Add message queue for SSE messages POST endpoint (#459)

Author: akash329d

README.md

@@ -412,6 +412,30 @@ app.router.routes.append(Host('mcp.acme.corp', app=mcp.sse_app()))
 
 For more information on mounting applications in Starlette, see the [Starlette documentation](https://www.starlette.io/routing/#submounting-routes).
 
+#### Message Dispatch Options
+
+By default, the SSE server uses an in-memory message dispatch system for incoming POST messages. For production deployments or distributed scenarios, you can use Redis or implement your own message dispatch system that conforms to the `MessageDispatch` protocol:
+
+```python
+# Using the built-in Redis message dispatch
+from mcp.server.fastmcp import FastMCP
+from mcp.server.message_queue import RedisMessageDispatch
+
+# Create a Redis message dispatch
+redis_dispatch = RedisMessageDispatch(
+    redis_url="redis://localhost:6379/0", prefix="mcp:pubsub:"
+)
+
+# Pass the message dispatch instance to the server
+mcp = FastMCP("My App", message_queue=redis_dispatch)
+```
+
+To use Redis, add the Redis dependency:
+
+```bash
+uv add "mcp[redis]"
+```
+
 ## Examples
 
 ### Echo Server

examples/servers/simple-prompt/mcp_simple_prompt/server.py

@@ -88,12 +88,15 @@ async def get_prompt(
         )
 
     if transport == "sse":
+        from mcp.server.message_queue.redis import RedisMessageDispatch
         from mcp.server.sse import SseServerTransport
         from starlette.applications import Starlette
         from starlette.responses import Response
         from starlette.routing import Mount, Route
 
-        sse = SseServerTransport("/messages/")
+        message_dispatch = RedisMessageDispatch("redis://localhost:6379/0")
+
+        sse = SseServerTransport("/messages/", message_dispatch=message_dispatch)
 
         async def handle_sse(request):
             async with sse.connect_sse(

pyproject.toml

@@ -37,6 +37,7 @@ dependencies = [
 rich = ["rich>=13.9.4"]
 cli = ["typer>=0.12.4", "python-dotenv>=1.0.0"]
 ws = ["websockets>=15.0.1"]
+redis = ["redis>=5.2.1", "types-redis>=4.6.0.20241004"]
 
 [project.scripts]
 mcp = "mcp.cli:app [cli]"
@@ -55,6 +56,7 @@ dev = [
     "pytest-xdist>=3.6.1",
     "pytest-examples>=0.0.14",
     "pytest-pretty>=1.2.0",
+    "fakeredis==2.28.1",
 ]
 docs = [
     "mkdocs>=1.6.1",

src/mcp/client/sse.py

@@ -98,7 +98,9 @@ async def sse_reader(
                                             await read_stream_writer.send(exc)
                                             continue
 
-                                        session_message = SessionMessage(message)
+                                        session_message = SessionMessage(
+                                            message=message
+                                        )
                                         await read_stream_writer.send(session_message)
                                     case _:
                                         logger.warning(
@@ -148,3 +150,5 @@ async def post_writer(endpoint_url: str):
         finally:
             await read_stream_writer.aclose()
             await write_stream.aclose()
+            await read_stream.aclose()
+            await write_stream_reader.aclose()

src/mcp/client/stdio/__init__.py

@@ -144,7 +144,7 @@ async def stdout_reader():
                             await read_stream_writer.send(exc)
                             continue
 
-                        session_message = SessionMessage(message)
+                        session_message = SessionMessage(message=message)
                         await read_stream_writer.send(session_message)
         except anyio.ClosedResourceError:
             await anyio.lowlevel.checkpoint()

src/mcp/client/streamable_http.py

@@ -153,7 +153,7 @@ async def _handle_sse_event(
                 ):
                     message.root.id = original_request_id
 
-                session_message = SessionMessage(message)
+                session_message = SessionMessage(message=message)
                 await read_stream_writer.send(session_message)
 
