Event Loops & Non-Blocking Execution Models

The Event Loop is an architectural pattern that coordinates non-blocking, asynchronous execution on a single main thread. Popularized by Node.js, browsers, Redis, and Python AsyncIO, it eliminates the need for thread-per-connection synchronization.

An event loop continuously evaluates three core structures:

1. Call Stack: The standard LIFO execution stack executing synchronous JavaScript or bytecode instructions.
2. Microtask Queue: High-priority queue containing process.nextTick(), Promise.then(), and queueMicrotask(). The event loop drains the microtask queue completely until empty after every single synchronous function or macrotask execution before moving to the next phase.
3. Macrotask Queue: Phased queue organized by event type:
   • Timers Phase: Executes callbacks scheduled by setTimeout() and setInterval().
   • Pending I/O Callbacks Phase: Executes deferred network and disk I/O errors and completions.
   • Poll Phase: Retrieves new I/O events from the OS kernel (via epoll_wait() / kevent()).
   • Check Phase: Executes setImmediate() callbacks.
   • Close Callbacks Phase: Executes cleanup handlers (e.g., socket.on('close')).