSubscribe` | Subscriber receives snapshot after each rotation |
+| `TestGRPCEndToEnd` | Two in-process fake collector servers; real aggregator dials them; TopN, Trend, StreamSnapshots verified |
+
+All existing collector tests must continue to pass after the `internal/store` refactor.
+
+---
+
+## Step 8 — Smoke test
+
+- Start two collector instances pointing at generated log files
+- Start the aggregator pointing at both
+- Use `grpcurl` to call `TopN` on the aggregator and confirm counts match the sum of the two
+  individual collector `TopN` results
+- Kill one collector; confirm the aggregator continues serving and logs a degraded warning
+- Restart the killed collector; confirm the aggregator recovers and resumes merging
+
+---
+
+## ✓ COMPLETE — Implementation notes
+
+### Deviations from the plan
+
+- **`TestMergerZeroNonexistent` added**: Plan listed 10 tests; an extra test was added to cover
+  `Zero()` on a source that never sent a snapshot (should be a no-op). Total: 13 tests.
+- **`TestDegradedCollector` in end-to-end section**: Rather than a separate block, degraded
+  behaviour is tested with one real fake collector + one unreachable port in the same test file.
+- **Race in `TestGRPCEndToEnd`**: The `cache.rotate()` call to trigger a broadcast needed a
+  50 ms sleep after `client.StreamSnapshots()` to allow the server goroutine to register its
+  subscriber before the broadcast fired. Without it the test was intermittently flaky under
+  the race detector and parallel test runs.
+- **`source` field not stored on `CollectorSub`**: Plan mentioned storing `source` from the first
+  snapshot, but `Apply` uses `snap.Source` directly (keying `perCollector` by address). The
+  `source` field was not needed on the struct.
+
+### Test results
+
+```
+$ go test ./... -count=1 -race -timeout 60s
+ok  git.ipng.ch/ipng/nginx-logtail/cmd/aggregator  4.1s
+ok  git.ipng.ch/ipng/nginx-logtail/cmd/collector   9.7s
+```
+
+All 13 aggregator tests and all 17 collector tests pass with `-race`.
+
+### Test inventory
+
+| Test | Package | What it covers |
+|------|---------|----------------|
+| `TestMergerApply` | aggregator | Two collectors sum correctly |
+| `TestMergerReplacement` | aggregator | Second snapshot replaces, not adds |
+| `TestMergerZero` | aggregator | Degraded collector removed from merged |
+| `TestMergerZeroNonexistent` | aggregator | Zero on unknown source is a no-op |
+| `TestMergerConcurrent` | aggregator | Apply + Zero from concurrent goroutines; -race |
+| `TestCacheRotation` | aggregator | Fine ring written after one ticker fire |
+| `TestCacheCoarseRing` | aggregator | Coarse ring written after 5 ticker fires |
+| `TestCacheQueryTopN` | aggregator | TopN returns correct merged rankings |
+| `TestCacheQueryTopNWithFilter` | aggregator | TopN with website filter |
+| `TestCacheQueryTrend` | aggregator | Trend per-bucket sums oldest-first |
+| `TestCacheSubscribe` | aggregator | Subscriber receives snapshot on rotation |
+| `TestGRPCEndToEnd` | aggregator | Two fake collectors; real gRPC TopN/Trend/Stream |
+| `TestDegradedCollector` | aggregator | Bad address zeroed; good collector still visible |
+
+---
+
+## Deferred (not in v0)
+
+- Per-source (busiest nginx) breakdown — requires adding `SOURCE` to the `GroupBy` proto enum
+  and encoding the source into the merged snapshot entries; deferred until the proto is stable
+- `cmd/cli` — covered in PLAN_CLI.md
+- `cmd/frontend` — covered in PLAN_FRONTEND.md
+- ClickHouse export
+- TLS / auth
+- Prometheus metrics endpoint
diff --git a/cmd/aggregator/aggregator_test.go b/cmd/aggregator/aggregator_test.go
new file mode 100644
index 0000000..c610299
--- /dev/null
+++ b/cmd/aggregator/aggregator_test.go
@@ -0,0 +1,425 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"sync"
+	"testing"
+	"time"
+
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+)
+
+// --- Merger tests ---
+
+func makeSnap(source string, entries map[string]int64) *pb.Snapshot {
+	snap := &pb.Snapshot{Source: source, Timestamp: time.Now().Unix()}
+	for label, count := range entries {
+		snap.Entries = append(snap.Entries, &pb.TopNEntry{Label: label, Count: count})
+	}
+	return snap
+}
+
+func TestMergerApply(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 10, "b": 20}))
+	m.Apply(makeSnap("c2", map[string]int64{"a": 5, "c": 15}))
+
+	top := m.TopK(10)
+	totals := map[string]int64{}
+	for _, e := range top {
+		totals[e.Label] = e.Count
+	}
+	if totals["a"] != 15 { // 10 + 5
+		t.Errorf("a = %d, want 15", totals["a"])
+	}
+	if totals["b"] != 20 {
+		t.Errorf("b = %d, want 20", totals["b"])
+	}
+	if totals["c"] != 15 {
+		t.Errorf("c = %d, want 15", totals["c"])
+	}
+}
+
+func TestMergerReplacement(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 100}))
+	// Second snapshot from same collector replaces the first, not adds.
+	m.Apply(makeSnap("c1", map[string]int64{"a": 50, "b": 30}))
+
+	top := m.TopK(10)
+	totals := map[string]int64{}
+	for _, e := range top {
+		totals[e.Label] = e.Count
+	}
+	if totals["a"] != 50 {
+		t.Errorf("a = %d, want 50 (not 150)", totals["a"])
+	}
+	if totals["b"] != 30 {
+		t.Errorf("b = %d, want 30", totals["b"])
+	}
+}
+
+func TestMergerZero(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 100}))
+	m.Apply(makeSnap("c2", map[string]int64{"a": 50}))
+
+	m.Zero("c1")
+
+	top := m.TopK(10)
+	if len(top) != 1 || top[0].Label != "a" || top[0].Count != 50 {
+		t.Errorf("after Zero(c1): %v", top)
+	}
+}
+
+func TestMergerZeroNonexistent(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 10}))
+	// Zeroing an unknown addr should not panic.
+	m.Zero("unknown")
+	top := m.TopK(10)
+	if len(top) != 1 || top[0].Count != 10 {
+		t.Errorf("unexpected: %v", top)
+	}
+}
+
+func TestMergerConcurrent(t *testing.T) {
+	m := NewMerger()
+	var wg sync.WaitGroup
+	for i := 0; i < 20; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			addr := fmt.Sprintf("c%d", i%3)
+			for j := 0; j < 100; j++ {
+				m.Apply(makeSnap(addr, map[string]int64{"x": int64(j)}))
+			}
+		}(i)
+	}
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		for i := 0; i < 30; i++ {
+			m.Zero(fmt.Sprintf("c%d", i%3))
+		}
+	}()
+	wg.Wait()
+	// No race, no panic — the race detector will catch issues if run with -race.
+}
+
+// --- Cache tests ---
+
+func TestCacheRotation(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 100, "b": 50}))
+
+	cache := NewCache(m, "test")
+	cache.rotate(time.Now())
+
+	cache.mu.RLock()
+	defer cache.mu.RUnlock()
+	if cache.fineFilled != 1 {
+		t.Fatalf("fineFilled = %d, want 1", cache.fineFilled)
+	}
+	snap := cache.fineRing[(cache.fineHead-1+st.FineRingSize)%st.FineRingSize]
+	if len(snap.Entries) != 2 {
+		t.Fatalf("got %d entries, want 2", len(snap.Entries))
+	}
+	if snap.Entries[0].Count != 100 {
+		t.Errorf("top count = %d, want 100", snap.Entries[0].Count)
+	}
+}
+
+func TestCacheCoarseRing(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"a": 10}))
+
+	cache := NewCache(m, "test")
+	now := time.Now()
+	for i := 0; i < st.CoarseEvery; i++ {
+		cache.rotate(now.Add(time.Duration(i) * time.Minute))
+	}
+
+	cache.mu.RLock()
+	defer cache.mu.RUnlock()
+	if cache.coarseFilled != 1 {
+		t.Fatalf("coarseFilled = %d, want 1", cache.coarseFilled)
+	}
+	coarse := cache.coarseRing[(cache.coarseHead-1+st.CoarseRingSize)%st.CoarseRingSize]
+	if len(coarse.Entries) == 0 {
+		t.Fatal("coarse snapshot is empty")
+	}
+	// 5 fine ticks × 10 counts = 50
+	if coarse.Entries[0].Count != 50 {
+		t.Errorf("coarse count = %d, want 50", coarse.Entries[0].Count)
+	}
+}
+
+func TestCacheQueryTopN(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{
+		st.EncodeTuple(st.Tuple4{"busy.com", "1.0.0.0/24", "/", "200"}): 300,
+		st.EncodeTuple(st.Tuple4{"quiet.com", "2.0.0.0/24", "/", "200"}): 50,
+	}))
+
+	cache := NewCache(m, "test")
+	cache.rotate(time.Now())
+
+	entries := cache.QueryTopN(nil, pb.GroupBy_WEBSITE, 2, pb.Window_W1M)
+	if len(entries) != 2 {
+		t.Fatalf("got %d entries, want 2", len(entries))
+	}
+	if entries[0].Label != "busy.com" || entries[0].Count != 300 {
+		t.Errorf("top = %+v, want {busy.com 300}", entries[0])
+	}
+}
+
+func TestCacheQueryTopNWithFilter(t *testing.T) {
+	m := NewMerger()
+	status429 := st.EncodeTuple(st.Tuple4{"example.com", "1.0.0.0/24", "/api", "429"})
+	status200 := st.EncodeTuple(st.Tuple4{"example.com", "2.0.0.0/24", "/api", "200"})
+	m.Apply(makeSnap("c1", map[string]int64{status429: 200, status200: 500}))
+
+	cache := NewCache(m, "test")
+	cache.rotate(time.Now())
+
+	f429 := int32(429)
+	entries := cache.QueryTopN(&pb.Filter{HttpResponse: &f429}, pb.GroupBy_WEBSITE, 10, pb.Window_W1M)
+	if len(entries) != 1 || entries[0].Label != "example.com" || entries[0].Count != 200 {
+		t.Errorf("filtered result: %v", entries)
+	}
+}
+
+func TestCacheQueryTrend(t *testing.T) {
+	m := NewMerger()
+	cache := NewCache(m, "test")
+	now := time.Now()
+
+	for i, count := range []int64{10, 20, 30} {
+		m.Apply(makeSnap("c1", map[string]int64{
+			st.EncodeTuple(st.Tuple4{"x.com", "1.0.0.0/24", "/", "200"}): count,
+		}))
+		cache.rotate(now.Add(time.Duration(i) * time.Minute))
+	}
+
+	points := cache.QueryTrend(nil, pb.Window_W5M)
+	if len(points) != 3 {
+		t.Fatalf("got %d points, want 3", len(points))
+	}
+	if points[0].Count != 10 || points[1].Count != 20 || points[2].Count != 30 {
+		t.Errorf("counts: %v %v %v", points[0].Count, points[1].Count, points[2].Count)
+	}
+}
+
+func TestCacheSubscribe(t *testing.T) {
+	m := NewMerger()
+	m.Apply(makeSnap("c1", map[string]int64{"x": 5}))
+	cache := NewCache(m, "test")
+
+	ch := cache.Subscribe()
+	cache.rotate(time.Now())
+
+	select {
+	case snap := <-ch:
+		if len(snap.Entries) == 0 {
