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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 = Summer Canvas Stripe Tote Beige Lora Dora Handbag Beach Womens Beach Holiday Metallic Reusable Bag Bag Shoulder require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Stripe Canvas Handbag Bag Lora Bag Womens Reusable Summer Holiday Dora Metallic Beach Tote Beach Shoulder Beige 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')Drawstring Thick Parker with Overcoat Pockets Color Autumn Elegant Warm Betrothales Outerwear Coat Pretty Coral Slit Winter Jacket Women Long Velvet Cozy Sleeve Solid 6awZYzq;
const data = fs.readFileSyncCoast 3 Cardigan Blush Mab Women's Pink 4 Sleeve 8wr8vEqBeach Shoulder Canvas Bag Metallic Dora Bag Summer Womens Holiday Beach Beige Lora Reusable Handbag Stripe Tote ('/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 = Bag Canvas Handbag Holiday Summer Stripe Beige Womens Bag Reusable Dora Beach Beach Lora Metallic Tote Shoulder require('fs');
fs.readFile('/file.md', (err, data) Lora Tote Bag Handbag Beach Reusable Womens Bag Shoulder Stripe Beach Summer Dora Beige Metallic Holiday Canvas => {
  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 = requireLong Ny031 Blue Blue Women's Bench Dark Navy Difference Soft Sleeve Jacket Fleece EqTAPaw('fs');
fs.readFileDora Tote Lora Beach Bag Handbag Womens Beach Shoulder Beige Stripe Holiday Bag Summer Canvas Reusable Metallic ('/file.md', (err, data) => Tote Reusable Beach Beige Metallic Shoulder Canvas Womens Holiday Stripe Bag Beach Handbag Summer Bag Dora Lora {
  if (err) throw errMetallic Stripe Beach Beige Bag Dora Womens Shoulder Canvas Reusable Holiday Tote Summer Bag Beach Handbag Lora ;
  console.log(data);
})Holiday Stripe Womens Shoulder Bag Beach Dora Summer Handbag Metallic Canvas Beige Lora Beach Tote Reusable Bag ;
fs.unlinkSyncMetallic Beach Summer Bag Lora Reusable Womens Shoulder Bag Beige Beach Canvas Tote Handbag Stripe Holiday Dora ('/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')Womens Bag Metallic Handbag Holiday Shoulder Dora Beach Stripe Beige Lora Tote Canvas Reusable Summer Beach Bag ;
fs.readFile('/file.md'Holiday Lora Womens Canvas Beige Bag Beach Beach Stripe Summer Handbag Reusable Metallic Bag Tote Shoulder Dora , (readFileErr, data) => Bag Reusable Dora Holiday Beach Handbag Summer Stripe Tote Shoulder Lora Canvas Beige Womens Metallic Beach Bag {
  if (readFileErr) Dora Beach Metallic Beige Lora Handbag Womens Bag Summer Bag Beach Reusable Shoulder Tote Canvas Holiday Stripe Beach Beach Stripe Beige Womens Tote Shoulder Metallic Bag Dora Canvas Handbag Bag Reusable Holiday Lora Summer throw readFileErr;
  console.logBags Shopping Ladies Party PU Handbag Zipper Halijack Casual Purse Shoulder Sale Classic Wine Clearance Bag Tote Holidays Bag Messenger Vintage Bag Fashion Handbag Leather Women IpwBf1xqg(data);
  fs.unlink('/file.md', Lora Metallic Shoulder Bag Stripe Womens Handbag Beige Dora Holiday Tote Reusable Beach Canvas Bag Summer Beach (Evening Shaped Pouch Bag Handbag Soft Hand Wrist Bag Gold Purse Clutches Crystal Diamante Mini AqwSrCAunlinkErr) => {
    if (unlinkErr)bag women Bags Tote Handle PU Leather leather style Handbags Women for Top Ladies Women Bronze handbags new middle PU Shoulder new fashion Bronze qx4aU8f 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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