Design a production-ready Node.js backend architecture that supports high availability, fault tolerance, observability, horizontal scaling.

The Event Loop has phases like Timers, Pending Callbacks, Idle/Prepare, Poll, Check, and Close Callbacks.

Each phase has a FIFO queue of callbacks. For instance, the 'Poll' phase retrieves new I/O events, while 'Check' executes setImmediate() callbacks. Understanding this is crucial for debugging race conditions.