Support streaming + process-based execution mechanism

storey/flow.py

@@ -1850,6 +1850,53 @@ def _static_run(*args, **kwargs):
     return _sval._run(*args, **kwargs)
 
 
+def _streaming_run_wrapper(
+    runnable: ParallelExecutionRunnable,
+    input,
+    path: str,
+    origin_name: Optional[str],
+    queue: multiprocessing.Queue,
+) -> None:
+    """Wrapper that runs a streaming runnable and sends chunks via queue.
+
+    This function runs in a child process and iterates over the generator,
+    sending each chunk through the multiprocessing queue.
+    """
+    try:
+        for chunk in runnable._run(input, path, origin_name):
+            queue.put(("chunk", chunk))
+        queue.put(("done", None))
+    except Exception as e:
+        queue.put(("error", (type(e).__name__, str(e), traceback.format_exc())))
+
+
+def _static_streaming_run(input, path: str, origin_name: Optional[str], queue: multiprocessing.Queue) -> None:
+    """Streaming wrapper for dedicated_process using the global runnable."""
+    global _sval
+    _streaming_run_wrapper(_sval, input, path, origin_name, queue)
+
+
+async def _async_read_streaming_queue(
+    queue: multiprocessing.Queue, loop: Optional[asyncio.AbstractEventLoop] = None
+) -> AsyncGenerator:
+    """Async generator that reads chunks from a multiprocessing queue.
+
+    This runs in the parent process and yields chunks sent by the child process,
+    without blocking the asyncio event loop.
+    """
+    loop = asyncio.get_running_loop()
+    while True:
+        # Use run_in_executor to avoid blocking the event loop
+        msg_type, payload = await loop.run_in_executor(None, queue.get)
+        if msg_type == "chunk":
+            yield payload
+        elif msg_type == "done":
+            break
+        elif msg_type == "error":
+            exc_type, exc_msg, exc_tb = payload
+            raise RuntimeError(f"{exc_type}: {exc_msg}\n\nOriginal traceback:\n{exc_tb}")
+
+
 class RunnableExecutor:
     """
     Manages and executes `ParallelExecutionRunnable` instances using various parallel execution mechanisms.
@@ -1877,6 +1924,8 @@ def __init__(
         self._process_executor_by_runnable_name = {}
         self._mp_context = multiprocessing.get_context("spawn")
         self._executors = {}
+        self._is_streaming_by_runnable_name: dict[str, bool] = {}
+        self._manager = None  # Lazy-initialized multiprocessing Manager for queue-based streaming
 
     def add_runnable(self, runnable: ParallelExecutionRunnable, execution_mechanism: str) -> None:
         """
@@ -1913,15 +1962,9 @@ def init_runnable(self, runnable: Union[str, ParallelExecutionRunnable]) -> None
 
         execution_mechanism = self._execution_mechanism_by_runnable_name[runnable.name]
 
-        # Check for streaming + process-based execution (incompatible combination)
-        if execution_mechanism in ParallelExecutionMechanisms.process():
-            is_streaming = inspect.isgeneratorfunction(runnable.run) or inspect.isasyncgenfunction(runnable.run_async)
-            if is_streaming:
-                raise StreamingError(
-                    f"Streaming is not supported with process-based execution mechanisms. "
-                    f"Runnable '{runnable.name}' uses '{execution_mechanism}'. "
-                    f"Use 'thread_pool', 'asyncio', or 'naive' for streaming runnables."
-                )
+        # Record whether this runnable is a streaming runnable (generator function)
+        is_streaming = inspect.isgeneratorfunction(runnable.run) or inspect.isasyncgenfunction(runnable.run_async)
+        self._is_streaming_by_runnable_name[runnable.name] = is_streaming
 
         if execution_mechanism == ParallelExecutionMechanisms.process_pool:
             self.num_processes += 1
@@ -1956,6 +1999,12 @@ def init_executors(self):
             if num_threads:
                 self._executors[ParallelExecutionMechanisms.thread_pool] = ThreadPoolExecutor(max_workers=num_threads)
 
