Add message queue for SSE messages POST endpoint

README.md

@@ -385,6 +385,30 @@ 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/fastmcp/unicode_example.py

@@ -4,8 +4,14 @@
 """
 
 from mcp.server.fastmcp import FastMCP
+from mcp.server.message_queue import RedisMessageQueue
 
-mcp = FastMCP()
+# Create a Redis message queue
+redis_queue = RedisMessageQueue(
+    redis_url="redis://localhost:6379/0", prefix="mcp:pubsub:"
+)
+
+mcp = FastMCP(message_queue=redis_queue)
 
 
 @mcp.tool(
@@ -61,4 +67,4 @@ def multilingual_hello() -> str:
 
 
 if __name__ == "__main__":
-    mcp.run()
+    mcp.run(transport="sse")

pyproject.toml

@@ -36,6 +36,7 @@ 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]"

src/mcp/server/fastmcp/server.py

@@ -32,6 +32,7 @@
 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
@@ -75,6 +76,11 @@ 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
 
@@ -466,6 +472,7 @@ async def run_stdio_async(self) -> None:
     async def run_sse_async(self) -> None:
         """Run the server using SSE transport."""
         import uvicorn
+
         starlette_app = self.sse_app()
 
         config = uvicorn.Config(
@@ -479,7 +486,16 @@ async def run_sse_async(self) -> None:
 
     def sse_app(self) -> Starlette:
         """Return an instance of the SSE server app."""
-        sse = SseServerTransport(self.settings.message_path)
+        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")
+
+        sse = SseServerTransport(
+            self.settings.message_path, message_dispatch=message_dispatch
+        )
 
         async def handle_sse(request: Request) -> None:
             async with sse.connect_sse(

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 +1,108 @@
+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
+
+import mcp.types as types
+
+logger = logging.getLogger(__name__)
+
+MessageCallback = Callable[[types.JSONRPCMessage | 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: types.JSONRPCMessage | 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 (JSONRPCMessage 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
+        """
+        ...
+
+
+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: types.JSONRPCMessage | 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 = types.JSONRPCMessage.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