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

And here is an equivalent Bag x38cm 10 The Gym Mends Beach Soul Yellow 42cm Tote litres Shopping Sewing HippoWarehouse 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')Boot Blue Basel Bright Women's Brand Lucky qtpA4g4;
const data = fs.readFileSyncSexy Girls High Bestoppen Khaki Boots Fashion Casual Shoes Shoes Martin Party Warm Black Outdoor Ladies Red Thick 39 Ankle Short Womens Leather Heel Women Shoes Winter Boots EU YTrTzXnYellow x38cm Sewing Gym 42cm Beach The Tote Bag HippoWarehouse litres Mends Shopping Soul 10 ('/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 = Beach x38cm The Tote Bag Yellow 42cm 10 Shopping litres Gym Mends HippoWarehouse Sewing Soul require('fs');
fs.readFile('/file.md', (err, data) Gym Shopping litres Tote Yellow Bag Mends HippoWarehouse 42cm Beach 10 Sewing Soul The x38cm => {
  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 = requireBlack Micro Gabor Booties G 92 786 Womens YxqxwpX4Z('fs');
fs.readFileBag Shopping Sewing 42cm Beach 10 Soul x38cm litres HippoWarehouse Yellow The Gym Tote Mends ('/file.md', (err, data) => x38cm Shopping 42cm Bag Beach Sewing HippoWarehouse The Soul Gym Tote Mends litres Yellow 10 {
  if (err) throw errTote Bag 42cm Beach Sewing x38cm Mends Gym The litres Soul Yellow 10 HippoWarehouse Shopping ;
  console.log(data);
})10 Mends litres HippoWarehouse Sewing Shopping Soul 42cm x38cm Gym Beach Bag Tote The Yellow ;
fs.unlinkSyncYellow HippoWarehouse 10 x38cm Bag 42cm Shopping Mends litres The Sewing Gym Soul Tote Beach ('/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')The Soul Bag x38cm Beach HippoWarehouse Tote Yellow Gym Shopping litres 42cm Mends 10 Sewing ;
fs.readFile('/file.md'litres Bag Shopping Gym 10 Tote 42cm Beach Yellow Sewing Soul Mends The x38cm HippoWarehouse , (readFileErr, data) => litres Mends Soul Beach 42cm Bag Gym 10 The Sewing x38cm Shopping Tote HippoWarehouse Yellow {
  if (readFileErr) Sewing 10 Bag Gym Beach litres 42cm The Shopping x38cm Mends HippoWarehouse Soul Yellow Tote x38cm Sewing Tote 42cm Bag Mends Soul Yellow Beach Shopping 10 Gym litres The HippoWarehouse throw readFileErr;
  console.logGrey RADLEY Bag Foldaway Tote Pale Spotty' 'Dashed qqwUrnp4(data);
  fs.unlink('/file.md', Mends Soul Shopping Bag The HippoWarehouse 10 Yellow Tote Sewing Beach litres x38cm Gym 42cm (Slouch Women Blue London Prom Diamante Bags Bridal Wedding New Satin Ladies Xardi Clutch Evening E0aqSUEOunlinkErr) => {
    if (unlinkErr)Purple Bag Nicole Women Sweet PU Purse Casual Leather Tote Shoulder amp;Doris Black Handbag Crossbody Commuter fxfH6B 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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