                 # Call resumption token callback if we have an ID
@@ -286,7 +286,7 @@ async def _handle_json_response(
         try:
             content = await response.aread()
             message = JSONRPCMessage.model_validate_json(content)
-            session_message = SessionMessage(message)
+            session_message = SessionMessage(message=message)
             await read_stream_writer.send(session_message)
         except Exception as exc:
             logger.error(f"Error parsing JSON response: {exc}")
@@ -333,7 +333,7 @@ async def _send_session_terminated_error(
             id=request_id,
             error=ErrorData(code=32600, message="Session terminated"),
         )
-        session_message = SessionMessage(JSONRPCMessage(jsonrpc_error))
+        session_message = SessionMessage(message=JSONRPCMessage(jsonrpc_error))
         await read_stream_writer.send(session_message)
 
     async def post_writer(

src/mcp/client/websocket.py

@@ -60,7 +60,7 @@ async def ws_reader():
                 async for raw_text in ws:
                     try:
                         message = types.JSONRPCMessage.model_validate_json(raw_text)
-                        session_message = SessionMessage(message)
+                        session_message = SessionMessage(message=message)
                         await read_stream_writer.send(session_message)
                     except ValidationError as exc:
                         # If JSON parse or model validation fails, send the exception

src/mcp/server/fastmcp/server.py

@@ -44,6 +44,7 @@
 from mcp.server.lowlevel.server import LifespanResultT
 from mcp.server.lowlevel.server import Server as MCPServer
 from mcp.server.lowlevel.server import lifespan as default_lifespan
+from mcp.server.message_queue import MessageDispatch
 from mcp.server.session import ServerSession, ServerSessionT
 from mcp.server.sse import SseServerTransport
 from mcp.server.stdio import stdio_server
@@ -90,6 +91,11 @@ class Settings(BaseSettings, Generic[LifespanResultT]):
     sse_path: str = "/sse"
     message_path: str = "/messages/"
 
+    # SSE message queue settings
+    message_dispatch: MessageDispatch | None = Field(
+        None, description="Custom message dispatch instance"
+    )
+
     # resource settings
     warn_on_duplicate_resources: bool = True
 
@@ -569,12 +575,21 @@ async def run_sse_async(self) -> None:
 
     def sse_app(self) -> Starlette:
         """Return an instance of the SSE server app."""
+        message_dispatch = self.settings.message_dispatch
+        if message_dispatch is None:
+            from mcp.server.message_queue import InMemoryMessageDispatch
+
+            message_dispatch = InMemoryMessageDispatch()
+            logger.info("Using default in-memory message dispatch")
+
         from starlette.middleware import Middleware
         from starlette.routing import Mount, Route
 
         # Set up auth context and dependencies
 
-        sse = SseServerTransport(self.settings.message_path)
+        sse = SseServerTransport(
+            self.settings.message_path, message_dispatch=message_dispatch
+        )
 
         async def handle_sse(scope: Scope, receive: Receive, send: Send):
             # Add client ID from auth context into request context if available
@@ -589,7 +604,14 @@ async def handle_sse(scope: Scope, receive: Receive, send: Send):
                     streams[1],
                     self._mcp_server.create_initialization_options(),
                 )
-            return Response()
+                return Response()
+
+        @asynccontextmanager
+        async def lifespan(app: Starlette):
+            try:
+                yield
+            finally:
+                await message_dispatch.close()
 
         # Create routes
         routes: list[Route | Mount] = []
@@ -666,7 +688,10 @@ async def sse_endpoint(request: Request) -> None:
 