+			t.Error("received empty snapshot")
+		}
+	case <-time.After(time.Second):
+		t.Fatal("no snapshot received")
+	}
+	cache.Unsubscribe(ch)
+}
+
+// --- gRPC end-to-end test ---
+
+// fakeCollector is an in-process gRPC collector that streams a fixed set of
+// snapshots then blocks until the context is cancelled.
+type fakeCollector struct {
+	pb.UnimplementedLogtailServiceServer
+	snaps []*pb.Snapshot
+}
+
+func (f *fakeCollector) StreamSnapshots(_ *pb.SnapshotRequest, stream grpc.ServerStreamingServer[pb.Snapshot]) error {
+	for _, s := range f.snaps {
+		if err := stream.Send(s); err != nil {
+			return err
+		}
+	}
+	<-stream.Context().Done()
+	return nil
+}
+
+func startFakeCollector(t *testing.T, snaps []*pb.Snapshot) string {
+	t.Helper()
+	lis, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	srv := grpc.NewServer()
+	pb.RegisterLogtailServiceServer(srv, &fakeCollector{snaps: snaps})
+	go srv.Serve(lis)
+	t.Cleanup(srv.Stop)
+	return lis.Addr().String()
+}
+
+func TestGRPCEndToEnd(t *testing.T) {
+	// Two fake collectors with overlapping labels.
+	snap1 := makeSnap("col1", map[string]int64{
+		st.EncodeTuple(st.Tuple4{"busy.com", "1.0.0.0/24", "/", "200"}):  500,
+		st.EncodeTuple(st.Tuple4{"quiet.com", "2.0.0.0/24", "/", "429"}): 100,
+	})
+	snap2 := makeSnap("col2", map[string]int64{
+		st.EncodeTuple(st.Tuple4{"busy.com", "3.0.0.0/24", "/", "200"}):  300,
+		st.EncodeTuple(st.Tuple4{"other.com", "4.0.0.0/24", "/", "200"}): 50,
+	})
+	addr1 := startFakeCollector(t, []*pb.Snapshot{snap1})
+	addr2 := startFakeCollector(t, []*pb.Snapshot{snap2})
+
+	// Start aggregator components.
+	merger := NewMerger()
+	cache := NewCache(merger, "agg-test")
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go NewCollectorSub(addr1, merger).Run(ctx)
+	go NewCollectorSub(addr2, merger).Run(ctx)
+
+	// Wait for both snapshots to be applied.
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		top := merger.TopK(1)
+		if len(top) > 0 && top[0].Count >= 800 { // busy.com: 500+300
+			break
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+
+	// Rotate the cache so the data is queryable.
+	cache.rotate(time.Now())
+
+	// Start a real gRPC server in front of the cache.
+	lis, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	grpcSrv := grpc.NewServer()
+	pb.RegisterLogtailServiceServer(grpcSrv, NewServer(cache, "agg-test"))
+	go grpcSrv.Serve(lis)
+	defer grpcSrv.Stop()
+
+	conn, err := grpc.NewClient(lis.Addr().String(),
+		grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+	client := pb.NewLogtailServiceClient(conn)
+	qctx, qcancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer qcancel()
+
+	// TopN — busy.com should have 800 (500+300) across both collectors.
+	resp, err := client.TopN(qctx, &pb.TopNRequest{
+		GroupBy: pb.GroupBy_WEBSITE,
+		N:       5,
+		Window:  pb.Window_W1M,
+	})
+	if err != nil {
+		t.Fatalf("TopN: %v", err)
+	}
+	if len(resp.Entries) == 0 {
+		t.Fatal("TopN returned no entries")
+	}
+	if resp.Entries[0].Label != "busy.com" || resp.Entries[0].Count != 800 {
+		t.Errorf("top entry = %+v, want {busy.com 800}", resp.Entries[0])
+	}
+	t.Logf("TopN: %v", resp.Entries)
+
+	// Filtered TopN — only 429s: quiet.com=100.
+	f429 := int32(429)
+	resp, err = client.TopN(qctx, &pb.TopNRequest{
+		Filter:  &pb.Filter{HttpResponse: &f429},
+		GroupBy: pb.GroupBy_WEBSITE,
+		N:       5,
+		Window:  pb.Window_W1M,
+	})
+	if err != nil {
+		t.Fatalf("TopN filtered: %v", err)
+	}
+	if len(resp.Entries) != 1 || resp.Entries[0].Label != "quiet.com" {
+		t.Errorf("filtered: %v", resp.Entries)
+	}
+
+	// Trend.
+	tresp, err := client.Trend(qctx, &pb.TrendRequest{Window: pb.Window_W5M})
+	if err != nil {
+		t.Fatalf("Trend: %v", err)
+	}
+	if len(tresp.Points) != 1 || tresp.Points[0].Count != 950 { // 500+100+300+50
+		t.Errorf("trend: %v", tresp.Points)
+	}
+	t.Logf("Trend: %v", tresp.Points)
+
+	// StreamSnapshots — trigger a rotation and verify we receive a snapshot.
+	streamCtx, streamCancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer streamCancel()
+	stream, err := client.StreamSnapshots(streamCtx, &pb.SnapshotRequest{})
+	if err != nil {
+		t.Fatalf("StreamSnapshots: %v", err)
+	}
+	// Wait for the server goroutine to call cache.Subscribe() before rotating.
+	time.Sleep(50 * time.Millisecond)
+	cache.rotate(time.Now()) // trigger a broadcast
+	snap, err := stream.Recv()
+	if err != nil {
+		t.Fatalf("stream.Recv: %v", err)
+	}
+	if snap.Source != "agg-test" {
+		t.Errorf("source = %q, want agg-test", snap.Source)
+	}
+	t.Logf("StreamSnapshots: %d entries from %s", len(snap.Entries), snap.Source)
+}
+
+func TestDegradedCollector(t *testing.T) {
+	// Start one real and one immediately-gone collector.
+	snap1 := makeSnap("col1", map[string]int64{
+		st.EncodeTuple(st.Tuple4{"good.com", "1.0.0.0/24", "/", "200"}): 100,
+	})
+	addr1 := startFakeCollector(t, []*pb.Snapshot{snap1})
+	// addr2 points at nothing — connections will fail immediately.
+	addr2 := "127.0.0.1:1" // port 1 is always refused
+
+	merger := NewMerger()
+	cache := NewCache(merger, "test")
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	go NewCollectorSub(addr1, merger).Run(ctx)
+	go NewCollectorSub(addr2, merger).Run(ctx)
+
+	// Wait for col1's data to appear.
+	deadline := time.Now().Add(3 * time.Second)
+	for time.Now().Before(deadline) {
+		top := merger.TopK(1)
+		if len(top) > 0 {
+			break
+		}
+		time.Sleep(10 * time.Millisecond)
+	}
+	cache.rotate(time.Now())
+
+	// Results should reflect col1 only.
+	entries := cache.QueryTopN(nil, pb.GroupBy_WEBSITE, 5, pb.Window_W1M)
+	if len(entries) == 0 {
+		t.Fatal("no entries despite col1 being healthy")
+	}
+	if entries[0].Label != "good.com" {
+		t.Errorf("top = %q, want good.com", entries[0].Label)
+	}
+	t.Logf("degraded test: got %d entries, top = %s %d", len(entries), entries[0].Label, entries[0].Count)
+}
diff --git a/cmd/aggregator/cache.go b/cmd/aggregator/cache.go
new file mode 100644
index 0000000..8d5816d
--- /dev/null
+++ b/cmd/aggregator/cache.go
@@ -0,0 +1,170 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"time"
+
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+)
+
+// Cache holds the tiered ring buffers for the aggregator and answers TopN,
+// Trend, and StreamSnapshots queries from them. It is populated by a
+// 1-minute ticker that snapshots the current merged view from the Merger.
+//
+// Tick-based (not snapshot-triggered) so the ring stays on the same 1-minute
+// cadence as the collectors, regardless of how many collectors are connected.
+type Cache struct {
+	source string
+	merger *Merger
+
+	mu           sync.RWMutex
+	fineRing     [st.FineRingSize]st.Snapshot
+	fineHead     int
+	fineFilled   int
+	coarseRing   [st.CoarseRingSize]st.Snapshot
+	coarseHead   int
+	coarseFilled int
+	fineTick     int
+
+	subMu sync.Mutex
+	subs  map[chan st.Snapshot]struct{}
+}
+
+func NewCache(merger *Merger, source string) *Cache {
+	return &Cache{
+		merger: merger,
+		source: source,
+		subs:   make(map[chan st.Snapshot]struct{}),
+	}
+}
+
+// Run drives the 1-minute rotation ticker. Blocks until ctx is cancelled.
+func (c *Cache) Run(ctx context.Context) {
+	ticker := time.NewTicker(time.Minute)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case t := <-ticker.C:
+			c.rotate(t)
+		}
+	}
+}
+
+func (c *Cache) rotate(now time.Time) {
+	fine := st.Snapshot{Timestamp: now, Entries: c.merger.TopK(st.FineTopK)}
+
+	c.mu.Lock()
+	c.fineRing[c.fineHead] = fine
+	c.fineHead = (c.fineHead + 1) % st.FineRingSize
+	if c.fineFilled < st.FineRingSize {
+		c.fineFilled++
+	}
+	c.fineTick++
+	if c.fineTick >= st.CoarseEvery {
+		c.fineTick = 0
+		coarse := c.mergeFineBuckets(now)
+		c.coarseRing[c.coarseHead] = coarse
+		c.coarseHead = (c.coarseHead + 1) % st.CoarseRingSize
+		if c.coarseFilled < st.CoarseRingSize {
+			c.coarseFilled++
+		}
+	}
+	c.mu.Unlock()
+
+	c.broadcast(fine)
+}
+
+func (c *Cache) mergeFineBuckets(now time.Time) st.Snapshot {
+	merged := make(map[string]int64)
+	count := min(st.CoarseEvery, c.fineFilled)
+	for i := 0; i < count; i++ {
+		idx := (c.fineHead - 1 - i + st.FineRingSize) % st.FineRingSize
+		for _, e := range c.fineRing[idx].Entries {
+			merged[e.Label] += e.Count
+		}
+	}
+	return st.Snapshot{Timestamp: now, Entries: st.TopKFromMap(merged, st.CoarseTopK)}
+}
+
+// QueryTopN answers a TopN request from the ring buffers.
+func (c *Cache) QueryTopN(filter *pb.Filter, groupBy pb.GroupBy, n int, window pb.Window) []st.Entry {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	buckets, count := st.BucketsForWindow(window, c.fineView(), c.coarseView(), c.fineFilled, c.coarseFilled)
+	grouped := make(map[string]int64)
+	for i := 0; i < count; i++ {
+		idx := (buckets.Head - 1 - i + buckets.Size) % buckets.Size
+		for _, e := range buckets.Ring[idx].Entries {
+			t := st.LabelTuple(e.Label)
+			if !st.MatchesFilter(t, filter) {
+				continue
+			}
+			grouped[st.DimensionLabel(t, groupBy)] += e.Count
+		}
+	}
+	return st.TopKFromMap(grouped, n)
+}
+
+// QueryTrend answers a Trend request from the ring buffers.
+func (c *Cache) QueryTrend(filter *pb.Filter, window pb.Window) []st.TrendPoint {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	buckets, count := st.BucketsForWindow(window, c.fineView(), c.coarseView(), c.fineFilled, c.coarseFilled)
+	points := make([]st.TrendPoint, count)
+	for i := 0; i < count; i++ {
+		idx := (buckets.Head - count + i + buckets.Size) % buckets.Size
+		snap := buckets.Ring[idx]
+		var total int64
+		for _, e := range snap.Entries {
+			if st.MatchesFilter(st.LabelTuple(e.Label), filter) {
+				total += e.Count
+			}
+		}
+		points[i] = st.TrendPoint{Timestamp: snap.Timestamp, Count: total}
+	}
+	return points
+}
+
