Implement queue backpressure with gradual message dropping

- Add gradual message dropping based on queue utilization
- Start dropping messages at 50% queue capacity
- Drop rate increases linearly from 0% at 50% to 100% at full
- Uses random drops to maintain fair distribution
- Helps prevent queue overflow under high load

internal/streamer/streamer.go

@@ -8,6 +8,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"math"
+	"math/rand"
 	"net/http"
 	"sync"
 	"sync/atomic"
@@ -29,6 +30,10 @@ const (
 	bytesPerKB            = 1024
 	bytesPerMB            = 1024 * 1024
 	maxConcurrentHandlers = 800 // Maximum number of concurrent message handlers
+
+	// Backpressure constants
+	backpressureThreshold = 0.5 // Start dropping at 50% queue utilization
+	backpressureSlope     = 2.0 // Slope for linear drop probability increase
 )
 
 // MessageHandler is an interface for handling RIS messages
@@ -74,7 +79,8 @@ type Streamer struct {
 	cancel       context.CancelFunc
 	running      bool
 	metrics      *metrics.Tracker
-	totalDropped uint64 // Total dropped messages across all handlers
+	totalDropped uint64     // Total dropped messages across all handlers
+	random       *rand.Rand // Random number generator for backpressure drops
 }
 
 // New creates a new RIS streamer
@@ -86,6 +92,8 @@ func New(logger *logger.Logger, metrics *metrics.Tracker) *Streamer {
 		},
 		handlers: make([]*handlerInfo, 0),
 		metrics:  metrics,
+		//nolint:gosec // Non-cryptographic randomness is fine for backpressure
+		random: rand.New(rand.NewSource(time.Now().UnixNano())),
 	}
 }
 
@@ -526,15 +534,26 @@ func (s *Streamer) stream(ctx context.Context) error {
 		// Dispatch to interested handlers
 		s.mu.RLock()
 		for _, info := range s.handlers {
-			if info.handler.WantsMessage(msg.Type) {
-				select {
-				case info.queue <- &msg:
-					// Message queued successfully
-				default:
-					// Queue is full, drop the message
-					atomic.AddUint64(&info.metrics.droppedCount, 1)
-					atomic.AddUint64(&s.totalDropped, 1)
-				}
+			if !info.handler.WantsMessage(msg.Type) {
+				continue
+			}
+
+			// Check if we should drop due to backpressure
+			if s.shouldDropForBackpressure(info) {
+				atomic.AddUint64(&info.metrics.droppedCount, 1)
+				atomic.AddUint64(&s.totalDropped, 1)
+
+				continue
+			}
+
+			// Try to queue the message
+			select {
+			case info.queue <- &msg:
+				// Message queued successfully
+			default:
+				// Queue is full, drop the message
+				atomic.AddUint64(&info.metrics.droppedCount, 1)
+				atomic.AddUint64(&s.totalDropped, 1)
 			}
 		}
 		s.mu.RUnlock()
@@ -546,3 +565,25 @@ func (s *Streamer) stream(ctx context.Context) error {
 
 	return nil
 }
+
+// shouldDropForBackpressure determines if a message should be dropped based on queue utilization
+func (s *Streamer) shouldDropForBackpressure(info *handlerInfo) bool {
+	// Calculate queue utilization
+	queueLen := len(info.queue)
+	queueCap := cap(info.queue)
+	utilization := float64(queueLen) / float64(queueCap)
+
+	// No drops below threshold
+	if utilization < backpressureThreshold {
+		return false
+	}
+
+	// Calculate drop probability (0.0 at threshold, 1.0 at 100% full)
+	dropProbability := (utilization - backpressureThreshold) * backpressureSlope
+	if dropProbability > 1.0 {
+		dropProbability = 1.0
+	}
+
+	// Random drop based on probability
+	return s.random.Float64() < dropProbability
+}