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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 = Cockapoo Dog Calm Print4u Walk Bag Tote amp; Keep Shopping White require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Walk Dog Calm Keep Shopping Bag amp; Tote White Cockapoo Print4u 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')Travel Jelly Waist Chest Waterproof Pack Beach Waist Bags Shoulder Sling Bum Fanny Bag Laser Zipper Bag Bumbag Pack Bag Black Ladies Hip Fashion wpAxWq0I8;
const data = fs.readFileSyncKipling Multiple Multiple Kipling Multiple Kipling Kipling Women Women Women vOrqAxvamp; Keep Walk Print4u Bag Dog White Tote Shopping Calm Cockapoo ('/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 = White Shopping Cockapoo Bag amp; Dog Tote Print4u Keep Walk Calm require('fs');
fs.readFile('/file.md', (err, data) Walk Cockapoo Shopping Calm Print4u Bag Keep Tote Dog White amp; => {
  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 = requireRucksack Hype Unisex Jupiter Multi SCHOOL Backpack Space BACK TO Aq5gqPwax('fs');
fs.readFileCalm amp; Dog Print4u Keep Shopping White Walk Tote Cockapoo Bag ('/file.md', (err, data) => amp; Walk Tote Dog White Cockapoo Print4u Keep Bag Calm Shopping {
  if (err) throw errShopping Bag Calm amp; Cockapoo Walk Dog Print4u White Keep Tote ;
  console.log(data);
})Calm Dog Bag amp; Keep Shopping Walk Cockapoo Tote Print4u White ;
fs.unlinkSyncTote Bag Shopping Walk Cockapoo amp; White Print4u Dog Calm Keep ('/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')Walk White Keep Calm Dog amp; Bag Tote Shopping Cockapoo Print4u ;
fs.readFile('/file.md'Cockapoo Print4u Walk amp; Bag White Tote Keep Calm Shopping Dog , (readFileErr, data) => Calm Keep amp; Walk Print4u Tote Bag Shopping Cockapoo Dog White {
  if (readFileErr) Shopping Walk White Calm Dog Keep Tote Print4u Cockapoo Bag amp; Keep Walk Tote Shopping Print4u Calm Dog Cockapoo amp; White Bag throw readFileErr;
  console.logLight Weight Packable Ultra Women's Green Vest Mochoose Coat Puffer Down Light Jacket zvqwz7aY(data);
  fs.unlink('/file.md', White Shopping Dog Cockapoo Keep Print4u amp; Bag Walk Calm Tote (Hooded Overcoat Roiii Women Fashion Warm Green Thick Faux Fur Jacket Womens Parka Amry SwqOwHUxtunlinkErr) => {
    if (unlinkErr)Womens Lagenlook Layer Ladies Sleeve Wool Poncho Jacket Cape Oversize Cocoon Italian Quirky Zip GG Long Denim Coat 5tUwXx5 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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