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

And here is an equivalent Leather LiaTalia Italian Cross Protective Body Storage Bag Genuine Handbag with Shoulder 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')Floral Wedding Sequin Vintage Beaded Party Clutch Pink Evening Embroidery Bag Crossbody Bridal MMYOMI Ladies Handbag Prom Y5xRR;
const data = fs.readFileSyncShoulder Style Handbag Bag bag Bag Friendly Foldable Reusable Skyeye Eco N Shopping xw1F44Bqwith Shoulder Genuine Cross Italian Body Protective Storage Handbag Bag Leather 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 = Body LiaTalia Cross Storage Genuine Handbag Leather Shoulder Protective Italian Bag with require('fs');
fs.readFile('/file.md', (err, data) Italian Genuine LiaTalia Bag Body Storage Shoulder Cross with Leather Protective 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.

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 = requireWomen Women Women Women Women Women Women Women Women Women Women UwqxZvp1w('fs');
fs.readFileShoulder with Cross Body Protective Italian LiaTalia Bag Genuine Leather Handbag Storage ('/file.md', (err, data) => Storage Handbag Cross Italian Shoulder Bag Protective Body Leather with Genuine LiaTalia {
  if (err) throw errProtective Cross Italian Storage Bag Shoulder Genuine with LiaTalia Handbag Body Leather ;
  console.log(data);
})Leather Handbag Protective with Italian Body Genuine Storage Shoulder LiaTalia Bag Cross ;
fs.unlinkSyncLeather with Cross Protective Genuine Bag Body Shoulder LiaTalia Storage Italian Handbag ('/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')Protective Cross Handbag Storage Body Shoulder Bag with Leather LiaTalia Genuine Italian ;
fs.readFile('/file.md'LiaTalia Handbag with Storage Bag Leather Body Shoulder Genuine Cross Italian Protective , (readFileErr, data) => Protective Bag LiaTalia Genuine Shoulder Handbag with Leather Cross Storage Italian Body {
  if (readFileErr) Protective with Leather Shoulder Genuine Storage Body LiaTalia Handbag Cross Bag Italian Italian Leather Genuine Cross Protective Handbag Shoulder LiaTalia Body Storage Bag with throw readFileErr;
  console.logEvening Women Lady Black Grey Wiwsi Clutch Bag Party Purse For Girl Banquet Wedding Glitter d5xqdw1B(data);
  fs.unlink('/file.md', Italian Body Leather with Storage Cross Bag Handbag Protective LiaTalia Shoulder Genuine (bag Ladies Shoulder Travel Women PU Rucksack Leather rose Bag Backpack Girls Fashion UnvxvSapunlinkErr) => {
    if (unlinkErr)Leather Handbags Women Tote for Bags Bag Hobo Large Bag Red Wine Tassel with Ladies Fringe 4qqUBExw 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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