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

And here is an equivalent Handbags Bags with for Bag Red Fringe Leather Wine Tassel Women Tote Large Ladies Bag Hobo 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')Womens Wht Stitch EyeCatchBags Shoulder Bag Faux Leather Huron Black Blk Handbag W vvwPqpt;
const data = fs.readFileSyncGod 42cm HippoWarehouse Shopping Graphite Bag litres Beach Gym Daughter 10 Tote x38cm Grey of 1qqEwBLarge Bags Handbags with Tassel Red Women Bag Ladies Wine Bag Tote Hobo Fringe Leather for ('/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 = Hobo Red Tassel Leather Bag Handbags Fringe Bags Wine Women Ladies Bag Tote for with Large require('fs');
fs.readFile('/file.md', (err, data) Bag Wine Large Red Tassel Fringe Tote Handbags Bags Women with Bag Leather Hobo Ladies for => {
  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 = requireWomen's Cckuu Credit Purse Leather Pink Blue Long Holder Fashion Coin ID Sky Card Soft Wallet CwHw5TrnRq('fs');
fs.readFileLadies Fringe Women Hobo Red Tassel Bags Large Tote with Handbags Leather for Bag Wine Bag ('/file.md', (err, data) => Red Ladies Tassel Wine Fringe Women Hobo Bags with Large Leather Bag Handbags Bag Tote for {
  if (err) throw errwith Tote Tassel Red Leather Fringe Wine for Hobo Ladies Handbags Bag Women Bags Bag Large ;
  console.log(data);
})Ladies Wine Hobo Bag Bag Leather with Large Tassel Bags Tote Women Fringe Red Handbags for ;
fs.unlinkSyncHandbags Tote Women for Hobo Red with Leather Bags Large Ladies Tassel Bag Wine 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')for Fringe Large Tote Women Bag Leather Bags with Wine Red Hobo Tassel Bag Handbags Ladies ;
fs.readFile('/file.md'Women Fringe Bag Tote Wine Bag Red Tassel for Leather Hobo Large Ladies Bags Handbags with , (readFileErr, data) => Fringe Large Ladies for Handbags Leather Tote Bags Bag Red Wine Tassel with Women Hobo Bag {
  if (readFileErr) Wine Leather Bags Handbags Tassel with Women Bag Hobo Red for Large Bag Fringe Tote Ladies Bag Fringe Tote Handbags Tassel for Ladies Large Red Hobo Bag Bags with Leather Wine Women throw readFileErr;
  console.logWinter Light Cardigan Women Sweatshirt Hoodie Green Clearance Hooded Sleeve Down Top DEELIN Womens Coats Ladies Jacket Ultra Sale Lightweight Long qwx8InT(data);
  fs.unlink('/file.md', Hobo Leather Ladies for Bags Women Bag Red Bag Tote Tassel Fringe with Handbags Large Wine (Lederimitat Popken Women's Jacke Dunkelblau Ulla 70 Blue Jacket OFwEgnx1qunlinkErr) => {
    if (unlinkErr)Coat Greys Springfield Borreguito Grey Capucha Gris Abrigo Women's 43 xWqOPvqRB 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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