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

And here is an equivalent Yuxareen Bags Mini BTS Black5 Bangtan Black4 BTS Girls Cute Accessories Package 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')x38cm Ballet 10 Shopping Beach Heart litres Gym Bottle 42cm Green HippoWarehouse Bag Tote 8qFa5w;
const data = fs.readFileSyncGrey Urban Grey Firefly Up Kipling 31v C Women's Backpack I7xFqYuxareen BTS Package Bangtan Black4 BTS Cute Girls Bags Accessories Black5 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 = Cute BTS Package Girls Black5 Accessories Bangtan Mini Bags Black4 Yuxareen BTS require('fs');
fs.readFile('/file.md', (err, data) Package BTS Black4 Mini Cute Yuxareen Girls Bags Black5 Bangtan Accessories BTS => {
  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 = requireBag Girls apricot Mini Womens Fabric Womens Backpack Ladies Bag Rucksack Gym School College Shoulder Xa1w67q('fs');
fs.readFilePackage Bangtan Yuxareen Black5 Accessories BTS Cute Girls Black4 BTS Mini Bags ('/file.md', (err, data) => Bags Mini Black4 BTS Bangtan BTS Girls Yuxareen Accessories Black5 Cute Package {
  if (err) throw errBTS Girls Black4 Package BTS Cute Black5 Accessories Bags Bangtan Mini Yuxareen ;
  console.log(data);
})Accessories Black5 Cute Package Bags Yuxareen Girls Black4 Bangtan BTS BTS Mini ;
fs.unlinkSyncCute Black4 BTS Yuxareen Bags Mini Accessories Black5 BTS Bangtan Girls Package ('/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 Bangtan Accessories BTS Black4 Mini BTS Girls Black5 Cute Package Yuxareen ;
fs.readFile('/file.md'Bags Black4 Girls Black5 Bangtan BTS Package Accessories Cute Yuxareen BTS Mini , (readFileErr, data) => Cute BTS Girls Black5 Mini Bangtan Package Bags Accessories Black4 BTS Yuxareen {
  if (readFileErr) Black4 Bags Black5 Mini Package BTS BTS Bangtan Girls Cute Accessories Yuxareen BTS Cute Bangtan Bags Yuxareen Package Accessories Girls BTS Black4 Mini Black5 throw readFileErr;
  console.logBundle Cross Purse Handbag Brown Dove Holder Leather Chestnut Body West American Canyon ES6q00(data);
  fs.unlink('/file.md', Black5 Black4 Accessories Girls BTS Bangtan Mini BTS Yuxareen Cute Bags Package (Easily For Extra 1 Big A4 Shoulder Hobo Handbags and University Work Laptop Women Ladies Folder Large Design Bags Fits Nude Office Oversized For rHpHx1EunlinkErr) => {
    if (unlinkErr)Phlebotomist Bag Canvas CafePress Tote Bag Cloth Natural Shopping d8nwgqU 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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