Modern web applications constantly monitor file changes, whether for code reloads, asset compilation, or development environment updates.

Rails achieves this efficiently using the EventedFileUpdateChecker, which relies on OS-level file system notifications instead of inefficient polling.

📌 In this guide, you’ll learn:
✔️ What is the Evented File Update Checker?
✔️ How it works in Rails
✔️ How to configure and optimize it
✔️ Common issues and debugging techniques

1. What Is the Evented File Update Checker?

Rails’ EventedFileUpdateChecker is a lightweight file monitoring mechanism that uses OS-specific event-driven libraries to detect file changes without active polling.

🔍 Key Benefits:

  • Efficient File Watching – Uses event-based notifications instead of CPU-intensive polling.
  • Reduces Load – Minimizes system resource usage.
  • Fast Response Time – Detects file changes instantly in development mode.

Supported File Change Detection Methods:
| OS | Backend Used | |——|————| | Linux | inotify via listen gem | | macOS | FSEvent via listen gem | | Windows | WDM via listen gem |

2. How Does Rails Use EventedFileUpdateChecker?

Rails enables the EventedFileUpdateChecker by default in development mode to monitor changes in:
✔️ Application code (app/, config/)
✔️ View templates (views/)
✔️ Locale files (config/locales/)

📌 Where Is It Defined?
In config/environments/development.rb, you’ll find:

config.file_watcher = ActiveSupport::EventedFileUpdateChecker

📌 How It Works:

  1. Rails registers directories/files to monitor.
  2. When a change occurs, the OS sends an event instead of Rails continuously checking files.
  3. Rails reloads only the modified files, improving efficiency.

3. Customizing and Using Evented File Update Checker

Adding Custom File Watchers

You can manually set up a watcher for additional directories:

watcher = ActiveSupport::EventedFileUpdateChecker.new(["custom_folder"]) do  
puts "Custom folder updated!"  
end

Thread.new { loop { watcher.execute_if_updated } }

🚀 Why?
This allows you to watch files outside Rails’ default directories for live updates.

4. Debugging File Change Detection Issues

Even though Rails optimizes file watching, you might encounter unexpected behavior. Here’s how to troubleshoot:

Issue: Files Not Reloading in Development

🔍 Solution:

  1. Check if the listen gem is installed: 
    bundle show listen
  2. Ensure OS-level dependencies are present (e.g., inotify-tools for Linux).
  3. Restart the Rails server (rails s).
Issue: High CPU Usage Due to File Watching

🔍 Solution:

  1. Limit the watched directories by excluding unnecessary folders: 
    config.file_watcher = ActiveSupport::EventedFileUpdateChecker.new(  
Rails.root.join("app"),  
Rails.root.join("config")  
)
  2. Use manual reload strategies for large applications.

5. When to Disable Evented File Update Checker

🚫 In production environments, file changes are rare, so watching files isn’t necessary.

📌 Disable it in config/environments/production.rb:

config.file_watcher = ActiveSupport::FileUpdateChecker

🚀 Why?

  • Saves system resources.
  • Prevents unnecessary background threads.

Conclusion

🚀 Key Takeaways:
✔️ EventedFileUpdateChecker improves Rails’ file watching efficiency.
✔️ Uses OS-level notifications instead of CPU-heavy polling.
✔️ Can be customized to watch additional files in your project.
✔️ Debugging file watching issues can prevent reload failures.
✔️ Disable in production to optimize performance.

🔗 Next Steps: Explore Rails internals with ActiveSupport::Dependencies!