aperture: internally aggregate session statistics to reduce cardinality

hashmail_server.go

@@ -11,7 +11,6 @@ import (
 	"github.com/btcsuite/btclog/v2"
 	"github.com/lightninglabs/lightning-node-connect/hashmailrpc"
 	"github.com/lightningnetwork/lnd/tlv"
-	"github.com/prometheus/client_golang/prometheus"
 	"golang.org/x/time/rate"
 	"google.golang.org/grpc/codes"
 	"google.golang.org/grpc/status"
@@ -35,6 +34,11 @@ const (
 	// DefaultBufSize is the default number of bytes that are read in a
 	// single operation.
 	DefaultBufSize = 4096
+
+	// streamTTL is the amount of time that a stream needs to be exist without
+	// reads for it to be considered for pruning. Otherwise, memory will grow
+	// unbounded.
+	streamTTL = 24 * time.Hour
 )
 
 // streamID is the identifier of a stream.
@@ -747,9 +751,7 @@ func (h *hashMailServer) RecvStream(desc *hashmailrpc.CipherBoxDesc,
 		streamID := newStreamID(desc.StreamId)
 		if streamID.isOdd() {
 			baseID := streamID.baseID()
-			mailboxReadCount.With(prometheus.Labels{
-				streamIDLabel: fmt.Sprintf("%x", baseID),
-			}).Inc()
+			streamActivityTracker.Record(fmt.Sprintf("%x", baseID))
 		}
 
 		err = reader.Send(&hashmailrpc.CipherBox{
@@ -766,6 +768,91 @@ func (h *hashMailServer) RecvStream(desc *hashmailrpc.CipherBoxDesc,
 
 var _ hashmailrpc.HashMailServer = (*hashMailServer)(nil)
 
+// streamActivity tracks per-session read activity for classifying mailbox
+// sessions as active, standby, or in-use. It maintains an in-memory map
+// of stream IDs to counters and timestamps.
+type streamActivity struct {
+	sync.Mutex
+	streams map[string]*activityEntry
+}
+
+// activityEntry holds the read count and last update time for a single mailbox
+// session.
+type activityEntry struct {
+	count      uint64
+	lastUpdate time.Time
+}
+
+// newStreamActivity creates a new streamActivity tracker used to monitor
+// mailbox read activity per stream ID.
+func newStreamActivity() *streamActivity {
+	return &streamActivity{
+		streams: make(map[string]*activityEntry),
+	}
+}
+
+// Record logs a read event for the given base stream ID.
+// It increments the read count and updates the last activity timestamp.
+func (sa *streamActivity) Record(baseID string) {
+	sa.Lock()
+	defer sa.Unlock()
+
+	entry, ok := sa.streams[baseID]
+	if !ok {
+		entry = &activityEntry{}
+		sa.streams[baseID] = entry
+	}
+	entry.count++
+	entry.lastUpdate = time.Now()
+}
+
+// ClassifyAndReset categorizes each tracked stream based on its recent read
+// rate and returns aggregate counts of active, standby, and in-use sessions.
+// A stream is classified as:
+// - In-use:   if read rate ≥ 0.5 reads/sec
+// - Standby:  if 0 < read rate < 0.5 reads/sec
+// - Active:   if read rate > 0 (includes standby and in-use)
+func (sa *streamActivity) ClassifyAndReset() (active, standby, inuse int) {
+	sa.Lock()
+	defer sa.Unlock()
+
+	now := time.Now()
+
+	for baseID, e := range sa.streams {
+		inactiveDuration := now.Sub(e.lastUpdate)
+
+		// Prune if idle for >24h and no new reads.
+		if e.count == 0 && inactiveDuration > streamTTL {
+			delete(sa.streams, baseID)
+			continue
+		}
+
+		elapsed := inactiveDuration.Seconds()
+		if elapsed <= 0 {
+			// Prevent divide-by-zero, treat as 1s interval.
+			elapsed = 1
+		}
+
+		rate := float64(e.count) / elapsed
+
+		switch {
+		case rate >= 0.5:
+			inuse++
+		case rate > 0:
+			standby++
+		}
+		if rate > 0 {
+			active++
+		}
+
+		// Reset for next window.
+		e.count = 0
+		e.lastUpdate = now
+	}
+
+	return active, standby, inuse
+}
+
 // streamStatus keeps track of the occupancy status of a stream's read and
 // write sub-streams. It is initialised with callback functions to call on the
 // event of the streams being occupied (either or both of the streams are

prometheus.go

@@ -3,13 +3,12 @@ package aperture
 import (
 	"fmt"
 	"net/http"
+	"time"
 
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const streamIDLabel = "streamID"
-
 var (
 	// mailboxCount tracks the current number of active mailboxes.
 	mailboxCount = prometheus.NewGauge(prometheus.GaugeOpts{
@@ -17,21 +16,31 @@ var (
 		Name:      "mailbox_count",
 	})
 
-	// mailboxReadCount counts each time a mailbox pair is being used.
-	// A session consists of a bidirectional stream each using a mailbox
-	// with an ID that overlaps for the first 63 bytes and differ for the
-	// last bit. So in order to obtain accurate data about a specific
-	// mailbox session, the stream ID that will be recorded is the first
-	// 16 bytes of the session ID and we will only record the odd stream's
-	// reads so that we don't duplicate the data.
-	mailboxReadCount = prometheus.NewCounterVec(
-		prometheus.CounterOpts{
+	// activeSessions tracks the active session count for mailbox
+	activeSessions = prometheus.NewGauge(prometheus.GaugeOpts{
 		Namespace: "hashmail",
-			Name:      "mailbox_read_count",
-		}, []string{streamIDLabel},
-	)
+		Name:      "mailbox_active_sessions",
+		Help:      "Number of active sessions",
+	})
+
+	// standbySessions tracks the standby session count for mailbox
+	standbySessions = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "hashmail",
+		Name:      "mailbox_standby_sessions",
+		Help:      "Number of standby sessions",
+	})
+
+	// inUseSessions tracks the in-use session count for mailbox
+	inUseSessions = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "hashmail",
+		Name:      "mailbox_inuse_sessions",
+		Help:      "Number of in-use sessions",
+	})
 )
 
+// streamActivityTracker handles the calculation of session statistics
+var streamActivityTracker = newStreamActivity()
+
 // PrometheusConfig is the set of configuration data that specifies if
 // Prometheus metric exporting is activated, and if so the listening address of
 // the Prometheus server.
@@ -55,7 +64,22 @@ func StartPrometheusExporter(cfg *PrometheusConfig) error {
 
 	// Next, we'll register all our metrics.
 	prometheus.MustRegister(mailboxCount)
-	prometheus.MustRegister(mailboxReadCount)
+	prometheus.MustRegister(activeSessions)
+	prometheus.MustRegister(standbySessions)
+	prometheus.MustRegister(inUseSessions)
+
+	// Periodically update session classification metrics from internal tracker
+	go func() {
+		ticker := time.NewTicker(10 * time.Second)
+		defer ticker.Stop()
+
+		for range ticker.C {
+			active, standby, inuse := streamActivityTracker.ClassifyAndReset()
+			activeSessions.Set(float64(active))
+			standbySessions.Set(float64(standby))
+			inUseSessions.Set(float64(inuse))
+		}
+	}()
 
 	// Finally, we'll launch the HTTP server that Prometheus will use to
 	// scape our metrics.