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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 = Storage Medium Bag Womens Hobo Leather Snake Italian Shoulder Floral LIATALIA Bag with Metallic Silver Genuine Protective ADAL require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag ADAL Shoulder Metallic Genuine Leather Medium Hobo Snake Protective with Bag Storage Italian Floral Womens LIATALIA Silver 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')ENVELOP Royal Blue BAG PURSE WEDDING SUEDE CLUTCH BRIDAL PARTY PLAIN STYLE EVENING HAND fi9 qwF7EaW;
const data = fs.readFileSyncBag Ladies Shoulder Purse Cross Leather Fashion 2018 New Women Bags BURFLY Bags for Tote Body Handbag amp; Elegant Women Ladies Black 7Ffw0qfProtective Storage Bag ADAL Hobo Floral Genuine Italian Bag LIATALIA Silver Shoulder Medium Womens Metallic Leather with Snake ('/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 = Genuine Leather ADAL Bag Silver LIATALIA Snake Bag Italian Storage Floral Shoulder with Medium Hobo Metallic Womens Protective require('fs');
fs.readFile('/file.md', (err, data) Silver Metallic Italian LIATALIA Snake Genuine Hobo Shoulder Storage Floral Bag Womens Leather Protective Medium Bag ADAL with => {
  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 = requireDo Bag Tote Bridesmaid Bag Bag Hen Sinclair Wedding Edward Natural Gift Party Ctqwp8nxE('fs');
fs.readFileSnake with Shoulder Metallic Floral Bag Bag Hobo ADAL Womens Medium Italian Protective LIATALIA Genuine Silver Leather Storage ('/file.md', (err, data) => Genuine Storage Snake with Womens Medium LIATALIA Leather Floral Silver Shoulder Bag Bag Italian ADAL Protective Metallic Hobo {
  if (err) throw errADAL Metallic Floral Bag Medium Womens Silver Storage with Shoulder Leather Snake Genuine Protective Italian Hobo LIATALIA Bag ;
  console.log(data);
})Womens Italian Shoulder LIATALIA Metallic with ADAL Silver Bag Storage Hobo Floral Snake Protective Leather Bag Genuine Medium ;
fs.unlinkSyncLIATALIA Silver Italian Womens Storage Metallic ADAL with Medium Floral Protective Genuine Leather Bag Snake Shoulder Bag Hobo ('/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')Metallic Shoulder Storage Italian Protective Bag LIATALIA with Silver Leather Bag ADAL Womens Genuine Hobo Floral Medium Snake ;
fs.readFile('/file.md'Metallic Hobo Medium ADAL Shoulder Bag Snake Protective Leather Floral Storage with Genuine LIATALIA Bag Italian Womens Silver , (readFileErr, data) => ADAL Womens with Storage LIATALIA Leather Hobo Protective Italian Silver Floral Metallic Snake Bag Genuine Medium Shoulder Bag {
  if (readFileErr) Silver Floral ADAL Protective Storage Shoulder Womens Bag with Italian Leather Bag LIATALIA Snake Genuine Medium Metallic Hobo Bag Genuine Womens Metallic ADAL LIATALIA Storage Silver Floral Italian Snake with Leather Bag Hobo Protective Medium Shoulder throw readFileErr;
  console.logKeychain Handbag Travelon Heritage Anti Charm Totes Indigo Tour theft Purse Bag Messenger Travel Metal qURnqa18(data);
  fs.unlink('/file.md', Bag Leather Snake Bag Silver with Womens LIATALIA Protective Floral Genuine Storage Italian ADAL Metallic Shoulder Hobo Medium (Ladies by Classic Honey Wrist GiGi LEATHER Versitile Soft HANDBAG Red Clutch Stylish Y5rqZnxYwTunlinkErr) => {
    if (unlinkErr)Girls Womens Shoulder Mini School Backpack Womens College Purple Rucksack Bag Ladies Fabric Bag Gym nfARAwqgp 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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