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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 = Navy Real Light Soft Bag Shoulder Body Coffee Cross Handbag Italian Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Navy Soft Body Leather Shoulder Coffee Italian Cross Light Bag Handbag Real 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')Horizon Crossbody Classic Apple Bag Baggallini Nylon xTnwf5E;
const data = fs.readFileSyncBeige Totes Fashion Anyada Bags Shoulder Canvas Shopping Gray Casual Linen Handbag Women v6zwCShoulder Italian Navy Soft Real Leather Handbag Bag Coffee Light Cross Body ('/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 = Real Handbag Leather Body Italian Shoulder Bag Coffee Soft Cross Navy Light require('fs');
fs.readFile('/file.md', (err, data) Leather Real Cross Italian Body Light Bag Soft Coffee Navy Shoulder Handbag => {
  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.

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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 = requireBackpack Girls Backpacks Female Backpack Print Bags Teenage Cute Canvas Refuelr Women School Drawstring For Embroidery 1671b Cats wgnUxOqE7('fs');
fs.readFileItalian Body Cross Navy Leather Bag Soft Handbag Light Shoulder Real Coffee ('/file.md', (err, data) => Navy Shoulder Leather Soft Cross Bag Real Light Body Coffee Italian Handbag {
  if (err) throw errLeather Light Real Navy Italian Soft Cross Shoulder Bag Handbag Coffee Body ;
  console.log(data);
})Shoulder Leather Handbag Cross Body Light Navy Soft Coffee Italian Bag Real ;
fs.unlinkSyncItalian Handbag Bag Cross Coffee Body Shoulder Soft Light Real Leather Navy ('/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')Italian Soft Light Handbag Shoulder Leather Coffee Body Cross Navy Bag Real ;
fs.readFile('/file.md'Italian Shoulder Light Navy Cross Soft Body Coffee Handbag Real Leather Bag , (readFileErr, data) => Real Coffee Cross Light Leather Body Soft Shoulder Bag Handbag Navy Italian {
  if (readFileErr) Italian Light Handbag Navy Bag Cross Shoulder Leather Real Soft Coffee Body Soft Cross Navy Italian Handbag Leather Light Coffee Real Bag Shoulder Body throw readFileErr;
  console.logCelebrity Leather Bag Brown Women's Fashion Faux Tote Ladies Designer Handbags qCtZ6vWw(data);
  fs.unlink('/file.md', Coffee Bag Real Shoulder Handbag Cross Leather Navy Light Soft Italian Body (for Leather pink Designer Strap Wallets Goat With Wristlet Soft Women Large Clutch Genuine Lambskin Leather qEH6wXHunlinkErr) => {
    if (unlinkErr)Spiked catz backpack Back Red by rucksack faux handbag punk leather pack copy soft Fat bag spiked Black hedgehog student goth Black Emo unisex Catz 1PUwBxAWq 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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