[Executor] Support node_concurrency when running an async flow (#1698)

# Description

Use asyncio.semaphore to support concurrency control.

When node_concurrency is set to 2:
sync_passthrough1 and async_passthrough1 can run concurrently:

![1705287434459](https://github.com/microsoft/promptflow/assets/39176492/487f1d03-34fa-4016-bf01-ff31d64d2c3f)


When node_concurrency is set to 1:
Only one of sync_passthrough1 and async_passthrough1 can at the same
time:

![1705288060087](https://github.com/microsoft/promptflow/assets/39176492/9ce31cd9-4af4-4451-a898-06cccf5c2321)

---------

Co-authored-by: Min Shi <[email protected]>

src/promptflow/promptflow/executor/_async_nodes_scheduler.py

@@ -34,7 +34,6 @@ def __init__(
         node_concurrency: int,
     ) -> None:
         self._tools_manager = tools_manager
-        # TODO: Add concurrency control in execution
         self._node_concurrency = node_concurrency
         self._task_start_time = {}
         self._task_last_log_time = {}
@@ -55,7 +54,9 @@ async def execute(
                 "Current thread is not main thread, skip signal handler registration in AsyncNodesScheduler."
             )
 
+        # Semaphore should be created in the loop, otherwise it will not work.
         loop = asyncio.get_running_loop()
+        self._semaphore = asyncio.Semaphore(self._node_concurrency, loop=loop)
         monitor = threading.Thread(
             target=monitor_long_running_coroutine,
             args=(loop, self._task_start_time, self._task_last_log_time, self._dag_manager_completed_event),
@@ -129,6 +130,10 @@ def _execute_nodes(
             self._create_node_task(node, dag_manager, context, executor): node for node in dag_manager.pop_ready_nodes()
         }
 
+    async def run_task_with_semaphore(self, coroutine):
+        async with self._semaphore:
+            return await coroutine
+
     def _create_node_task(
         self,
         node: Node,
@@ -139,12 +144,17 @@ def _create_node_task(
         f = self._tools_manager.get_tool(node.name)
         kwargs = dag_manager.get_node_valid_inputs(node, f)
         if inspect.iscoroutinefunction(f):
+            # For async task, it will not be executed before calling create_task.
             task = context.invoke_tool_async(node, f, kwargs)
         else:
+            # For sync task, convert it to async task and run it in executor thread.
+            # Even though the task is put to the thread pool, thread.start will only be triggered after create_task.
             task = self._sync_function_to_async_task(executor, context, node, f, kwargs)
         # Set the name of the task to the node name for debugging purpose
         # It does not need to be unique by design.
-        return asyncio.create_task(task, name=node.name)
+        # Wrap the coroutine in a task with asyncio.create_task to schedule it for event loop execution
+        # The task is created and added to the event loop, but the exact execution depends on loop's scheduling
+        return asyncio.create_task(self.run_task_with_semaphore(task), name=node.name)
 
     @staticmethod
     async def _sync_function_to_async_task(
@@ -154,6 +164,7 @@ async def _sync_function_to_async_task(
         f,
         kwargs,
     ):
+        # The task will not be executed before calling create_task.
         return await asyncio.get_running_loop().run_in_executor(executor, context.invoke_tool, node, f, kwargs)
 
 

src/promptflow/tests/executor/e2etests/test_async.py

@@ -0,0 +1,42 @@
+import os
+import pytest
+from promptflow.executor import FlowExecutor
+from ..utils import get_flow_folder, get_yaml_file
+
+
+@pytest.mark.e2etest
+class TestAsync:
+    @pytest.mark.parametrize(
+        "folder_name, concurrency_levels, expected_concurrency",
+        [
+            ("async_tools", [1, 2, 3], [1, 2, 2]),
+            ("async_tools_with_sync_tools", [1, 2, 3], [1, 2, 2]),
+        ],
+    )
+    def test_executor_node_concurrency(self, folder_name, concurrency_levels, expected_concurrency):
+        os.chdir(get_flow_folder(folder_name))
+        executor = FlowExecutor.create(get_yaml_file(folder_name), {})
+
+        def calculate_max_concurrency(flow_result):
+            timeline = []
+            api_calls = flow_result.run_info.api_calls[0]["children"]
+            for api_call in api_calls:
+                timeline.append(("start", api_call["start_time"]))
+                timeline.append(("end", api_call["end_time"]))
+            timeline.sort(key=lambda x: x[1])
+            current_concurrency = 0
+            max_concurrency = 0
+            for event, _ in timeline:
+                if event == "start":
+                    current_concurrency += 1
+                    max_concurrency = max(max_concurrency, current_concurrency)
+                elif event == "end":
+                    current_concurrency -= 1
+            return max_concurrency
+
+        for i in range(len(concurrency_levels)):
+            concurrency = concurrency_levels[i]
+            flow_result = executor.exec_line({"input_str": "Hello"}, node_concurrency=concurrency)
+            max_concurrency = calculate_max_concurrency(flow_result)
+            assert max_concurrency == expected_concurrency[i]
+            assert max_concurrency <= concurrency