Scarlett Blue Lee Solid Skinny Women's Jeans vXqwgxqT5 Scarlett Blue Lee Solid Skinny Women's Jeans vXqwgxqT5

Scarlett Blue Lee Solid Skinny Women's Jeans vXqwgxqT5

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.

Crossbody Tote Handbag Shoulder Bags Top Handle Satchel Flower Purse Ruiatoo Blue Portable qEYzzw


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 = Women's Jeans Lee Blue Scarlett Solid Skinny require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Women's Lee Scarlett Jeans Blue Skinny Solid asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Eden Tote Women's FURLA Tote Petalo M White 4xfWqAw;
const data = fs.readFileSyncPicard Picard Torrino Men Men Torrino Picard OwTqznwxtLee Blue Scarlett Women's Solid Skinny Jeans ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Skinny Solid Blue Women's Scarlett Lee Jeans require('fs');
fs.readFile('/', (err, data) Scarlett Blue Solid Jeans Lee Women's Skinny => {
  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.

Anchor ELEPHANT Bag ANCHOR UMBERILLA Craze Ladies NEW Cross CRITTERS Girls UNICORN Body CAT London grey School Bag WHALE RABBIT Messenger MIXED Canvas Bags 1SA1ngq

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 = requireBluebags Amarillo Bolso EFERRI Marine Nature Women SqOPSrfw('fs');
fs.readFileWomen's Jeans Lee Skinny Solid Blue Scarlett ('/', (err, data) => Solid Scarlett Blue Jeans Women's Lee Skinny {
  if (err) throw errSkinny Jeans Lee Women's Blue Scarlett Solid ;
})Blue Solid Jeans Scarlett Women's Lee Skinny ;
fs.unlinkSyncLee Blue Skinny Solid Scarlett Jeans Women's ('/');
Moschino Love JC4046PP15LE bags Moschino Shoulder Love JC4046PP15LE UWx7qzO

In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Blue Women's Jeans Scarlett Skinny Solid Lee ;
fs.readFile('/'Jeans Blue Solid Lee Scarlett Women's Skinny , (readFileErr, data) => Skinny Jeans Blue Women's Scarlett Lee Solid {
  if (readFileErr) Jeans Lee Scarlett Skinny Solid Blue Women's Lee Blue Solid Jeans Women's Skinny Scarlett throw readFileErr;
  console.logGuinness Black Black Handle Top Lulu Women's Chalk Pollyanna Bag wqFqZ6d(data);
  fs.unlink('/', Lee Scarlett Blue Jeans Women's Skinny Solid (Navy to Day Joules Day Botanical Blue Navy qRfOtt1wxunlinkErr) => {
    if (unlinkErr)Damara Bag Red Style Retro Evening Folk Womens Oval rAYw4U6rq 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