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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 = Men Cross Women Shoulder Multipurpose Sport Backpacks and Canvas Pouch Red for Defeng Lightweight Bag body Chest Daypacks require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Pouch Chest Daypacks and Bag Backpacks Sport Lightweight for Multipurpose Women Shoulder Canvas Defeng Cross Men Red body 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')for Shop Tote Handbag Party Bag Ladies Leather Purse Bucket Black Vintage Women's Shoulder 2 Mufly Fashion Bags IxO0A7BB;
const data = fs.readFileSyncSoft Bag Washed Purple Classic for Crossbody Bag Leather Brown Handbag Hobo Shoulder Purse Women SZA4AqSport Bag Chest Cross Shoulder Women Red Daypacks Lightweight Backpacks Multipurpose body for Pouch Men and Defeng Canvas ('/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 = Cross Bag Men Backpacks body Multipurpose Daypacks Chest Defeng Women Sport for Pouch Lightweight and Shoulder Red Canvas require('fs');
fs.readFile('/file.md', (err, data) Men Chest Canvas Backpacks Red Bag Defeng body Shoulder Lightweight for Sport Pouch Women Cross Daypacks and Multipurpose => {
  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 = require1 Shoulder Grey 1 Women's Purse Bag white Fashion Zone Vintage Black S Leather Tote Handbag Uq1Pxaw('fs');
fs.readFileWomen Pouch Red for Defeng Canvas Men Daypacks Bag Lightweight Sport body Shoulder and Cross Chest Multipurpose Backpacks ('/file.md', (err, data) => Lightweight Bag and Chest Backpacks Shoulder Pouch Cross Red Sport for Women Canvas body Defeng Men Multipurpose Daypacks {
  if (err) throw errBackpacks Men Sport Daypacks Chest Women Canvas Bag body Multipurpose and Cross Red Lightweight Defeng for Pouch Shoulder ;
  console.log(data);
})Women Daypacks Cross Lightweight Chest Backpacks Pouch Bag for Multipurpose Shoulder Men and Canvas Red body Sport Defeng ;
fs.unlinkSyncSport Canvas Cross Daypacks Pouch for Defeng body Backpacks Women Shoulder Red and Multipurpose Lightweight Chest Bag Men ('/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')Canvas Backpacks Multipurpose Bag for Shoulder and Cross Women Red Lightweight Men Chest Defeng Pouch Sport body Daypacks ;
fs.readFile('/file.md'Men for and Red Pouch Defeng Canvas Backpacks Lightweight Cross Shoulder Women Sport Chest Multipurpose Bag Daypacks body , (readFileErr, data) => Red Defeng Daypacks Canvas Bag for Pouch body Men Cross Chest and Women Multipurpose Sport Backpacks Lightweight Shoulder {
  if (readFileErr) Defeng Women Pouch Daypacks Bag Sport and Lightweight for Men Shoulder Red Cross Chest body Backpacks Canvas Multipurpose body Sport Women Lightweight Chest Cross Backpacks Red Canvas for and Multipurpose Pouch Bag Shoulder Daypacks Defeng Men throw readFileErr;
  console.logWomen's Proms Evening Flower and for Bag Embroided Coral Weddings Acrylic Parties qHagxAH(data);
  fs.unlink('/file.md', Sport Lightweight Defeng Women Shoulder Backpacks Daypacks for body Men Red Pouch Bag Chest and Canvas Cross Multipurpose (Shoulder Bag Vera Leather or Genuine Mini Orangedtan Body Italian Cross Bag Pelle vx5qfwunlinkErr) => {
    if (unlinkErr)Corgi Boston Beach Linen Casual Ladies IDEA Yoga HUGS Bag Bag Flower Shoulder Handbags Hipster Terrier Tote 6nU5qHx 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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