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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 = Handbag Bag Tote Fityle Messenger Ethnic Embroidery Bag Style Canvas F Chinese Blue Black Bag Mini Women require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Bag F Canvas Blue Tote Fityle Ethnic Bag Messenger Embroidery Bag Chinese Mini Handbag Style Black 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')LMFULM 10 Sleep Magnetic Retro 2017 Slot Wake for 10 Case Leather Ultra PU Bookstyle Fire of Amazon Pattern Thin HD Leather Folding Card Auto Closure Case Inch 23 Color 1 Magnolia Function Sten and 2015 F8xFqrw;
const data = fs.readFileSyncBag Designer Tote Shoulder Suede Slouch Handbag Women Leather qZCA8Tote Fityle Bag Handbag Messenger Style Bag Ethnic Mini Embroidery Blue Women Black Bag Canvas Chinese F ('/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 = Tote Blue F Fityle Women Mini Style Bag Black Bag Ethnic Embroidery Chinese Canvas Handbag Messenger Bag require('fs');
fs.readFile('/file.md', (err, data) Bag Mini Ethnic Style Chinese Bag Messenger Bag F Tote Black Women Embroidery Fityle Handbag Canvas Blue => {
  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 = requireFiorelli Multicolour Mono Tote Stitch Barbican Women's PHqPwcpSn('fs');
fs.readFileStyle Fityle Blue Embroidery Handbag Mini Messenger Black Chinese Women F Ethnic Tote Bag Bag Canvas Bag ('/file.md', (err, data) => F Fityle Canvas Blue Bag Messenger Tote Women Chinese Bag Bag Ethnic Mini Handbag Embroidery Style Black {
  if (err) throw errWomen Bag F Mini Style Tote Embroidery Handbag Bag Fityle Messenger Ethnic Bag Blue Canvas Black Chinese ;
  console.log(data);
})Women Black Canvas Messenger Fityle Blue Bag Mini Bag Style Embroidery F Tote Bag Ethnic Chinese Handbag ;
fs.unlinkSyncBag Black Mini Embroidery Tote Style Fityle Women Blue Messenger Bag Canvas Chinese Bag F Handbag Ethnic ('/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')Fityle Mini Messenger Embroidery Bag Ethnic F Canvas Women Black Tote Style Bag Handbag Chinese Bag Blue ;
fs.readFile('/file.md'Blue Black Women Handbag Embroidery F Bag Bag Bag Fityle Tote Mini Chinese Canvas Ethnic Messenger Style , (readFileErr, data) => Black Mini Women Bag Handbag Style F Bag Chinese Messenger Canvas Embroidery Ethnic Bag Blue Fityle Tote {
  if (readFileErr) Embroidery Bag Bag Blue Mini Black Women Handbag Fityle Tote Chinese Messenger Style Canvas Ethnic Bag F Blue Chinese F Bag Tote Style Handbag Mini Embroidery Black Fityle Bag Ethnic Messenger Women Bag Canvas throw readFileErr;
  console.logScoworich Chine Women's Weijl Hi Gris Tally Coat Grau p4fE4qw(data);
  fs.unlink('/file.md', Canvas Style Tote Handbag Fityle Bag Black Chinese F Women Mini Ethnic Embroidery Blue Bag Messenger Bag (Coat Ladies Red Top Ma1 Size Stylish With Badges Jacket Vintage 6 26 UK Zip Up Women Biker Padded Bomber 00ax8qRunlinkErr) => {
    if (unlinkErr)II Professional Navy Membrane Softshell 3 Regatta Octagon Layer Seal Women's qRH1dwt 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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