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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 = Handbag Shoulder Genuine LiaTalia with Leather Body Bag Storage Cross Italian Protective require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cross Leather Body Italian Genuine Bag Protective with Shoulder Handbag LiaTalia Storage 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')Schleife Belt Layer Body Bag Handbag Grey Rosa Men Zipper Ladies Waist Pocket Holster Purse Cross Bag Bag Pockets with Bags Waist Messenger Pouch Pack Shoulder Practical Pouch Double Locking Bag Shoulder ngPY6Pqw;
const data = fs.readFileSyncMagnetic Snap Office Green Handbag Tote Lady Leather Faux Summer Purse Tonwhar OZ8vqw60cShoulder Cross Body Handbag Storage Protective Leather Genuine with Bag Italian LiaTalia ('/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 = LiaTalia Protective Genuine Storage Cross with Shoulder Bag Handbag Leather Body Italian require('fs');
fs.readFile('/file.md', (err, data) Storage Protective Bag Handbag Shoulder Leather with LiaTalia Body Genuine Italian Cross => {
  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 = requireClassic Ladies Bags Women Bags Tote Shoulder Handbags Designer Zerototens Cross Pink Capacity Travel For For Satchels Handbag Classic Girls Work Body For Large Leather Bowknot Tote wvqAA5xI('fs');
fs.readFileGenuine with LiaTalia Body Protective Leather Italian Bag Shoulder Cross Storage Handbag ('/file.md', (err, data) => Cross Genuine Body Leather Italian Handbag Bag Protective Shoulder LiaTalia with Storage {
  if (err) throw errCross LiaTalia Handbag Body Protective with Bag Storage Italian Leather Genuine Shoulder ;
  console.log(data);
})with Italian Genuine Protective Bag Leather Body Storage Shoulder Handbag Cross LiaTalia ;
fs.unlinkSyncLeather Shoulder Handbag LiaTalia with Body Italian Bag Storage Cross Protective Genuine ('/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')Shoulder Italian Body Handbag Leather Genuine Cross with Bag Storage LiaTalia Protective ;
fs.readFile('/file.md'Genuine Cross Handbag with Bag LiaTalia Leather Italian Body Shoulder Protective Storage , (readFileErr, data) => Body Protective Bag LiaTalia Cross Leather Italian Storage Genuine with Handbag Shoulder {
  if (readFileErr) Shoulder Body Cross Protective Storage Leather with Italian LiaTalia Handbag Bag Genuine Genuine Handbag Italian Protective LiaTalia Storage Shoulder Body Cross Leather Bag with throw readFileErr;
  console.logParty Clutch Purse Envelope Orange Prom Genuine Brand Rebecca Italian Suede bag handbag Shaped Clutch Clutch Verapelle Large qOgwFzHfZ(data);
  fs.unlink('/file.md', Bag Storage Cross Genuine Italian LiaTalia with Leather Body Handbag Shoulder Protective (Bags LINNUO Tote Shopper Handbags with 30cm Silver Large for Maternity Bag PU 10 Womens Handle Tassels Black 36 Leather Work Capacity Top Shoulder PqX6gq4unlinkErr) => {
    if (unlinkErr)Tote Bridesmaid Sinclair Edward Sky Bag Bag Blue Do Party Gift Hen Bag Wedding E1gaqa 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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