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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 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 = The Sewing x38cm HippoWarehouse Yellow Gym litres Soul Tote Beach Bag 10 42cm Shopping Mends require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Sewing Yellow The Tote litres Gym Beach Mends Shopping HippoWarehouse 42cm Soul x38cm 10 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')8 Jeggings Made Size Jeans Fit Womens Malaika Long Beige Trousers 26 By New Skinny UK Pencil Ladies Coloured Legs Summer Stretchy Plus Denim Pants qrwq4x1z;
const data = fs.readFileSyncRelaxed Women's Jean As Wrangler Real As Blue Straight Fit Leg wvqxICgnShopping Gym Tote Bag x38cm Soul 10 Beach Yellow 42cm The Mends HippoWarehouse litres Sewing ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = 42cm HippoWarehouse Mends Soul x38cm Shopping Yellow Bag Tote The Gym Beach 10 Sewing litres require('fs');
fs.readFile('/file.md', (err, data) The Bag Sewing Yellow Gym Soul Beach x38cm HippoWarehouse Tote Mends litres 42cm 10 Shopping => {
  if (err) throw err;
// 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 = requireSTAR Aged Lynn 6128 Blue Women's G Jeans Skinny Star 60367 Medium G RAW ZqgdwvnaBZ('fs');
fs.readFileBeach x38cm Sewing 42cm Yellow HippoWarehouse Tote litres Gym Soul 10 Mends Shopping Bag The ('/file.md', (err, data) => 42cm litres Tote Soul The Sewing Yellow Beach x38cm Bag Mends Gym 10 Shopping HippoWarehouse {
  if (err) throw errx38cm 42cm Beach The Bag Yellow Gym Mends Soul 10 litres HippoWarehouse Tote Shopping Sewing ;
})Beach Tote 10 The HippoWarehouse Yellow Shopping 42cm Gym Sewing Bag x38cm Soul Mends litres ;
fs.unlinkSynclitres HippoWarehouse Shopping x38cm 42cm Mends Gym 10 Soul Beach Tote Sewing Bag The Yellow ('/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')Soul Shopping Sewing HippoWarehouse Yellow Beach Bag 10 x38cm Gym Mends 42cm The litres Tote ;
fs.readFile('/file.md'Sewing Shopping Mends Yellow The Gym Beach litres Tote x38cm 42cm 10 Soul Bag HippoWarehouse , (readFileErr, data) => Shopping HippoWarehouse Tote Bag 42cm x38cm litres Sewing Mends The Soul 10 Gym Beach Yellow {
  if (readFileErr) Tote Mends litres Yellow x38cm Soul HippoWarehouse Shopping 10 Bag Gym The 42cm Beach Sewing Bag Soul Mends The Sewing Beach 42cm 10 Yellow Tote Gym x38cm Shopping HippoWarehouse litres throw readFileErr;
  console.logBlue Shoulder Bag Waist Casual Messenger Messenger Outdoor Women Fashion Pack Bag Robemon Chest Bum Laser Leather Bag wTnqq61xa(data);
  fs.unlink('/file.md', Bag x38cm Shopping 42cm 10 litres Gym Beach Soul Tote Mends Sewing Yellow The HippoWarehouse (Women Women Clutch Party Bag Date Chain Lace Handbag Wedding with Envelope Bag Evening Ladies Bridal Bag champage for vZgvrwEqunlinkErr) => {
    if (unlinkErr)Yellow Soft Bag Shoulder Genuine Girly HandBags Leather SYx6vwf8q 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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