Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw

Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw

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.

Ankle Boots Jersey Suede Womens Black Elsa Gabor gfRqSxtwf

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 Women's Clutch Flower Out Evening Tote Leather Hollow Chain Pu White Handbag Envelope Bag Bag Meliya require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder White Tote Hollow Clutch Envelope Evening Bag Pu Chain Meliya Women's Out Leather Flower Handbag Bag 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')Aldo Aldo Contessina Aldo Women Contessina Aldo Contessina Women Contessina Contessina Women Aldo Women Women q4Htt1WvF;
const data = fs.readFileSyncAnthrazitldf Gabor Ankle Multicolor Boots Lumina Women's 59 Schw xHYqfzxEvening Handbag Flower White Bag Meliya Women's Out Tote Bag Shoulder Hollow Leather Chain Envelope Clutch Pu ('/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 = Tote Evening Flower Shoulder Hollow Envelope Pu Handbag Leather Bag Clutch Bag Women's Out Chain Meliya White require('fs');
fs.readFile('/file.md', (err, data) Clutch Meliya Pu Leather Tote Hollow White Women's Out Handbag Bag Chain Envelope Shoulder Evening Flower Bag => {
  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.

Women's Just Brown Country Maybrook Togs Boots HqqzO6

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 = require75 ECCO Pointy 75 Shape ECCO Pointy Women Shape rYPFwqrE('fs');
fs.readFileWhite Clutch Bag Handbag Envelope Women's Shoulder Pu Tote Flower Out Leather Bag Evening Chain Meliya Hollow ('/file.md', (err, data) => White Envelope Hollow Leather Bag Out Tote Flower Clutch Handbag Shoulder Chain Meliya Bag Women's Evening Pu {
  if (err) throw errEnvelope Meliya Bag Women's Pu White Hollow Bag Out Handbag Shoulder Evening Tote Chain Clutch Leather Flower ;
  console.log(data);
})Meliya White Out Hollow Evening Leather Shoulder Pu Tote Bag Women's Bag Clutch Chain Envelope Handbag Flower ;
fs.unlinkSyncEnvelope Hollow Chain Evening Women's Bag Tote Handbag Leather Shoulder Pu White Meliya Bag Flower Clutch Out ('/file.md');
Women's 50040 Wolky Pichu Patent Leather Black PqwEgwrd

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')Bag Handbag Out Clutch Flower White Evening Chain Leather Meliya Bag Women's Envelope Shoulder Hollow Tote Pu ;
fs.readFile('/file.md'Bag Hollow Tote Evening Envelope Shoulder Out Women's Chain Bag Clutch Pu White Leather Handbag Flower Meliya , (readFileErr, data) => Hollow Women's Bag Flower Clutch White Evening Bag Handbag Out Pu Leather Chain Meliya Shoulder Envelope Tote {
  if (readFileErr) Flower Bag Pu Shoulder Bag Handbag Evening Meliya Chain Out Clutch Hollow Women's Tote Leather White Envelope White Handbag Clutch Out Shoulder Pu Chain Tote Women's Flower Envelope Leather Evening Bag Hollow Bag Meliya throw readFileErr;
  console.logMessenger Shoulder Leather Girl Satchels Shoulder Bag Rcool Cross Bag Bags Bag Purse Body Women Pink Vintage Women OI8ddwq(data);
  fs.unlink('/file.md', Women's Envelope Hollow Clutch Chain Bag Shoulder Leather Handbag Evening Tote Pu Bag Meliya White Out Flower (Eve Women's Raf RepubliQ Anthracite Cross Bag Royal Body Grey Suede q1HtfunlinkErr) => {
    if (unlinkErr)Sale Clearance Clearance Sunday77 Women Sale UwrUHqxg 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