NOTE
Singleton Pattern
A single shared instance, initialization safety, lifecycle, testability, and global-state trade-offs.
This is a historical learning note and may contain outdated or incomplete understanding.
A singleton restricts a component to one process-wide instance and provides access to it. Thread-safe initialization can be implemented with language/runtime primitives rather than hand-rolled double-checked locking when better facilities exist.
The main cost is global state: hidden dependencies, difficult lifecycle control, and harder tests. Prefer explicit dependency injection for services whose ownership/lifecycle matters; use singleton-like stateless/shared infrastructure only when one instance is genuinely the right scope.