Explain the concept of 'Non-blocking I/O'.

A thread does not wait for a resource (like disk or network) to respond; it moves to other tasks and is notified when data is ready.

Reactive systems are designed to handle the uncertainty of distributed environments. While reactive programming (like RxJS) helps manage local state changes, reactive systems focus on the whole architecture. The message-driven nature is the foundation that enables elasticity and resilience.