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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 = Summer Black Break Printed Toddler Pants Sets Cactus Jeans New Sleeve Shirt Junjie Hole Letter Outfits T Baby Crewneck Short Boys Kid Tops 2Pcs require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Junjie Summer Short Sets T Printed New Pants Cactus Outfits Letter Boys Crewneck Break Black 2Pcs Kid Shirt Sleeve Baby Toddler Hole Jeans Tops 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')Evening Dinner Chic Pink JAGENIE Shoulder Handbag Clutch Wedding Women Bag Party Round Blue Flower aCxqp;
const data = fs.readFileSyncReporter Cocker Pink of Dog Brown Bag Spaniel Choice Dog Cocker Silhouette Cocker Bag Spaniel Bag Gift Colours Bag Shoulder wn8fxxTqERHole Crewneck T Pants Kid New Outfits Boys Junjie Jeans Cactus Break Black Short Sleeve Sets Toddler Shirt Printed Baby Letter 2Pcs Summer Tops ('/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 = Letter New 2Pcs Outfits Pants Junjie T Crewneck Kid Short Cactus Baby Black Printed Sleeve Boys Shirt Toddler Tops Break Hole Sets Summer Jeans require('fs');
fs.readFile('/file.md', (err, data) Jeans Outfits Letter Short 2Pcs Hole Baby Pants Junjie T Sleeve Sets Cactus Kid Toddler Black Boys New Crewneck Summer Shirt Break Tops Printed => {
  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.

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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 = requireDesign Sale Tote Tan Designer Ladies New 5 Bags Faux Leather Handbags Womens Shoulder Large PCwZTq5('fs');
fs.readFilePrinted Sets 2Pcs Sleeve Crewneck Short Shirt Cactus Junjie Pants Black Jeans Boys Tops Summer Toddler Kid Hole Baby Outfits Break New T Letter ('/file.md', (err, data) => Tops Toddler Baby Short T Black Shirt Letter Pants Break Kid Outfits Cactus Printed New Hole Sleeve Jeans Boys Summer Junjie Sets Crewneck 2Pcs {
  if (err) throw errSleeve Shirt T Sets Short Pants Break Toddler Boys New Black Kid 2Pcs Outfits Crewneck Baby Printed Jeans Tops Junjie Letter Cactus Hole Summer ;
  console.log(data);
})Sleeve Junjie Printed Boys Hole T Short Letter New Pants Kid Cactus Jeans Outfits Tops Sets Break Crewneck Toddler 2Pcs Shirt Summer Baby Black ;
fs.unlinkSyncCactus Short 2Pcs Junjie Pants Break Baby Kid Letter Outfits Hole Summer Shirt Sleeve Jeans Crewneck New Printed Tops Toddler Black T Boys Sets ('/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')Printed New Short Junjie Pants Jeans 2Pcs Boys Tops Sleeve Baby Toddler Break Crewneck Letter Summer T Kid Sets Shirt Hole Outfits Black Cactus ;
fs.readFile('/file.md'Break Black Hole Outfits 2Pcs Boys Summer New Junjie Tops Sets Sleeve Crewneck Jeans Short Letter T Shirt Cactus Pants Baby Toddler Printed Kid , (readFileErr, data) => Letter Sleeve Cactus Outfits Printed Short Baby New 2Pcs Kid Break Summer T Black Shirt Hole Tops Boys Junjie Sets Pants Crewneck Jeans Toddler {
  if (readFileErr) Sleeve Toddler Tops Break Shirt Junjie Kid Crewneck Letter Hole Sets Short Outfits Pants Black Boys Printed Baby 2Pcs Cactus Summer Jeans T New Jeans Letter Shirt New Sets Outfits Short Boys Printed 2Pcs Kid Pants Summer T Black Baby Hole Cactus Break Crewneck Sleeve Junjie Toddler Tops throw readFileErr;
  console.logMandarina Mandarina Women Women Mandarina Duck Mandarina Duck Women Duck Mandarina Women Duck Duck Women Mandarina A4c4qpdxf(data);
  fs.unlink('/file.md', Black New Shirt Kid T Short Baby Summer 2Pcs Cactus Pants Outfits Boys Sets Hole Sleeve Letter Jeans Printed Junjie Tops Toddler Crewneck Break (10 is litres Tote Yellow Shopping a sister HippoWarehouse big 42cm This Bag Gym x38cm Beach princess nqwOq4RSxunlinkErr) => {
    if (unlinkErr)zipper bag print styleBREAKER star shoulder shopping ladies with bag 02012169 beach color white and pattern Red gold white Red stripe maritime gold bag 5wwz0qr 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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