mcp-hub-003: Client SSE transport and message relay

src/relay.js

@@ -0,0 +1,56 @@
+const { v4: uuidv4 } = require('uuid');
+const registry = require('./backend-registry');
+
+const pendingRequests = new Map();
+
+function sendToBackend(serviceId, message, clientSessionId) {
+  const ws = registry.get(serviceId);
+  if (!ws) return null;
+
+  const requestId = uuidv4();
+  pendingRequests.set(requestId, { serviceId, clientSessionId });
+
+  ws.send(JSON.stringify({
+    type: 'mcp-request',
+    requestId,
+    clientSessionId,
+    payload: message,
+  }));
+
+  return requestId;
+}
+
+function handleBackendMessage(serviceId, data, sessions) {
+  let msg;
+  try {
+    msg = JSON.parse(data);
+  } catch (e) {
+    console.error('[relay] invalid JSON from backend ' + serviceId);
+    return;
+  }
+
+  if (msg.type === 'mcp-response') {
+    const pending = pendingRequests.get(msg.requestId);
+    if (!pending) {
+      console.warn('[relay] no pending request for requestId ' + msg.requestId);
+      return;
+    }
+    pendingRequests.delete(msg.requestId);
+
+    const session = sessions.get(pending.clientSessionId);
+    if (!session) {
+      console.warn('[relay] no session for clientSessionId ' + pending.clientSessionId);
+      return;
+    }
+
+    session.res.write('event: message\ndata: ' + JSON.stringify(msg.payload) + '\n\n');
+  } else if (msg.type === 'mcp-notification') {
+    for (const [, session] of sessions) {
+      if (session.serviceId === serviceId) {
+        session.res.write('event: message\ndata: ' + JSON.stringify(msg.payload) + '\n\n');
+      }
+    }
+  }
+}
+
+module.exports = { sendToBackend, handleBackendMessage };

src/routes/mcp-proxy.js

@@ -0,0 +1,51 @@
+const { Router } = require('express');
+const { v4: uuidv4 } = require('uuid');
+const registry = require('../backend-registry');
+const relay = require('../relay');
+
+const router = Router();
+const sessions = new Map();
+
+router.get('/:serviceId/sse', (req, res) => {
+  const { serviceId } = req.params;
+
+  if (!registry.get(serviceId)) {
+    return res.status(502).json({ error: 'backend not connected' });
+  }
+
+  res.setHeader('Content-Type', 'text/event-stream');
+  res.setHeader('Cache-Control', 'no-cache');
+  res.setHeader('Connection', 'keep-alive');
+  res.flushHeaders();
+
+  const clientSessionId = uuidv4();
+  sessions.set(clientSessionId, { res, serviceId });
+
+  res.write('event: endpoint\ndata: /' + serviceId + '/message?sessionId=' + clientSessionId + '\n\n');
+
+  req.on('close', () => {
+    sessions.delete(clientSessionId);
+    console.log('[sse] client disconnected: ' + clientSessionId + ' (' + serviceId + ')');
+  });
+});
+
+router.post('/:serviceId/message', (req, res) => {
+  const { serviceId } = req.params;
+  const { sessionId } = req.query;
+
+  const session = sessions.get(sessionId);
+  if (!session) {
+    return res.status(404).json({ error: 'session not found' });
+  }
+
+  if (!registry.get(serviceId)) {
+    return res.status(502).json({ error: 'backend not connected' });
+  }
+
+  relay.sendToBackend(serviceId, req.body, sessionId);
+
+  return res.status(202).json({ status: 'accepted' });
+});
+
+module.exports = router;
+module.exports.sessions = sessions;

src/server.js

@@ -1,5 +1,6 @@
 const express = require('express');
 const registry = require('./backend-registry');
+const mcpProxy = require('./routes/mcp-proxy');
 
 const app = express();
 
@@ -8,7 +9,7 @@ app.use(express.json());
 app.use((req, res, next) => {
   const start = Date.now();
   res.on('finish', () => {
-    console.log(`${req.method} ${req.path} ${res.statusCode} ${Date.now() - start}ms`);
+    console.log(req.method + ' ' + req.path + ' ' + res.statusCode + ' ' + (Date.now() - start) + 'ms');
   });
   next();
 });
@@ -21,4 +22,6 @@ app.get('/health', (req, res) => {
   });
 });
 
+app.use('/', mcpProxy);
+
 module.exports = app;

src/ws-server.js

@@ -1,6 +1,7 @@
 const WebSocket = require('ws');
 const config = require('./config');
 const registry = require('./backend-registry');
+const relay = require('./relay');
 
 const PING_INTERVAL_MS = 30000;
 const MAX_MISSED_PONGS = 2;
@@ -43,7 +44,8 @@ function setupWsServer(httpServer) {
       registry.register(serviceId, ws);
 
       ws.on('message', (payload) => {
-        // MCP responses — stored for relay (handled in next task)
+        const { sessions } = require('./routes/mcp-proxy');
+        relay.handleBackendMessage(serviceId, payload, sessions);
       });
 
       ws.on('pong', () => {