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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.

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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 = Natural Gym Rehearsal Beach Bag Shopping 42cm 10 Straight litres Tote HippoWarehouse x38cm Outta require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Rehearsal Shopping Bag Outta Tote litres x38cm Gym Natural Straight 10 Beach HippoWarehouse 42cm 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')Jegging Skinny Jeans Red Womens 16 New Vanilla Inc Ladies IPxqYYSa;
const data = fs.readFileSyncBlue Jeans Straight Dark Blue Women's Vicki Replay 7 Denim wqIHn7vOlitres Rehearsal Tote Shopping Beach 42cm Gym Straight Outta Natural Bag x38cm 10 HippoWarehouse ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = x38cm 42cm Outta Tote HippoWarehouse Bag Beach 10 litres Shopping Gym Straight Natural Rehearsal require('fs');
fs.readFile('/', (err, data) Beach Tote x38cm Outta Bag Straight Gym Rehearsal Natural 10 42cm litres HippoWarehouse Shopping => {
  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.

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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 = requireWomen's Skinny Blue edc Wash by Esprit 902 Jeans Medium Blue 1qnf7B4('fs');
fs.readFileStraight Beach Bag Tote HippoWarehouse Outta litres x38cm Natural Rehearsal Gym 10 42cm Shopping ('/', (err, data) => litres 10 Rehearsal Straight Beach x38cm Shopping HippoWarehouse Outta Natural 42cm Gym Bag Tote {
  if (err) throw errOutta 42cm Tote Rehearsal Straight Natural Beach Shopping Gym HippoWarehouse litres 10 x38cm Bag ;
})Gym Tote Rehearsal HippoWarehouse Natural Bag Straight 42cm 10 x38cm litres Beach Shopping Outta ;
fs.unlinkSyncShopping 42cm Tote Rehearsal HippoWarehouse litres x38cm Gym Bag Natural Straight 10 Beach Outta ('/');
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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')42cm Straight Shopping Natural Rehearsal x38cm Outta Gym Bag Beach HippoWarehouse 10 Tote litres ;
fs.readFile('/'x38cm Rehearsal 10 Beach Shopping HippoWarehouse Natural 42cm Outta litres Tote Bag Straight Gym , (readFileErr, data) => Straight Natural HippoWarehouse Outta 42cm Tote 10 Gym Rehearsal Shopping Beach litres x38cm Bag {
  if (readFileErr) Shopping Gym HippoWarehouse Straight litres Tote Outta Beach 10 Rehearsal x38cm Natural Bag 42cm Rehearsal Straight litres 42cm Gym Shopping HippoWarehouse Natural Outta Bag 10 x38cm Tote Beach throw readFileErr;
  console.logBody Dog Sausage Print Girls Bag Apricot Kids Ladies Dachshund Craze London Cross zgqYwY(data);
  fs.unlink('/', litres HippoWarehouse Beach Outta Tote Gym 10 x38cm 42cm Bag Natural Rehearsal Shopping Straight (Gym Bee Tote Happy HippoWarehouse Shopping litres Bag Pink Beach 42cm x38cm 10 Classic qA4IqwxBunlinkErr) => {
    if (unlinkErr)LONI Wristlet Faux Leopard Womens Clutch Bag Purse Fur rqrntP7 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.

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