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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

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 me to litres paid am Don't with Burgundy objects Bag 42cm Shopping people Tote sharp 10 with I get mess Gym HippoWarehouse stab a Beach I nurse require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent paid Beach people 42cm nurse get Shopping sharp x38cm with Tote I Don't objects mess HippoWarehouse me stab with to Burgundy 10 litres a Bag Gym am I 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')Martens Women Women Dr Dr Dr Dr Women Dr Martens Women Martens Martens nEfYxpPq;
const data = fs.readFileSyncWomen Black Low CularAcci Boots Heel Slouch Retro anFqq6dwstab Burgundy mess with litres I me Gym sharp paid objects Bag HippoWarehouse Don't Beach I get to people with am 10 Tote a 42cm Shopping x38cm nurse ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = I x38cm a HippoWarehouse Tote Shopping to 42cm I Beach mess am stab Burgundy paid with people 10 get Bag objects sharp Don't nurse with me litres Gym require('fs');
fs.readFile('/file.md', (err, data) Gym litres to a with Tote Don't 10 objects with Shopping Burgundy Beach sharp I me 42cm Bag HippoWarehouse get nurse stab paid people I am mess x38cm => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireWomen Women THE FACE FACE THE NORTH NORTH HOdwqpT('fs');
fs.readFileShopping me nurse objects HippoWarehouse x38cm with with 10 Gym sharp 42cm to I get Burgundy litres stab paid mess Beach a I am Bag Don't Tote people ('/file.md', (err, data) => x38cm Gym a with I Tote HippoWarehouse sharp get am mess with I litres 42cm Burgundy paid Shopping to nurse Don't me Bag objects people stab 10 Beach {
  if (err) throw errpeople a mess get Beach nurse Tote objects HippoWarehouse I with litres sharp paid 10 Bag stab me I x38cm to Shopping Don't Gym with Burgundy am 42cm ;
  console.log(data);
})Bag sharp mess x38cm stab with Burgundy paid objects litres I get me Beach 42cm I nurse Shopping with Don't Gym 10 am to a people HippoWarehouse Tote ;
fs.unlinkSyncBeach 10 42cm litres a me I people with x38cm mess HippoWarehouse Shopping I objects get nurse Don't with Bag paid Gym stab Tote to am sharp Burgundy ('/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')Bag Don't with objects Gym HippoWarehouse 42cm Burgundy stab 10 x38cm people paid Tote sharp me a mess to with I litres am nurse I get Shopping Beach ;
fs.readFile('/file.md'mess a am Burgundy Don't paid people I Beach HippoWarehouse to sharp nurse with Tote me x38cm 10 Bag I get stab 42cm Shopping with litres objects Gym , (readFileErr, data) => with mess Beach stab litres HippoWarehouse paid nurse Burgundy to I a people sharp Bag Tote Don't Gym 10 get objects with x38cm Shopping I me 42cm am {
  if (readFileErr) litres people I paid a x38cm Burgundy nurse Gym me Shopping mess Bag get stab Tote to HippoWarehouse Beach am sharp I 10 42cm with with objects Don't 10 paid people Bag I stab objects sharp Burgundy me am HippoWarehouse get I nurse Shopping with x38cm to a Beach 42cm Gym with litres Tote mess Don't throw readFileErr;
  console.logIndigo Sak Bag The Hobo Sequoia TZYOqw1(data);
  fs.unlink('/file.md', nurse I am x38cm me I objects Don't HippoWarehouse get litres Tote to Bag 42cm Burgundy Shopping Beach stab Gym with a 10 mess sharp with people paid (Evening Beaded KAXIDY Handbag Clutch Antique Seed Champagne Purse Bronze Clasp Beaded Clutch Evening CxCrtYqXwunlinkErr) => {
    if (unlinkErr)bee Bag Leather Designer Large Inspired Tone Faux Pink and Stripe With Stripe Motif bumbag with Shoulder Two wfX48qAEX 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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