perf(go): cache realtime plate lookups

go/vehicle-gateway/internal/realtime/snapshot_writer.go

@@ -5,6 +5,7 @@ import (
 	"database/sql"
 	"errors"
 	"strings"
+	"sync"
 	"time"
 
 	"lingniu-vehicle-ingest/go/vehicle-gateway/internal/envelope"
@@ -18,6 +19,69 @@ type PlateResolver interface {
 	PlateByVIN(context.Context, string) (string, error)
 }
 
+type CachedPlateResolver struct {
+	delegate PlateResolver
+	ttl      time.Duration
+	now      func() time.Time
+	mu       sync.Mutex
+	entries  map[string]cachedPlateEntry
+}
+
+type cachedPlateEntry struct {
+	plate     string
+	notFound  bool
+	expiresAt time.Time
+}
+
+func NewCachedPlateResolver(delegate PlateResolver, ttl time.Duration) *CachedPlateResolver {
+	if delegate == nil {
+		panic("cached plate resolver delegate must not be nil")
+	}
+	if ttl <= 0 {
+		ttl = 10 * time.Minute
+	}
+	return &CachedPlateResolver{
+		delegate: delegate,
+		ttl:      ttl,
+		now:      time.Now,
+		entries:  map[string]cachedPlateEntry{},
+	}
+}
+
+func (r *CachedPlateResolver) PlateByVIN(ctx context.Context, vin string) (string, error) {
+	vin = strings.TrimSpace(vin)
+	if vin == "" {
+		return "", sql.ErrNoRows
+	}
+	now := r.now()
+	r.mu.Lock()
+	entry, ok := r.entries[vin]
+	if ok && now.Before(entry.expiresAt) {
+		r.mu.Unlock()
+		if entry.notFound {
+			return "", sql.ErrNoRows
+		}
+		return entry.plate, nil
+	}
+	r.mu.Unlock()
+
+	plate, err := r.delegate.PlateByVIN(ctx, vin)
+	if err != nil && !errors.Is(err, sql.ErrNoRows) {
+		return "", err
+	}
+	r.mu.Lock()
+	r.entries[vin] = cachedPlateEntry{
+		plate:     strings.TrimSpace(plate),
+		notFound:  errors.Is(err, sql.ErrNoRows),
+		expiresAt: now.Add(r.ttl),
+	}
+	r.mu.Unlock()
+	if err != nil {
+		return "", err
+	}
+	return strings.TrimSpace(plate), nil
+}
+
 type SnapshotWriter struct {
 	exec          SnapshotExecer
 	plateResolver PlateResolver