Add SSE endpoint for streaming vertex build

src/backend/langflow/api/v1/chat.py

@@ -10,12 +10,14 @@ from fastapi import (
     WebSocketException,
     status,
 )
+from fastapi.responses import StreamingResponse
 from loguru import logger
 from sqlmodel import Session
 
 from langflow.api.utils import build_and_cache_graph, format_elapsed_time
 from langflow.api.v1.schemas import (
     ResultData,
+    StreamData,
     VertexBuildResponse,
     VerticesOrderResponse,
 )
@@ -204,3 +206,51 @@ async def build_vertex(
         logger.error(f"Error building vertex: {exc}")
         logger.exception(exc)
         raise HTTPException(status_code=500, detail=str(exc)) from exc
+
+
+# Now onto an endpoint that is an SSE endpoint
+# it will receive a component_id and a flow_id
+#
+@router.get("/build/{flow_id}/{vertex_id}/stream", response_class=StreamingResponse)
+async def build_vertex_stream(
+    flow_id: str,
+    vertex_id: str,
+    chat_service: "ChatService" = Depends(get_chat_service),
+):
+    """Build a vertex instead of the entire graph."""
+    try:
+
+        async def stream_vertex():
+            try:
+                cache = chat_service.get_cache(flow_id)
+                if not cache:
+                    # If there's no cache
+                    raise ValueError(f"No cache found for {flow_id}.")
+                else:
+                    graph = cache.get("result")
+
+                vertex = graph.get_vertex(vertex_id)
+                if not vertex.pinned or not vertex._built:
+                    stream_data = StreamData(
+                        event="message",
+                        data={"message": "Building vertex"},
+                    )
+                    yield str(stream_data)
+
+                    async for chunk in vertex.stream():
+                        stream_data = StreamData(
+                            event="message",
+                            data={"chunk": chunk},
+                        )
+                        yield str(stream_data)
+                else:
+                    raise ValueError(f"No result found for vertex {vertex_id}")
+
+            except Exception as exc:
+                yield str(StreamData(event="error", data={"error": str(exc)}))
+
+            yield str(StreamData(event="close", data={"message": "Stream closed"}))
+
+        return StreamingResponse(stream_vertex(), media_type="text/event-stream")
+    except Exception as exc:
+        raise HTTPException(status_code=500, detail="Error building vertex") from exc