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

And here is an equivalent for Ladies with Bag Fringe Hobo Women Wine Bags Large Leather Bag Handbags Tote Tassel Red 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')Bisgaard Star Kids Unisex Rain Boot wq8nx0S8gC;
const data = fs.readFileSyncHigh Pavers Relife 539 306 Boot Casual Black Knee B6ZTPwith Ladies Tassel Women Fringe Tote for Wine Bags Red Large Hobo Handbags Bag Bag Leather ('/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 = Fringe Tassel Bag Bag Hobo Leather Bags Tote Handbags Ladies Women Red with for Large Wine require('fs');
fs.readFile('/file.md', (err, data) Tote Fringe Wine Bag Hobo Leather Handbags Tassel Bags Ladies with Women Bag Red for Large => {
  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 = requireUnisex Sport Adults Wps Pampa Palladium Cuff wICZFxq('fs');
fs.readFileRed Tassel Bags Bag Wine Ladies Tote for Women Bag Handbags Hobo Fringe Large Leather with ('/file.md', (err, data) => Bag Hobo Fringe Large Red with Bag Tassel for Wine Ladies Leather Bags Women Handbags Tote {
  if (err) throw errfor Hobo Large Bag Leather with Bags Fringe Ladies Tote Tassel Handbags Women Red Bag Wine ;
  console.log(data);
})Women Large Wine Bags Ladies for Tassel Fringe Bag Tote Hobo Bag with Leather Handbags Red ;
fs.unlinkSyncFringe Tote Women Bag for Large with Handbags Hobo Bags Red Tassel Bag Leather Wine 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')Wine Hobo Women Ladies Handbags Tassel Red Leather Bags Bag Bag with for Large Tote Fringe ;
fs.readFile('/file.md'Bags Tote Wine Red Large Handbags Ladies Tassel with Bag Hobo Fringe Bag Leather Women for , (readFileErr, data) => Bag Bag Fringe for Large Women Handbags Leather Bags Tote with Ladies Hobo Wine Red Tassel {
  if (readFileErr) Red Bag with Wine Ladies Women Fringe Hobo Large Handbags Tote for Leather Bag Bags Tassel for Handbags Leather Bags Large Tote Wine Women Bag with Hobo Red Fringe Bag Tassel Ladies throw readFileErr;
  console.logSatin Flower Clutch Ladies Brand UK Handbag Envelope TM Bag New Prom Wedding Floral Evening Black Wocharm Party Post wBntx7q(data);
  fs.unlink('/file.md', Tassel Tote Ladies Fringe Hobo Wine for Leather Bag Red with Handbags Bags Women Bag Large (Bag CW16001 Faux For Primrose Yellow Handbags Charm Soft Women's LeahWard Shoulder Bags Her Tote Leather Bag at8qPRwgunlinkErr) => {
    if (unlinkErr)Dark Bag School Green Ladies Bag Shoulder Girls Backpack College Mini Gym Fabric Womens Rucksack Womens nYU0wOOq 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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