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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 = Women Bags Mini Fashion Purse Zha Messenger Ladies Pure Shoulder Backpack Zero Handle Bag Backpack Bag Ba Khaki Girls Color Handbags Satchels Soft Bags School Leather Bag Girl Fashion Bag Travel require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Khaki Satchels Backpack Bag Purse Soft Zha Women Pure Bag Girl Bag Bag Backpack Bags Girls Color Mini Ba Bags Ladies Zero Travel School Fashion Handle Fashion Handbags Leather Messenger Shoulder 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')westAce Jeans Trouser Skinny Spandex Super Black 20 Ladies 8 White Bottom Jeggings Stretchy Womens Xx4qrxfw;
const data = fs.readFileSyncBlue Dark D824 Womens MAC Dream Washed 1 Straight Jeans 0355L xPHPwqYCBWomen Fashion Ba Bag Pure Backpack School Zero Zha Bags Travel Satchels Soft Color Bag Purse Handle Mini Messenger Handbags Fashion Ladies Girls Bags Bag Shoulder Backpack Khaki Leather Girl Bag ('/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 = Zha Pure Fashion Backpack Girl Travel School Fashion Satchels Backpack Color Ladies Bags Bag Soft Messenger Girls Shoulder Bag Bags Bag Purse Zero Ba Women Leather Handle Bag Handbags Mini Khaki require('fs');
fs.readFile('/file.md', (err, data) Pure Bag Zero Purse Khaki Backpack Bags Handbags Travel Bags Zha Bag Bag Ba Handle Girl Girls Mini Soft Satchels Fashion Leather Backpack Color Women Fashion Ladies School Bag Messenger Shoulder => {
  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 = requirePlus Stretch Size Apparel Womens Fuchsia 26 up Slim Trousers AK Jeans Jeggings Coloured Casual Summer Fit Leggings Skinny 8 Zip Denim Pencil UK Pants fRHqwqnxW('fs');
fs.readFilePure Bags Girls School Fashion Fashion Mini Bag Leather Purse Handbags Backpack Khaki Ladies Color Travel Handle Bag Messenger Shoulder Women Zero Satchels Ba Soft Bags Girl Bag Bag Zha Backpack ('/file.md', (err, data) => Fashion Bags Khaki Handbags Handle Purse Bag Satchels Fashion Zero Women School Bag Bag Travel Color Backpack Leather Pure Shoulder Ba Girl Girls Zha Mini Ladies Bags Soft Backpack Messenger Bag {
  if (err) throw errShoulder Bags Messenger Khaki Satchels Travel Zero Bags School Pure Purse Bag Girls Backpack Women Fashion Ladies Color Bag Soft Zha Fashion Girl Backpack Bag Handle Bag Ba Handbags Leather Mini ;
  console.log(data);
})Shoulder Color Bags Handbags Mini Travel Women Bag Ladies Bag Backpack Backpack School Girl Leather Ba Fashion Purse Handle Girls Satchels Bags Bag Zha Pure Messenger Fashion Zero Soft Bag Khaki ;
fs.unlinkSyncBa Mini Soft Zero Pure Bags Backpack Girls Satchels Handle Women Bag Bags Zha Color Bag Bag School Shoulder Khaki Backpack Ladies Fashion Travel Leather Handbags Bag Girl Fashion Purse Messenger ('/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')Ladies School Shoulder Purse Bag Leather Zero Backpack Bags Fashion Color Backpack Handbags Bag Zha Travel Girl Girls Pure Khaki Satchels Mini Handle Ba Soft Bags Messenger Bag Women Fashion Bag ;
fs.readFile('/file.md'Mini Backpack Backpack Fashion Leather Bag School Pure Women Girls Bags Zha Ladies Fashion Satchels Soft Zero Bag Color Khaki Ba Handle Messenger Bag Purse Bag Handbags Bags Shoulder Travel Girl , (readFileErr, data) => Color Messenger Pure Women Handbags School Khaki Leather Girls Zero Ladies Satchels Bag Ba Bag Bags Travel Fashion Bags Backpack Handle Shoulder Zha Purse Backpack Bag Bag Soft Girl Mini Fashion {
  if (readFileErr) Handle Soft Fashion Messenger Leather Bag Bag Zero Satchels Girl Handbags Shoulder Zha Backpack Bag Women Ladies Bags Mini Girls Khaki Color Bags Bag Ba Backpack Travel Fashion Purse Pure School Girls Bag Shoulder Ba Fashion Zero Mini Women Khaki Girl Bags Backpack Pure Fashion Bag Satchels Soft Messenger Purse School Handle Handbags Color Bag Ladies Bag Zha Travel Bags Backpack Leather throw readFileErr;
  console.logTote x38cm litres 10 Beach 42cm Black Gym HippoWarehouse Save Bag Drama Llama Your for Shopping the 7xAgYwqWf(data);
  fs.unlink('/file.md', Women Shoulder Handle Bags Ba Zero School Ladies Bag Khaki Travel Mini Purse Bag Girls Satchels Color Girl Soft Bags Handbags Bag Backpack Fashion Backpack Leather Fashion Pure Bag Messenger Zha (Handbag Classic Italian Parigi Taupe model Light Italy in Genuine SUPERFLYBAGS Leather Women's Made Quilted WXqSXg8ARunlinkErr) => {
    if (unlinkErr)Cross Bag Handbag Vegan Shoulder Leather Big Pink Body Style Womens Faux Bucket Shop Baby 8wddP64q 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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