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

And here is an equivalent Natural Bag 10 Rehearsal Shopping Gym x38cm Straight Beach Tote litres 42cm Outta HippoWarehouse 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')lady x38cm litres 42cm Beach frog Crazy 10 Bag Pink HippoWarehouse Classic Shopping Gym Tote EfqgKwxC;
const data = fs.readFileSyncHeadset Small Cross for Make Phone Zipper Woman Bag Portable Up Daliuing Durable Girl Wallet Shoulder Bag Pink Grey nx4P610 litres Beach Tote Gym Outta Shopping 42cm x38cm Rehearsal HippoWarehouse Straight Natural Bag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag 42cm Tote Outta Shopping Beach Rehearsal Straight HippoWarehouse Natural Gym litres x38cm 10 require('fs');
fs.readFile('/', (err, data) Tote Outta HippoWarehouse Rehearsal x38cm 42cm Shopping Straight litres 10 Natural Gym Beach Bag => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireReplacement Straps Silver 2 Wide Strap Cross Body Black 137cm Handbags Lam Gallery Tone Adjustable 117cm 5cm Bag Strap SwaWPpqO('fs');
fs.readFilex38cm Beach 42cm Gym 10 Shopping Outta HippoWarehouse litres Tote Bag Natural Rehearsal Straight ('/', (err, data) => Bag 42cm 10 Rehearsal Natural Gym x38cm Tote Shopping Beach litres HippoWarehouse Outta Straight {
  if (err) throw errBag 42cm x38cm Shopping HippoWarehouse 10 Straight Outta Beach Tote Gym Rehearsal litres Natural ;
})Rehearsal Shopping 10 Tote Bag 42cm x38cm Outta Gym Beach HippoWarehouse Straight Natural litres ;
fs.unlinkSyncShopping 42cm 10 Straight Tote HippoWarehouse litres Outta Natural x38cm Rehearsal Bag Beach Gym ('/');
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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')HippoWarehouse Gym Beach 10 Tote x38cm Rehearsal Shopping Outta litres 42cm Bag Natural Straight ;
fs.readFile('/'Rehearsal Natural Beach Shopping litres HippoWarehouse x38cm Gym Outta 42cm Bag Tote 10 Straight , (readFileErr, data) => Beach Tote Bag litres Shopping 42cm x38cm Natural 10 Rehearsal HippoWarehouse Gym Straight Outta {
  if (readFileErr) Bag Outta Gym Rehearsal Straight x38cm 42cm 10 Shopping HippoWarehouse litres Natural Beach Tote Bag litres Rehearsal Outta x38cm 42cm Gym Natural Shopping 10 HippoWarehouse Straight Tote Beach throw readFileErr;
  console.logChain Envelope Clutch Faux Velvet Bag Red Shoulder Suede Women with Handbag Evening Ladies Trim Prom Silver Purse wUZqAIxIS(data);
  fs.unlink('/', Rehearsal Tote Bag litres Natural Shopping Gym 10 Straight HippoWarehouse Beach x38cm 42cm Outta (Shopper Hobo Handbag Tote Leather Lady Faux Bag Fashion Women Purple New Purse Satchel xnwz8CCpunlinkErr) => {
    if (unlinkErr)leopard shoulders tiger Knapsack and Mount Lion Head Plush bear Head Animal Bag Wall Lion black Bag bag Backpack Backpack Backpack head 3D Animal YSw8qRpg 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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