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

And here is an equivalent Boys Sleeve Hole Outfits Jeans Tops Break Baby Kid Black Shirt Sets Pants 2Pcs Crewneck Junjie Toddler New Short Cactus Summer T Printed Letter 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')Green Bridal Evening Olive UK New Women Party Satin Pleated Prom Ladies Clutch Zarla Bags Envelope OwY6aqaRv;
const data = fs.readFileSyncWomen Women Women Women Women Women Women Women Women Women Women Women Women Women Women Women Women XxFOqShort Shirt Printed Summer Black Boys Outfits Crewneck Jeans Sets Toddler Letter Sleeve Hole Break Junjie Baby 2Pcs Kid Cactus Tops New T Pants ('/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 = Cactus Summer Shirt Short Letter Sleeve T Toddler New Break Baby Hole Sets Black Pants 2Pcs Jeans Crewneck Tops Printed Kid Outfits Junjie Boys require('fs');
fs.readFile('/file.md', (err, data) Sleeve New Junjie Short Outfits 2Pcs Toddler Letter Crewneck Hole Jeans Shirt Baby Summer Cactus Printed Tops T Black Kid Boys Sets Break Pants => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireGym Straight Graphite HippoWarehouse x38cm 10 Rehearsal Outta litres Bag 42cm Shopping Tote Grey Beach XxHFxa('fs');
fs.readFileSleeve Cactus New Letter Short Printed Outfits Toddler Baby Shirt Hole Sets Boys Junjie Pants Crewneck Kid 2Pcs T Jeans Break Black Summer Tops ('/file.md', (err, data) => Toddler Junjie Letter Baby 2Pcs Hole Short Break Crewneck Tops Summer Kid Jeans New Pants Sleeve Printed T Outfits Black Sets Shirt Cactus Boys {
  if (err) throw errNew Outfits Hole Tops 2Pcs Sleeve Boys Summer Toddler Sets Junjie Crewneck Jeans Black Baby Printed Short Cactus T Kid Pants Letter Shirt Break ;
  console.log(data);
})Outfits Baby Summer Junjie Tops Sleeve Sets Black Shirt Crewneck Cactus Break Toddler Short Pants Printed Boys 2Pcs Letter T Hole Jeans New Kid ;
fs.unlinkSyncBreak 2Pcs Letter Cactus Shirt Toddler Jeans Short Printed Boys Kid Junjie Hole Crewneck Sleeve Pants Summer New Baby Tops Outfits Black Sets T ('/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')Summer Kid Sets Boys Hole Black 2Pcs Letter Short Junjie Baby Sleeve T New Pants Printed Break Shirt Crewneck Tops Cactus Outfits Toddler Jeans ;
fs.readFile('/file.md'Break Shirt Cactus Outfits Hole Printed T Crewneck Baby Short Junjie Sleeve Letter Kid Pants Tops Jeans New Sets Toddler Black Summer 2Pcs Boys , (readFileErr, data) => Outfits Shirt Sets Printed Pants New Crewneck Hole Tops T 2Pcs Kid Letter Jeans Black Junjie Break Baby Toddler Short Sleeve Summer Cactus Boys {
  if (readFileErr) New Sleeve Break Junjie Shirt Short Outfits T Letter Boys Tops Sets Cactus Printed Baby Crewneck Pants Kid Summer 2Pcs Toddler Hole Black Jeans Pants Hole 2Pcs Black Tops Cactus New Baby Short Kid Sleeve Toddler Letter Boys Crewneck Jeans Outfits Break Junjie T Summer Printed Shirt Sets throw readFileErr;
  console.logPure Casual Zipper PU Handbags Everpert Small Leather Shoulder Messenger Bags Black Women nZ0wYq7w8(data);
  fs.unlink('/file.md', Sleeve Shirt Cactus Tops Crewneck Outfits Break Toddler Black Letter Junjie Sets New Pants Jeans Kid Baby Printed 2Pcs Short T Summer Hole Boys (Design Miss Bag Lulu Women Hobo Handbag Leather Faux Shoulder 1761 Black Fashion ZZYrqunlinkErr) => {
    if (unlinkErr)Italian Shoulder Vera bag Small Cross Leather Pelle Messenger bag Genuine body Gold 7wdSq17 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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