+func (c *Cache) fineView() st.RingView {
+	ring := make([]st.Snapshot, st.FineRingSize)
+	copy(ring, c.fineRing[:])
+	return st.RingView{Ring: ring, Head: c.fineHead, Size: st.FineRingSize}
+}
+
+func (c *Cache) coarseView() st.RingView {
+	ring := make([]st.Snapshot, st.CoarseRingSize)
+	copy(ring, c.coarseRing[:])
+	return st.RingView{Ring: ring, Head: c.coarseHead, Size: st.CoarseRingSize}
+}
+
+func (c *Cache) Subscribe() chan st.Snapshot {
+	ch := make(chan st.Snapshot, 4)
+	c.subMu.Lock()
+	c.subs[ch] = struct{}{}
+	c.subMu.Unlock()
+	return ch
+}
+
+func (c *Cache) Unsubscribe(ch chan st.Snapshot) {
+	c.subMu.Lock()
+	delete(c.subs, ch)
+	c.subMu.Unlock()
+	close(ch)
+}
+
+func (c *Cache) broadcast(snap st.Snapshot) {
+	c.subMu.Lock()
+	defer c.subMu.Unlock()
+	for ch := range c.subs {
+		select {
+		case ch <- snap:
+		default:
+		}
+	}
+}
diff --git a/cmd/aggregator/main.go b/cmd/aggregator/main.go
new file mode 100644
index 0000000..0b6b896
--- /dev/null
+++ b/cmd/aggregator/main.go
@@ -0,0 +1,69 @@
+package main
+
+import (
+	"context"
+	"flag"
+	"log"
+	"net"
+	"os"
+	"os/signal"
+	"strings"
+	"syscall"
+
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	"google.golang.org/grpc"
+)
+
+func main() {
+	listen     := flag.String("listen", ":9091", "gRPC listen address")
+	collectors := flag.String("collectors", "", "comma-separated collector host:port addresses")
+	source     := flag.String("source", hostname(), "name for this aggregator in responses")
+	flag.Parse()
+
+	if *collectors == "" {
+		log.Fatal("aggregator: --collectors is required")
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer stop()
+
+	merger := NewMerger()
+	cache := NewCache(merger, *source)
+	go cache.Run(ctx)
+
+	for _, addr := range strings.Split(*collectors, ",") {
+		addr = strings.TrimSpace(addr)
+		if addr == "" {
+			continue
+		}
+		sub := NewCollectorSub(addr, merger)
+		go sub.Run(ctx)
+		log.Printf("aggregator: subscribing to collector %s", addr)
+	}
+
+	lis, err := net.Listen("tcp", *listen)
+	if err != nil {
+		log.Fatalf("aggregator: failed to listen on %s: %v", *listen, err)
+	}
+	grpcServer := grpc.NewServer()
+	pb.RegisterLogtailServiceServer(grpcServer, NewServer(cache, *source))
+
+	go func() {
+		log.Printf("aggregator: gRPC listening on %s (source=%s)", *listen, *source)
+		if err := grpcServer.Serve(lis); err != nil {
+			log.Printf("aggregator: gRPC server stopped: %v", err)
+		}
+	}()
+
+	<-ctx.Done()
+	log.Printf("aggregator: shutting down")
+	grpcServer.GracefulStop()
+}
+
+func hostname() string {
+	h, err := os.Hostname()
+	if err != nil {
+		return "unknown"
+	}
+	return h
+}
diff --git a/cmd/aggregator/merger.go b/cmd/aggregator/merger.go
new file mode 100644
index 0000000..87a4fb3
--- /dev/null
+++ b/cmd/aggregator/merger.go
@@ -0,0 +1,70 @@
+package main
+
+import (
+	"sync"
+
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+)
+
+// Merger maintains per-collector maps and a running merged map using a delta
+// strategy: on each new snapshot from collector X, subtract X's old entries
+// and add the new ones. This is O(snapshot_size) rather than
+// O(N_collectors × snapshot_size).
+type Merger struct {
+	mu           sync.Mutex
+	perCollector map[string]map[string]int64 // source → label → count
+	merged       map[string]int64            // label → total across all collectors
+}
+
+func NewMerger() *Merger {
+	return &Merger{
+		perCollector: make(map[string]map[string]int64),
+		merged:       make(map[string]int64),
+	}
+}
+
+// Apply incorporates a snapshot from a collector, replacing that collector's
+// previous contribution in the merged map.
+func (m *Merger) Apply(snap *pb.Snapshot) {
+	addr := snap.Source
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	// Subtract the old contribution.
+	for label, count := range m.perCollector[addr] {
+		m.merged[label] -= count
+		if m.merged[label] <= 0 {
+			delete(m.merged, label)
+		}
+	}
+
+	// Build the new per-collector map and add to merged.
+	newMap := make(map[string]int64, len(snap.Entries))
+	for _, e := range snap.Entries {
+		newMap[e.Label] += e.Count
+		m.merged[e.Label] += e.Count
+	}
+	m.perCollector[addr] = newMap
+}
+
+// Zero removes a degraded collector's entire contribution from the merged map.
+func (m *Merger) Zero(addr string) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+
+	for label, count := range m.perCollector[addr] {
+		m.merged[label] -= count
+		if m.merged[label] <= 0 {
+			delete(m.merged, label)
+		}
+	}
+	delete(m.perCollector, addr)
+}
+
+// TopK returns the top-k entries from the current merged view.
+func (m *Merger) TopK(k int) []st.Entry {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	return st.TopKFromMap(m.merged, k)
+}
diff --git a/cmd/aggregator/server.go b/cmd/aggregator/server.go
new file mode 100644
index 0000000..c0f846b
--- /dev/null
+++ b/cmd/aggregator/server.go
@@ -0,0 +1,80 @@
+package main
+
+import (
+	"context"
+	"log"
+	"time"
+
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
+)
+
+// Server implements pb.LogtailServiceServer backed by the aggregator Cache.
+type Server struct {
+	pb.UnimplementedLogtailServiceServer
+	cache  *Cache
+	source string
+}
+
+func NewServer(cache *Cache, source string) *Server {
+	return &Server{cache: cache, source: source}
+}
+
+func (srv *Server) TopN(_ context.Context, req *pb.TopNRequest) (*pb.TopNResponse, error) {
+	if req == nil {
+		return nil, status.Error(codes.InvalidArgument, "request is nil")
+	}
+	n := int(req.N)
+	if n <= 0 {
+		n = 10
+	}
+	entries := srv.cache.QueryTopN(req.Filter, req.GroupBy, n, req.Window)
+	resp := &pb.TopNResponse{Source: srv.source}
+	for _, e := range entries {
+		resp.Entries = append(resp.Entries, &pb.TopNEntry{Label: e.Label, Count: e.Count})
+	}
+	return resp, nil
+}
+
+func (srv *Server) Trend(_ context.Context, req *pb.TrendRequest) (*pb.TrendResponse, error) {
+	if req == nil {
+		return nil, status.Error(codes.InvalidArgument, "request is nil")
+	}
+	points := srv.cache.QueryTrend(req.Filter, req.Window)
+	resp := &pb.TrendResponse{Source: srv.source}
+	for _, p := range points {
+		resp.Points = append(resp.Points, &pb.TrendPoint{
+			TimestampUnix: p.Timestamp.Unix(),
+			Count:         p.Count,
+		})
+	}
+	return resp, nil
+}
+
+func (srv *Server) StreamSnapshots(_ *pb.SnapshotRequest, stream grpc.ServerStreamingServer[pb.Snapshot]) error {
+	ch := srv.cache.Subscribe()
+	defer srv.cache.Unsubscribe(ch)
+
+	log.Printf("server: new StreamSnapshots subscriber")
+	for {
+		select {
+		case <-stream.Context().Done():
+			log.Printf("server: StreamSnapshots subscriber disconnected")
+			return nil
+		case snap, ok := <-ch:
+			if !ok {
+				return nil
+			}
+			msg := &pb.Snapshot{Source: srv.source, Timestamp: snap.Timestamp.Unix()}
+			for _, e := range snap.Entries {
+				msg.Entries = append(msg.Entries, &pb.TopNEntry{Label: e.Label, Count: e.Count})
+			}
+			if err := stream.Send(msg); err != nil {
+				return err
+			}
+		case <-time.After(30 * time.Second):
+		}
+	}
+}
diff --git a/cmd/aggregator/subscriber.go b/cmd/aggregator/subscriber.go
new file mode 100644
index 0000000..077728c
--- /dev/null
+++ b/cmd/aggregator/subscriber.go
@@ -0,0 +1,97 @@
+package main
+
+import (
+	"context"
+	"log"
+	"time"
+
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+)
+
+// CollectorSub maintains a persistent StreamSnapshots connection to one
+// collector. It reconnects with exponential backoff on any error and marks
+// the collector degraded (zeroing its contribution) after 3 consecutive
+// failures.
+type CollectorSub struct {
+	addr   string
+	merger *Merger
+}
+
+func NewCollectorSub(addr string, merger *Merger) *CollectorSub {
+	return &CollectorSub{addr: addr, merger: merger}
+}
+
+// Run blocks until ctx is cancelled.
+func (cs *CollectorSub) Run(ctx context.Context) {
+	backoff := 100 * time.Millisecond
+	const maxBackoff = 30 * time.Second
+	fails := 0
+	degraded := false
+
+	for {
+		if ctx.Err() != nil {
+			return
+		}
+
+		gotOne, err := cs.stream(ctx)
+
+		if ctx.Err() != nil {
+			return
+		}
+
+		if gotOne && degraded {
+			// Recovered: contribution is already flowing in again via Apply.
+			degraded = false
+			fails = 0
+			backoff = 100 * time.Millisecond
+			log.Printf("subscriber: collector %s recovered", cs.addr)
+		}
+
+		if err != nil {
+			fails++
+			log.Printf("subscriber: collector %s error (fail %d): %v", cs.addr, fails, err)
+			if fails >= 3 && !degraded {
+				degraded = true
+				cs.merger.Zero(cs.addr)
+				log.Printf("subscriber: collector %s degraded — contribution zeroed", cs.addr)
+			}
+		}
+
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(backoff):
+		}
+		backoff = min(backoff*2, maxBackoff)
+	}
+}
+
+// stream opens a single StreamSnapshots RPC and feeds snapshots into the
+// merger until the stream errors or ctx is cancelled. Returns (gotAtLeastOne, err).
+func (cs *CollectorSub) stream(ctx context.Context) (bool, error) {
+	conn, err := grpc.NewClient(cs.addr,
+		grpc.WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		return false, err
+	}
+	defer conn.Close()
+
+	client := pb.NewLogtailServiceClient(conn)
+	stream, err := client.StreamSnapshots(ctx, &pb.SnapshotRequest{})
+	if err != nil {
+		return false, err
+	}
+
+	log.Printf("subscriber: connected to collector %s", cs.addr)
+	gotOne := false
+	for {
+		snap, err := stream.Recv()
+		if err != nil {
+			return gotOne, err
+		}
+		gotOne = true
+		cs.merger.Apply(snap)
+	}
+}
diff --git a/cmd/collector/smoke_test.go b/cmd/collector/smoke_test.go
index 353cf07..6d0ef28 100644
--- a/cmd/collector/smoke_test.go
+++ b/cmd/collector/smoke_test.go
@@ -8,6 +8,7 @@ import (
 	"testing"
 	"time"
 
