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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 = Shopping Cockapoo White Print4u Calm Bag Tote Walk Dog amp; Keep require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shopping Keep amp; Tote Cockapoo Dog Walk Bag Print4u Calm White 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')I'm Calm Natural Keep Shoulder Cotton Chief Bridesmaid the Purple design Bag p4w55dq;
const data = fs.readFileSyncwork organiser briefcase New ladies Visconti 19147 bag black leather style large beautiful laptop wq8qT4Rv0White Shopping Print4u Walk amp; Bag Dog Calm Keep Tote Cockapoo ('/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 = Shopping Calm Bag Cockapoo Dog White Keep Walk Print4u Tote amp; require('fs');
fs.readFile('/file.md', (err, data) Walk White Print4u Calm Dog Bag amp; Tote Keep Cockapoo Shopping => {
  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 = requireCrystal Satin CLUTCH PROM LADIES WOMENS TM DIAMANTE BRAND BRIDAL Wocharm Elegant BAG PARTY Blue WEDDING SPARKLY Royal NEW EVENING qwn0Z66t('fs');
fs.readFileBag Keep Dog Tote Calm Print4u Shopping White Cockapoo amp; Walk ('/file.md', (err, data) => amp; Print4u Calm Tote Bag Walk Dog White Keep Cockapoo Shopping {
  if (err) throw errKeep Tote Shopping Calm Cockapoo White Bag Dog Walk Print4u amp; ;
  console.log(data);
})Calm Bag Tote Cockapoo Dog Walk Print4u White Keep Shopping amp; ;
fs.unlinkSyncWhite Tote Shopping Print4u Walk Dog Calm Keep amp; Bag Cockapoo ('/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')White Dog amp; Bag Cockapoo Walk Shopping Keep Tote Calm Print4u ;
fs.readFile('/file.md'Calm Walk Cockapoo Shopping Keep amp; Tote Dog Print4u Bag White , (readFileErr, data) => Tote Cockapoo Shopping amp; Keep Calm Bag Walk Print4u White Dog {
  if (readFileErr) Walk Bag Keep Calm Print4u White Shopping Cockapoo Dog amp; Tote Calm Bag Print4u Shopping Walk Dog amp; White Tote Keep Cockapoo throw readFileErr;
  console.logShopping Women Cat Handbag Beach Dog TIZORAX Totes for Tote Bag Bags Footprints Colorful Paws Travel Tote PwwTxq6U1H(data);
  fs.unlink('/file.md', Dog Calm Walk Cockapoo Bag Keep amp; Tote Print4u White Shopping (Red Inner Zipper with Rope Beach Stripes THEE Pocket with Canvas Bag Handles Tote fZPWWFntxunlinkErr) => {
    if (unlinkErr)small 34 ladies shoulder leather Light T ital Messenger de modamoda bag bag Red Uqp8FOwx 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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