Index: flexoentity/runtime_backend.py
===================================================================
--- flexoentity/runtime_backend.py	(revision a8dca95d801a30f5bc870b245936258d47288325)
+++ flexoentity/runtime_backend.py	(revision a8dca95d801a30f5bc870b245936258d47288325)
@@ -0,0 +1,38 @@
+from .persistance_backend import PersistanceBackend
+from .flexo_entity import FlexoEntity
+
+
+class RuntimeBackend(PersistanceBackend):
+    """
+    Runtime backend.
+
+    - Stores entity *instances*
+    - Guarantees identity stability
+    - No serialization
+    - No reconstruction
+    """
+
+    def __init__(self, entity_class):
+        if not issubclass(entity_class, FlexoEntity):
+            raise TypeError("entity_class must be a subclass of FlexoEntity")
+
+        super().__init__(entity_class=entity_class)
+        self._store: dict[str, FlexoEntity] = {}
+
+    def save(self, entity):
+        self._store[entity.flexo_id] = entity
+
+    def update(self, entity):
+        self._store[entity.flexo_id] = entity
+
+    def delete(self, flexo_id: str):
+        self._store.pop(flexo_id, None)
+
+    def load(self, flexo_id: str):
+        return self._store.get(flexo_id)
+
+    def load_all(self):
+        return list(self._store.values())
+
+    def clear(self):
+        self._store.clear()