         # Create Starlette app with routes and middleware
         return Starlette(
-            debug=self.settings.debug, routes=routes, middleware=middleware
+            debug=self.settings.debug,
+            routes=routes,
+            middleware=middleware,
+            lifespan=lifespan,
         )
 
     async def list_prompts(self) -> list[MCPPrompt]:

src/mcp/server/message_queue/__init__.py

@@ -0,0 +1,16 @@
+"""
+Message Dispatch Module for MCP Server
+
+This module implements dispatch interfaces for handling
+messages between clients and servers.
+"""
+
+from mcp.server.message_queue.base import InMemoryMessageDispatch, MessageDispatch
+
+# Try to import Redis implementation if available
+try:
+    from mcp.server.message_queue.redis import RedisMessageDispatch
+except ImportError:
+    RedisMessageDispatch = None
+
+__all__ = ["MessageDispatch", "InMemoryMessageDispatch", "RedisMessageDispatch"]

src/mcp/server/message_queue/base.py

@@ -0,0 +1,116 @@
+import logging
+from collections.abc import Awaitable, Callable
+from contextlib import asynccontextmanager
+from typing import Protocol, runtime_checkable
+from uuid import UUID
+
+from pydantic import ValidationError
+
+from mcp.shared.message import SessionMessage
+
+logger = logging.getLogger(__name__)
+
+MessageCallback = Callable[[SessionMessage | Exception], Awaitable[None]]
+
+
+@runtime_checkable
+class MessageDispatch(Protocol):
+    """Abstract interface for SSE message dispatching.
+
+    This interface allows messages to be published to sessions and callbacks to be
+    registered for message handling, enabling multiple servers to handle requests.
+    """
+
+    async def publish_message(
+        self, session_id: UUID, message: SessionMessage | str
+    ) -> bool:
+        """Publish a message for the specified session.
+
+        Args:
+            session_id: The UUID of the session this message is for
+            message: The message to publish (SessionMessage or str for invalid JSON)
+
+        Returns:
+            bool: True if message was published, False if session not found
+        """
+        ...
+
+    @asynccontextmanager
+    async def subscribe(self, session_id: UUID, callback: MessageCallback):
+        """Request-scoped context manager that subscribes to messages for a session.
+
+        Args:
+            session_id: The UUID of the session to subscribe to
+            callback: Async callback function to handle messages for this session
+        """
+        yield
+
+    async def session_exists(self, session_id: UUID) -> bool:
+        """Check if a session exists.
+
+        Args:
+            session_id: The UUID of the session to check
+
+        Returns:
+            bool: True if the session is active, False otherwise
+        """
+        ...
+
+    async def close(self) -> None:
+        """Close the message dispatch."""
+        ...
+
+
+class InMemoryMessageDispatch:
+    """Default in-memory implementation of the MessageDispatch interface.
+
+    This implementation immediately dispatches messages to registered callbacks when
+    messages are received without any queuing behavior.
+    """
+
+    def __init__(self) -> None:
+        self._callbacks: dict[UUID, MessageCallback] = {}
+
+    async def publish_message(
+        self, session_id: UUID, message: SessionMessage | str
+    ) -> bool:
+        """Publish a message for the specified session."""
+        if session_id not in self._callbacks:
+            logger.warning(f"Message dropped: unknown session {session_id}")
+            return False
+
+        # Parse string messages or recreate original ValidationError
+        if isinstance(message, str):
+            try:
+                callback_argument = SessionMessage.model_validate_json(message)
+            except ValidationError as exc:
+                callback_argument = exc
+        else:
+            callback_argument = message
+
+        # Call the callback with either valid message or recreated ValidationError
+        await self._callbacks[session_id](callback_argument)
+
+        logger.debug(f"Message dispatched to session {session_id}")
+        return True
+
+    @asynccontextmanager
+    async def subscribe(self, session_id: UUID, callback: MessageCallback):
+        """Request-scoped context manager that subscribes to messages for a session."""
+        self._callbacks[session_id] = callback
+        logger.debug(f"Subscribing to messages for session {session_id}")
+
+        try:
+            yield
+        finally:
+            if session_id in self._callbacks:
+                del self._callbacks[session_id]
+            logger.debug(f"Unsubscribed from session {session_id}")
+
+    async def session_exists(self, session_id: UUID) -> bool:
+        """Check if a session exists."""
+        return session_id in self._callbacks
+
+    async def close(self) -> None:
+        """Close the message dispatch."""
+        pass