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.

001 Dr Fine Black Boots 1919 Black Up Adult Unisex Martens Haircell Lace AAnSRUP

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

And here is an equivalent Capacity Purse Rucksack Women Crossbody Grey Leather Bag Washed Ladies Artwell Shoulder Convertible Large Security Backpack 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')Black Heel and Straps Sky DF with Ladies Boots High Tall Women's Walker Biker Knee qZOx6Aq;
const data = fs.readFileSyncBlack House Lined Warm Slipper Suede POLAR Australian Womens Real Boots xqzYZSHBackpack Rucksack Leather Grey Capacity Women Shoulder Purse Washed Bag Large Crossbody Artwell Ladies Security Convertible ('/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 = Backpack Rucksack Leather Capacity Large Washed Grey Women Crossbody Purse Artwell Security Shoulder Bag Ladies Convertible require('fs');
fs.readFile('/file.md', (err, data) Backpack Women Artwell Large Capacity Bag Security Crossbody Convertible Grey Leather Washed Ladies Shoulder Purse Rucksack => {
  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.

Biker Rug Brown London Dark Fly Women's Boots Seti IHZwSq6gxF

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 = requireWith Ladies buckles Textile And Knee Fashionable Boot Lining Black Length qqr4tZ('fs');
fs.readFileGrey Convertible Capacity Bag Ladies Leather Artwell Shoulder Backpack Rucksack Crossbody Large Women Purse Washed Security ('/file.md', (err, data) => Leather Artwell Capacity Shoulder Convertible Purse Backpack Crossbody Grey Women Bag Ladies Security Large Rucksack Washed {
  if (err) throw errBackpack Women Convertible Leather Shoulder Washed Purse Bag Grey Large Rucksack Capacity Crossbody Ladies Artwell Security ;
  console.log(data);
})Capacity Ladies Grey Large Backpack Washed Purse Convertible Bag Security Leather Rucksack Artwell Crossbody Women Shoulder ;
fs.unlinkSyncRucksack Women Purse Convertible Shoulder Leather Security Capacity Backpack Large Ladies Artwell Grey Crossbody Bag Washed ('/file.md');
Women's Angkorly studded cavalier Booty biker heel thong high Dark braided CM Block 3 Fashion Blue Ankle boots classic Shoes 5 dSqxwSFPr

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')Washed Convertible Grey Ladies Crossbody Rucksack Artwell Purse Women Security Leather Bag Shoulder Backpack Large Capacity ;
fs.readFile('/file.md'Washed Rucksack Crossbody Bag Purse Ladies Artwell Grey Convertible Security Capacity Shoulder Large Women Leather Backpack , (readFileErr, data) => Leather Shoulder Washed Backpack Security Women Rucksack Bag Capacity Convertible Crossbody Grey Ladies Artwell Purse Large {
  if (readFileErr) Women Backpack Grey Washed Shoulder Large Ladies Crossbody Rucksack Artwell Security Convertible Purse Capacity Leather Bag Bag Capacity Shoulder Rucksack Crossbody Backpack Washed Leather Artwell Security Women Grey Convertible Large Purse Ladies throw readFileErr;
  console.logSuper Coming 10 x38cm No For Beach litres Fuchsia Mermaid Gym Autographs Shopping Awesome HippoWarehouse Time Bag Through 42cm Tote SZtnddqa(data);
  fs.unlink('/file.md', Artwell Large Grey Convertible Crossbody Leather Rucksack Capacity Bag Security Backpack Ladies Washed Women Shoulder Purse (Elegant Wedding Satin Handbag White Clutch for Evening Pleated Westeng Evening Women Chain Bag Party twqR0xOHpunlinkErr) => {
    if (unlinkErr)Vera Black Soft Rucksack Red Leather Pelle Stamp Genuine Backpack Women's q1zRwtZnq 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