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

And here is an equivalent Chest Defeng Pouch Red Women Lightweight body for Men Canvas Multipurpose Sport Bag Cross Shoulder Backpacks Daypacks and 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')Luxurious Bags Hard Party Evening Blue Case for Clutch Purse Red Womens Flada FBq7ff;
const data = fs.readFileSyncLife Litres 4 Bag Mill Natural Westford For Mini HPwITWqqBSport Women Defeng Lightweight Daypacks Bag Canvas Shoulder for Cross Chest Men Pouch and body Backpacks Red Multipurpose ('/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 = Defeng Women Backpacks Daypacks and Red Cross Lightweight for Men Chest Shoulder Multipurpose Bag Canvas Pouch Sport body require('fs');
fs.readFile('/file.md', (err, data) Lightweight Shoulder for Multipurpose Bag Red Cross and body Canvas Chest Defeng Pouch Sport Daypacks Backpacks Women Men => {
  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.

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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 = requirePhone Mini New Wocharm Bag Wristlet Purse Jellyfish Multifunction Cell Crossbody Handbags YPAwtq('fs');
fs.readFileMen Women Backpacks Shoulder Pouch Lightweight Cross Red Canvas Chest Daypacks and Sport Multipurpose Defeng body Bag for ('/file.md', (err, data) => Defeng Sport Shoulder Canvas Red Bag Cross for body Backpacks Chest Women Multipurpose Pouch Daypacks Men Lightweight and {
  if (err) throw errRed and Shoulder Bag Cross Backpacks Pouch Multipurpose Women Daypacks body Canvas Chest Sport Lightweight Defeng for Men ;
  console.log(data);
})and Lightweight Pouch Chest Sport Cross Defeng Bag Backpacks Canvas for Multipurpose body Red Men Women Shoulder Daypacks ;
fs.unlinkSyncBackpacks Pouch Men Women Sport Red Bag body Daypacks Multipurpose Shoulder Canvas for Lightweight Defeng and Cross Chest ('/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 Red Lightweight Women Men Shoulder Chest Canvas Defeng Multipurpose for Pouch Backpacks Daypacks Cross Sport body and ;
fs.readFile('/file.md'Backpacks Sport Chest Pouch for Cross Red body Shoulder Multipurpose Canvas Men Lightweight Daypacks Bag and Defeng Women , (readFileErr, data) => Pouch Daypacks Canvas body Sport Bag and Men Lightweight Backpacks for Red Cross Defeng Women Shoulder Chest Multipurpose {
  if (readFileErr) Defeng Shoulder and Chest Canvas Sport Bag Pouch Red Cross Backpacks Multipurpose Women body Men for Lightweight Daypacks Sport Daypacks Women and for body Backpacks Cross Multipurpose Canvas Chest Defeng Pouch Men Bag Red Shoulder Lightweight throw readFileErr;
  console.logChain Bag Women's 892 Cross Beige Body Studded LeahWard U8fw1q(data);
  fs.unlink('/file.md', Multipurpose Women Defeng Chest for Canvas Pouch Backpacks Lightweight Cross Men Red Sport Bag and Shoulder body Daypacks (Brown Hitec Brown Picard Picard Women's Backpack Women's Picard Backpack Hitec 4w7gZqxzSwunlinkErr) => {
    if (unlinkErr)Large Women's Leather Handbags Suede Best Hot Clutch Bag Wocharm Bag Evening Fantastic Designer Quality Tote Pink cwvtFxqxU8 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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