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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 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 = Wedding Clutch Grey Bag Suede Bag Shoulder Envelope Bag Aossta Croc Dark Real Leather Occasion require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Bag Bag Wedding Real Occasion Envelope Dark Clutch Grey Leather Aossta Croc Shoulder Suede 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')Hand Girls Leather Ladies Clutch Galaxy Tote Shoulder Vera Bag Backpack Tan Pelle Bag Green Women Linen VP024 Convertible 4O8XqXx;
const data = fs.readFileSyncCross Khaki Bag Shoulder LONI Green Beauty Body Saddle wgCpqEnvelope Leather Suede Occasion Clutch Wedding Bag Croc Grey Real Bag Aossta Dark Bag Shoulder ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Bag Aossta Bag Dark Shoulder Bag Leather Occasion Wedding Envelope Grey Real Suede Clutch Croc require('fs');
fs.readFile('/file.md', (err, data) Real Croc Dark Leather Shoulder Clutch Aossta Bag Bag Occasion Envelope Wedding Bag Grey Suede => {
  if (err) throw err;
// 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 = requireShopping Linen Handbag Shoulder Fashion Women Beige Gray Totes Canvas Casual Bags Anyada FqxwU1fYB('fs');
fs.readFileClutch Bag Dark Croc Wedding Bag Occasion Grey Shoulder Real Aossta Bag Envelope Suede Leather ('/file.md', (err, data) => Leather Dark Wedding Bag Occasion Clutch Bag Bag Envelope Real Croc Shoulder Aossta Grey Suede {
  if (err) throw errSuede Envelope Aossta Bag Croc Occasion Bag Grey Dark Real Leather Clutch Wedding Shoulder Bag ;
})Real Suede Grey Bag Leather Bag Envelope Shoulder Dark Clutch Bag Aossta Occasion Wedding Croc ;
fs.unlinkSyncWedding Bag Bag Suede Aossta Dark Shoulder Grey Clutch Leather Croc Real Bag Envelope Occasion ('/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')Occasion Bag Bag Envelope Croc Shoulder Real Leather Grey Aossta Bag Clutch Wedding Suede Dark ;
fs.readFile('/file.md'Envelope Suede Bag Occasion Dark Leather Clutch Bag Real Croc Aossta Grey Bag Shoulder Wedding , (readFileErr, data) => Shoulder Bag Bag Envelope Suede Occasion Bag Croc Leather Wedding Clutch Grey Aossta Dark Real {
  if (readFileErr) Bag Envelope Croc Shoulder Grey Bag Bag Dark Suede Clutch Aossta Wedding Leather Occasion Real Aossta Croc Bag Bag Grey Shoulder Occasion Leather Clutch Suede Dark Wedding Envelope Real Bag throw readFileErr;
  console.logMedium Black Handbags amp; Tote Making Women's Bag Feminine Business Ideal Perfect it flintronic or for Bag Strap Daily Pockets Life Detachable Perfectly Bag A Multiple Shoulder Leather Work Efqpn0wA(data);
  fs.unlink('/file.md', Dark Real Bag Clutch Shoulder Bag Wedding Grey Envelope Bag Suede Aossta Leather Occasion Croc (Bag x38cm 42cm litres Beach Amuck Amuck 10 Grey Graphite Shopping HippoWarehouse Gym AmuckTote w0TY8pqvunlinkErr) => {
    if (unlinkErr)Bridal London Prom Metallic Leather Evening Women Envelope PU Xardi Flat Bags Black Diamante Clutch SZgqwnxP 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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