backward compatible server example

Author: ihrpr

src/examples/server/sseAndStreamableHttpCompatibleServer.ts

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+import express, { Request, Response } from 'express';
+import { randomUUID } from "node:crypto";
+import { McpServer } from '../../server/mcp.js';
+import { EventStore, StreamableHTTPServerTransport } from '../../server/streamableHttp.js';
+import { SSEServerTransport } from '../../server/sse.js';
+import { z } from 'zod';
+import { CallToolResult, isInitializeRequest, JSONRPCMessage } from '../../types.js';
+
+/**
+ * This example server demonstrates backwards compatibility with both:
+ * 1. The deprecated HTTP+SSE transport (protocol version 2024-11-05)
+ * 2. The Streamable HTTP transport (protocol version 2025-03-26)
+ * 
+ * It maintains a single MCP server instance but exposes two transport options:
+ * - /mcp: The new Streamable HTTP endpoint (supports GET/POST/DELETE)
+ * - /sse: The deprecated SSE endpoint for older clients (GET to establish stream)
+ * - /request: The deprecated POST endpoint for older clients (POST to send messages)
+ */
+
+// Simple in-memory event store for resumability
+class InMemoryEventStore implements EventStore {
+  private events: Map<string, { streamId: string, message: JSONRPCMessage }> = new Map();
+
+  /**
+   * Generates a unique event ID for a given stream ID
+   */
+  private generateEventId(streamId: string): string {
+    return `${streamId}_${Date.now()}_${Math.random().toString(36).substring(2, 10)}`;
+  }
+
+  private getStreamIdFromEventId(eventId: string): string {
+    const parts = eventId.split('_');
+    return parts.length > 0 ? parts[0] : '';
+  }
+
+  /**
+   * Stores an event with a generated event ID
+   * Implements EventStore.storeEvent
+   */
+  async storeEvent(streamId: string, message: JSONRPCMessage): Promise<string> {
+    const eventId = this.generateEventId(streamId);
+    console.log(`Storing event ${eventId} for stream ${streamId}`);
+    this.events.set(eventId, { streamId, message });
+    return eventId;
+  }
+
+  /**
+   * Replays events that occurred after a specific event ID
+   * Implements EventStore.replayEventsAfter
+   */
+  async replayEventsAfter(lastEventId: string,
+    { send }: { send: (eventId: string, message: JSONRPCMessage) => Promise<void> }
+  ): Promise<string> {
+    if (!lastEventId || !this.events.has(lastEventId)) {
+      console.log(`No events found for lastEventId: ${lastEventId}`);
+      return '';
+    }
+
+    const streamId = this.getStreamIdFromEventId(lastEventId);
+    if (!streamId) {
+      console.log(`Could not extract streamId from lastEventId: ${lastEventId}`);
+      return '';
+    }
+
+    let foundLastEvent = false;
+    let eventCount = 0;
+
+    // Sort events by eventId for chronological ordering
+    const sortedEvents = [...this.events.entries()].sort((a, b) => a[0].localeCompare(b[0]));
+
+    for (const [eventId, { streamId: eventStreamId, message }] of sortedEvents) {
+      // Only include events from the same stream
+      if (eventStreamId !== streamId) {
+        continue;
+      }
+
+      // Start sending events after we find the lastEventId
+      if (eventId === lastEventId) {
+        foundLastEvent = true;
+        continue;
+      }
+
+      if (foundLastEvent) {
+        await send(eventId, message);
+        eventCount++;
+      }
+    }
+
+    console.log(`Replayed ${eventCount} events after ${lastEventId} for stream ${streamId}`);
+    return streamId;
+  }
+}
+
+// Create a shared MCP server instance
+const server = new McpServer({
+  name: 'backwards-compatible-server',
+  version: '1.0.0',
+}, { capabilities: { logging: {} } });
+
+// Register a simple tool that sends notifications over time
+server.tool(
+  'start-notification-stream',
+  'Starts sending periodic notifications for testing resumability',
+  {
+    interval: z.number().describe('Interval in milliseconds between notifications').default(100),
+    count: z.number().describe('Number of notifications to send (0 for 100)').default(50),
+  },
+  async ({ interval, count }, { sendNotification }): Promise<CallToolResult> => {
+    const sleep = (ms: number) => new Promise(resolve => setTimeout(resolve, ms));
+    let counter = 0;
+
+    while (count === 0 || counter < count) {
+      counter++;
+      try {
+        await sendNotification({
+          method: "notifications/message",
+          params: {
+            level: "info",
+            data: `Periodic notification #${counter} at ${new Date().toISOString()}`
+          }
+        });
+      }
+      catch (error) {
+        console.error("Error sending notification:", error);
+      }
+      // Wait for the specified interval
+      await sleep(interval);
+    }
+
+    return {
+      content: [
+        {
+          type: 'text',
+          text: `Started sending periodic notifications every ${interval}ms`,
+        }
+      ],
+    };
+  }
+);
+
+// Create Express application
+const app = express();
+app.use(express.json());
+
+// Store transports by session ID
+const transports: Record<string, StreamableHTTPServerTransport | SSEServerTransport> = {};
+
+//=============================================================================
+// STREAMABLE HTTP TRANSPORT (PROTOCOL VERSION 2025-03-26)
+//=============================================================================
+
+// Handle all MCP Streamable HTTP requests (GET, POST, DELETE) on a single endpoint
