Let's imagine we're building a robust configuration management system. This isn't about deploying VMs or managing infrastructure at a massive scale (that's HLD). Instead, we're diving into the code-level challenge of managing application configurations within a single application instance. Think about a complex application that needs to adjust its behavior based on various settings, feature flags, and thresholds. These configurations might come from different sources (files, databases, environment variables), and we need a system to handle updates, validations, and efficient access to these settings. The system should also allow for multiple subscribers to be notified when configuration changes occur.

We need a flexible, extensible, and thread-safe design that allows new configuration sources and types to be added easily without modifying the core components. We also need a mechanism to validate configurations before they are applied, preventing the application from entering an invalid state. Finally, we need to allow components to subscribe to configuration changes and receive notifications when updates occur.

Requirements