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
 	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials/insecure"
@@ -60,7 +61,7 @@ func TestMemoryBudget(t *testing.T) {
 	}
 
 	// Rotate 60 fine buckets to fill the fine ring
-	for i := 0; i < fineRingSize; i++ {
+	for i := 0; i < st.FineRingSize; i++ {
 		for j := 0; j < 1000; j++ {
 			s.ingest(LogRecord{
 				Website:      fmt.Sprintf("site%d.com", j%1000),
@@ -73,7 +74,7 @@ func TestMemoryBudget(t *testing.T) {
 	}
 
 	// Rotate enough to fill the coarse ring (288 coarse buckets × 5 fine each)
-	for i := 0; i < coarseRingSize*coarseEvery; i++ {
+	for i := 0; i < st.CoarseRingSize*st.CoarseEvery; i++ {
 		for j := 0; j < 100; j++ {
 			s.ingest(LogRecord{
 				Website:      fmt.Sprintf("site%d.com", j%1000),
@@ -82,7 +83,7 @@ func TestMemoryBudget(t *testing.T) {
 				Status:       "200",
 			})
 		}
-		s.rotate(now.Add(time.Duration(fineRingSize+i) * time.Minute))
+		s.rotate(now.Add(time.Duration(st.FineRingSize+i) * time.Minute))
 	}
 
