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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 = x38cm The HippoWarehouse 10 Bag Gym Soul Shopping Mends Beach 42cm Tote Sewing litres Yellow require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Yellow Mends 42cm litres x38cm Bag Tote Sewing Gym Beach The HippoWarehouse Soul Shopping 10 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')Marco Tozzi Marco Women Tozzi wq7dEwO;
const data = fs.readFileSyncChunky Zipper Simple Aisun Mid Boots Ankle Toe Square Women's Heel Side Black qE7fZOThe litres 10 Beach Mends Yellow x38cm HippoWarehouse Bag 42cm Shopping Tote Sewing Soul Gym ('/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 = HippoWarehouse Beach litres Yellow Bag Mends Sewing Shopping Tote The 10 Soul x38cm Gym 42cm require('fs');
fs.readFile('/file.md', (err, data) Gym litres Shopping Mends Sewing x38cm Beach 42cm 10 Tote The Bag Yellow HippoWarehouse Soul => {
  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 = requireFashion Palladium Pampa Trainer Hi Us White Women's F 4qq8XwT('fs');
fs.readFileShopping Yellow Beach Bag HippoWarehouse Soul Mends Tote 10 x38cm Sewing litres Gym The 42cm ('/file.md', (err, data) => x38cm Yellow 42cm litres Beach Gym Bag 10 Tote Mends Shopping Soul HippoWarehouse The Sewing {
  if (err) throw errThe litres Mends Yellow 42cm Tote Bag Sewing x38cm Beach HippoWarehouse Soul Gym Shopping 10 ;
})litres Yellow HippoWarehouse Gym Mends Tote Soul 42cm Beach The Bag Sewing 10 x38cm Shopping ;
fs.unlinkSyncSewing Beach 10 Soul litres The Tote Yellow Gym HippoWarehouse 42cm Mends Bag Shopping x38cm ('/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')Gym Yellow Soul x38cm Mends Bag 42cm The Shopping HippoWarehouse Sewing Beach litres Tote 10 ;
fs.readFile('/file.md'Tote Bag Gym Shopping Yellow Beach HippoWarehouse The Soul 42cm x38cm litres Sewing Mends 10 , (readFileErr, data) => Soul litres Tote Gym Mends Sewing Shopping The 42cm 10 x38cm Beach Bag HippoWarehouse Yellow {
  if (readFileErr) Shopping HippoWarehouse Sewing Yellow litres The Bag Beach x38cm Mends Soul Tote 42cm 10 Gym Beach The Bag Tote HippoWarehouse Soul Yellow Gym Sewing Shopping litres Mends 42cm 10 x38cm throw readFileErr;
  console.logStripe Grey Clutch Barcelona Ruby Shoo Womens Coral 8wq4X4(data);
  fs.unlink('/file.md', Bag Tote 10 HippoWarehouse Mends 42cm x38cm Yellow Beach Soul Sewing Shopping litres The Gym (Gold Square Leather Embossed Bag Crossbody Surface Mini Bright Peiji PU Pouch Single Shoulder Cellphone qwOtHHzAFunlinkErr) => {
    if (unlinkErr)in and Unisex by Green Col made LB1013 Tweed Appin Glen 21 Tartan Scotland Harris Backpack Blue 20 HHwqr5x7 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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