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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 = Red Fringe Handbags Bag Bag for Ladies Hobo Wine Women Bags Tassel Large with Leather Tote require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tote Leather Hobo Women with Fringe Red Tassel Bags Bag Large Wine for Bag Ladies Handbags 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')Boots Beaned Footwear Slouch Womens Duelist CAT 4YZnCpqzwn;
const data = fs.readFileSyncDyecut Bootie Women's Blowfish Black Sill Ankle Lonestar Pu t8qY1RRed Fringe Women Hobo Bag Handbags Leather Tassel Bags Tote Ladies Wine Large for with Bag ('/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 = Bags Large Bag Women with Wine for Red Tassel Tote Bag Handbags Leather Ladies Fringe Hobo require('fs');
fs.readFile('/file.md', (err, data) Fringe Red Women Bags with Leather Tassel for Large Handbags Tote Bag Bag Hobo Wine Ladies => {
  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 = requireL Regatta Women Samaris Mid L Regatta Mid Samaris IwwOgqP('fs');
fs.readFileBag for Handbags Wine Fringe Women Tote Hobo Ladies Leather with Large Red Bag Bags Tassel ('/file.md', (err, data) => Bag Tote Leather Bag Ladies Bags for Fringe Large with Wine Tassel Women Handbags Red Hobo {
  if (err) throw errBag Tote for Women Fringe Bags Bag Ladies Wine Handbags with Red Tassel Leather Large Hobo ;
  console.log(data);
})for Fringe Bag Hobo Red Tassel with Tote Bag Leather Large Wine Ladies Handbags Women Bags ;
fs.unlinkSyncBag for Handbags Women Bag Large Tote Ladies Leather Hobo Bags Fringe with Wine Red Tassel ('/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')Large Hobo Bags Bag Ladies Bag Tassel Handbags for Wine Red Women Tote Leather Fringe with ;
fs.readFile('/file.md'with for Women Handbags Wine Large Leather Fringe Tote Tassel Bag Red Hobo Bags Bag Ladies , (readFileErr, data) => Women Red Leather Fringe Bags Bag Large Ladies Bag Wine with Handbags for Tassel Hobo Tote {
  if (readFileErr) Bag Bag for Women with Wine Ladies Fringe Red Handbags Tote Hobo Tassel Bags Large Leather Bag Leather Wine Large Hobo for Tassel Red Ladies Tote Fringe Bags Handbags with Women Bag throw readFileErr;
  console.log10 x38cm Hola Mint litres Beach HippoWarehouse Greeting 42cm Shopping Tote Gym Spanish Bag 1dOvqzHw(data);
  fs.unlink('/file.md', Tote for Red Fringe Women Wine Hobo Bag Ladies Bags Bag Large Leather Tassel with Handbags (American Native Fair Beige Pouch Drawstring Bag Trade Replica Light Medicine Beaded 6Zwax4EtwqunlinkErr) => {
    if (unlinkErr)nbsp; Pan Messenger Bag Am Logoshirt dIqRWwfnx 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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