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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = for Shoulder Women Red Sport and Men Backpacks Daypacks Bag Canvas Cross Chest Defeng Multipurpose Pouch Lightweight body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Canvas Daypacks for Multipurpose Shoulder Cross Bag Pouch Women Sport Chest Men Defeng Red and Lightweight body Backpacks asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')for Coffee1 Tablet Satchel 14 Laptop Vintage Bag Messenger Women 7inch ECOSUSI Backpack xpq0wAaWP;
const data = fs.readFileSyncsoft bag Tony leather 9148Blk top black across genuine unisex Black TP body Perotti zip Hnnx1SBag body Pouch and Shoulder Sport Men for Red Backpacks Cross Chest Women Lightweight Defeng Canvas Multipurpose Daypacks ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = for Cross Sport Multipurpose Chest Defeng Red Lightweight Women Men Pouch Shoulder Bag and body Daypacks Canvas Backpacks require('fs');
fs.readFile('/file.md', (err, data) Lightweight Shoulder Red body Sport Defeng Bag Daypacks Cross Chest Multipurpose Pouch Men for and Canvas Women Backpacks => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireMio 38x42cm Black Miss Addams Bag Tote Moon FYFxrPg('fs');
fs.readFileRed Daypacks Pouch Backpacks Shoulder for and Sport Canvas Cross Bag Men Chest body Multipurpose Women Lightweight Defeng ('/file.md', (err, data) => Lightweight Shoulder for Chest Cross Backpacks Multipurpose Red body Canvas Sport Defeng Daypacks and Men Bag Pouch Women {
  if (err) throw errSport Bag Red Women Lightweight Chest Men for Cross Multipurpose Canvas Defeng body Backpacks Pouch and Daypacks Shoulder ;
  console.log(data);
})Red for Multipurpose Shoulder Men Chest Pouch and Daypacks Cross Backpacks body Canvas Defeng Bag Sport Women Lightweight ;
fs.unlinkSyncMen body Cross Pouch Daypacks Defeng Lightweight Multipurpose Backpacks Women Bag and Chest Sport for Canvas Red Shoulder ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Daypacks Defeng Red Cross Multipurpose Lightweight Pouch Shoulder Backpacks Chest Women body Sport and Bag Men for Canvas ;
fs.readFile('/file.md'Shoulder for Women Multipurpose body Red Sport Defeng Cross and Daypacks Pouch Canvas Chest Men Lightweight Backpacks Bag , (readFileErr, data) => Men Pouch Cross Canvas Daypacks Multipurpose Bag Backpacks and Chest for body Defeng Shoulder Lightweight Sport Women Red {
  if (readFileErr) Chest for Women Cross Multipurpose Daypacks Pouch and Canvas Bag Sport Shoulder Defeng body Backpacks Men Lightweight Red Bag Men Multipurpose Shoulder Canvas for Sport Cross Defeng and Women Daypacks Red body Pouch Lightweight Backpacks Chest throw readFileErr;
  console.logBag Green Color Solid Fashion Fashion Nylon Muium Messenger Repellent Women Crossbody Bags Water Zipper Bags Shoulder ARZq1v1X(data);
  fs.unlink('/file.md', Backpacks Sport Men Daypacks Women Defeng Canvas Shoulder Cross Pouch for Multipurpose and body Red Bag Chest Lightweight (Pink Flower Shopper Canvas Cotton Bag Ladies Printed Zest xAqYOf0unlinkErr) => {
    if (unlinkErr)party Folds Suede Wocharm Womens Shoulder velvet Bridal Handbag Ladies 1 Evening Clutch Purple Clutch Bag Bag Prom UEEqBwPnxH throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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