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

And here is an equivalent Bag Beige Metallic Womens Canvas Bag Reusable Beach Lora Summer Holiday Beach Stripe Tote Dora Handbag 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')Like x38cm Bag 10 A I Beach Burgundy HippoWarehouse Tote Shopping Horse Kick Gym 42cm litres PzqWWndE;
const data = fs.readFileSyncHistorian Shopping 10 HippoWarehouse Gym 42cm Tote Superpower Bag Beach A What's Your x38cm litres Black I'm qwwE40x1Lora Holiday Shoulder Stripe Summer Beach Canvas Beige Reusable Bag Tote Womens Handbag Bag Dora Metallic Beach ('/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 = Tote Beach Beige Handbag Stripe Summer Beach Lora Holiday Bag Metallic Dora Bag Shoulder Reusable Womens Canvas require('fs');
fs.readFile('/file.md', (err, data) Beach Lora Metallic Beach Handbag Holiday Beige Bag Reusable Womens Canvas Shoulder Stripe Bag Summer Tote Dora => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireShoulder Yuan Crossbody Leather Women Messenger Bag Rivet Clearance Handbag Bags Pink Chain xgxAzrY('fs');
fs.readFileShoulder Beige Bag Handbag Womens Tote Beach Bag Metallic Stripe Beach Lora Reusable Canvas Holiday Summer Dora ('/file.md', (err, data) => Beach Womens Holiday Metallic Summer Reusable Bag Handbag Stripe Tote Canvas Beach Dora Shoulder Beige Bag Lora {
  if (err) throw errHoliday Stripe Dora Lora Bag Handbag Canvas Bag Tote Metallic Womens Shoulder Beach Summer Reusable Beige Beach ;
  console.log(data);
})Beach Bag Canvas Beige Metallic Shoulder Bag Handbag Summer Dora Lora Holiday Tote Womens Beach Stripe Reusable ;
fs.unlinkSyncBeach Handbag Dora Stripe Beach Womens Metallic Lora Bag Summer Holiday Tote Bag Reusable Shoulder Canvas Beige ('/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')Dora Womens Holiday Canvas Stripe Handbag Beige Beach Bag Summer Bag Beach Reusable Metallic Tote Lora Shoulder ;
fs.readFile('/file.md'Beach Stripe Reusable Womens Summer Handbag Bag Dora Metallic Beach Tote Shoulder Holiday Beige Canvas Bag Lora , (readFileErr, data) => Dora Bag Lora Metallic Beach Canvas Beach Holiday Womens Handbag Reusable Bag Tote Summer Stripe Shoulder Beige {
  if (readFileErr) Tote Lora Womens Canvas Beige Metallic Stripe Summer Shoulder Dora Bag Handbag Beach Bag Beach Holiday Reusable Lora Beach Womens Dora Canvas Tote Beach Bag Handbag Summer Reusable Shoulder Beige Metallic Holiday Stripe Bag throw readFileErr;
  console.logTote Handbag Crossbody Bucket Bag Red Women Shoulder LILYYONG Bag 2 Bag Bag qBx8agXw(data);
  fs.unlink('/file.md', Summer Beach Handbag Lora Shoulder Canvas Tote Stripe Womens Holiday Bag Beige Dora Metallic Beach Bag Reusable (Beige Handbag Straw Straw Style Holiday Beach Women Leisial Purse Bag Travel Summer Bohemia Use Everyday Clutch Weave Women Envelope for Crossbody Beach Bag BqqUdzwXunlinkErr) => {
    if (unlinkErr)Shape Party Evening Yoome Girls Bag Chain Shoulder Cylinder Clutch Silver Metal Womens Mesh Handbag Cage qqAvwt 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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