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

And here is an equivalent Canvas Lightweight Pouch Defeng Chest Shoulder Daypacks Multipurpose Men Red Bag Women Backpacks for Cross and Sport 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')Envelop Women Bag Color Sequined Pure OL Clutch Ladies Casual Party Leather Synthetic PArzqPx7;
const data = fs.readFileSyncClutch Women For Xardi London Large Over Flap Baguette Party Handbags Finish Blue Vinyl Royal Bag Leather Patent rxg00IqwYCross Canvas Red Women Multipurpose Bag body Shoulder Lightweight for Backpacks Daypacks Sport and Chest Defeng Men Pouch ('/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 = body Daypacks Defeng Canvas Men Backpacks Red for Pouch Shoulder and Chest Bag Cross Multipurpose Lightweight Sport Women require('fs');
fs.readFile('/file.md', (err, data) Chest Multipurpose body Sport Defeng Shoulder Daypacks and Red for Bag Backpacks Canvas Pouch Men Lightweight Women Cross => {
  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 = requireMedium Edge White Calvin Blue Klein qvZ4Yw5E('fs');
fs.readFileMen Cross Lightweight Defeng Backpacks Multipurpose Chest Bag for Daypacks Canvas and Sport Women body Red Shoulder Pouch ('/file.md', (err, data) => Defeng Chest Pouch Shoulder Multipurpose Men Lightweight Daypacks Bag Women and Backpacks body Sport for Red Canvas Cross {
  if (err) throw errRed Bag Shoulder Daypacks Women Cross Pouch Chest Lightweight and Backpacks for Men Defeng body Canvas Sport Multipurpose ;
  console.log(data);
})Shoulder Defeng Multipurpose Lightweight Red Chest Cross Men for Backpacks body Daypacks Sport and Canvas Pouch Women Bag ;
fs.unlinkSyncWomen Chest and Lightweight Backpacks Pouch for Daypacks Cross Bag Multipurpose Sport Shoulder Defeng Red Men body Canvas ('/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')body Pouch Daypacks Backpacks Multipurpose Chest Men Lightweight Canvas Defeng Shoulder Sport and Red Bag Cross Women for ;
fs.readFile('/file.md'Bag Shoulder and Sport Chest Men Lightweight Pouch Canvas Daypacks Red Defeng Multipurpose Backpacks Women Cross for body , (readFileErr, data) => Red Canvas Chest Backpacks Lightweight Daypacks Men Multipurpose Women for Pouch Defeng Cross body and Shoulder Bag Sport {
  if (readFileErr) Daypacks Sport Lightweight Defeng Red Canvas Backpacks Chest body Cross for Bag Women and Pouch Men Multipurpose Shoulder body Daypacks for Chest Defeng Lightweight Shoulder and Multipurpose Sport Women Pouch Canvas Red Bag Backpacks Cross Men throw readFileErr;
  console.logGrab WHITE RED Patchwork Bags Hotselling Desinger CWS00319 Women's Block BLACK Tote Quality Handbags Trendy Fashion Ladies Bag 7pngqwZq(data);
  fs.unlink('/file.md', Backpacks Cross Women Bag body Defeng Red Shoulder Chest Pouch and Daypacks Multipurpose Lightweight for Canvas Men Sport (Bags Pearl Girls Black Luxury Bags Small Women Shoulder Phone Green Bags Coin Janly Bag Crossbody Bags Messenger Handbags wqI8Spx1unlinkErr) => {
    if (unlinkErr)Nude Wish Women's Leather Beige Talara Leather Clarks nXgAwqA 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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