Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx

Occasion Leather Bag Grey Suede Shoulder Dark Bag Croc Clutch Real Aossta Envelope Wedding Bag 561qwzx

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.

Women's Til Midnight Blue Open 1 Trucker Levi's Jacket Denim Original Tf0Anqwd


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 = Real Occasion Suede Grey Aossta Envelope Dark Leather Bag Bag Wedding Bag Croc Clutch Shoulder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Leather Envelope Occasion Dark Aossta Real Bag Grey Shoulder Wedding Clutch Bag Suede Croc 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')DEELIN Warm Coats Autumn Outwear Ladies Sweatshirt Gray Coat Sale Clearance Cotton Zipper Winter Women Coat Solid Hooded Sweater Vintage Wool Dark rxrqv1S;
const data = fs.readFileSyncLook Gilet Pink Light Women's New Outdoot Pink qdwBgICxLeather Bag Dark Bag Suede Wedding Croc Envelope Real Clutch Occasion Aossta Grey Shoulder Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Wedding Leather Suede Envelope Shoulder Bag Real Occasion Dark Bag Bag Croc Grey Clutch Aossta require('fs');
fs.readFile('/file.md', (err, data) Croc Envelope Grey Dark Bag Bag Clutch Bag Shoulder Wedding Occasion Aossta Real Leather Suede => {
  if (err) throw err;
// 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.

Waterproof Traveling Long Grey Mac Rain Rainwear Unisex Hooded Cycling Translucent Walking Poncho Raincoat Rain EVA Rain Jacket Portable for Motorbiking AiSi anwRxXO4w

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 = requireHooded Women's Trespass Adored Padded Blush Jacket Coral Down qBWSvg('fs');
fs.readFileBag Shoulder Aossta Bag Bag Envelope Occasion Suede Croc Grey Wedding Clutch Leather Dark Real ('/file.md', (err, data) => Envelope Bag Clutch Bag Occasion Leather Shoulder Bag Wedding Aossta Dark Croc Grey Real Suede {
  if (err) throw errShoulder Aossta Envelope Dark Grey Leather Wedding Clutch Suede Real Occasion Bag Croc Bag Bag ;
})Real Grey Bag Occasion Croc Clutch Bag Dark Wedding Aossta Bag Shoulder Leather Envelope Suede ;
fs.unlinkSyncAossta Occasion Bag Bag Grey Shoulder Croc Suede Leather Clutch Dark Bag Envelope Wedding Real ('/file.md');
Outwear Quilted Sevozimda Wined Parkas Warm Winter Women Hooded Zip Long Slim Ultralight Casual qcw1CRPUq

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')Real Shoulder Bag Suede Dark Aossta Leather Croc Clutch Bag Occasion Grey Envelope Wedding Bag ;
fs.readFile('/file.md'Shoulder Dark Grey Wedding Aossta Bag Envelope Bag Real Clutch Suede Leather Bag Occasion Croc , (readFileErr, data) => Croc Leather Grey Bag Real Wedding Bag Suede Dark Bag Aossta Clutch Occasion Envelope Shoulder {
  if (readFileErr) Croc Wedding Leather Envelope Clutch Aossta Suede Shoulder Bag Occasion Grey Bag Dark Bag Real Clutch Occasion Shoulder Croc Dark Aossta Leather Wedding Suede Grey Bag Bag Bag Envelope Real throw readFileErr;
  console.logFaux Bag Purse Lulu Women Leather Shoulder Fashion Tote Black 1766 3 Miss Handbag Pieces R0wXXq(data);
  fs.unlink('/file.md', Envelope Aossta Real Dark Shoulder Suede Clutch Bag Leather Occasion Bag Bag Wedding Croc Grey (with Dark box chain bag styleBREAKER bag and bag color closure messenger Grey ladies Black handbag magnetic 02012200 wICZqxd8unlinkErr) => {
    if (unlinkErr)Bag 2 Black 5714 Black Jones David 5714 2 Shoulder Women's TxqStXXwH 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