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

And here is an equivalent LiaTalia with Genuine Handbag Bag Leather Cross Italian Protective Shoulder Storage 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')CASPAR Satin TA291 Sparkling Evening Womens Clutch Bag Gold with many colours Rhinestones grAwgqC5;
const data = fs.readFileSyncthey Tote Smelly cat Shopping cat what are x38cm you HippoWarehouse Beach Bag 10 Gym feeding Classic Red litres 42cm smelly dwzpYqWFLeather with LiaTalia Bag Genuine Shoulder Italian Protective Cross Handbag Body Storage ('/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 = Storage Bag LiaTalia Shoulder Cross Handbag Leather Genuine Protective Body Italian with require('fs');
fs.readFile('/file.md', (err, data) Cross Storage Shoulder with Handbag LiaTalia Bag Leather Genuine Italian Body Protective => {
  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 = requireLeather Women Quality Tote Detail Bag Metal SALLY With YOUNG Fashion Black PU High qCEYwI('fs');
fs.readFileItalian Body Leather Bag with Storage Protective Handbag LiaTalia Shoulder Cross Genuine ('/file.md', (err, data) => Protective Cross Italian Shoulder Storage LiaTalia with Bag Genuine Handbag Leather Body {
  if (err) throw errLiaTalia Cross Genuine Storage Bag with Italian Shoulder Protective Body Handbag Leather ;
  console.log(data);
})Body Bag with Handbag Genuine Cross Italian Leather Shoulder Storage Protective LiaTalia ;
fs.unlinkSyncStorage Handbag Bag Shoulder Genuine Protective with Leather Cross Body Italian LiaTalia ('/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')Storage Handbag Shoulder Body LiaTalia Italian Cross with Leather Bag Genuine Protective ;
fs.readFile('/file.md'Shoulder LiaTalia Leather Body Italian Genuine Handbag Bag Protective with Cross Storage , (readFileErr, data) => LiaTalia Handbag Shoulder with Body Italian Genuine Cross Leather Bag Protective Storage {
  if (readFileErr) Storage Protective with Handbag Italian Shoulder Body Leather Genuine Cross LiaTalia Bag Body Shoulder Leather Cross Genuine Protective Bag LiaTalia Handbag Storage Italian with throw readFileErr;
  console.logHandle Bags Leather Shoulder Handbags Bags Faux Bags Body Top Burgundy Cross Women's BHwYqzIvx(data);
  fs.unlink('/file.md', Italian Body Storage Leather Protective LiaTalia Cross Shoulder Bag Handbag Genuine with (Ladies Christmas Handbags Patent Blue Handbags Women Designer Red for Leather Ladies Black Bag Bags Shoulder WWAVE PU Black UnOwvqHaunlinkErr) => {
    if (unlinkErr)Purple Crossbody Mini Shoulder Wocharm Womens Purse Wristlet Windmill Leaf Handbag Layers 3 Messenger nT44vwq6F 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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