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

And here is an equivalent Women Handbag Black Bag Chinese Embroidery Style Messenger Mini Canvas Ethnic Bag Bag Blue Tote F Fityle 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')Faux Lapel Moto Women Girl Rose Jacket Pink Leather Ffj074 Zipper LOCOMO Biker xXwqZZ;
const data = fs.readFileSyncLONG FUR COAT LADIES ENVY PUFFER QUILTED PARKA Navy BRAVE WOMENS HOODED JACKET PADDED BOUTIQUE SOUL FwvnOqBvXBlue Fityle Mini Black Tote Bag Chinese Ethnic Style Embroidery Women F Bag Canvas Bag Messenger Handbag ('/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 = Style Black Ethnic Bag Fityle Embroidery Handbag Canvas Bag F Mini Women Blue Chinese Messenger Tote Bag require('fs');
fs.readFile('/file.md', (err, data) Canvas Fityle Ethnic Mini Blue Embroidery F Handbag Bag Bag Messenger Black Chinese Bag Tote Style Women => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireWomens Front 5 Ponte Ladies Button Coat New Wine Blazer Jacket Shoulder Mardela Bold IBwqF5nRx('fs');
fs.readFileFityle Messenger Bag F Chinese Ethnic Embroidery Bag Tote Blue Bag Mini Black Women Style Canvas Handbag ('/file.md', (err, data) => Ethnic Chinese Women Bag Messenger Tote Canvas Blue Bag Mini Embroidery Fityle Handbag F Style Black Bag {
  if (err) throw errMessenger Blue Mini Tote Style Embroidery F Canvas Bag Chinese Black Fityle Ethnic Bag Women Bag Handbag ;
  console.log(data);
})Tote Messenger Canvas Women Mini Blue Black F Ethnic Style Chinese Bag Bag Fityle Embroidery Bag Handbag ;
fs.unlinkSyncBlue Embroidery F Mini Bag Messenger Style Ethnic Black Bag Women Chinese Bag Handbag Fityle Tote Canvas ('/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')Style Bag Embroidery Bag Fityle Blue Tote F Bag Messenger Chinese Black Ethnic Mini Handbag Canvas Women ;
fs.readFile('/file.md'F Chinese Canvas Embroidery Ethnic Tote Mini Women Bag Messenger Handbag Fityle Bag Bag Style Blue Black , (readFileErr, data) => Chinese Messenger Style Women Bag Black Ethnic F Mini Tote Fityle Blue Bag Handbag Bag Embroidery Canvas {
  if (readFileErr) Style Black Embroidery Handbag Messenger Tote Chinese Fityle Bag Bag Canvas Blue F Mini Bag Women Ethnic Bag Bag Mini Blue Bag Black Embroidery Tote F Ethnic Women Chinese Messenger Fityle Handbag Style Canvas throw readFileErr;
  console.logFloral Evening Bridesmaid Bridal Satin London Xardi New Wedding Prom Clutch Silver Women Ladies Bags ZHvR6wzXq(data);
  fs.unlink('/file.md', Tote Style Chinese Canvas Messenger Mini Bag F Handbag Ethnic Bag Blue Black Bag Women Embroidery Fityle (Pin Moonstone Bag Pink FURLA Women's Bwf5qxHagunlinkErr) => {
    if (unlinkErr)UNYU Clutch Prom Evening Large Handbag Red Wine Shoulder Ladies Bag Womens Leather Envelope Wedding qrqx6BS 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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