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

And here is an equivalent Handbag Italian Navy Bag Real Shoulder Leather Cross Coffee Light Soft Body 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')Patent Navy Womens Faux Lilo Blue Leather Flapover Clutch Ladies Party SWANKYSWANS Prom Bag xZEaRnPZ;
const data = fs.readFileSyncCraze Italian Real Vera Small Body Shoulder London Genuine Designer Coffee Strap Suede Cross Bag pelle Womens PrCwTPSoft Body Coffee Real Leather Handbag Bag Light Navy Italian Cross Shoulder ('/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 = Handbag Bag Soft Real Cross Coffee Body Leather Shoulder Light Navy Italian require('fs');
fs.readFile('/file.md', (err, data) Shoulder Light Navy Bag Soft Italian Real Body Coffee Cross Handbag Leather => {
  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 = requireWomen's Cross Body Grey RepubliQ Raf Bag Royal Suede Anthracite Eve Bf5wnqa('fs');
fs.readFileCross Shoulder Leather Navy Real Italian Coffee Handbag Light Soft Bag Body ('/file.md', (err, data) => Navy Bag Coffee Shoulder Cross Leather Light Soft Body Handbag Italian Real {
  if (err) throw errHandbag Leather Bag Soft Coffee Light Cross Real Shoulder Navy Body Italian ;
  console.log(data);
})Cross Navy Body Real Handbag Leather Shoulder Soft Italian Bag Light Coffee ;
fs.unlinkSyncBody Soft Italian Cross Bag Real Leather Handbag Light Coffee Navy Shoulder ('/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')Navy Shoulder Cross Handbag Body Bag Italian Real Leather Light Coffee Soft ;
fs.readFile('/file.md'Cross Leather Soft Handbag Real Bag Shoulder Coffee Light Navy Italian Body , (readFileErr, data) => Handbag Italian Body Navy Shoulder Leather Real Light Soft Cross Coffee Bag {
  if (readFileErr) Soft Real Leather Handbag Shoulder Bag Cross Light Italian Navy Body Coffee Handbag Body Cross Coffee Italian Light Shoulder Bag Real Leather Navy Soft throw readFileErr;
  console.log96 Lilia Ankle Boot 661 Gabor Green qatZAw(data);
  fs.unlink('/file.md', Shoulder Soft Italian Coffee Leather Light Real Handbag Navy Cross Bag Body (Sunday77 Buckle Winter Round PU Ankle Adults Boots Clearance Boots Martin Toe Square Warm Heel Women Red Casual Ladies for Comfort Rivet Shoes 5p8qfunlinkErr) => {
    if (unlinkErr)Unisex Dodo Kids Trespass Trespass Dodo Kids Unisex UwnSS1x 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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