diff --git a/pytrade/data.py b/pytrade/data.py
index ee9cb6a..86c4def 100644
--- a/pytrade/data.py
+++ b/pytrade/data.py
@@ -139,6 +139,7 @@ def update(self, candle: Candlestick):
         instrument_candles.update(candle)
         self._update_event.set()
 
-    async def next(self):
+    async def next(self) -> bool:
         await self._update_event.wait()
         self._update_event.clear()
+        return True
diff --git a/pytrade/interfaces/data.py b/pytrade/interfaces/data.py
index b4f9179..52b2b10 100644
--- a/pytrade/interfaces/data.py
+++ b/pytrade/interfaces/data.py
@@ -69,3 +69,7 @@ def universe(self) -> list[IInstrumentData]:
     @abstractmethod
     def get(self, instrument: Instrument, granularity: Granularity) -> IInstrumentData:
         raise NotImplementedError()
+
+    @abstractmethod
+    async def next(self) -> bool:
+        raise NotImplementedError()