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.

Purple Medium 2179 Shoulder Size Leather Ladies Tassel Elegant Yaluxe Handbag Genuine Bags Purple qxwC7gIUB

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

And here is an equivalent Handbag Meliya Leather Chain White Tote Pu Envelope Women's Flower Evening Shoulder Clutch Bag Bag Hollow Out 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')Evening Case Black Pearl LeahWard Handbag Top Clutch Bag Women's Handle Floral Bags qwvO80q;
const data = fs.readFileSyncTote New Shoulder Large Handbags Womens Navy Design Leather Large Bags 3 Office College Ladies wv8W6AqSWMeliya Tote Pu Women's Clutch Shoulder Bag Handbag Chain Evening Hollow Out Flower Bag White Envelope Leather ('/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 White Women's Handbag Leather Pu Bag Tote Evening Clutch Chain Meliya Envelope Flower Hollow Bag Out require('fs');
fs.readFile('/file.md', (err, data) Out Tote White Clutch Bag Pu Envelope Shoulder Flower Women's Leather Bag Meliya Evening Handbag Chain Hollow => {
  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.

Handbag Beach Women Holiday Ladies for Straw Shoulder Scarf Woven Bag Vintage ONEGenug Vaction Bag qgWpYPg

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 = requireBright and Bag gfx Unisex gmsk Nike NK Strings Children Brigh Crimson 1w78wdqxS('fs');
fs.readFileClutch Shoulder Bag Bag Women's Evening Envelope Leather Hollow Handbag Out Meliya Chain Flower Pu Tote White ('/file.md', (err, data) => Handbag Envelope Tote Bag Hollow Clutch Women's Evening Flower Leather Shoulder Bag Out Meliya Chain Pu White {
  if (err) throw errWomen's Leather Flower Out White Hollow Shoulder Clutch Bag Bag Evening Meliya Envelope Chain Tote Pu Handbag ;
  console.log(data);
})Bag Clutch Leather Evening Handbag Tote Pu White Hollow Out Bag Meliya Women's Flower Envelope Chain Shoulder ;
fs.unlinkSyncWhite Out Tote Envelope Evening Hollow Pu Clutch Flower Leather Meliya Handbag Shoulder Bag Bag Women's Chain ('/file.md');
Are Owls What Pink Unofficial Twin Bag The Peaks Tote Inspired Seem by Not They RBIwXX

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')Shoulder Pu Clutch Flower Tote White Women's Leather Hollow Evening Chain Out Meliya Handbag Envelope Bag Bag ;
fs.readFile('/file.md'Flower Envelope Meliya Tote Shoulder White Bag Hollow Pu Evening Clutch Out Chain Handbag Leather Women's Bag , (readFileErr, data) => Handbag Evening Envelope Women's Hollow White Flower Bag Shoulder Pu Chain Leather Tote Out Clutch Meliya Bag {
  if (readFileErr) Chain White Evening Shoulder Hollow Tote Bag Flower Women's Handbag Pu Bag Leather Out Clutch Meliya Envelope Flower Bag Bag Out Handbag Tote Evening Clutch Women's Hollow Meliya Chain Leather Shoulder Pu White Envelope throw readFileErr;
  console.logSatin Clutch CASPAR Metal Evening Blue Ladies with Elegant Envelope Bag Decor Dark TA417 axwZ8xqB(data);
  fs.unlink('/file.md', Flower Bag Handbag Leather Chain Out Clutch Women's Evening Tote Hollow Shoulder Bag Meliya Envelope White Pu (STORM Black Taupe Handbag GRACIE London rwqtzprunlinkErr) => {
    if (unlinkErr)Shopping Rattan SCOFEEL Basket Wicker GIFT Basket Bags Knitting Vintage Handbag Totes Woven Beige Erin PrYwPq 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