+    def _get_manager(self):
+        """Get or create the multiprocessing Manager for queue-based streaming."""
+        if self._manager is None:
+            self._manager = self._mp_context.Manager()
+        return self._manager
+
     def run_executor(
         self,
         runnable: Union[ParallelExecutionRunnable, str],
@@ -1985,36 +2034,49 @@ def run_executor(
             raise ValueError(f"select_runnables() returned more than one outlet named '{runnable.name}'")
 
         execution_mechanism = self._execution_mechanism_by_runnable_name[runnable.name]
+        is_streaming = self._is_streaming_by_runnable_name.get(runnable.name, False)
 
         input = (
             event.body if execution_mechanism in ParallelExecutionMechanisms.process() else copy.deepcopy(event.body)
         )
 
+        loop = asyncio.get_running_loop()
+
         if execution_mechanism == ParallelExecutionMechanisms.asyncio:
-            future = asyncio.get_running_loop().create_task(
-                runnable._async_run(input, event.path, origin_runnable_name)
-            )
+            future = loop.create_task(runnable._async_run(input, event.path, origin_runnable_name))
         elif execution_mechanism == ParallelExecutionMechanisms.naive:
-            future = asyncio.get_running_loop().create_future()
+            future = loop.create_future()
             future.set_result(runnable._run(input, event.path, origin_runnable_name))
-        elif execution_mechanism == ParallelExecutionMechanisms.dedicated_process:
-            executor = self._process_executor_by_runnable_name[runnable.name]
-            future = asyncio.get_running_loop().run_in_executor(
-                executor,
-                _static_run,
-                input,
-                event.path,
-                origin_runnable_name,
-            )
+        elif execution_mechanism in ParallelExecutionMechanisms.process():
+            # Get the appropriate executor for this process mechanism
+            if execution_mechanism == ParallelExecutionMechanisms.dedicated_process:
+                executor = self._process_executor_by_runnable_name[runnable.name]
+            else:  # process_pool
+                executor = self._executors[execution_mechanism]
+
+            if is_streaming:
+                # Use Manager's queue for cross-process streaming (regular queues can't be passed to executor)
+                queue = self._get_manager().Queue()
+                # Use appropriate streaming function based on mechanism
+                if execution_mechanism == ParallelExecutionMechanisms.dedicated_process:
+                    loop.run_in_executor(
+                        executor, _static_streaming_run, input, event.path, origin_runnable_name, queue
+                    )
+                else:
+                    loop.run_in_executor(
+                        executor, _streaming_run_wrapper, runnable, input, event.path, origin_runnable_name, queue
+                    )
+                future = loop.create_future()
+                future.set_result(_async_read_streaming_queue(queue))
+            else:
+                # Use appropriate run function based on mechanism
+                if execution_mechanism == ParallelExecutionMechanisms.dedicated_process:
+                    future = loop.run_in_executor(executor, _static_run, input, event.path, origin_runnable_name)
+                else:
+                    future = loop.run_in_executor(executor, runnable._run, input, event.path, origin_runnable_name)
         else:
             executor = self._executors[execution_mechanism]
-            future = asyncio.get_running_loop().run_in_executor(
-                executor,
-                runnable._run,
-                input,
-                event.path,
-                origin_runnable_name,
-            )
+            future = loop.run_in_executor(executor, runnable._run, input, event.path, origin_runnable_name)
         return future
 
 

tests/test_streaming.py

@@ -54,6 +54,14 @@ async def run_async(self, body, path: str, origin_name: Optional[str] = None) ->
             yield f"{body}_chunk_{i}"
 
 
+class ErrorStreamingRunnable(ParallelExecutionRunnable):
+    """A streaming runnable that yields one chunk then raises an error."""
+
+    def run(self, body, path: str, origin_name: Optional[str] = None) -> Generator:
+        yield f"{body}_chunk_0"
+        raise ValueError("Simulated streaming error")
+
+
 class TestStreamingPrimitives:
     """Tests for streaming primitive classes."""
 
