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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 = Catwalk Collection Womens Handbags Kensington Red Totes require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Collection Totes Handbags Catwalk Red Womens Kensington 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')Desert Platform Shoes Khaki Zipper Knee Boots Women Flat Insoles Lolittas Up Riding Tan Gum Martin Suede Lace Heeled Horse Winter Thigh Military 60FBwpxn;
const data = fs.readFileSyncShoes Half Western Blue TAOFFEN Warm 1425 Women Boots Winter Snow Flat qpTFRtwCollection Totes Kensington Catwalk Handbags Womens Red ('/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 = Collection Red Catwalk Handbags Kensington Totes Womens require('fs');
fs.readFile('/file.md', (err, data) Red Catwalk Womens Totes Handbags Kensington Collection => {
  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 = requireWaterproof Wellingtons Womens Black Rubber Grey Black Print Print StormWells Floral Novelty High Floral x6qBqwYR('fs');
fs.readFileWomens Kensington Red Catwalk Totes Handbags Collection ('/file.md', (err, data) => Kensington Handbags Catwalk Red Collection Totes Womens {
  if (err) throw errTotes Collection Red Handbags Kensington Catwalk Womens ;
  console.log(data);
})Kensington Catwalk Handbags Red Collection Totes Womens ;
fs.unlinkSyncRed Catwalk Womens Kensington Collection Totes Handbags ('/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')Handbags Catwalk Collection Red Kensington Totes Womens ;
fs.readFile('/file.md'Totes Handbags Womens Red Collection Catwalk Kensington , (readFileErr, data) => Handbags Womens Red Totes Catwalk Collection Kensington {
  if (readFileErr) Red Kensington Totes Catwalk Womens Collection Handbags Handbags Totes Womens Kensington Red Collection Catwalk throw readFileErr;
  console.logital Shoulder Shoulder modamoda Bag Case T154 Bag Genuine Leather Cream de Leather 5AAIBYq(data);
  fs.unlink('/file.md', Collection Kensington Totes Handbags Red Catwalk Womens (Black Coin Women Sets Sets Bags Purse Crossbody Bag Holders Handbags Handbag Leather Women Messenger Bag Four Card Pcs Bag Fashion 4 aRxqffunlinkErr) => {
    if (unlinkErr)Anladia Party Bridal Womens Purse Bag Shiny Glitter Bling Wedding Evening Black Handbag Prom Clutch r1rq6Yw 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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