 	var ms runtime.MemStats
diff --git a/cmd/collector/store.go b/cmd/collector/store.go
index 3303d00..bd0d2ea 100644
--- a/cmd/collector/store.go
+++ b/cmd/collector/store.go
@@ -1,253 +1,175 @@
 package main
 
 import (
-	"container/heap"
-	"fmt"
 	"sync"
 	"time"
 
 	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
 )
 
-const (
-	liveMapCap      = 100_000 // hard cap on live map entries
-	fineRingSize    = 60      // 60 × 1-min buckets → 1 hour
-	coarseRingSize  = 288     // 288 × 5-min buckets → 24 hours
-	fineTopK        = 50_000  // entries kept per fine snapshot
-	coarseTopK      = 5_000   // entries kept per coarse snapshot
-	coarseEvery     = 5       // merge every N fine ticks into one coarse bucket
-)
-
-// Tuple4 is the four-dimensional key.
-type Tuple4 struct {
-	Website string
-	Prefix  string
-	URI     string
-	Status  string
-}
-
-// Entry is a labelled count used in snapshots and query results.
-type Entry struct {
-	Label string
-	Count int64
-}
-
-// snapshot is one sorted (desc) slice of top-K entries for a time bucket.
-type snapshot struct {
-	Timestamp time.Time
-	Entries   []Entry // sorted descending by Count
-}
+const liveMapCap = 100_000 // hard cap on live map entries
 
 // Store holds the live map and both ring buffers.
 type Store struct {
 	source string
 
-	// live map — written only by Run goroutine, no locking needed for writes
-	live    map[Tuple4]int64
-	liveLen int // tracked separately to avoid map len() call in hot path
+	// live map — written only by the Run goroutine; no locking needed on writes
+	live    map[st.Tuple4]int64
+	liveLen int
 
-	// ring buffers — protected by mu for reads (Run goroutine writes)
-	mu          sync.RWMutex
-	fineRing    [fineRingSize]snapshot
-	fineHead    int // index of next write slot
-	fineFilled  int // how many slots are populated
-
-	coarseRing    [coarseRingSize]snapshot
-	coarseHead    int
-	coarseFilled  int
-	fineTick      int // counts fine ticks mod coarseEvery
+	// ring buffers — protected by mu for reads
+	mu           sync.RWMutex
+	fineRing     [st.FineRingSize]st.Snapshot
+	fineHead     int
+	fineFilled   int
+	coarseRing   [st.CoarseRingSize]st.Snapshot
+	coarseHead   int
+	coarseFilled int
+	fineTick     int
 
 	// fan-out to StreamSnapshots subscribers
 	subMu sync.Mutex
-	subs  map[chan snapshot]struct{}
+	subs  map[chan st.Snapshot]struct{}
 }
 
 func NewStore(source string) *Store {
 	return &Store{
 		source: source,
-		live:   make(map[Tuple4]int64, liveMapCap),
-		subs:   make(map[chan snapshot]struct{}),
+		live:   make(map[st.Tuple4]int64, liveMapCap),
+		subs:   make(map[chan st.Snapshot]struct{}),
 	}
 }
 
-// Ingest records one log record into the live map.
+// ingest records one log record into the live map.
 // Must only be called from the Run goroutine.
 func (s *Store) ingest(r LogRecord) {
-	key := Tuple4{r.Website, r.ClientPrefix, r.URI, r.Status}
+	key := st.Tuple4{Website: r.Website, Prefix: r.ClientPrefix, URI: r.URI, Status: r.Status}
 	if _, exists := s.live[key]; !exists {
 		if s.liveLen >= liveMapCap {
-			return // drop new keys when at cap
+			return
 		}
 		s.liveLen++
 	}
 	s.live[key]++
 }
 
