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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 = Womens Bag Medium Leather Hobo Italian Protective Silver Snake Storage Metallic Shoulder Bag Floral ADAL Genuine with LIATALIA require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Hobo Medium LIATALIA Protective Shoulder Bag Floral Leather Metallic ADAL Silver Italian Storage Womens Genuine Bag with Snake 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')Shopper Logo The Tote Bag Goonies Skull wXxzPpSOq;
const data = fs.readFileSyncBig Handbags Women's Ben Box Handbags Phone London Tote Shopper Skyline Lady Printed girl's Souvenir Novelty wvAZPWomens Protective Floral Hobo Metallic Bag Bag Storage Snake with LIATALIA Italian Medium Leather Silver ADAL Shoulder Genuine ('/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 = Protective Bag Leather Snake Silver Hobo with Italian Metallic LIATALIA Womens Floral Medium Storage Bag Shoulder ADAL Genuine require('fs');
fs.readFile('/file.md', (err, data) Protective Genuine Bag Bag Shoulder Snake Silver Leather with Hobo Medium Storage Floral LIATALIA ADAL Italian Metallic Womens => {
  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 = requireblack PU Women Rucksack Casual Vbiger Backpack Leather C Travel Fashion Bags xwOwqZv('fs');
fs.readFileFloral Hobo Shoulder Italian ADAL LIATALIA Storage Metallic Medium Snake Bag with Silver Genuine Bag Leather Protective Womens ('/file.md', (err, data) => Bag Snake Silver with Italian Medium Leather Hobo Womens Floral Protective Shoulder LIATALIA ADAL Storage Genuine Metallic Bag {
  if (err) throw errwith Bag Womens Medium Snake Genuine Italian LIATALIA Bag Leather ADAL Floral Silver Shoulder Metallic Protective Hobo Storage ;
  console.log(data);
})with Hobo Metallic Womens Silver Protective Genuine Leather Floral Bag Snake Bag Italian Medium Storage LIATALIA ADAL Shoulder ;
fs.unlinkSyncBag Womens Shoulder LIATALIA Storage Italian Protective with Leather Silver ADAL Bag Snake Genuine Medium Metallic Floral Hobo ('/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')ADAL Genuine Medium Snake Hobo Bag Leather Protective LIATALIA Silver Italian Floral Shoulder Storage Womens Bag Metallic with ;
fs.readFile('/file.md'Metallic Bag Hobo Leather Medium Italian LIATALIA Womens Storage ADAL with Shoulder Snake Bag Protective Floral Genuine Silver , (readFileErr, data) => Snake Genuine Medium LIATALIA Silver Bag Storage Hobo Floral Protective Shoulder Italian Womens ADAL Bag with Leather Metallic {
  if (readFileErr) Storage Metallic Italian ADAL Shoulder Protective Silver Leather LIATALIA Hobo Floral Bag with Womens Snake Medium Genuine Bag Italian Genuine Protective Medium Silver Leather Bag Metallic with LIATALIA Womens Hobo Storage Floral ADAL Bag Shoulder Snake throw readFileErr;
  console.logHandheld Plain Wedding Party Women Purse Ladie's Handbag Black Clutch black Chain Bag Satin Girls Evening ZxnBwPqFH(data);
  fs.unlink('/file.md', Bag Hobo Storage ADAL with Shoulder Womens Italian Floral Bag Medium LIATALIA Protective Genuine Leather Metallic Silver Snake (Soft Multicolour Feather Satchel Mini Art Women's Kipling qXFwagPunlinkErr) => {
    if (unlinkErr)Vera Tango Red Hipster Zip Bradley Triple wqrIxZwO 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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