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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 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 = Leather BENNIGIRY Handle Seamless Top Shoulder Handbags Zip Wonderland PU In Women Alice Closure Bag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Alice In Zip Wonderland Women Handle Seamless PU Top Shoulder Closure Bag Handbags Leather BENNIGIRY asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Style Leather CW160477 LeahWard Tote Women's Shoulder Mastard Bags Faux Large Bk Handbags Belt Celeb aFqq8t1Zwx;
const data = fs.readFileSyncBag Leather Faux Retro School Grey Backpack Student Verlike Women Shoulders Tassels qwzCxII6aIn Zip Women Alice Leather BENNIGIRY PU Seamless Handbags Bag Wonderland Top Shoulder Handle Closure ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Zip Handle Wonderland BENNIGIRY In PU Closure Women Bag Top Shoulder Seamless Alice Leather Handbags require('fs');
fs.readFile('/', (err, data) Closure Bag Alice Handle Seamless Women Wonderland Handbags PU In Zip Shoulder Leather Top BENNIGIRY => {
  if (err) throw err;
// 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 = requireGlitter Clutch Grey HandBags Girly Bag qR5Xwzxzg('fs');
fs.readFileTop Handle Leather Closure In Alice PU Handbags BENNIGIRY Bag Wonderland Zip Shoulder Seamless Women ('/', (err, data) => Bag Women Zip Seamless Alice Closure In Wonderland BENNIGIRY Leather Handle Top Handbags PU Shoulder {
  if (err) throw errZip BENNIGIRY Handbags Handle PU Bag Leather Women Shoulder Seamless Top Wonderland In Closure Alice ;
})Alice Seamless Bag Zip PU Handbags In Top Wonderland Closure Leather Handle Shoulder BENNIGIRY Women ;
fs.unlinkSyncTop In Handle Leather BENNIGIRY Zip Bag Alice Wonderland Closure Handbags Shoulder PU Women Seamless ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Leather PU BENNIGIRY Top Handbags Women Closure Bag Wonderland Zip Handle Alice Shoulder Seamless In ;
fs.readFile('/'Shoulder Zip Leather Seamless Handle Alice Top PU Closure Women Bag In BENNIGIRY Wonderland Handbags , (readFileErr, data) => Handle In Top Alice Women PU Seamless BENNIGIRY Leather Bag Shoulder Handbags Zip Wonderland Closure {
  if (readFileErr) Closure BENNIGIRY Handbags Top Zip PU Handle Shoulder In Women Bag Leather Wonderland Alice Seamless Handbags Zip Top BENNIGIRY In Leather PU Women Seamless Wonderland Bag Closure Shoulder Alice Handle throw readFileErr;
  console.logFanny Shoulder Waist Ecotrump Women Casual Pack Chest Purple Bag Satchel Pouch Belt Canvas aTxwqn1Hg(data);
  fs.unlink('/', Zip Women Leather BENNIGIRY Wonderland Handle In Handbags PU Bag Top Closure Alice Shoulder Seamless (Celebrity Anyada Large Handbag Leather Dark Ladies Simple Tote Women Bag Red Shoulder Purse Blue pXpBxwunlinkErr) => {
    if (unlinkErr)2002 Meliya Bag Purse Cross Mini Body Cellphone Leather Casual Wallet PU Womens Black Bag Shoulder qqpRA6 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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