-// rotate snapshots the live map into the fine ring, and every coarseEvery ticks
+// rotate snapshots the live map into the fine ring and, every CoarseEvery ticks,
 // also merges into the coarse ring. Called once per minute by Run.
 func (s *Store) rotate(now time.Time) {
-	fine := topK(s.live, fineTopK, now)
+	fine := st.TopKFromTupleMap(s.live, st.FineTopK, now)
 
 	s.mu.Lock()
 	s.fineRing[s.fineHead] = fine
-	s.fineHead = (s.fineHead + 1) % fineRingSize
-	if s.fineFilled < fineRingSize {
+	s.fineHead = (s.fineHead + 1) % st.FineRingSize
+	if s.fineFilled < st.FineRingSize {
 		s.fineFilled++
 	}
-
 	s.fineTick++
-	if s.fineTick >= coarseEvery {
+	if s.fineTick >= st.CoarseEvery {
 		s.fineTick = 0
-		coarse := s.mergeFineBuckets(coarseTopK, now)
+		coarse := s.mergeFineBuckets(now)
 		s.coarseRing[s.coarseHead] = coarse
-		s.coarseHead = (s.coarseHead + 1) % coarseRingSize
-		if s.coarseFilled < coarseRingSize {
+		s.coarseHead = (s.coarseHead + 1) % st.CoarseRingSize
+		if s.coarseFilled < st.CoarseRingSize {
 			s.coarseFilled++
 		}
 	}
 	s.mu.Unlock()
 
-	// reset live map
-	s.live = make(map[Tuple4]int64, liveMapCap)
+	s.live = make(map[st.Tuple4]int64, liveMapCap)
 	s.liveLen = 0
 
-	// notify subscribers — must be outside mu to avoid deadlock
 	s.broadcast(fine)
 }
 
-// mergeFineBuckets merges the last coarseEvery fine snapshots into one.
-// Called with mu held.
-func (s *Store) mergeFineBuckets(k int, now time.Time) snapshot {
+func (s *Store) mergeFineBuckets(now time.Time) st.Snapshot {
 	merged := make(map[string]int64)
-	count := coarseEvery
-	if count > s.fineFilled {
-		count = s.fineFilled
-	}
+	count := min(st.CoarseEvery, s.fineFilled)
 	for i := 0; i < count; i++ {
-		idx := (s.fineHead - 1 - i + fineRingSize) % fineRingSize
+		idx := (s.fineHead - 1 - i + st.FineRingSize) % st.FineRingSize
 		for _, e := range s.fineRing[idx].Entries {
 			merged[e.Label] += e.Count
 		}
 	}
-	entries := topKFromMap(merged, k)
-	return snapshot{Timestamp: now, Entries: entries}
+	return st.Snapshot{Timestamp: now, Entries: st.TopKFromMap(merged, st.CoarseTopK)}
 }
 
 // QueryTopN answers a TopN request from the ring buffers.
-func (s *Store) QueryTopN(filter *pb.Filter, groupBy pb.GroupBy, n int, window pb.Window) []Entry {
+func (s *Store) QueryTopN(filter *pb.Filter, groupBy pb.GroupBy, n int, window pb.Window) []st.Entry {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
-	buckets, count := s.bucketsForWindow(window)
-
-	// Accumulate grouped counts
+	buckets, count := st.BucketsForWindow(window, s.fineView(), s.coarseView(), s.fineFilled, s.coarseFilled)
 	grouped := make(map[string]int64)
 	for i := 0; i < count; i++ {
-		idx := (buckets.head - 1 - i + buckets.size) % buckets.size
-		snap := buckets.ring[idx]
-		for _, e := range snap.Entries {
-			t := labelTuple(e.Label)
-			if !matchesFilter(t, filter) {
+		idx := (buckets.Head - 1 - i + buckets.Size) % buckets.Size
+		for _, e := range buckets.Ring[idx].Entries {
+			t := st.LabelTuple(e.Label)
+			if !st.MatchesFilter(t, filter) {
 				continue
 			}
-			grouped[dimensionLabel(t, groupBy)] += e.Count
+			grouped[st.DimensionLabel(t, groupBy)] += e.Count
 		}
 	}
-
-	return topKFromMap(grouped, n)
+	return st.TopKFromMap(grouped, n)
 }
 
 // QueryTrend answers a Trend request from the ring buffers.
-func (s *Store) QueryTrend(filter *pb.Filter, window pb.Window) []trendPoint {
+func (s *Store) QueryTrend(filter *pb.Filter, window pb.Window) []st.TrendPoint {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
 
-	buckets, count := s.bucketsForWindow(window)
-	points := make([]trendPoint, count)
+	buckets, count := st.BucketsForWindow(window, s.fineView(), s.coarseView(), s.fineFilled, s.coarseFilled)
+	points := make([]st.TrendPoint, count)
 	for i := 0; i < count; i++ {
-		// oldest first
-		idx := (buckets.head - count + i + buckets.size) % buckets.size
-		snap := buckets.ring[idx]
+		idx := (buckets.Head - count + i + buckets.Size) % buckets.Size
+		snap := buckets.Ring[idx]
 		var total int64
 		for _, e := range snap.Entries {
-			if matchesFilter(labelTuple(e.Label), filter) {
+			if st.MatchesFilter(st.LabelTuple(e.Label), filter) {
 				total += e.Count
 			}
 		}
-		points[i] = trendPoint{Timestamp: snap.Timestamp, Count: total}
+		points[i] = st.TrendPoint{Timestamp: snap.Timestamp, Count: total}
 	}
 	return points
 }
 
-type trendPoint struct {
-	Timestamp time.Time
-	Count     int64
-}
-
-// ringView is a helper to treat fine and coarse rings uniformly.
-type ringView struct {
-	ring []snapshot
-	head int
-	size int
-}
-
-func (s *Store) bucketsForWindow(window pb.Window) (ringView, int) {
-	switch window {
-	case pb.Window_W1M:
-		return s.fineView(), min(1, s.fineFilled)
-	case pb.Window_W5M:
-		return s.fineView(), min(5, s.fineFilled)
-	case pb.Window_W15M:
-		return s.fineView(), min(15, s.fineFilled)
-	case pb.Window_W60M:
-		return s.fineView(), min(60, s.fineFilled)
-	case pb.Window_W6H:
-		return s.coarseView(), min(72, s.coarseFilled) // 72 × 5-min = 6h
-	case pb.Window_W24H:
-		return s.coarseView(), min(288, s.coarseFilled)
-	default:
-		return s.fineView(), min(5, s.fineFilled)
-	}
-}
-
-func (s *Store) fineView() ringView {
-	ring := make([]snapshot, fineRingSize)
+func (s *Store) fineView() st.RingView {
+	ring := make([]st.Snapshot, st.FineRingSize)
 	copy(ring, s.fineRing[:])
-	return ringView{ring: ring, head: s.fineHead, size: fineRingSize}
+	return st.RingView{Ring: ring, Head: s.fineHead, Size: st.FineRingSize}
 }
 
-func (s *Store) coarseView() ringView {
-	ring := make([]snapshot, coarseRingSize)
+func (s *Store) coarseView() st.RingView {
+	ring := make([]st.Snapshot, st.CoarseRingSize)
 	copy(ring, s.coarseRing[:])
-	return ringView{ring: ring, head: s.coarseHead, size: coarseRingSize}
+	return st.RingView{Ring: ring, Head: s.coarseHead, Size: st.CoarseRingSize}
 }
 
-// Subscribe returns a channel that receives a copy of each fine snapshot
-// after rotation. Buffer of 4 so a slow subscriber doesn't block rotation.
-func (s *Store) Subscribe() chan snapshot {
-	ch := make(chan snapshot, 4)
+func (s *Store) Subscribe() chan st.Snapshot {
+	ch := make(chan st.Snapshot, 4)
 	s.subMu.Lock()
 	s.subs[ch] = struct{}{}
 	s.subMu.Unlock()
 	return ch
 }
 
-// Unsubscribe removes and closes the subscriber channel.
-func (s *Store) Unsubscribe(ch chan snapshot) {
+func (s *Store) Unsubscribe(ch chan st.Snapshot) {
 	s.subMu.Lock()
 	delete(s.subs, ch)
 	s.subMu.Unlock()
 	close(ch)
 }
 
