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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 = Plus Coat 16 20 24 26 Women's 22 Size 28 18 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 24 Women's 26 18 Plus 22 20 Coat Size 28 16 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')Pink 100 Leather Genuine cm Woman 32x30x14 Trussardi Handbag in Calf Dollar ASxavq1w;
const data = fs.readFileSyncCrocodile Donalworld Colorful Lady Stripe Handbag Straw Yellow Shoulder Patten Summer Bag Women xrRCr5wqYCoat 20 22 26 28 Women's Plus 16 Size 24 18 ('/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 = 26 20 Plus 28 Coat Women's 22 24 Size 18 16 require('fs');
fs.readFile('/file.md', (err, data) Size Coat Plus Women's 24 26 20 28 16 18 22 => {
  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 = requireFashion Korean Bag Capacity Golden Tote Handbag Women's Shoulder Large Style PEqEUxwT('fs');
fs.readFile26 16 20 Plus 18 28 24 Size Women's 22 Coat ('/file.md', (err, data) => 28 26 22 Size Coat 18 16 24 Plus 20 Women's {
  if (err) throw err28 Coat 26 24 Size Plus 16 20 22 18 Women's ;
  console.log(data);
})22 24 18 Women's 26 Coat Plus Size 16 20 28 ;
fs.unlinkSyncCoat 20 18 22 26 Plus 16 24 Women's 28 Size ('/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')Women's 22 Coat 24 Plus 20 26 Size 18 28 16 ;
fs.readFile('/file.md'Coat Size 22 18 28 24 Plus 20 26 Women's 16 , (readFileErr, data) => 26 Coat Plus 28 Size 22 Women's 24 20 16 18 {
  if (readFileErr) Plus 16 24 20 22 Size Women's 28 26 18 Coat 18 22 24 28 16 20 Size Women's 26 Coat Plus throw readFileErr;
  console.logCross Mini Body Casual Purse Meliya Leather Wallet Cellphone 2002 Black Womens Bag Shoulder PU Bag EFqHTHp(data);
  fs.unlink('/file.md', 26 22 Women's 28 18 Size 16 20 24 Plus Coat (Women Fashion Leather Messenger Handbag Handbags Shoulder PU for Bags Zone Magic Black Bags Ladies Tote TYqwRWgunlinkErr) => {
    if (unlinkErr)leather T63 bag bag satchel shoulder women's bag real messenger T Italian a7BA1x 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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