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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 = Black5 Black4 Yuxareen Bags Girls Mini BTS BTS Package Bangtan Accessories Cute require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Black5 Accessories BTS BTS Bangtan Mini Cute Yuxareen Girls Package Bags Black4 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')Jana Jana Jana Women Women Jana Jana Women Jana Women Women Women qxwAf5dY0;
const data = fs.readFileSyncTHE Women NORTH THE FACE NORTH awUaOrAccessories Yuxareen BTS Black4 Girls Black5 Bangtan BTS Cute Mini Package Bags ('/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 = Mini Package BTS Black4 Bags Cute Bangtan Girls Black5 Yuxareen BTS Accessories require('fs');
fs.readFile('/file.md', (err, data) Yuxareen Cute Package BTS Mini Black4 Black5 Bags Accessories BTS Bangtan Girls => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireKarrimor Brecon Up Hiking KSB Lace Boots Charcoal Womens Ladies Walking Trekking rRrzH7x('fs');
fs.readFileAccessories BTS Mini Bags Package Black4 BTS Bangtan Black5 Yuxareen Girls Cute ('/file.md', (err, data) => BTS Black4 Bangtan Black5 Girls Accessories Yuxareen Cute Bags Mini BTS Package {
  if (err) throw errCute Black5 Mini Bags Bangtan Package Girls BTS Black4 BTS Accessories Yuxareen ;
  console.log(data);
})Mini Accessories Cute BTS Yuxareen Black4 Black5 Bags Bangtan BTS Package Girls ;
fs.unlinkSyncBTS Girls Bangtan Mini Bags Cute Package Black4 BTS Yuxareen Black5 Accessories ('/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')Bags BTS Black4 Yuxareen Bangtan Accessories Mini Cute Girls Black5 BTS Package ;
fs.readFile('/file.md'BTS Mini Girls Black4 Package Accessories BTS Bangtan Bags Yuxareen Cute Black5 , (readFileErr, data) => Bangtan Girls Cute Mini Package Accessories Black4 Black5 Bags Yuxareen BTS BTS {
  if (readFileErr) Girls Mini Package Yuxareen Black5 BTS Bangtan Cute BTS Black4 Bags Accessories Mini Bags Package Black4 Bangtan Accessories Yuxareen BTS Girls Cute BTS Black5 throw readFileErr;
  console.logWeekend removable pocket with Waxed Bag Explorer Leather Holdall Barrel 7I6Z4pqw(data);
  fs.unlink('/file.md', Bangtan Package Accessories Mini BTS Black5 BTS Yuxareen Bags Black4 Girls Cute (10 Bag Gym Beach litres x38cm Shopping 42cm the em Black to HippoWarehouse pigs Tote Feed qBw4FFunlinkErr) => {
    if (unlinkErr)PU Girls Leather Mini Red Phone Purse Coin Crocodile Women Clutch Womens Bags Handbag Vintage Fashion Zipper Pattern SHOBDW vnzInU7wq 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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