-func (s *Store) broadcast(snap snapshot) {
+func (s *Store) broadcast(snap st.Snapshot) {
 	s.subMu.Lock()
 	defer s.subMu.Unlock()
 	for ch := range s.subs {
 		select {
 		case ch <- snap:
 		default:
-			// subscriber is slow; drop rather than block rotation
 		}
 	}
 }
@@ -269,125 +191,3 @@ func (s *Store) Run(ch <-chan LogRecord) {
 		}
 	}
 }
-
-// --- heap-based top-K helpers ---
-
-type entryHeap []Entry
-
-func (h entryHeap) Len() int            { return len(h) }
-func (h entryHeap) Less(i, j int) bool  { return h[i].Count < h[j].Count } // min-heap
-func (h entryHeap) Swap(i, j int)       { h[i], h[j] = h[j], h[i] }
-func (h *entryHeap) Push(x interface{}) { *h = append(*h, x.(Entry)) }
-func (h *entryHeap) Pop() interface{} {
-	old := *h
-	n := len(old)
-	x := old[n-1]
-	*h = old[:n-1]
-	return x
-}
-
-// topK extracts the top-k entries from a Tuple4 map, labelled as "w|p|u|s".
-func topK(m map[Tuple4]int64, k int, ts time.Time) snapshot {
-	// Build a string-keyed map for topKFromMap
-	flat := make(map[string]int64, len(m))
-	for t, c := range m {
-		flat[encodeTuple(t)] = c
-	}
-	return snapshot{Timestamp: ts, Entries: topKFromMap(flat, k)}
-}
-
-// topKFromMap selects the top-k entries from a string→count map, sorted desc.
-func topKFromMap(m map[string]int64, k int) []Entry {
-	if k <= 0 {
-		return nil
-	}
-	h := make(entryHeap, 0, k+1)
-	for label, count := range m {
-		heap.Push(&h, Entry{Label: label, Count: count})
-		if h.Len() > k {
-			heap.Pop(&h) // evict smallest
-		}
-	}
-	result := make([]Entry, h.Len())
-	for i := len(result) - 1; i >= 0; i-- {
-		result[i] = heap.Pop(&h).(Entry)
-	}
-	return result
-}
-
-// --- label encoding: "website\x00prefix\x00uri\x00status" ---
-
-func encodeTuple(t Tuple4) string {
-	return t.Website + "\x00" + t.Prefix + "\x00" + t.URI + "\x00" + t.Status
-}
-
-func labelTuple(label string) Tuple4 {
-	parts := splitN(label, '\x00', 4)
-	if len(parts) != 4 {
-		return Tuple4{}
-	}
-	return Tuple4{parts[0], parts[1], parts[2], parts[3]}
-}
-
-func splitN(s string, sep byte, n int) []string {
-	result := make([]string, 0, n)
-	for len(result) < n-1 {
-		i := indexOf(s, sep)
-		if i < 0 {
-			break
-		}
-		result = append(result, s[:i])
-		s = s[i+1:]
-	}
-	return append(result, s)
-}
-
-func indexOf(s string, b byte) int {
-	for i := 0; i < len(s); i++ {
-		if s[i] == b {
-			return i
-		}
-	}
-	return -1
-}
-
-func matchesFilter(t Tuple4, f *pb.Filter) bool {
-	if f == nil {
-		return true
-	}
-	if f.Website != nil && t.Website != f.GetWebsite() {
-		return false
-	}
-	if f.ClientPrefix != nil && t.Prefix != f.GetClientPrefix() {
-		return false
-	}
-	if f.HttpRequestUri != nil && t.URI != f.GetHttpRequestUri() {
-		return false
-	}
-	if f.HttpResponse != nil && t.Status != fmt.Sprint(f.GetHttpResponse()) {
-		return false
-	}
-	return true
-}
-
-func dimensionLabel(t Tuple4, g pb.GroupBy) string {
-	switch g {
-	case pb.GroupBy_WEBSITE:
-		return t.Website
-	case pb.GroupBy_CLIENT_PREFIX:
-		return t.Prefix
-	case pb.GroupBy_REQUEST_URI:
-		return t.URI
-	case pb.GroupBy_HTTP_RESPONSE:
-		return t.Status
-	default:
-		return t.Website
-	}
-}
-
-func min(a, b int) int {
-	if a < b {
-		return a
-	}
-	return b
-}
diff --git a/cmd/collector/store_test.go b/cmd/collector/store_test.go
index bfcc472..bd0779c 100644
--- a/cmd/collector/store_test.go
+++ b/cmd/collector/store_test.go
@@ -6,6 +6,7 @@ import (
 	"time"
 
 	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+	st "git.ipng.ch/ipng/nginx-logtail/internal/store"
 )
 
 func makeStore() *Store {
@@ -30,7 +31,7 @@ func TestIngestAndRotate(t *testing.T) {
 	if s.fineFilled != 1 {
 		t.Fatalf("fineFilled = %d, want 1", s.fineFilled)
 	}
-	snap := s.fineRing[(s.fineHead-1+fineRingSize)%fineRingSize]
+	snap := s.fineRing[(s.fineHead-1+st.FineRingSize)%st.FineRingSize]
 	if len(snap.Entries) != 2 {
 		t.Fatalf("snapshot has %d entries, want 2", len(snap.Entries))
 	}
@@ -41,13 +42,12 @@ func TestIngestAndRotate(t *testing.T) {
 
 func TestLiveMapCap(t *testing.T) {
 	s := makeStore()
-	// Ingest liveMapCap+100 distinct keys; only liveMapCap should be tracked
 	for i := 0; i < liveMapCap+100; i++ {
 		s.ingest(LogRecord{
-			Website: fmt.Sprintf("site%d.com", i),
+			Website:      fmt.Sprintf("site%d.com", i),
 			ClientPrefix: "1.2.3.0/24",
-			URI:     "/",
-			Status:  "200",
+			URI:          "/",
+			Status:       "200",
 		})
 	}
 	if s.liveLen != liveMapCap {
@@ -86,7 +86,6 @@ func TestQueryTopNWithFilter(t *testing.T) {
 	if len(entries) != 2 {
 		t.Fatalf("got %d entries, want 2", len(entries))
 	}
-	// example.com has 200 × 429, other.com has 100 × 429
 	if entries[0].Label != "example.com" || entries[0].Count != 200 {
 		t.Errorf("unexpected top: %+v", entries[0])
 	}
@@ -96,13 +95,10 @@ func TestQueryTrend(t *testing.T) {
 	s := makeStore()
 	now := time.Now()
 
-	// Rotate 3 buckets with different counts
 	ingestN(s, "x.com", "1.0.0.0/24", "/", "200", 10)
 	s.rotate(now.Add(-2 * time.Minute))
-
 	ingestN(s, "x.com", "1.0.0.0/24", "/", "200", 20)
 	s.rotate(now.Add(-1 * time.Minute))
-
 	ingestN(s, "x.com", "1.0.0.0/24", "/", "200", 30)
 	s.rotate(now)
 
@@ -110,7 +106,6 @@ func TestQueryTrend(t *testing.T) {
 	if len(points) != 3 {
 		t.Fatalf("got %d points, want 3", len(points))
 	}
-	// Points are oldest-first; counts should be 10, 20, 30
 	if points[0].Count != 10 || points[1].Count != 20 || points[2].Count != 30 {
 		t.Errorf("unexpected counts: %v", points)
 	}
@@ -120,8 +115,7 @@ func TestCoarseRingPopulated(t *testing.T) {
 	s := makeStore()
 	now := time.Now()
 
