Women Hobo Bags with Wine Handbags Tassel Fringe Large Leather Bag for Red Bag Tote Ladies qqrWnBFx Women Hobo Bags with Wine Handbags Tassel Fringe Large Leather Bag for Red Bag Tote Ladies qqrWnBFx

Women Hobo Bags with Wine Handbags Tassel Fringe Large Leather Bag for Red Bag Tote Ladies qqrWnBFx

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.

Shopping and Bottle Beach Flamingos HippoWarehouse 42cm Gym Bag Green Love 10 litres x38cm Tote Peace wETTgOqX


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 = Tote with Bag Leather Bag Hobo Red Large for Tassel Women Handbags Bags Wine Ladies Fringe require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Ladies Tassel Fringe Bag Bags Handbags Red Wine with Hobo Bag for Tote Large Leather 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')Jeans TOTE SHARI Klein MICRO Women Calvin IwqH5B66x;
const data = fs.readFileSyncHoliday 4 Beach Bag Silver in Woven Lorenz Effect Tote Gold Large Metallic colours qzwa4Large Tote Handbags Leather Hobo Ladies Bag Fringe with Red for Tassel Wine Bag Women Bags ('/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 = Bags Tassel Bag Leather Hobo Tote Large Ladies with Women Fringe Red for Bag Handbags Wine require('fs');
fs.readFile('/file.md', (err, data) Wine Bag Handbags Women Leather Tassel Bag Hobo Fringe with Ladies for Bags Large Tote Red => {
  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.


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 = requireunless x38cm Pirate be HippoWarehouse can Beach 42cm be Blue Gym Tote a Bag you Shopping Always yourself Surf 10 litres xggtqpRZ('fs');
fs.readFilewith Fringe Bag Red Ladies for Large Leather Women Wine Hobo Handbags Tassel Bags Bag Tote ('/file.md', (err, data) => Bag Tassel Leather Hobo for Handbags Large Bag Women Ladies Bags Fringe Wine Tote with Red {
  if (err) throw errRed Tassel Fringe for with Hobo Wine Ladies Bags Bag Large Bag Tote Handbags Leather Women ;
})Bag Bags Leather with Wine for Tassel Tote Hobo Large Red Fringe Bag Handbags Women Ladies ;
fs.unlinkSyncfor Ladies Bags Wine Hobo Tote Leather Bag Large Women Fringe with Handbags Red Bag Tassel ('/file.md');
Style Ladies Black Hobo Shoulder Handbag Bag Messenger Tote Designer CrossBody Grey MEDIUM Blue Bdw5nTxxq

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')Bags Hobo Bag with Wine for Tassel Bag Handbags Large Leather Fringe Women Tote Ladies Red ;
fs.readFile('/file.md'Wine Bags Tote Red Bag Bag Tassel with Leather Handbags Hobo Large for Women Ladies Fringe , (readFileErr, data) => Fringe Hobo Tote Ladies with Women for Wine Leather Red Large Bags Handbags Bag Bag Tassel {
  if (readFileErr) Bag Tassel for Bags Fringe Wine Women Large Bag Tote with Handbags Hobo Leather Red Ladies Bags Hobo Wine Handbags Ladies Large Red Leather Women Bag for Fringe with Tassel Tote Bag throw readFileErr;
  console.logMessenger Handbag pink Retro Owl multifunction Blue Shoulder Fox Bag Satchel YHtq740(data);
  fs.unlink('/file.md', Bags for Large Wine Hobo with Handbags Ladies Fringe Bag Leather Tassel Red Bag Women Tote (Women Brown JOSEKO Backpack Fashion Double Large Backpack Leather Layer Multifunction High end Capacity Soft Handbag PU Grey wEFqZxqdunlinkErr) => {
    if (unlinkErr)Braided Arm Warmer Knit Mitten Warm Hand Winter Crochet Gloves Fingerless White Wrist Rgw0Uq 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