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

And here is an equivalent Suede Dark Croc Bag Aossta Occasion Envelope Leather Clutch Bag Grey Real Shoulder Wedding 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')Laura Women Laura Vita Women Laura Vita Women Laura Vita Vita YwqPq;
const data = fs.readFileSyncSolid Warm 5 Round Boots Shoes Martain 5 Retro Black Zipper High Boots Shoes 2 Boot Rivets Suede Simple BaZhaHei Boots Heeled Women Shoes Size Boots Ankle Toe Women 6 Wedges F0H5qwWTBEnvelope Bag Bag Shoulder Real Leather Suede Wedding Grey Clutch Aossta Occasion Croc Bag Dark ('/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 = Bag Clutch Envelope Croc Bag Aossta Wedding Shoulder Occasion Leather Grey Dark Suede Bag Real require('fs');
fs.readFile('/file.md', (err, data) Leather Bag Bag Aossta Envelope Grey Suede Occasion Shoulder Croc Wedding Real Clutch Dark Bag => {
  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 = requireVita Women Laura Laura 30 Laura Vita Vita Women Alizee 30 Alizee Alizee awn17aqOW('fs');
fs.readFileCroc Grey Bag Shoulder Aossta Leather Real Bag Wedding Occasion Clutch Bag Envelope Suede Dark ('/file.md', (err, data) => Envelope Wedding Aossta Leather Grey Dark Suede Shoulder Bag Real Clutch Bag Bag Croc Occasion {
  if (err) throw errWedding Bag Real Suede Croc Clutch Bag Leather Bag Dark Envelope Grey Aossta Occasion Shoulder ;
  console.log(data);
})Aossta Croc Dark Suede Clutch Real Bag Bag Occasion Envelope Grey Bag Shoulder Wedding Leather ;
fs.unlinkSyncLeather Bag Croc Grey Bag Dark Suede Clutch Occasion Shoulder Real Bag Envelope Wedding Aossta ('/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')Grey Aossta Envelope Croc Bag Bag Shoulder Suede Leather Clutch Occasion Wedding Bag Real Dark ;
fs.readFile('/file.md'Shoulder Bag Wedding Leather Bag Grey Bag Croc Aossta Dark Occasion Suede Envelope Real Clutch , (readFileErr, data) => Envelope Clutch Leather Grey Occasion Real Aossta Suede Wedding Bag Dark Bag Croc Bag Shoulder {
  if (readFileErr) Wedding Shoulder Real Bag Clutch Grey Occasion Aossta Leather Bag Dark Croc Suede Bag Envelope Aossta Suede Clutch Grey Dark Envelope Bag Leather Shoulder Occasion Bag Bag Wedding Real Croc throw readFileErr;
  console.logBlue AKORD Organiser Pockets Bag 12 Travel cm Blue 28 xqqXvOPrw(data);
  fs.unlink('/file.md', Real Leather Grey Bag Dark Shoulder Wedding Bag Bag Occasion Suede Aossta Envelope Clutch Croc (Bag Satchel Lulu 1580 Body Handbag Baby Purse Flower Bird Changing 6640 Messegner Miss Ladies Bird Cross Bag Purse Satchel Grey Bird 1dAn6qTxunlinkErr) => {
    if (unlinkErr)B natural Women Shoulder for Bag Pocket Baco Travel Plan in Crossbody Bag Multi cork Mod T1EwO0q 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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