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import { future } from "@mdx-deck/themes"; import { syntaxHighlighter } from "mdx-deck/themes"; export const themes = [future, syntaxHighlighter]; import { SplitRight } from "mdx-deck/layouts"; import { Steps, Appear } from "@mdx-deck/components"; import { Image, Notes } from "mdx-deck";

Event Loop

NodeJS Asynchronicity

Basics, Common Pitfalls & Solutions

What is the

Event Loop?

<Steps length={2} render={({ step }) => ( <img src={"./static/el" + step + ".png"} /> )} />

  • Event loop can be visualized like so

  • Next slide: better to show it like this

  • Next slide: or maybe like Life Cycle Events

  • Next slide: Event Loop is a flat circle

Basics

  • NodeJS / V8 is single threaded
  • Async = non-blocking
  • Promises are async
    • but are they non-blocking?
  • In all seriousness...

  • Using this visualization of the Event Loop

  • Go over each "Phase"

Phases - Timers

  • setTimeout() / setInterval() calls
    • Timer ms must have passed before evaluating
  • Evaluated once per loop in NodeJS
    • Brower handles this differently (but that's a different talk)
  • setTimeout(, 0) === setTimeout(, 1)
    • Always at least 1ms wait

Phases - Poll

  • nextTick / micro tasks
    • "micro tasks" = Promise callbacks
  • Occurs between each Event Loop "Phase"
    • Node v11 and up handles this differently 😱
  • Phase ends when queue exhausted
  • Reolved Pomises in this phase are added back into this queue
    • Can result in "I/0 starvation", e.g. blocking the event loop

Phases - I/O

  • e.g. handle incoming API requests, disk I/O, DNS lookups
  • Complex and varies based on host OS

Phases - Immediates

  • setImmediate() calls
  • Callbacks added to setImmediate queue during this phase will be evaluated on next pass of the event loop

Phases - Close Handlers

  • process.exit() and crashes of NodeJS process
  • Important for scripts, but generally not important in context of an API server
  • Last opportunity to cleanup before Node process is terminated
    • e.g. Draining database connections

Async Resolution Order

DEMO: Promise.resolve / setTimeout / setImmediate

- Browser: nextTick callbacks and microtasks will run between each individual setTimeout and setImmediate callbacks

Demo

Blocking Event Loop

with Promises

demo blocking event loop on API server here and how to solve it 
Ping API server every second:
while true; do date && curl -m 5 http://localhost:5000/api/auth && echo; sleep 1; done


Blocking v Non-Blocking API requests:
START=$(date +%s) && curl http://localhost:3000/loop && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"
START=$(date +%s) && curl http://localhost:3000/loop/bmap && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"
START=$(date +%s) && curl http://localhost:3000/loop/seti && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"
START=$(date +%s) && curl http://localhost:3000/loop/bird && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"
START=$(date +%s) && curl http://localhost:3000/loop/native && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"
START=$(date +%s) && curl http://localhost:3000/loop/sett && END=$(date +%s) && echo "Took: $(expr $END - $START) sec"

"The fair treatment of clients is thus the responsibility of your application." -NodeJS Docs

Gather Reports

Partitioning on the Event Loop Paradigm

show refactor of Admins Partners Report here

import { Split } from "mdx-deck/layouts";

Questions?

Helpful Articles

Upcoming Talk

  • Event Loop: NodeJS vs Browser
  • How NodeJS v11 changes async resolution