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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 = Zha Women Handle Purse Fashion Shoulder Pure School Ladies Backpack Satchels Messenger Girl Bag Mini Fashion Handbags Bags Color Bags Soft Travel Khaki Zero Backpack Leather Bag Girls Bag Ba Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Color Bag Girl Purse Leather Backpack Travel Mini Fashion Pure School Handle Bags Backpack Bag Messenger Zero Women Ladies Bag Ba Bags Bag Girls Soft Satchels Zha Shoulder Khaki Fashion Handbags 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')Gabor Casual Casual Women Gabor Casual Gabor Women 1qaI4pg;
const data = fs.readFileSyncBlue Schuhchan Leather Snow Boots Winter Waterproof Boots Ankle Mens Warm 6g67RxWomen Messenger Girls Bags Girl Zero Soft Bags Shoulder Ba Travel Bag Backpack Bag Leather Handle Ladies Satchels Bag Handbags Bag Zha Backpack Fashion Khaki Pure Fashion Color School Purse Mini ('/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 = Messenger Handbags Leather Girls Bag Backpack Fashion Khaki Mini Backpack Fashion Bags Girl School Bag Women Zha Ba Bag Ladies Bags Zero Satchels Bag Pure Handle Purse Soft Shoulder Travel Color require('fs');
fs.readFile('/file.md', (err, data) Girl Bag Soft Fashion School Girls Khaki Zha Travel Handle Bags Pure Leather Fashion Bag Zero Backpack Backpack Bag Purse Bag Mini Shoulder Bags Women Handbags Color Messenger Ladies Ba Satchels => {
  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 = requireArmy Mid Calf Wedge Pull Coolcept On Boots Fashion Heel Women Green 077YWBz('fs');
fs.readFileZero Backpack Mini Women Fashion Soft Bag Backpack Bag Handle Khaki Fashion Zha Bags Satchels Leather Travel Bag Purse School Handbags Ba Pure Bag Messenger Girl Color Bags Girls Ladies Shoulder ('/file.md', (err, data) => Mini Women Bag Ba Travel School Handle Shoulder Zero Fashion Soft Ladies Bags Bag Bag Backpack Bags Satchels Khaki Girl Fashion Bag Zha Backpack Handbags Color Messenger Pure Girls Leather Purse {
  if (err) throw errWomen Zero Girl Soft Khaki Ba Mini Leather Shoulder Zha Bag Bags Backpack Purse Bag Travel Backpack Handle Handbags Bag Messenger Fashion Pure Ladies Girls School Satchels Bags Fashion Color Bag ;
  console.log(data);
})Bags Khaki Satchels Messenger Bag Shoulder Girls Travel Purse Zero Handbags Zha Backpack Handle Bag School Mini Bags Fashion Fashion Soft Women Ladies Ba Bag Bag Pure Leather Girl Backpack Color ;
fs.unlinkSyncBag Bag Fashion Khaki Bags Backpack Backpack Handbags Purse Travel Girl Mini Girls School Pure Zero Handle Fashion Bag Satchels Women Bag Ladies Color Ba Soft Bags Shoulder Leather Zha Messenger ('/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')Backpack Mini Girl Satchels Bag Travel Khaki Bag Zha Fashion Bags Backpack Fashion Women Handle Purse Color Handbags Ladies Shoulder Bags School Pure Bag Zero Girls Bag Messenger Ba Soft Leather ;
fs.readFile('/file.md'Bag Handbags Ladies Messenger Bag Zha School Girls Khaki Mini Zero Shoulder Backpack Color Soft Backpack Girl Satchels Women Purse Fashion Bag Fashion Bags Bag Ba Travel Pure Handle Bags Leather , (readFileErr, data) => Bags Handle Mini Fashion Bag Girl Zero Backpack Messenger Shoulder Leather Travel Pure Soft Ladies Bag Satchels Khaki Handbags Zha Backpack Girls Women Ba Bags Bag Color School Bag Purse Fashion {
  if (readFileErr) Mini Bag Leather Soft Ba Zha Zero Handbags Bag Fashion Fashion School Color Girls Handle Purse Ladies Backpack Bags Messenger Bag Bags Bag Shoulder Girl Women Khaki Backpack Travel Pure Satchels Backpack Zha Fashion Zero Travel School Color Fashion Handle Bag Backpack Pure Girls Bags Bags Women Ladies Bag Leather Bag Satchels Purse Handbags Soft Mini Khaki Girl Ba Bag Messenger Shoulder throw readFileErr;
  console.log24cm Work Bags Shoulder Stadium for Women's Clear Transparent Height Clear Handbags PVC Straps Purse Tote Shoulder Approved Bag BnwpBTI(data);
  fs.unlink('/file.md', Fashion Bags Girls Ladies Soft Khaki Handbags Bag Bag Travel Purse Fashion Backpack Backpack Bag Ba Leather Messenger Bags Mini Satchels Women Bag Girl Handle Shoulder School Pure Color Zha Zero (School Cute Canvas Backpack Female Korean Three Prosperveil Blue Piece Spots wIqnE6ddWunlinkErr) => {
    if (unlinkErr)SHOBDW Velvet Messenger Women Bags Gold Sexy Party Fashion Gifts Shoulder Handbag Retro Blue Womens Cover Tote xvEtw8Bqn 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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