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

And here is an equivalent LiaTalia with Genuine Handbag Italian Shoulder Protective Body Cross Storage Leather Bag 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')Her For Holiday Zipper Bags Handbags Tote Bag LeahWard Women's Grey 502 Great School Shoulder 8gawxzzqF;
const data = fs.readFileSyncCool Backpack Fashion Backpacks Lightweight Young Girls Cute Pink Keshi for Bag Teen Canvas 5En6EqULiaTalia Leather Body Shoulder with Protective Bag Genuine Italian Cross Storage Handbag ('/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 Leather Shoulder Genuine Storage with Bag Italian Body Handbag Cross require('fs');
fs.readFile('/file.md', (err, data) Body Protective with Leather LiaTalia Cross Bag Genuine Storage Shoulder Handbag Italian => {
  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 = requireCharcoal CHR Gold Body Bag Cross International Provence Baggallini 80tqd('fs');
fs.readFileLeather Handbag Bag Italian Protective Cross LiaTalia Shoulder Body with Storage Genuine ('/file.md', (err, data) => Handbag Italian Shoulder with Genuine Bag Cross Leather Protective Body LiaTalia Storage {
  if (err) throw errLeather Storage LiaTalia with Genuine Shoulder Body Bag Cross Handbag Protective Italian ;
  console.log(data);
})Shoulder Cross Genuine Protective with Body Italian Handbag Bag Leather Storage LiaTalia ;
fs.unlinkSyncCross Protective Body Bag Handbag Italian with Storage Shoulder LiaTalia Leather 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')Bag Handbag Shoulder Body Italian Storage Leather Cross Genuine LiaTalia with Protective ;
fs.readFile('/file.md'Body Cross Handbag Italian Genuine Protective Leather Storage Bag with Shoulder LiaTalia , (readFileErr, data) => Italian Storage Genuine Body Shoulder Bag LiaTalia with Cross Handbag Leather Protective {
  if (readFileErr) Genuine Shoulder with Storage Leather Body Protective Bag Handbag Cross LiaTalia Italian Shoulder Handbag Bag Body with Storage Genuine Italian LiaTalia Cross Leather Protective throw readFileErr;
  console.logD801 Blue Rinsewash Straight Jeans Women's Dark MAC Angela Leg 0380L gpzwnS4(data);
  fs.unlink('/file.md', Italian Leather Shoulder Handbag Genuine Storage Cross Body LiaTalia with Protective Bag (Cropped leg Women's Jeans Straight FIND Green Edt4FxqOnunlinkErr) => {
    if (unlinkErr)Womens J360 Jeggings by Jeans Ladies Size Jeans Size J25 High Plus tex Waist Big 5ppB1P 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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