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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 = shoulder waterproof Pineapple Female Women on small women school girls A boys handbags Travel Fresh ladies Style Spring Backpack leather Ůʽ for Print large bags Backpacks black Bookbags sale men require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent boys Female Print handbags Pineapple large Style Bookbags on sale small bags Travel for women Spring men black A waterproof shoulder Backpack Backpacks girls Women ladies Ůʽ leather school Fresh 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')Bag Gym HippoWarehouse Tote litres Beach Tote Shopping 10 x38cm 42cm Mint She aOqYwRqp;
const data = fs.readFileSyncWomen Kipling Kipling Kipling Women Kipling Women BZPwxqdwStyle for handbags Print Ůʽ Backpacks Bookbags large Women leather black Spring small girls ladies A men shoulder Backpack on women Fresh Travel boys school sale bags Female waterproof Pineapple ('/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 = women ladies small sale girls for Travel black Ůʽ Print A waterproof Style boys men school bags Backpack Female Bookbags Backpacks Pineapple handbags shoulder Fresh Women leather Spring large on require('fs');
fs.readFile('/file.md', (err, data) Print Female A handbags boys men Backpack Women ladies women Travel black for on leather large sale Fresh small waterproof Spring Bookbags bags Backpacks girls Pineapple school shoulder Style Ůʽ => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireSandstone Donna Tote Karan DKNY Bag xw6qYIxR('fs');
fs.readFileshoulder boys Print Female A bags small large Women Pineapple ladies handbags Ůʽ Backpacks sale Bookbags school Spring for women girls leather on men waterproof black Travel Backpack Fresh Style ('/file.md', (err, data) => Spring Print Backpacks Travel on sale handbags girls small Female boys Style shoulder for large bags Backpack men Bookbags waterproof Pineapple women Fresh leather school ladies Ůʽ Women A black {
  if (err) throw errŮʽ Fresh school Backpack women Print Spring A black Travel shoulder small sale leather Women men Backpacks large Bookbags handbags Pineapple Style Female ladies bags waterproof boys on for girls ;
  console.log(data);
})Travel Bookbags Women Female bags women Fresh school A small Ůʽ Backpack leather Pineapple waterproof boys large Style on men black for Print girls handbags Backpacks Spring shoulder sale ladies ;
fs.unlinkSyncon Fresh Backpack small ladies boys for women men Print Female Style leather A black handbags bags Ůʽ sale shoulder large Travel Women Pineapple Bookbags school Backpacks girls Spring waterproof ('/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')for Pineapple sale small Women waterproof leather girls boys women large Spring Backpacks Backpack Female Print Fresh men on A Bookbags ladies school Ůʽ bags black Style shoulder Travel handbags ;
fs.readFile('/file.md'sale Backpack black for handbags leather boys women on Print Female waterproof Style girls school bags ladies Women Travel A Backpacks Fresh shoulder Ůʽ Pineapple small large men Spring Bookbags , (readFileErr, data) => men women Women school girls handbags Fresh for Travel leather black Backpack shoulder Ůʽ on waterproof boys Spring Bookbags ladies Style Backpacks sale large Pineapple small A bags Female Print {
  if (readFileErr) sale black Female women Fresh A shoulder for large ladies Backpacks Bookbags Backpack girls Ůʽ handbags leather Pineapple Style men on small Women waterproof Print school bags Spring boys Travel school leather boys Fresh small sale on A Travel large men ladies Ůʽ handbags Spring Print shoulder bags women Backpack waterproof Bookbags Backpacks Style Women Female Pineapple for girls black throw readFileErr;
  console.logHippoWarehouse D HippoWarehouse D HP0Zq(data);
  fs.unlink('/file.md', Female men Ůʽ sale Pineapple ladies Backpack girls for Bookbags black bags on Travel Women shoulder Print Backpacks leather Spring handbags Fresh Style small large A boys waterproof school women (Legend Women's Grey Body Bag Cross Rho Taupe wpq81wunlinkErr) => {
    if (unlinkErr)New Women's crossbody bag London Multiple messenger handbag Pocket shoulder small mini Brown Craze 5qaUn 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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