@@ -1090,29 +1098,6 @@ def double(x):
 class TestParallelExecutionStreaming:
     """Tests for ParallelExecution streaming support."""
 
-    def test_parallel_execution_single_runnable_streaming(self):
-        """Test streaming with a single runnable."""
-        runnable = StreamingRunnable(name="streamer")
-        controller = build_flow(
-            [
-                SyncEmitSource(),
-                ParallelExecution(
-                    runnables=[runnable],
-                    execution_mechanism_by_runnable_name={"streamer": ParallelExecutionMechanisms.naive},
-                ),
-                Complete(),
-            ]
-        ).run()
-
-        try:
-            awaitable = controller.emit("test")
-            result = awaitable.await_result()
-            assert inspect.isgenerator(result)
-            assert list(result) == ["test_chunk_0", "test_chunk_1", "test_chunk_2"]
-        finally:
-            controller.terminate()
-            controller.await_termination()
-
     def test_parallel_execution_async_runnable_streaming(self):
         """Test streaming with an async runnable."""
         runnable = AsyncStreamingRunnable(name="async_streamer")
@@ -1190,15 +1175,24 @@ async def _test():
 
         asyncio.run(_test())
 
-    def test_parallel_execution_streaming_with_thread_pool(self):
-        """Test streaming works with thread_pool execution mechanism."""
+    @pytest.mark.parametrize(
+        "execution_mechanism",
+        [
+            ParallelExecutionMechanisms.naive,
+            ParallelExecutionMechanisms.thread_pool,
+            ParallelExecutionMechanisms.process_pool,
+            ParallelExecutionMechanisms.dedicated_process,
+        ],
+    )
+    def test_parallel_execution_streaming_with_executor(self, execution_mechanism):
+        """Test streaming works with various execution mechanisms."""
         runnable = StreamingRunnable(name="streamer")
         controller = build_flow(
             [
                 SyncEmitSource(),
                 ParallelExecution(
                     runnables=[runnable],
-                    execution_mechanism_by_runnable_name={"streamer": ParallelExecutionMechanisms.thread_pool},
+                    execution_mechanism_by_runnable_name={"streamer": execution_mechanism},
                 ),
                 Complete(),
             ]
@@ -1213,14 +1207,20 @@ def test_parallel_execution_streaming_with_thread_pool(self):
             controller.terminate()
             controller.await_termination()
 
-    def test_parallel_execution_streaming_with_shared_executor_thread_based(self):
-        """Test streaming works with shared_executor when the shared executor uses threads."""
-        # Create a shared executor with a thread-based runnable
+    @pytest.mark.parametrize(
+        "execution_mechanism",
+        [
+            ParallelExecutionMechanisms.thread_pool,
+            ParallelExecutionMechanisms.process_pool,
+            ParallelExecutionMechanisms.dedicated_process,
+        ],
+    )
+    def test_parallel_execution_streaming_with_shared_executor(self, execution_mechanism):
+        """Test streaming works with shared_executor using different underlying mechanisms."""
         shared_executor = RunnableExecutor()
         shared_runnable = StreamingRunnable(name="shared_streamer")
-        shared_executor.add_runnable(shared_runnable, ParallelExecutionMechanisms.thread_pool)
+        shared_executor.add_runnable(shared_runnable, execution_mechanism)
 
-        # Create a proxy runnable that references the shared executor
         proxy_runnable = StreamingRunnable(name="proxy", shared_runnable_name="shared_streamer")
 
         class ContextWithExecutor:
@@ -1251,67 +1251,45 @@ def __init__(self, executor):
             controller.await_termination()
 
