event-loop-and-libuv.md

Event Loop, Call Stack, and libuv

Goal

Understand why Node can handle many concurrent connections despite JavaScript being single-threaded.

Core Pieces

• Call Stack: where JS functions execute
• Event Loop: scheduler that decides what callback runs next
• libuv: C library handling async I/O and thread pool tasks

Event Loop Phases (Simplified)

1. Timers: setTimeout and setInterval callbacks
2. Pending callbacks: deferred system-level callbacks
3. Poll: receive new I/O events, execute I/O callbacks
4. Check: setImmediate callbacks
5. Close callbacks: socket close handlers, etc.

In practice, Poll and Check behavior is key for server responsiveness.

Why This Matters for Taskflow

Taskflow server handles network I/O efficiently because:

• HTTP socket handling is asynchronous
• callbacks are queued and processed without blocking on slow clients

If you add CPU-heavy synchronous logic, you block the loop and all clients feel delay.

Practical Example

Bad pattern:

• request handler runs a huge synchronous loop

Result:

• all requests stall while loop runs

Better pattern:

• offload CPU-heavy work to Worker Threads (covered later)

Quick Experiment

1. Add an endpoint that computes Fibonacci recursively with large n.
2. Open two terminals.
3. Hit CPU endpoint in terminal A.
4. Call GET /tasks in terminal B.

Observation:

• GET /tasks slows dramatically while CPU endpoint runs.

Lesson:

• Node scales in I/O concurrency, not CPU-bound work on main thread.