feat: add PoC of stateless HTTP transport

This is an implementation of a stateless transport over HTTP
for MCP. It's not part of the official spec, but something
similar will probably be added in the future.

Functionally, it just presents a single HTTP endpoint
(conventionally at `/mcp`) that accepts JSONRPC messages
as POST bodies, and returns JSONRPC messages as responses.

Each inbound message is validated and gets its own 'server'
instance. The handler then takes care of initializing the
server (which in a stateless protocol is the responsibility
of the client), then forwards the inbound message to the
server, and returns the server's response.

We could easily add middleware for things like authentication,
similar to in #21.

This also comes with a 'client' implementation which is similar
to the stdio and sse clients in the official SDK and returns a
pair of read/write streams for use with a `ClientSession`. This
is useful for testing or writing example MCP clients.

The idea is that this transport will be compatible with the
[HTTP transport in the unofficial Go SDK][go-sdk], so we can
start building examples of usage. Notably this client also
supports custom headers, which will be useful for authentication.

[go-sdk]: https://github.com/metoro-io/mcp-golang/blob/main/examples/http_example/README.md

Author: sd2k

pyproject.toml

@@ -34,3 +34,7 @@ build-backend = "hatchling.build"
 
 [tool.pytest.ini_options]
 asyncio_default_fixture_loop_scope = "session"
+
+[tool.ruff]
+line-length = 88
+target-version = "py310"

src/mcp_grafana/__init__.py

@@ -1,7 +1,73 @@
+import enum
+from typing import Literal
+
+import anyio
+import uvicorn
 from mcp.server import FastMCP
+from starlette.requests import Request
 
 from .tools import add_tools
 
+
+class Transport(enum.StrEnum):
+    http = "http"
+    stdio = "stdio"
+    sse = "sse"
+
+
+class GrafanaMCP(FastMCP):
+    async def run_http_async(self) -> None:
+        from starlette.applications import Starlette
+        from starlette.routing import Route
+
+        from .transports.http import handle_message
+
+        async def handle_http(request: Request):
+            async with handle_message(
+                request.scope, request.receive, request._send
+            ) as (
+                read_stream,
+                write_stream,
+            ):
+                await self._mcp_server.run(
+                    read_stream,
+                    write_stream,
+                    self._mcp_server.create_initialization_options(),
+                )
+
+        starlette_app = Starlette(
+            debug=self.settings.debug,
+            routes=[
+                Route("/mcp", endpoint=handle_http, methods=["POST"]),
+            ],
+        )
+
+        config = uvicorn.Config(
+            starlette_app,
+            host=self.settings.host,
+            port=self.settings.port,
+            log_level=self.settings.log_level.lower(),
+        )
+        server = uvicorn.Server(config)
+        await server.serve()
+
+    def run(self, transport: Literal["http", "stdio", "sse"] = "stdio") -> None:
+        """Run the FastMCP server. Note this is a synchronous function.
+
+        Args:
+            transport: Transport protocol to use ("stdio" or "sse")
+        """
+        if transport not in Transport:
+            raise ValueError(f"Unknown transport: {transport}")
+
+        if transport == "stdio":
+            anyio.run(self.run_stdio_async)
+        elif transport == "sse":
+            anyio.run(self.run_sse_async)
+        else:
+            anyio.run(self.run_http_async)
+
+
 # Create an MCP server
-mcp = FastMCP("Grafana", log_level="DEBUG")
+mcp = GrafanaMCP("Grafana", log_level="DEBUG")
 add_tools(mcp)

src/mcp_grafana/cli.py

@@ -1,17 +1,10 @@
-import enum
-
 import typer
 
-from . import mcp
+from . import mcp, Transport
 
 app = typer.Typer()
 
 
-class Transport(enum.StrEnum):
-    stdio = "stdio"
-    sse = "sse"
-
-
 @app.command()
 def run(transport: Transport = Transport.stdio):
     mcp.run(transport.value)

