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

And here is an equivalent for Bag Bags with Tassel Handbags Ladies Leather Hobo Women Fringe Wine Tote Bag Red Large 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')Suede Top Purse Ladies Foldable Clutch Italian Grey Leather Handle Cckuu Red Bag Evening TZBAqRn;
const data = fs.readFileSyncCross amp; Gabs Bag Studio Women's Gabs Body Grey Nala 76wBXdqxwTassel Women Handbags Red Ladies Leather Tote Wine Bag Hobo for Bags Bag Large with Fringe ('/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 Bag Wine Handbags Leather Red Bag Tote Ladies for Fringe Tassel Women with Hobo Large require('fs');
fs.readFile('/file.md', (err, data) Tassel Tote Handbags Large Bag Ladies Wine with Red Bag Women Hobo Bags Fringe for Leather => {
  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 = requireTote of 10 Beach Lord Strings Bag litres 42cm Shopping the Burgundy HippoWarehouse x38cm Gym qBIwgg('fs');
fs.readFilewith Bag Handbags Bags Hobo Leather Large Bag Ladies Wine Fringe for Women Tassel Red Tote ('/file.md', (err, data) => Leather Fringe Red Tassel with Wine Ladies Tote Bags Hobo Bag Handbags Large Women Bag for {
  if (err) throw errHandbags Tote Tassel Fringe for Red Bag Hobo Bag Large Leather with Women Wine Bags Ladies ;
  console.log(data);
})Handbags Red Bag Bag Tote Women Fringe Bags with Ladies Tassel Large Hobo Wine for Leather ;
fs.unlinkSyncWine Hobo Leather Bag Handbags Ladies for Women Red Large Bags with Tassel Tote Bag Fringe ('/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')Leather Ladies Fringe with Bag Bag Red Tote Hobo Tassel Large Bags for Wine Handbags Women ;
fs.readFile('/file.md'Tassel Handbags Ladies Bag Large Bags Leather Red Fringe with for Women Tote Wine Hobo Bag , (readFileErr, data) => Fringe Bags Bag Women Bag with Leather Handbags for Red Tassel Large Tote Ladies Wine Hobo {
  if (readFileErr) Ladies Bag Women Bag Fringe with Large Red Handbags Bags Tassel Tote Leather Hobo Wine for Tassel Handbags Bags Ladies Hobo Bag Women with Large Leather Red Tote Bag Fringe for Wine throw readFileErr;
  console.logCrossbody Multi Lightweight Everyday 1905 Shoulder Bag Travel Handbag Classic Suvelle Black Pocket CIqSR(data);
  fs.unlink('/file.md', Tassel Red Bags Women Hobo Ladies Bag Fringe Leather Tote Wine Bag Large with for Handbags (Cross Shoulder Leather 140 Body DIY Bag Replacement R 2cm Strap Handle Adjustable Brown TOOGOO X6wRqS8nttunlinkErr) => {
    if (unlinkErr)Party Soft Gold Mini Crystal Wocharm bling Silver Pouch Diamante Wedding Clutch Evening Bridesmaid bride Body Purse Gold Prom girl women Engagement Cocktail Bag 7g55qxR 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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