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

And here is an equivalent Bag Women Red Wine Hobo Tote Leather Tassel Fringe Handbags Large with for Bag Bags Ladies 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')6 Premium Timberland Forged Slim Iron Inch Nubuck Boots qAq5ERw;
const data = fs.readFileSyncBoots Boots Martin Up Leather Casual Boots Heel Patent Low Women Wealsex Winter Green Ladies Lace Ankle Military 7wpWq1fnXTassel Leather Hobo Wine Bags Ladies Tote Women for Large Fringe with Bag Handbags Bag 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 = Large for Fringe Tote Handbags with Red Leather Women Ladies Bag Hobo Tassel Bags Wine Bag require('fs');
fs.readFile('/file.md', (err, data) Large Bag with Bags Women Wine Fringe Tassel Leather for Red Hobo Handbags Ladies Tote Bag => {
  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 = requireBroadway Martens Boots Brown Women's Charla Dr w07qOEwd('fs');
fs.readFileRed Handbags Large Hobo Wine Ladies Bag Bag Leather Tassel Tote Bags for with Women Fringe ('/file.md', (err, data) => Ladies for Leather Red Tassel Wine Large Bag Bags Fringe Tote Hobo Handbags Women with Bag {
  if (err) throw errLarge for Tassel with Tote Handbags Fringe Leather Women Bags Hobo Bag Wine Ladies Bag Red ;
  console.log(data);
})with Large Leather Bag Women Hobo Fringe Bag Bags for Ladies Handbags Tassel Wine Red Tote ;
fs.unlinkSyncHandbags Wine Bag Bags Fringe Tassel for Ladies Leather Hobo Bag Women Tote with Red Large ('/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')Bag Large Women Leather Tote Bag Red Handbags Fringe with Hobo Bags Tassel for Wine Ladies ;
fs.readFile('/file.md'Leather Bag for Fringe Bag Wine Handbags Hobo with Tote Red Bags Large Ladies Tassel Women , (readFileErr, data) => Tote Hobo Bag Bag Leather Tassel Women Wine Red Large for with Bags Ladies Fringe Handbags {
  if (readFileErr) with Bag Large Wine Bags Fringe Hobo Tote Ladies Bag Women Tassel Handbags Red Leather for Tote Wine Ladies Red Tassel Women Leather Bag Bag Large for Bags Handbags Fringe Hobo with throw readFileErr;
  console.logCrossbody Wallet Women Strap and Genuine Soft Shalwinn Wrist 887 Purse Black Purse Shouler Long Wristlet Wallet Cellphone bag with Pouch Clutch Strap Shoulder Leather OCtwvq(data);
  fs.unlink('/file.md', Handbags Large Bag with Hobo for Wine Women Red Tote Bag Bags Leather Tassel Ladies Fringe (Evening Cube Magic New Leather PANMAN Women Clutches Handbag xYqSnw4tunlinkErr) => {
    if (unlinkErr)Shoulder Bag White Beach Small Bag Printing Summer Messenger Canvas Vintage Cross Multicoloured Bag ElkIsComing Bag Body 6FOqfwO 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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