Experimental MCP reaction

[danielgerlag]

Drasi MCP Reaction

This is a Drasi reaction that provides a Model Context Protocol (MCP) server. MCP clients can connect using HTTP transport and use the resource subscription feature to subscribe to one or more queries for real-time updates.

Features

• MCP Server: Provides an HTTP-based MCP server using Server-Sent Events (SSE)
• Resource Subscriptions: Clients can subscribe to specific query resources for real-time updates
• Real-time Notifications: When Drasi query results change, subscribed clients receive notifications
• Template Support: Uses Handlebars templates to format notification payloads
• Multiple Change Types: Supports added, updated, and deleted notifications

Architecture

The reaction integrates two main components:

1. Drasi Reaction SDK: Receives change events from Drasi queries
2. MCP SDK: Provides the MCP server and resource subscription functionality

When Drasi sends change events, the reaction processes them through Handlebars templates and sends notifications to subscribed MCP clients.

Configuration

Environment Variables

• port: HTTP server port (default: 3000)

Query Configuration

Each query can be configured with templates for different change types:

# Example query configuration
added:
  template: '{"type": "item_added", "data": {{json after}}}'
updated:
  template: '{"type": "item_updated", "before": {{json before}}, "after": {{json after}}}'
deleted:
  template: '{"type": "item_deleted", "data": {{json before}}}'

Usage

Running the Reaction

# Install dependencies
npm install

# Build the project
npm run build

# Start the server
npm start

MCP Client Connection

MCP clients can connect to the server using Streamable HTTP transport:

HTTP Endpoint: http://localhost:3000/
Transport: Streamable HTTP (preferred) with SSE fallback
Protocol Version: 2025-03-26 (with 2024-11-05 compatibility)

Connection Process:

1. Initialization: POST to / with MCP initialization request
2. Session Management: Server generates session ID and tracks transport
3. Message Exchange: POST requests for client-to-server messages
4. Notifications: GET requests establish SSE streams for server-to-client messages

Resource URIs

Query resources are accessible via URIs in the format:

drasi://query/{queryId}

Subscription Workflow

1. Client connects to the MCP server via SSE
2. Client subscribes to specific query resources using resources/subscribe
3. When Drasi sends change events for subscribed queries, the server sends notifications/{query}/added, notifications/{query}/updated and notifications/{query}/deleted to clients

Development

Project Structure

src/
  index.ts        # Main application entry point
package.json      # Node.js dependencies
tsconfig.json     # TypeScript configuration
Dockerfile.*      # Docker build files
Makefile         # Build automation

Key Dependencies

• @drasi/reaction-sdk: Core Drasi reaction functionality
• @modelcontextprotocol/sdk: Official MCP SDK for TypeScript
• handlebars: Template engine for formatting notifications

Protocol Support

This implementation follows the MCP specification and supports:

• Modern Streamable HTTP Transport (2025-03-26): Primary transport method
• Legacy SSE Transport (2024-11-05): Backward compatibility support
• Session Management: Proper session ID generation and tracking
• Resource Management: Dynamic resource registration and subscription
• Real-time Notifications: Both custom query-specific and standard MCP notifications
• Error Handling: Comprehensive error responses with proper JSON-RPC format

[amansinghoriginal]

LGTM!
We can revisit later if we want more features or flexibility.