mcp-hub-009: Graceful backend failure — JSON-RPC error responses and session cleanup

src/relay.js

@@ -3,9 +3,21 @@ const registry = require('./backend-registry');
 
 const pendingRequests = new Map();
 
-function sendToBackend(serviceId, message, clientSessionId) {
+function sendToBackend(serviceId, message, clientSessionId, sessions) {
   const ws = registry.get(serviceId);
-  if (!ws) return null;
+  if (!ws) {
+    if (sessions && message.id !== undefined && message.id !== null) {
+      const session = sessions.get(clientSessionId);
+      if (session) {
+        session.res.write('event: message\ndata: ' + JSON.stringify({
+          jsonrpc: '2.0',
+          id: message.id,
+          error: { code: -32603, message: 'Backend unavailable' }
+        }) + '\n\n');
+      }
+    }
+    return null;
+  }
 
   const requestId = uuidv4();
   pendingRequests.set(requestId, { serviceId, clientSessionId });
@@ -53,4 +65,37 @@ function handleBackendMessage(serviceId, data, sessions) {
   }
 }
 
-module.exports = { sendToBackend, handleBackendMessage };
+function cleanupBackend(serviceId, sessions) {
+  // Clean up pending requests for this backend
+  for (const [requestId, pending] of pendingRequests) {
+    if (pending.serviceId === serviceId) {
+      const session = sessions.get(pending.clientSessionId);
+      if (session) {
+        // Write error notification event
+        session.res.write('event: message\ndata: ' + JSON.stringify({
+          jsonrpc: '2.0',
+          id: null,
+          error: { code: -32603, message: 'Backend disconnected' }
+        }) + '\n\n');
+      }
+      pendingRequests.delete(requestId);
+    }
+  }
+
+  // Clean up active SSE sessions for this backend
+  for (const [clientSessionId, session] of sessions) {
+    if (session.serviceId === serviceId) {
+      // Write final error event
+      session.res.write('event: message\ndata: ' + JSON.stringify({
+        jsonrpc: '2.0',
+        id: null,
+        error: { code: -32603, message: 'Backend disconnected' }
+      }) + '\n\n');
+      // End the response
+      session.res.end();
+      sessions.delete(clientSessionId);
+    }
+  }
+}
+
+module.exports = { sendToBackend, handleBackendMessage, cleanupBackend };

src/routes/mcp-proxy.js

@@ -42,7 +42,7 @@ router.post('/:serviceId/message', (req, res) => {
     return res.status(502).json({ error: 'backend not connected' });
   }
 
-  relay.sendToBackend(serviceId, req.body, sessionId);
+  relay.sendToBackend(serviceId, req.body, sessionId, sessions);
 
   return res.status(202).json({ status: 'accepted' });
 });

src/ws-server.js

@@ -78,6 +78,9 @@ function setupWsServer(httpServer) {
       if (pingTimer) clearInterval(pingTimer);
       if (authenticated && serviceId) {
         registry.unregister(serviceId);
+        const { cleanupBackend } = require('./relay');
+        const { sessions } = require('./routes/mcp-proxy');
+        cleanupBackend(serviceId, sessions);
       }
     });