src/mcp_grafana/transports/http.py

@@ -0,0 +1,182 @@
+import logging
+from contextlib import asynccontextmanager
+from json import JSONDecodeError
+from typing import Any, Tuple
+
+import anyio
+from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
+import httpx
+from mcp import types
+from pydantic import ValidationError
+from starlette.requests import Request
+from starlette.responses import JSONResponse, Response
+from starlette.types import Receive, Scope, Send
+
+
+logger = logging.getLogger(__name__)
+
+
+ReadStream = MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
+ReadStreamWriter = MemoryObjectSendStream[types.JSONRPCMessage | Exception]
+WriteStream = MemoryObjectSendStream[types.JSONRPCMessage]
+WriteStreamReader = MemoryObjectReceiveStream[types.JSONRPCMessage]
+
+
+def make_streams() -> Tuple[
+    ReadStream, ReadStreamWriter, WriteStream, WriteStreamReader
+]:
+    read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception]
+    read_stream_writer: MemoryObjectSendStream[types.JSONRPCMessage | Exception]
+
+    write_stream: MemoryObjectSendStream[types.JSONRPCMessage]
+    write_stream_reader: MemoryObjectReceiveStream[types.JSONRPCMessage]
+
+    read_stream_writer, read_stream = anyio.create_memory_object_stream(0)
+    write_stream, write_stream_reader = anyio.create_memory_object_stream(0)
+    return read_stream, read_stream_writer, write_stream, write_stream_reader
+
+
+async def initialize(
+    read_stream_writer: ReadStreamWriter,
+    write_stream_reader: WriteStreamReader,
+):
+    """
+    Initialize the MCP server for this request.
+
+    In a stateful transport (e.g. stdio or sse) the client would
+    send an initialize request to the server, and the server would send
+    an 'initialized' response back to the client.
+
+    In the HTTP transport we're trying to be stateless, so we'll have to
+    handle the initialization ourselves.
+
+    This function handles that initialization by sending the required
+    messages to the server and consuming the response.
+    """
+    # First construct the initialize request.
+    initialize_request = types.InitializeRequest(
+        method="initialize",
+        params=types.InitializeRequestParams(
+            protocolVersion=types.LATEST_PROTOCOL_VERSION,
+            capabilities=types.ClientCapabilities(
+                experimental=None,
+                roots=None,
+                sampling=None,
+            ),
+            # TODO: get the name and version from the package metadata.
+            clientInfo=types.Implementation(name="mcp-grafana", version="0.1.2"),
+        ),
+    )
+    initialize_request = types.JSONRPCRequest(
+        jsonrpc="2.0",
+        id=1,
+        **initialize_request.model_dump(by_alias=True, mode="json"),
+    )
+    # Send it to the server.
+    await read_stream_writer.send(types.JSONRPCMessage(initialize_request))
+    # We can ignore the response since we're not sending it back to the client.
+    await write_stream_reader.receive()
+
+    # Next we need to notify the server that we're initialized.
+    initialize_notification = types.JSONRPCNotification(
+        jsonrpc="2.0",
+        **types.ClientNotification(
+            types.InitializedNotification(method="notifications/initialized"),
+        ).model_dump(by_alias=True, mode="json"),
+    )
+    await read_stream_writer.send(types.JSONRPCMessage(initialize_notification))
+    # Notifications don't have a response, so we don't need to await the
+    # write stream reader.
+
+
+@asynccontextmanager
+async def handle_message(scope: Scope, receive: Receive, send: Send):
+    """
+    ASGI application for handling MCP messages using the stateless HTTP transport.
+
+    This function is called for each incoming message. It creates a new
+    stream for reading and writing messages, which will be used by the
+    MCP server, and handles:
+
+        - decoding the client message from JSON into internal types
+        - validating the client message
+        - initializing the MCP server, which must be done on every request
+          (since this is a stateless transport)
+        - sending the client message to the MCP server
+        - receiving the server's response
+        - encoding the server's response into JSON and sending it back to the client
+
+    The returned read and write streams are intended to be passed to
+    `mcp.server.lowlevel.Server.run()` as the `read_stream` and `write_stream`
+    arguments.
+    """
+    read_stream, read_stream_writer, write_stream, write_stream_reader = make_streams()
+
+    async def handle_post_message():
+        request = Request(scope, receive)
+        try:
+            json = await request.json()
+        except JSONDecodeError as err:
+            logger.error(f"Failed to parse message: {err}")
+            response = Response("Could not parse message", status_code=400)
+            await response(scope, receive, send)
+            return
+        try:
+            client_message = types.JSONRPCMessage.model_validate(json)
+            logger.debug(f"Validated client message: {client_message}")
+        except ValidationError as err:
+            logger.error(f"Failed to parse message: {err}")
+            response = Response("Could not parse message", status_code=400)
+            await response(scope, receive, send)
+            return
+
+        # As part of the MCP spec we need to initialize first.
+        # In a stateful flow (e.g. stdio or sse transports) the client would
+        # send an initialize request to the server, and the server would send
+        # a response back to the client. In this case we're trying to be stateless,
+        # so we'll handle the initialization ourselves.
+        logger.debug("Initializing server")
+        await initialize(read_stream_writer, write_stream_reader)
+
+        # Alright, now we can send the client message.
+        logger.debug("Sending client message")
+        await read_stream_writer.send(client_message)
+        # Wait for the server's response, and forward it to the client.
+        server_message = await write_stream_reader.receive()
+        obj = server_message.model_dump(by_alias=True, mode="json", exclude_none=True)
+        response = JSONResponse(obj)
+        await response(scope, receive, send)
+
+    async with anyio.create_task_group() as tg:
+        tg.start_soon(handle_post_message)
+        yield (read_stream, write_stream)
+
+
+@asynccontextmanager
+async def http_client(url: str, headers: dict[str, Any] | None = None):
+    read_stream, read_stream_writer, write_stream, write_stream_reader = make_streams()
+
+    async with anyio.create_task_group() as tg:
+        try:
+
+            async def http_rw():
+                logger.debug("Waiting for request body")
+                body = await write_stream_reader.receive()
+
+                logger.debug(f"Connecting to HTTP endpoint: {url}")
+                async with httpx.AsyncClient(headers=headers) as client:
+                    response = await client.post(
+                        url, content=body.model_dump_json(by_alias=True)
+                    )
+                    logger.debug(f"Received response: {response.status_code}")
+                    message = types.JSONRPCMessage.model_validate_json(response.content)
+                    await read_stream_writer.send(message)
+
+            tg.start_soon(http_rw)
+            try:
+                yield read_stream, write_stream
+            finally:
+                tg.cancel_scope.cancel()
+        finally:
+            await read_stream_writer.aclose()
+            await write_stream_reader.aclose()

tests/conftest.py

@@ -1,3 +1,6 @@
+import pytest
+
+
 def pytest_addoption(parser):
     """
     Add command line options for integration and cloud tests.
@@ -29,3 +32,8 @@ def pytest_addoption(parser):
         default=False,
         help="enable cloud integration tests",
     )
+
+
+@pytest.fixture
+def anyio_backend():
+    return "asyncio"