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layout default
title Chapter 4: Notifications, Logging, and Observability
nav_order 4
parent MCP Ruby SDK Tutorial

Chapter 4: Notifications, Logging, and Observability

Welcome to Chapter 4: Notifications, Logging, and Observability. In this part of MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby, you will build an intuitive mental model first, then move into concrete implementation details and practical production tradeoffs.

This chapter explains runtime observability patterns for Ruby MCP servers.

Learning Goals

  • send list-change notifications correctly for tools/prompts/resources
  • configure and emit structured log messages across severity levels
  • align server logging behavior with client logging/setLevel controls
  • avoid noisy or sensitive telemetry in production environments

Notification and Logging Surfaces

Surface Method
Tool list changes notify_tools_list_changed
Prompt list changes notify_prompts_list_changed
Resource list changes notify_resources_list_changed
Structured logs notify_log_message

Operational Notes

  • notifications require an active transport/session context.
  • logging emission is filtered by client-selected log level.
  • redact secrets before serialization into log message payloads.

Source References

Summary

You now have a practical observability model for Ruby MCP services.

Next: Chapter 5: Transports: stdio, Streamable HTTP, and Session Modes

Depth Expansion Playbook

Source Code Walkthrough

dev.yml

The dev module in dev.yml handles a key part of this chapter's functionality:

name: mcp-ruby

type: ruby

up:
  - ruby
  - bundler

commands:
  console:
    desc: Open console with the gem loaded
    run: bin/console
  build:
    desc: Build the gem using rake build
    run: bin/rake build
  test:
    desc: Run tests
    syntax:
      argument: file
      optional: args...
    run:  |
      if [[ $# -eq 0 ]]; then
        bin/rake test
      else
        bin/rake -I test "$@"
      fi
  style:
    desc: Run rubocop
    aliases: [rubocop, lint]
    run: bin/rake rubocop

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

examples/streamable_http_server.rb

The streamable_http_server module in examples/streamable_http_server.rb handles a key part of this chapter's functionality:

# frozen_string_literal: true

$LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
require "mcp"
require "rack/cors"
require "rackup"
require "json"
require "logger"

# Create a logger for SSE-specific logging
sse_logger = Logger.new($stdout)
sse_logger.formatter = proc do |severity, datetime, _progname, msg|
  "[SSE] #{severity} #{datetime.strftime("%H:%M:%S.%L")} - #{msg}\n"
end

# Tool that returns a response that will be sent via SSE if a stream is active
class NotificationTool < MCP::Tool
  tool_name "notification_tool"
  description "Returns a notification message that will be sent via SSE if stream is active"
  input_schema(
    properties: {
      message: { type: "string", description: "Message to send via SSE" },
      delay: { type: "number", description: "Delay in seconds before returning (optional)" },
    },
    required: ["message"],
  )

  class << self
    attr_accessor :logger

    def call(message:, delay: 0)
      sleep(delay) if delay > 0

      logger&.info("Returning notification message: #{message}")

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

.rubocop.yml

The .rubocop module in .rubocop.yml handles a key part of this chapter's functionality:

inherit_gem:
  rubocop-shopify: rubocop.yml

plugins:
  - rubocop-minitest
  - rubocop-rake

AllCops:
  TargetRubyVersion: 2.7

Gemspec/DevelopmentDependencies:
  Enabled: true

Lint/IncompatibleIoSelectWithFiberScheduler:
  Enabled: true

Minitest/LiteralAsActualArgument:
  Enabled: true

This module is important because it defines how MCP Ruby SDK Tutorial: Building MCP Servers and Clients in Ruby implements the patterns covered in this chapter.

How These Components Connect

flowchart TD
    A[dev]
    B[streamable_http_server]
    C[.rubocop]
    A --> B
    B --> C
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