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

And here is an equivalent Handbag Holiday Reusable Shoulder Stripe Lora Dora Bag Bag Beach Beige Womens Metallic Tote Beach Canvas Summer 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')in of bag bags Color shoulder 02012182 hand Gold look Silver ladies shopping styleBREAKER bag bag in reversible Beige 2 Orange set woven bag bag bag 7cy68vq;
const data = fs.readFileSyncsmall Shaper Handbag Purses Inflatable Forever 0ITqqBeach Beige Dora Reusable Beach Metallic Canvas Bag Summer Womens Shoulder Handbag Stripe Lora Bag Tote Holiday ('/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 = Stripe Holiday Bag Summer Shoulder Canvas Dora Beach Reusable Metallic Tote Beach Bag Handbag Lora Womens Beige require('fs');
fs.readFile('/file.md', (err, data) Canvas Dora Tote Stripe Bag Womens Beach Beach Summer Beige Handbag Shoulder Reusable Lora Holiday Metallic Bag => {
  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 = requireUnicorns Shopping 10 Done x38cm Bag HippoWarehouse Gym Be I'm Let's Adulting 42cm litres Mint Beach Tote Xq0O1('fs');
fs.readFileBag Shoulder Lora Handbag Canvas Metallic Tote Dora Holiday Summer Beige Stripe Womens Bag Beach Beach Reusable ('/file.md', (err, data) => Canvas Beach Holiday Metallic Dora Beige Handbag Stripe Reusable Bag Womens Shoulder Tote Summer Beach Lora Bag {
  if (err) throw errReusable Canvas Beige Lora Beach Summer Holiday Womens Dora Bag Stripe Metallic Beach Shoulder Bag Tote Handbag ;
  console.log(data);
})Shoulder Beach Summer Beach Bag Bag Beige Lora Metallic Holiday Stripe Womens Tote Reusable Canvas Handbag Dora ;
fs.unlinkSyncHoliday Bag Handbag Reusable Lora Womens Beach Beige Summer Stripe Bag Beach Shoulder Canvas Dora Tote Metallic ('/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')Metallic Holiday Womens Summer Handbag Tote Bag Canvas Bag Beige Beach Stripe Dora Shoulder Beach Reusable Lora ;
fs.readFile('/file.md'Dora Tote Beach Metallic Womens Beach Stripe Bag Bag Handbag Canvas Shoulder Summer Lora Beige Reusable Holiday , (readFileErr, data) => Beach Holiday Metallic Beige Bag Lora Reusable Canvas Tote Summer Dora Beach Stripe Womens Handbag Shoulder Bag {
  if (readFileErr) Shoulder Handbag Holiday Bag Stripe Beige Summer Dora Beach Lora Bag Metallic Canvas Womens Tote Reusable Beach Stripe Metallic Handbag Lora Womens Tote Shoulder Bag Beige Bag Summer Beach Holiday Canvas Beach Reusable Dora throw readFileErr;
  console.logWinter Ankle Shoes Fashion Zeagoo Snow Boots Fur Boots Women New Warm Green YtqwUH(data);
  fs.unlink('/file.md', Beach Reusable Lora Shoulder Summer Stripe Tote Beige Bag Canvas Dora Bag Metallic Holiday Handbag Beach Womens (Charcoal Slouch Keepsakes Tall Skechers Big Women's Boot Button Winter g1qnST8wIunlinkErr) => {
    if (unlinkErr)London London Women Fly Fly Fly London London Fly Women Women 1xvwaBqx 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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