diff --git a/sdk/event_relays.go b/sdk/event_relays.go
new file mode 100644
index 0000000..4716eaa
--- /dev/null
+++ b/sdk/event_relays.go
@@ -0,0 +1,115 @@
+package sdk
+
+import (
+	"encoding/binary"
+	"encoding/hex"
+	"fmt"
+)
+
+const eventRelayPrefix = byte('r')
+
+func makeEventRelayKey(eventID []byte) []byte {
+	// format: 'r' + first 8 bytes of event ID
+	key := make([]byte, 9)
+	key[0] = eventRelayPrefix
+	copy(key[1:], eventID[:8])
+	return key
+}
+
+func encodeRelayList(relays []string) []byte {
+	totalSize := 0
+	for _, relay := range relays {
+		totalSize += 2 + len(relay) // 2 bytes for length prefix
+	}
+
+	buf := make([]byte, totalSize)
+	offset := 0
+
+	for _, relay := range relays {
+		binary.LittleEndian.PutUint16(buf[offset:], uint16(len(relay)))
+		offset += 2
+		copy(buf[offset:], relay)
+		offset += len(relay)
+	}
+
+	return buf
+}
+
+func decodeRelayList(data []byte) []string {
+	relays := make([]string, 0)
+	offset := 0
+
+	for offset < len(data) {
+		if offset+2 > len(data) {
+			return nil // malformed
+		}
+
+		length := int(binary.LittleEndian.Uint16(data[offset:]))
+		offset += 2
+
+		if offset+length > len(data) {
+			return nil // malformed
+		}
+
+		relay := string(data[offset : offset+length])
+		relays = append(relays, relay)
+		offset += length
+	}
+
+	return relays
+}
+
+func (sys *System) trackEventRelayCommon(eventID string, relay string) {
+	// decode the event ID hex into bytes
+	idBytes, err := hex.DecodeString(eventID)
+	if err != nil || len(idBytes) < 8 {
+		return
+	}
+
+	// get the key for this event
+	key := makeEventRelayKey(idBytes)
+
+	// update the relay list atomically
+	sys.KVStore.Update(key, func(data []byte) ([]byte, error) {
+		var relays []string
+		if data != nil {
+			relays = decodeRelayList(data)
+		} else {
+			relays = make([]string, 0, 1)
+		}
+
+		// check if relay is already in list
+		for _, r := range relays {
+			if r == relay {
+				return data, nil // no change needed
+			}
+		}
+
+		// append new relay
+		relays = append(relays, relay)
+		return encodeRelayList(relays), nil
+	})
+}
+
+// GetEventRelays returns all known relay URLs that have been seen to carry the given event.
+func (sys *System) GetEventRelays(eventID string) ([]string, error) {
+	// decode the event ID hex into bytes
+	idBytes, err := hex.DecodeString(eventID)
+	if err != nil || len(idBytes) < 8 {
+		return nil, fmt.Errorf("invalid event id")
+	}
+
+	// get the key for this event
+	key := makeEventRelayKey(idBytes)
+
+	// get stored relay list
+	data, err := sys.KVStore.Get(key)
+	if err != nil {
+		return nil, err
+	}
+	if data == nil {
+		return nil, nil
+	}
+
+	return decodeRelayList(data), nil
+}
diff --git a/sdk/kvstore/badger/store.go b/sdk/kvstore/badger/store.go
index db957ea..57b55c4 100644
--- a/sdk/kvstore/badger/store.go
+++ b/sdk/kvstore/badger/store.go
@@ -58,27 +58,31 @@ func (s *Store) Close() error {
 	return s.db.Close()
 }
 
