FastMcp PubSub

Multi-worker cluster support extension for fast-mcp gem.

This gem extends the FastMcp gem to work with multiple Puma workers by adding PostgreSQL NOTIFY/LISTEN clustering support for FastMcp::Transports::RackTransport in Rails applications.

Problem

FastMcp::Transports::RackTransport stores SSE clients in an in-memory hash @sse_clients. In cluster mode (multiple Puma workers), messages don't reach between workers because each has its own memory space.

Solution

This gem provides PostgreSQL NOTIFY/LISTEN system for broadcasting messages between workers:

1. send_message → PostgreSQL NOTIFY
2. Listener thread in each worker → PostgreSQL LISTEN
3. On notification → send_local_message to local clients

Installation

Prerequisites: This gem requires the fast-mcp gem to be installed first.

Add both gems to your application's Gemfile:

gem 'fast-mcp', '~> 1.5.0'      # Required base gem
gem 'fast_mcp_pubsub'           # This extension

And then execute:

bundle install

Note: The fast-mcp gem provides the core MCP (Model Context Protocol) server functionality, while this gem extends it with multi-worker support.

Usage

Automatic Integration

No configuration needed! Just add the gem to your Gemfile and it works automatically.

The gem will:

• ✅ Automatically patch FastMcp::Transports::RackTransport during Rails initialization
• ✅ Start listener automatically when Rails server starts
• ✅ Use Rails.logger for logging (no configuration required)
• ✅ Work in both single-worker and cluster mode

Optional Configuration

If you need custom settings:

# config/initializers/fast_mcp_pubsub.rb (optional)
FastMcpPubsub.configure do |config|
  config.enabled = Rails.env.production?    # Enable only in production
  config.channel_name = 'my_custom_channel' # Custom PostgreSQL NOTIFY channel
  config.auto_start = true                  # Start listener automatically (default: true)
  config.connection_pool_size = 10          # Database connection pool size
end

Puma Cluster Mode

For cluster mode (multiple workers), you need to manually start the listener in each worker process:

# config/puma/production.rb
workers ENV.fetch("WEB_CONCURRENCY") { 2 }
preload_app!

on_worker_boot do
  Rails.logger.info "MCP Transport: Starting PubSub listener for cluster mode worker #{Process.pid}"
  
  # Start FastMcpPubsub listener in each worker
  FastMcpPubsub::Service.start_listener
  
  # Your other worker boot code...
end

Why manual setup is required:

• 🔧 Master process automatically detects cluster mode and skips listener startup
• 👷 Worker processes need explicit listener startup in on_worker_boot hook
• 📡 Each worker gets its own listener thread for receiving broadcasts
• 🔄 Automatic cleanup happens on worker shutdown

Manual Control

# Manually start/stop listener
FastMcpPubsub::Service.start_listener
FastMcpPubsub::Service.stop_listener

# Check listener status
FastMcpPubsub::Service.listener_thread&.alive?

How It Works

1. Patches FastMcp::Transports::RackTransport: Overrides send_message method
2. Broadcasts via PostgreSQL: Uses NOTIFY channel, payload
3. Listener threads: Each worker has a dedicated listener thread
4. Local delivery: Messages are delivered to local SSE clients in each worker

Configuration Options

Option	Default	Description
enabled	true	Enable/disable PubSub functionality
channel_name	'mcp_broadcast'	PostgreSQL NOTIFY channel name
auto_start	true	Start listener automatically
connection_pool_size	5	Database connection pool size

Note: Logging is handled automatically via FastMcpPubsub.logger which returns Rails.logger.

Error Handling

• Payload size limit: 7800 bytes (PostgreSQL NOTIFY limit)
• Fallback mechanism: Falls back to local delivery if PubSub fails
• Automatic restart: Listener restarts on connection errors
• Graceful shutdown: Proper cleanup on process exit

Requirements

This gem is an extension for Rails applications using FastMcp and requires:

• FastMcp gem ~> 1.5.0 (the base MCP server this gem extends)
• Rails 7.0+ (required for Railtie integration)
• PostgreSQL database (for NOTIFY/LISTEN functionality)
• Puma web server in cluster mode (multi-worker setup)

Important Notes:

• This gem will not work in standalone Ruby applications or non-Rails frameworks, as it relies heavily on Rails infrastructure (ActiveRecord, Railtie, Rails.logger, etc.)
• PostgreSQL is mandatory - this gem will NOT work with MySQL, SQLite, or other databases as it requires PostgreSQL's NOTIFY/LISTEN functionality. Support for other databases would require significant additional development.

Thread Safety

All operations are thread-safe and designed for multi-worker environments:

• Connection pooling for database operations
• Proper thread lifecycle management
• Automatic cleanup on process termination

Development

After checking out the repo, run:

bin/setup      # Install dependencies
rake test      # Run tests
bin/console    # Interactive prompt

To install locally:

bundle exec rake install

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/jchsoft/fast_mcp_pubsub.

License

The gem is available as open source under the terms of the MIT License.