Exploration of async/await blocking

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node_modules/

README.md

gist-async-await-blocking-explore

Exploration of async/await blocking

We've been curious how much async/await blocks other actions in Node.js so exploring it

Summarized conclusion:

• await only blocks the local function, just as nesting all code inside a callback blocks
  • Context/call stack is maintained so nothing to worry about
• Demos include express so we can 100% verify it doesn't block requests

basic-await.js

// Behavior:
// Timeout reached
// End of main

// Explanation:
// Our function halts execution of the rest of the function until the timeout is resolved

// Define our main function
async function main() {
  await new Promise((resolve) => {
    setTimeout(() => {
      console.log('Timeout reached');
      resolve();
    }, 1e3);
  });
  console.log('End of main');
}

// Run our main function
main();

express.js

// Behavior:
// Submitted 5x requests in parallel
// $ curl http://localhost:3000/ &
// Saw all responses way quicker than with 5s delays
// Verifying `await` works just fine in `express
// $ node express.js
// App listening at http://localhost:3000/
// Timeout reached 1602293925326
// Timeout reached 1602293925920
// Timeout reached 1602293926345
// Timeout reached 1602293926752
// Timeout reached 1602293927150
// Timeout reached 1602293927597

// Load in our dependencies
const express = require('express');

// Build out our app
let app = express();
app.get('/', async function (req, res) {
  await new Promise((resolve) => {
    setTimeout(() => {
      console.log('Timeout reached', Date.now());
      resolve();
    }, 5e3);
  });
  res.end('Hello world!');
});

// Start our listener inside of a `main()` function
function main() {
  app.listen(3000);
  console.log('App listening at http://localhost:3000/');
}
main();

multiple-functions-async.js

// Behavior:
// Start: 1602292970255 -- 0255ms
// End of main
// Timeout 2 reached 1602292971263 -- 1263ms (so 1s)
// End of timeout 2
// Timeout 1 reached 1602292973265 -- 3265ms (so 3s)
// End of timeout 1
// Command runtime: 3s

// Explanation:
// Each function operates in a callback esque fashion
// due to using generators under the hood
// So `timeout1` runs, hits into `await` and that pauses its execution in the event loop until the `yield` runs
// but that doesn't stop other invocations
// Just as `setTimeout()` doesn't block the event loop
// It persists the context for a later invocation

// Define our nested awaiting functions
async function timeout1() {
  await new Promise((resolve) => {
    setTimeout(() => {
      console.log('Timeout 1 reached', Date.now());
      resolve();
    }, 3e3);
  });
  console.log('End of timeout 1');
}
async function timeout2() {
  await new Promise((resolve) => {
    setTimeout(() => {
      console.log('Timeout 2 reached', Date.now());
      resolve();
    }, 1e3);
  });
  console.log('End of timeout 2');
}


// Define our main function
async function main() {
  console.log('Start:', Date.now());
  timeout1();
  timeout2();
  console.log('End of main');
}

// Run our main function
main();

package.json

{
  "name": "gist-async-await-blocking-explore",
  "version": "1.0.0",
  "description": "Exploration of async/await blocking",
  "main": "basic-await.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "repository": {
    "type": "git",
    "url": "git+ssh://git@gist.github.com/8a283fda713a9ae6597748900a05008c.git"
  },
  "author": "Todd Wolfson <todd@twolfson.com> (http://twolfson.com/)",
  "license": "Unlicense",
  "bugs": {
    "url": "https://gist.github.com/8a283fda713a9ae6597748900a05008c"
  },
  "homepage": "https://gist.github.com/8a283fda713a9ae6597748900a05008c",
  "dependencies": {
    "express": "~4.17.1"
  }
}