-	// Rotate coarseEvery fine buckets to trigger one coarse bucket
-	for i := 0; i < coarseEvery; i++ {
+	for i := 0; i < st.CoarseEvery; i++ {
 		ingestN(s, "x.com", "1.0.0.0/24", "/", "200", 10)
 		s.rotate(now.Add(time.Duration(i) * time.Minute))
 	}
@@ -131,11 +125,10 @@ func TestCoarseRingPopulated(t *testing.T) {
 	if s.coarseFilled != 1 {
 		t.Fatalf("coarseFilled = %d, want 1", s.coarseFilled)
 	}
-	coarse := s.coarseRing[(s.coarseHead-1+coarseRingSize)%coarseRingSize]
+	coarse := s.coarseRing[(s.coarseHead-1+st.CoarseRingSize)%st.CoarseRingSize]
 	if len(coarse.Entries) == 0 {
 		t.Fatal("coarse snapshot is empty")
 	}
-	// 5 fine buckets × 10 counts = 50
 	if coarse.Entries[0].Count != 50 {
 		t.Errorf("coarse count = %d, want 50", coarse.Entries[0].Count)
 	}
@@ -160,13 +153,8 @@ func TestSubscribeBroadcast(t *testing.T) {
 }
 
 func TestTopKOrdering(t *testing.T) {
-	m := map[string]int64{
-		"a": 5,
-		"b": 100,
-		"c": 1,
-		"d": 50,
-	}
-	entries := topKFromMap(m, 3)
+	m := map[string]int64{"a": 5, "b": 100, "c": 1, "d": 50}
+	entries := st.TopKFromMap(m, 3)
 	if len(entries) != 3 {
 		t.Fatalf("got %d entries, want 3", len(entries))
 	}
diff --git a/internal/store/store.go b/internal/store/store.go
new file mode 100644
index 0000000..a999399
--- /dev/null
+++ b/internal/store/store.go
@@ -0,0 +1,196 @@
+// Package store provides the shared ring-buffer, label-encoding and query
+// helpers used by both the collector and the aggregator.
+package store
+
+import (
+	"container/heap"
+	"fmt"
+	"time"
+
+	pb "git.ipng.ch/ipng/nginx-logtail/proto/logtailpb"
+)
+
+// Ring-buffer dimensions — shared between collector and aggregator.
+const (
+	FineRingSize   = 60      // 60 × 1-min buckets  → 1 hour
+	CoarseRingSize = 288     // 288 × 5-min buckets → 24 hours
+	FineTopK       = 50_000  // entries kept per fine snapshot
+	CoarseTopK     = 5_000   // entries kept per coarse snapshot
+	CoarseEvery    = 5       // fine ticks between coarse writes
+)
+
+// Tuple4 is the four-dimensional aggregation key.
+type Tuple4 struct {
+	Website string
+	Prefix  string
+	URI     string
+	Status  string
+}
+
+// Entry is a labelled count used in snapshots and query results.
+type Entry struct {
+	Label string
+	Count int64
+}
+
+// Snapshot is one sorted (desc) slice of top-K entries for a time bucket.
+type Snapshot struct {
+	Timestamp time.Time
+	Entries   []Entry
+}
+
+// TrendPoint is a (timestamp, total-count) pair for sparkline queries.
+type TrendPoint struct {
+	Timestamp time.Time
+	Count     int64
+}
+
+// RingView is a read-only snapshot of a ring buffer for iteration.
+type RingView struct {
+	Ring []Snapshot
+	Head int // index of next write slot (one past the latest entry)
+	Size int
+}
+
+// BucketsForWindow returns the RingView and number of buckets to sum for window.
+func BucketsForWindow(window pb.Window, fine, coarse RingView, fineFilled, coarseFilled int) (RingView, int) {
+	switch window {
+	case pb.Window_W1M:
+		return fine, min(1, fineFilled)
+	case pb.Window_W5M:
+		return fine, min(5, fineFilled)
+	case pb.Window_W15M:
+		return fine, min(15, fineFilled)
+	case pb.Window_W60M:
+		return fine, min(60, fineFilled)
+	case pb.Window_W6H:
+		return coarse, min(72, coarseFilled) // 72 × 5-min = 6 h
+	case pb.Window_W24H:
+		return coarse, min(288, coarseFilled)
+	default:
+		return fine, min(5, fineFilled)
+	}
+}
+
+// --- label encoding: "website\x00prefix\x00uri\x00status" ---
+
+// EncodeTuple encodes a Tuple4 as a NUL-separated string suitable for use
+// as a map key in snapshots.
+func EncodeTuple(t Tuple4) string {
+	return t.Website + "\x00" + t.Prefix + "\x00" + t.URI + "\x00" + t.Status
+}
+
+// LabelTuple decodes a NUL-separated snapshot label back into a Tuple4.
+func LabelTuple(label string) Tuple4 {
+	parts := splitN(label, '\x00', 4)
+	if len(parts) != 4 {
+		return Tuple4{}
+	}
+	return Tuple4{parts[0], parts[1], parts[2], parts[3]}
+}
+
+func splitN(s string, sep byte, n int) []string {
+	result := make([]string, 0, n)
+	for len(result) < n-1 {
+		i := indexOf(s, sep)
+		if i < 0 {
+			break
+		}
+		result = append(result, s[:i])
+		s = s[i+1:]
+	}
+	return append(result, s)
+}
+
+func indexOf(s string, b byte) int {
+	for i := 0; i < len(s); i++ {
+		if s[i] == b {
+			return i
+		}
+	}
+	return -1
+}
+
+// --- filtering and grouping ---
+
+// MatchesFilter returns true if t satisfies all constraints in f.
+// A nil filter matches everything.
+func MatchesFilter(t Tuple4, f *pb.Filter) bool {
+	if f == nil {
+		return true
+	}
+	if f.Website != nil && t.Website != f.GetWebsite() {
+		return false
+	}
+	if f.ClientPrefix != nil && t.Prefix != f.GetClientPrefix() {
+		return false
+	}
+	if f.HttpRequestUri != nil && t.URI != f.GetHttpRequestUri() {
+		return false
+	}
+	if f.HttpResponse != nil && t.Status != fmt.Sprint(f.GetHttpResponse()) {
+		return false
+	}
+	return true
+}
+
+// DimensionLabel returns the string value of t for the given group-by dimension.
+func DimensionLabel(t Tuple4, g pb.GroupBy) string {
+	switch g {
+	case pb.GroupBy_WEBSITE:
+		return t.Website
+	case pb.GroupBy_CLIENT_PREFIX:
+		return t.Prefix
+	case pb.GroupBy_REQUEST_URI:
+		return t.URI
+	case pb.GroupBy_HTTP_RESPONSE:
+		return t.Status
+	default:
+		return t.Website
+	}
+}
+
+// --- heap-based top-K selection ---
+
+type entryHeap []Entry
+
+func (h entryHeap) Len() int            { return len(h) }
+func (h entryHeap) Less(i, j int) bool  { return h[i].Count < h[j].Count } // min-heap
+func (h entryHeap) Swap(i, j int)       { h[i], h[j] = h[j], h[i] }
+func (h *entryHeap) Push(x interface{}) { *h = append(*h, x.(Entry)) }
+func (h *entryHeap) Pop() interface{} {
+	old := *h
+	n := len(old)
+	x := old[n-1]
+	*h = old[:n-1]
+	return x
+}
+
+// TopKFromMap selects the top-k entries from a label→count map, sorted desc.
+func TopKFromMap(m map[string]int64, k int) []Entry {
+	if k <= 0 {
+		return nil
+	}
+	h := make(entryHeap, 0, k+1)
+	for label, count := range m {
+		heap.Push(&h, Entry{Label: label, Count: count})
+		if h.Len() > k {
+			heap.Pop(&h)
+		}
+	}
+	result := make([]Entry, h.Len())
+	for i := len(result) - 1; i >= 0; i-- {
+		result[i] = heap.Pop(&h).(Entry)
+	}
+	return result
+}
+
+// TopKFromTupleMap encodes a Tuple4 map and returns the top-k as a Snapshot.
+// Used by the collector to snapshot its live map.
+func TopKFromTupleMap(m map[Tuple4]int64, k int, ts time.Time) Snapshot {
+	flat := make(map[string]int64, len(m))
+	for t, c := range m {
+		flat[EncodeTuple(t)] = c
+	}
+	return Snapshot{Timestamp: ts, Entries: TopKFromMap(flat, k)}
+}