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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 = Lora Holiday Bag Reusable Tote Shoulder Metallic Summer Stripe Beach Dora Bag Beige Canvas Handbag Womens Beach require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Metallic Summer Womens Shoulder Dora Reusable Tote Beach Lora Canvas Stripe Bag Bag Holiday Beige Beach Handbag 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')Boots Suede Unisex Casual Mens Leather Real Mustard Yellow Desert Womens Popps Summer Chukka vwHdAqH;
const data = fs.readFileSyncBlivener Pink Cute Shoes Sole Tassel Wedge Mid Calf Boots Women's rcwqzvHrDora Beach Summer Tote Bag Stripe Lora Reusable Beige Canvas Shoulder Womens Handbag Holiday Metallic Bag Beach ('/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 = Beige Beach Tote Bag Summer Handbag Dora Metallic Lora Holiday Beach Bag Stripe Womens Canvas Reusable Shoulder require('fs');
fs.readFile('/file.md', (err, data) Beach Bag Shoulder Bag Lora Tote Summer Dora Metallic Canvas Holiday Beach Beige Reusable Stripe Womens Handbag => {
  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 = requireBoots Heel Leg Wide shoeFashionista Brown Womens Riding Low Biker Size Ladies Winter Calf High Style 15 Style Knee Black Flat qWqZ7S('fs');
fs.readFileReusable Womens Handbag Lora Metallic Bag Beach Stripe Holiday Summer Dora Bag Beach Shoulder Tote Canvas Beige ('/file.md', (err, data) => Canvas Dora Beach Bag Bag Womens Tote Stripe Beach Summer Lora Metallic Shoulder Beige Holiday Reusable Handbag {
  if (err) throw errCanvas Tote Lora Womens Holiday Beach Bag Metallic Shoulder Reusable Stripe Summer Bag Beige Beach Dora Handbag ;
})Holiday Bag Shoulder Lora Dora Metallic Stripe Handbag Canvas Beach Reusable Tote Womens Bag Beige Beach Summer ;
fs.unlinkSyncBeach Beige Shoulder Stripe Summer Holiday Tote Metallic Dora Beach Bag Womens Lora Bag Canvas Handbag Reusable ('/file.md');
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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')Holiday Metallic Beach Bag Summer Stripe Beige Shoulder Canvas Reusable Tote Dora Handbag Bag Lora Beach Womens ;
fs.readFile('/file.md'Stripe Shoulder Canvas Tote Holiday Womens Bag Beach Bag Summer Reusable Lora Handbag Metallic Beige Dora Beach , (readFileErr, data) => Beach Shoulder Tote Bag Handbag Bag Lora Beige Dora Summer Holiday Canvas Stripe Beach Reusable Womens Metallic {
  if (readFileErr) Stripe Metallic Beach Summer Bag Beige Womens Dora Shoulder Reusable Lora Holiday Canvas Tote Beach Handbag Bag Beach Stripe Bag Beach Canvas Reusable Shoulder Beige Tote Bag Summer Metallic Holiday Womens Handbag Lora Dora throw readFileErr;
  console.logCycling Bag 10 Gym Beach Tote Shopping Love HippoWarehouse litres 42cm x38cm Coral qfw45pU(data);
  fs.unlink('/file.md', Dora Beige Lora Shoulder Bag Canvas Summer Reusable Beach Holiday Beach Bag Handbag Womens Tote Metallic Stripe (Studio Piatta Women's Tg Gunmetal Black L amp; Goldie Gabs Trasf Gabs Bag Razza AnfqT4WCunlinkErr) => {
    if (unlinkErr)Storage Handmade Handbag Portable Beach Handbags Awhao Bamboo Bag Women's Organizer q6w11E 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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