-func (s *Store) Scan(prefix []byte, fn func(key []byte, value []byte) bool) error {
-	return s.db.View(func(txn *badger.Txn) error {
-		it := txn.NewIterator(badger.DefaultIteratorOptions)
-		defer it.Close()
-
-		for it.Seek(prefix); it.ValidForPrefix(prefix); it.Next() {
-			item := it.Item()
-			err := item.Value(func(v []byte) error {
-				k := item.Key()
-				if !fn(k, v) {
-					return badger.ErrStopIteration
-				}
+func (s *Store) Update(key []byte, f func([]byte) ([]byte, error)) error {
+	return s.db.Update(func(txn *badger.Txn) error {
+		var val []byte
+		item, err := txn.Get(key)
+		if err == nil {
+			err = item.Value(func(v []byte) error {
+				val = make([]byte, len(v))
+				copy(val, v)
 				return nil
 			})
-			if err == badger.ErrStopIteration {
-				break
-			}
 			if err != nil {
 				return err
 			}
+		} else if err != badger.ErrKeyNotFound {
+			return err
 		}
-		return nil
+
+		newVal, err := f(val)
+		if err != nil {
+			return err
+		}
+
+		if newVal == nil {
+			return txn.Delete(key)
+		}
+		return txn.Set(key, newVal)
 	})
 }
diff --git a/sdk/kvstore/interface.go b/sdk/kvstore/interface.go
index c963a4c..c16b50a 100644
--- a/sdk/kvstore/interface.go
+++ b/sdk/kvstore/interface.go
@@ -14,7 +14,9 @@ type KVStore interface {
 	// Close releases any resources held by the store
 	Close() error
 
-	// Scan iterates through all keys with the given prefix.
-	// For each key-value pair, fn is called. If fn returns false, iteration stops.
-	Scan(prefix []byte, fn func(key []byte, value []byte) bool) error
+	// Update atomically modifies a value for a given key.
+	// The function f receives the current value (nil if not found)
+	// and returns the new value to be set.
+	// If f returns nil, the key is deleted.
+	Update(key []byte, f func([]byte) ([]byte, error)) error
 }
diff --git a/sdk/kvstore/lmdb/store.go b/sdk/kvstore/lmdb/store.go
index f8374d6..1be0e21 100644
--- a/sdk/kvstore/lmdb/store.go
+++ b/sdk/kvstore/lmdb/store.go
@@ -91,22 +91,26 @@ func (s *Store) Close() error {
 	return nil
 }
 
-func (s *Store) Scan(prefix []byte, fn func(key []byte, value []byte) bool) error {
-	return s.env.View(func(txn *lmdb.Txn) error {
-		cursor, err := txn.OpenCursor(s.dbi)
+func (s *Store) Update(key []byte, f func([]byte) ([]byte, error)) error {
+	return s.env.Update(func(txn *lmdb.Txn) error {
+		var val []byte
+		v, err := txn.Get(s.dbi, key)
+		if err == nil {
+			// make a copy since v is only valid during the transaction
+			val = make([]byte, len(v))
+			copy(val, v)
+		} else if !lmdb.IsNotFound(err) {
+			return err
+		}
+
+		newVal, err := f(val)
 		if err != nil {
 			return err
 		}
-		defer cursor.Close()
 
-		for k, v, err := cursor.Get(prefix, nil, lmdb.SetRange); err == nil; k, v, err = cursor.Get(nil, nil, lmdb.Next) {
-			if !bytes.HasPrefix(k, prefix) {
-				break
-			}
-			if !fn(k, v) {
-				break
-			}
+		if newVal == nil {
+			return txn.Del(s.dbi, key, nil)
 		}
-		return nil
+		return txn.Put(s.dbi, key, newVal, 0)
 	})
 }
diff --git a/sdk/kvstore/memory/store.go b/sdk/kvstore/memory/store.go
index 524083a..ff8a807 100644
--- a/sdk/kvstore/memory/store.go
+++ b/sdk/kvstore/memory/store.go
@@ -57,17 +57,29 @@ func (s *Store) Close() error {
 	return nil
 }
 
-func (s *Store) Scan(prefix []byte, fn func(key []byte, value []byte) bool) error {
-	s.RLock()
-	defer s.RUnlock()
+func (s *Store) Update(key []byte, f func([]byte) ([]byte, error)) error {
+	s.Lock()
+	defer s.Unlock()
 
