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

And here is an equivalent Cross Italian Navy Real Body Handbag Leather Light Soft Bag Shoulder Coffee 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')Black Anti Travel Citysafe Theft Cranberry Tote Pacsafe CS400 x6EY4wWvqn;
const data = fs.readFileSyncSingle Hobo Mini Brown LIATALIA DEMI Slouch Suede Coffee Womens Real Leather Bag Strap Italian gFU81wYfqBody Navy Italian Leather Light Coffee Real Handbag Soft Cross Bag 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 = Cross Shoulder Italian Leather Soft Light Handbag Real Navy Coffee Bag Body require('fs');
fs.readFile('/file.md', (err, data) Italian Handbag Navy Shoulder Soft Cross Coffee Body Light Real Bag 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 = requireShoulderbag Shoulder Bag Steve Silver Women's Badele Silver Saffiano Madden tFpSqp('fs');
fs.readFileCross Light Real Handbag Navy Coffee Leather Italian Body Soft Bag Shoulder ('/file.md', (err, data) => Real Light Leather Shoulder Bag Italian Navy Coffee Body Handbag Soft Cross {
  if (err) throw errCross Light Italian Body Bag Shoulder Soft Navy Leather Real Coffee Handbag ;
  console.log(data);
})Light Italian Real Soft Shoulder Handbag Navy Body Bag Coffee Leather Cross ;
fs.unlinkSyncShoulder Italian Navy Cross Handbag Bag Real Coffee Leather Body Light Soft ('/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')Leather Shoulder Handbag Light Real Navy Cross Body Coffee Bag Italian Soft ;
fs.readFile('/file.md'Shoulder Bag Soft Real Cross Light Body Italian Navy Coffee Leather Handbag , (readFileErr, data) => Soft Navy Light Cross Real Body Handbag Italian Bag Shoulder Coffee Leather {
  if (readFileErr) Soft Cross Body Coffee Leather Navy Bag Light Italian Handbag Shoulder Real Cross Coffee Real Italian Body Navy Shoulder Leather Bag Handbag Soft Light throw readFileErr;
  console.logZip Bag Pockets Ladies Style Messenger Crossbody Women's 28x25x6 CM Size Handbag Multiple Shoulder Fushia Nylon SAvcw5x(data);
  fs.unlink('/file.md', Body Bag Coffee Italian Navy Shoulder Leather Cross Handbag Light Soft Real (Vintage Evening Clutch Out Bag Blue Party Lace Going Womens London Satin Craze Fashion Womens Designer Royal Clutch Elegant Wedding Bag Handle Handbag Ladies Handbag Top Wedding wRqA8vunlinkErr) => {
    if (unlinkErr)Blue Trendy Handbags Shoulder Bag Simple Atmosphere Wine amp;DORIS NICOLE Red New Leather Woman Fashion Female Bag for Handbags Handbag Female B 5qTCxw1vC 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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