     @pytest.mark.parametrize(
-        "mechanism",
-        [ParallelExecutionMechanisms.process_pool, ParallelExecutionMechanisms.dedicated_process],
+        "execution_mechanism,expected_error",
+        [
+            (ParallelExecutionMechanisms.naive, ValueError),
+            (ParallelExecutionMechanisms.thread_pool, ValueError),
+            # Process-based mechanisms wrap errors in RuntimeError
+            (ParallelExecutionMechanisms.process_pool, RuntimeError),
+            (ParallelExecutionMechanisms.dedicated_process, RuntimeError),
+        ],
     )
-    def test_parallel_execution_streaming_with_process_based_fails_at_init(self, mechanism):
-        """Test that StreamingError is raised at init time when streaming runnable uses process-based mechanism."""
-        runnable = StreamingRunnable(name="streamer")
-
-        flow = build_flow(
+    def test_parallel_execution_streaming_error_propagation(self, execution_mechanism, expected_error):
+        """Test that errors in streaming are propagated correctly."""
+        runnable = ErrorStreamingRunnable(name="error_streamer")
+        controller = build_flow(
             [
                 SyncEmitSource(),
                 ParallelExecution(
                     runnables=[runnable],
-                    execution_mechanism_by_runnable_name={"streamer": mechanism},
+                    execution_mechanism_by_runnable_name={"error_streamer": execution_mechanism},
                 ),
                 Complete(),
             ]
-        )
-
-        expected_error_message = (
-            "Streaming is not supported with process-based execution mechanisms. "
-            f"Runnable 'streamer' uses '{mechanism}'. "
-            "Use 'thread_pool', 'asyncio', or 'naive' for streaming runnables."
-        )
-        with pytest.raises(StreamingError, match=expected_error_message):
-            flow.run()
-
-    def test_parallel_execution_streaming_with_shared_executor_process_based_fails_at_init(self):
-        """Test that StreamingError is raised at init when shared_executor uses process-based mechanism."""
-        # Create a shared executor with a process-based runnable
-        shared_executor = RunnableExecutor()
-        shared_runnable = StreamingRunnable(name="shared_streamer")
-        shared_executor.add_runnable(shared_runnable, ParallelExecutionMechanisms.process_pool)
-
-        # Create a proxy runnable that references the shared executor
-        proxy_runnable = StreamingRunnable(name="proxy", shared_runnable_name="shared_streamer")
-
-        class ContextWithExecutor:
-            def __init__(self, executor):
-                self.executor = executor
-
-        context = ContextWithExecutor(shared_executor)
+        ).run()
 
-        flow = build_flow(
-            [
-                SyncEmitSource(),
-                ParallelExecution(
-                    runnables=[proxy_runnable],
-                    execution_mechanism_by_runnable_name={"proxy": ParallelExecutionMechanisms.shared_executor},
-                    context=context,
-                ),
-                Complete(),
-            ]
-        )
-
-        expected_error_message = (
-            "Streaming is not supported with process-based execution mechanisms. "
-            "Runnable 'shared_streamer' uses 'process_pool'. "
-            "Use 'thread_pool', 'asyncio', or 'naive' for streaming runnables."
-        )
-        with pytest.raises(StreamingError, match=expected_error_message):
-            flow.run()
+        try:
+            awaitable = controller.emit("test")
+            result = awaitable.await_result()
+            assert inspect.isgenerator(result)
+            # Should get first chunk, then error
+            chunks = []
+            with pytest.raises(expected_error, match="Simulated streaming error"):
+                for chunk in result:
+                    chunks.append(chunk)
+            # Verify we got the first chunk before the error
+            assert chunks == ["test_chunk_0"]
+        finally:
+            controller.terminate()
+            # Error is also propagated through termination
+            with pytest.raises(expected_error, match="Simulated streaming error"):
+                controller.await_termination()
 
 
 class TestStreamingGraphSplits: