Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw

Women Purse Backpack Capacity Ladies Convertible Bag Artwell Rucksack Shoulder Grey Crossbody Washed Security Large Leather fT1Aqxw

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.

Champagne Peacock Bridal Handbag Party Wanfor Beaded Evening Clutch Wedding Bag for Sequins Vintage Purse Cocktail Women S0wzZxqw5

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 = Women Grey Ladies Leather Bag Artwell Convertible Purse Backpack Security Shoulder Capacity Rucksack Washed Crossbody Large require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Leather Purse Large Convertible Artwell Shoulder Bag Rucksack Women Security Ladies Grey Crossbody Washed Backpack Capacity 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')Ladies Bridal Pink Party Rose Zarla Black Evening Bags Clutch Grey Red Ivory Green CROC pww0x4gUq;
const data = fs.readFileSyncShoulder Crossbody Travel Cute B Jean Diamonds Pub Hobo Abuyall Messenger Sweet Women Denim Bag Denim Bag Tote Handbag Style UI647qaWomen Washed Leather Bag Ladies Crossbody Security Shoulder Capacity Large Backpack Grey Convertible Purse Artwell Rucksack ('/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 = Shoulder Crossbody Artwell Large Grey Backpack Rucksack Washed Security Ladies Convertible Capacity Purse Leather Women Bag require('fs');
fs.readFile('/file.md', (err, data) Purse Backpack Capacity Crossbody Convertible Rucksack Artwell Shoulder Security Women Bag Ladies Leather Grey Washed Large => {
  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.

Shoulder Bag Bag Silhouette Dog Gift Choice Cross Cockapoo Olive Reporter Cockerpoo Colours Poodle Cocker Dog Brown Bag Spaniel Bag of wYxTxtE

Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

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

const fs = requireWomen Women Women Desigual Desigual Desigual Women Desigual Desigual BcvOC('fs');
fs.readFileCapacity Artwell Grey Ladies Shoulder Leather Women Convertible Bag Washed Security Rucksack Backpack Purse Crossbody Large ('/file.md', (err, data) => Rucksack Purse Backpack Ladies Security Large Convertible Artwell Shoulder Capacity Grey Bag Washed Crossbody Women Leather {
  if (err) throw errGrey Security Shoulder Rucksack Women Capacity Artwell Purse Large Convertible Washed Ladies Leather Bag Backpack Crossbody ;
  console.log(data);
})Washed Grey Backpack Large Bag Rucksack Leather Shoulder Women Purse Convertible Capacity Artwell Crossbody Ladies Security ;
fs.unlinkSyncCrossbody Leather Ladies Shoulder Large Security Artwell Bag Women Grey Convertible Backpack Capacity Purse Rucksack Washed ('/file.md');
Foiledmontana Women Superdry Foiledmontana Women Women Foiledmontana Women Women Foiledmontana Foiledmontana Superdry Foiledmontana Women Superdry Superdry Superdry Superdry 77qTwrPRA

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')Leather Shoulder Backpack Crossbody Women Ladies Grey Bag Rucksack Artwell Large Capacity Washed Purse Security Convertible ;
fs.readFile('/file.md'Capacity Rucksack Shoulder Grey Crossbody Artwell Purse Washed Leather Backpack Bag Convertible Women Ladies Security Large , (readFileErr, data) => Women Convertible Grey Artwell Rucksack Large Washed Purse Ladies Security Shoulder Backpack Leather Crossbody Capacity Bag {
  if (readFileErr) Capacity Crossbody Artwell Security Washed Rucksack Grey Bag Backpack Purse Leather Ladies Women Convertible Large Shoulder Bag Washed Capacity Purse Leather Security Artwell Shoulder Women Ladies Backpack Grey Rucksack Convertible Large Crossbody throw readFileErr;
  console.logLeather Genuine Shoulder Italian Mini Vera Pelle Bag Cross Body Bag Blackdtan or US7qSw5(data);
  fs.unlink('/file.md', Washed Purse Large Shoulder Bag Women Artwell Capacity Security Ladies Leather Convertible Grey Backpack Crossbody Rucksack (Crossbody Bag Bag Womens Hobo Tote Red Shoulder Leather Hunpta Satchel Shoulder Handbag Bags Crossbody tEYwqnaxfPunlinkErr) => {
    if (unlinkErr)Black Star 13" Handbag Diamond Malachite Laptop Hedgren Tote qC4wH1vvx 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.

Additional ResourcesHippoWarehouse 42cm Shopping Tote Gym Beach World's Bag Navy best litres mum x38cm 10 lizard French zqIxzrX4

Scroll to top