+app.all('/mcp', async (req: Request, res: Response) => {
+  console.log(`Received ${req.method} request to /mcp`);
+
+  try {
+    // Check for existing session ID
+    const sessionId = req.headers['mcp-session-id'] as string | undefined;
+    let transport: StreamableHTTPServerTransport;
+
+    if (sessionId && transports[sessionId]) {
+      // Check if the transport is of the correct type
+      const existingTransport = transports[sessionId];
+      if (existingTransport instanceof StreamableHTTPServerTransport) {
+        // Reuse existing transport
+        transport = existingTransport;
+      } else {
+        // Transport exists but is not a StreamableHTTPServerTransport (could be SSEServerTransport)
+        res.status(400).json({
+          jsonrpc: '2.0',
+          error: {
+            code: -32000,
+            message: 'Bad Request: Session exists but uses a different transport protocol',
+          },
+          id: null,
+        });
+        return;
+      }
+    } else if (!sessionId && req.method === 'POST' && isInitializeRequest(req.body)) {
+      const eventStore = new InMemoryEventStore();
+      transport = new StreamableHTTPServerTransport({
+        sessionIdGenerator: () => randomUUID(),
+        eventStore, // Enable resumability
+        onsessioninitialized: (sessionId) => {
+          // Store the transport by session ID when session is initialized
+          console.log(`StreamableHTTP session initialized with ID: ${sessionId}`);
+          transports[sessionId] = transport;
+        }
+      });
+
+      // Set up onclose handler to clean up transport when closed
+      transport.onclose = () => {
+        const sid = transport.sessionId;
+        if (sid && transports[sid]) {
+          console.log(`Transport closed for session ${sid}, removing from transports map`);
+          delete transports[sid];
+        }
+      };
+
+      // Connect the transport to the MCP server
+      await server.connect(transport);
+    } else {
+      // Invalid request - no session ID or not initialization request
+      res.status(400).json({
+        jsonrpc: '2.0',
+        error: {
+          code: -32000,
+          message: 'Bad Request: No valid session ID provided',
+        },
+        id: null,
+      });
+      return;
+    }
+
+    // Handle the request with the transport
+    await transport.handleRequest(req, res, req.body);
+  } catch (error) {
+    console.error('Error handling MCP request:', error);
+    if (!res.headersSent) {
+      res.status(500).json({
+        jsonrpc: '2.0',
+        error: {
+          code: -32603,
+          message: 'Internal server error',
+        },
+        id: null,
+      });
+    }
+  }
+});
+
+//=============================================================================
+// DEPRECATED HTTP+SSE TRANSPORT (PROTOCOL VERSION 2024-11-05)
+//=============================================================================
+
+app.get('/sse', async (req: Request, res: Response) => {
+  console.log('Received GET request to /sse (deprecated SSE transport)');
+  const transport = new SSEServerTransport('/messages', res);
+  transports[transport.sessionId] = transport;
+  res.on("close", () => {
+    delete transports[transport.sessionId];
+  });
+  await server.connect(transport);
+});
+
+app.post("/messages", async (req: Request, res: Response) => {
+  const sessionId = req.query.sessionId as string;
+  let transport: SSEServerTransport;
+  const existingTransport = transports[sessionId];
+  if (existingTransport instanceof SSEServerTransport) {
+    // Reuse existing transport
+    transport = existingTransport;
+  } else {
+    // Transport exists but is not a SSEServerTransport (could be StreamableHTTPServerTransport)
+    res.status(400).json({
+      jsonrpc: '2.0',
+      error: {
+        code: -32000,
+        message: 'Bad Request: Session exists but uses a different transport protocol',
+      },
+      id: null,
+    });
+    return;
+  }
+  if (transport) {
+    await transport.handlePostMessage(req, res);
+  } else {
+    res.status(400).send('No transport found for sessionId');
+  }
+});
+
+
+// Start the server
+const PORT = 3000;
+app.listen(PORT, () => {
+  console.log(`Backwards compatible MCP server listening on port ${PORT}`);
+  console.log(`
+==============================================
+SUPPORTED TRANSPORT OPTIONS:
+
+1. Streamable Http(Protocol version: 2025-03-26)
+   Endpoint: /mcp
+   Methods: GET, POST, DELETE
+   Usage: 
+     - Initialize with POST to /mcp
+     - Establish SSE stream with GET to /mcp
+     - Send requests with POST to /mcp
+     - Terminate session with DELETE to /mcp
+
+2. Http + SSE (Protocol version: 2024-11-05)
+   Endpoints: /sse (GET) and /request (POST)
+   Usage:
+     - Establish SSE stream with GET to /sse
+     - Send requests with POST to /message?sessionId=<id>
+==============================================
+`);
+});
+
+// Handle server shutdown
+process.on('SIGINT', async () => {
+  console.log('Shutting down server...');
+
+  // Close all active transports to properly clean up resources
+  for (const sessionId in transports) {
+    try {
+      console.log(`Closing transport for session ${sessionId}`);
+      await transports[sessionId].close();
+      delete transports[sessionId];
+    } catch (error) {
+      console.error(`Error closing transport for session ${sessionId}:`, error);
+    }
+  }
+  await server.close();
+  console.log('Server shutdown complete');
+  process.exit(0);
+});