-	prefixStr := string(prefix)
-	for k, v := range s.data {
-		if strings.HasPrefix(k, prefixStr) {
-			if !fn([]byte(k), v) {
-				break
-			}
-		}
+	var val []byte
+	if v, ok := s.data[string(key)]; ok {
+		// Return a copy to prevent modification of stored data
+		val = make([]byte, len(v))
+		copy(val, v)
+	}
+
+	newVal, err := f(val)
+	if err != nil {
+		return err
+	}
+
+	if newVal == nil {
+		delete(s.data, string(key))
+	} else {
+		// Store a copy to prevent modification of stored data
+		cp := make([]byte, len(newVal))
+		copy(cp, newVal)
+		s.data[string(key)] = cp
 	}
 	return nil
 }
diff --git a/sdk/system.go b/sdk/system.go
index 7976db5..541b191 100644
--- a/sdk/system.go
+++ b/sdk/system.go
@@ -139,33 +139,6 @@ func (sys *System) Close() {
 	}
 }
 
-// GetEventRelays returns all known relay URLs that have been seen to carry the given event.
-func (sys *System) GetEventRelays(eventID string) ([]string, error) {
-	// decode the event ID hex into bytes
-	idBytes, err := hex.DecodeString(eventID)
-	if err != nil || len(idBytes) < 8 {
-		return nil, fmt.Errorf("invalid event id")
-	}
-
-	// create prefix for scanning: 'r' + first 8 bytes of event ID
-	prefix := make([]byte, 9)
-	prefix[0] = eventRelayPrefix
-	copy(prefix[1:], idBytes[:8])
-
-	relays := make([]string, 0)
-	err = sys.KVStore.Scan(prefix, func(key []byte, value []byte) bool {
-		// extract relay URL from key (everything after prefix)
-		relay := string(key[9:])
-		relays = append(relays, relay)
-		return true
-	})
-	if err != nil {
-		return nil, err
-	}
-
-	return relays, nil
-}
-
 func WithHintsDB(hdb hints.HintsDB) SystemModifier {
 	return func(sys *System) {
 		sys.Hints = hdb
diff --git a/sdk/tracker.go b/sdk/tracker.go
index ce32047..78362b0 100644
--- a/sdk/tracker.go
+++ b/sdk/tracker.go
@@ -1,7 +1,6 @@
 package sdk
 
 import (
-	"encoding/hex"
 	"net/url"
 
 	"github.com/nbd-wtf/go-nostr"
@@ -110,37 +109,10 @@ func (sys *System) TrackEventHints(ie nostr.RelayEvent) {
 	}
 }
 
-const eventRelayPrefix = byte('r')
-
-func makeEventRelayKey(eventID []byte, relay string) []byte {
-	// Format: 'r' + first 8 bytes of event ID + relay URL
-	key := make([]byte, 1+8+len(relay))
-	key[0] = eventRelayPrefix
-	copy(key[1:], eventID[:8])
-	copy(key[9:], relay)
-	return key
-}
-
 func (sys *System) TrackEventRelays(ie nostr.RelayEvent) {
-	// decode the event ID hex into bytes
-	idBytes, err := hex.DecodeString(ie.ID)
-	if err != nil || len(idBytes) < 8 {
-		return
-	}
-
-	// store with prefix + eventid + relay format
-	key := makeEventRelayKey(idBytes, ie.Relay.URL)
-	sys.KVStore.Set(key, nil) // value is not needed since relay is in key
+	sys.trackEventRelayCommon(ie.ID, ie.Relay.URL)
 }
 
 func (sys *System) TrackEventRelaysD(relay, id string) {
-	// decode the event ID hex into bytes
-	idBytes, err := hex.DecodeString(id)
-	if err != nil || len(idBytes) < 8 {
-		return
-	}
-
-	// store with prefix + eventid + relay format
-	key := makeEventRelayKey(idBytes, relay)
-	sys.KVStore.Set(key, nil) // value is not needed since relay is in key
+	sys.trackEventRelayCommon(id, relay)
 }