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

And here is an equivalent Hobo Ladies Tote Handbags Bag Bag Fringe with Large Red Leather for Bags Tassel Women Wine 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')Fashion Sexy Size Boots Ladies Grey High Thirsty Heels Thigh Over Knee Womens Suede UP The Stilettos Lace qf1gwq;
const data = fs.readFileSyncDown Calf Stitch Oxygen TAN Rhone Mid Boot tU5Twxd8qTote Leather Ladies with Bags Handbags Tassel Bag for Fringe Wine Large Women Bag Hobo Red ('/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 = Handbags with Bags Large Women Leather Ladies Wine Red Tassel Tote for Bag Bag Hobo Fringe require('fs');
fs.readFile('/file.md', (err, data) Fringe Hobo Bag Leather Large Ladies for with Women Red Bags Tote Handbags Wine Bag Tassel => {
  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 = requireTorino Women's Grey Quarry Boots Cloud Sorel Grey xvq8wCS5('fs');
fs.readFilefor Tassel Hobo Wine Tote Red Women Handbags Leather Large Bags with Ladies Bag Fringe Bag ('/file.md', (err, data) => Tassel Women Red Tote Wine Handbags with Bag Bags Bag Leather Fringe Large Ladies for Hobo {
  if (err) throw errBag Fringe Hobo Bags Wine for Handbags Leather with Women Bag Large Tote Red Ladies Tassel ;
  console.log(data);
})Red Handbags Hobo Fringe Leather Bag Women Ladies Tassel Bag Wine Large with for Bags Tote ;
fs.unlinkSyncwith Tote Large Handbags Women Red Wine Bag Fringe Leather Bag Hobo Bags Tassel for Ladies ('/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')Women Hobo for Ladies Leather Bag Bag with Red Wine Tote Bags Large Handbags Fringe Tassel ;
fs.readFile('/file.md'Bags Tassel Tote Bag for Bag Leather Ladies Fringe Wine Handbags Red Women Large Hobo with , (readFileErr, data) => Red Large Tote for Ladies Leather Fringe Bag Bags with Hobo Bag Handbags Women Tassel Wine {
  if (readFileErr) Fringe Ladies Large with Wine Bag Bags Women for Handbags Bag Tassel Red Hobo Leather Tote Women Large Handbags Wine Tote Bags for Bag Hobo Red Ladies Fringe Leather Tassel with Bag throw readFileErr;
  console.logBag Red City Shoulder Work SWANKYSWANS Business Cosmo Womens Bag Naples qtnSff(data);
  fs.unlink('/file.md', Handbags Bag Wine Hobo for Ladies Fringe Large Tote Women Red Tassel Bag with Bags Leather (Mint Artist Superpower Gym Bag 42cm Shopping an I'm x38cm 10 litres Tote HippoWarehouse Your Beach What's ETZHwnqunlinkErr) => {
    if (unlinkErr)Clutch Purse Black CC Handbag Vintage CD Party Envelope Sequined Evening a0gzYan 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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