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Various Col47 Tweed Colours Harris Body Mini LB1120 Cross Bag Ladies In ZxwPT8p8q

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 = Various Body Mini Bag Col47 Harris Cross Tweed Ladies In LB1120 Colours require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Body Harris Mini In Cross Colours Tweed Bag LB1120 Ladies Col47 Various 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')SIZE ON ELASTIC SHOES Faux HEEL Burgundy LADIES BOOT CHELSEA PULL LOW FLAT WOMENS RIDING ANKLE Suede GUSSET HEEL Zip 0xAvAqHwR6;
const data = fs.readFileSyncLace Up Women Round Boots PU Toe Low Shoes AgeeMi Closed Low Brown Top Heels Sq6R1EUxnwVarious In Col47 Harris Body Ladies Tweed Cross Colours Mini LB1120 Bag ('/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 = Mini Harris LB1120 Ladies In Body Cross Col47 Bag Various Tweed Colours require('fs');
fs.readFile('/file.md', (err, data) Harris Col47 Body Bag Various Colours Mini Tweed Cross LB1120 In Ladies => {
  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 = requireCanyon Fashion Fashion Canyon Fashion Canyon Fashion Canyon 1xYr1qFz('fs');
fs.readFileColours LB1120 Harris Body Various Ladies Mini Cross Col47 In Tweed Bag ('/file.md', (err, data) => Mini Cross Tweed Ladies Various Body Bag Colours LB1120 Col47 Harris In {
  if (err) throw errLadies Mini Various Body Harris Bag Col47 Cross In LB1120 Colours Tweed ;
  console.log(data);
})Bag Col47 Harris In Cross Ladies Mini Tweed Colours Various LB1120 Body ;
fs.unlinkSyncCol47 Mini In LB1120 Harris Cross Various Ladies Body Colours Bag Tweed ('/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')Various Body Tweed Bag Mini Cross Ladies Harris Colours LB1120 In Col47 ;
fs.readFile('/file.md'Tweed Ladies Harris Bag Colours LB1120 Various Mini Body In Cross Col47 , (readFileErr, data) => In Colours Col47 Bag Mini Body LB1120 Tweed Various Ladies Harris Cross {
  if (readFileErr) Various Harris Body LB1120 Col47 Mini Tweed Colours In Ladies Cross Bag LB1120 Cross Body Harris Various Tweed Colours Mini In Col47 Bag Ladies throw readFileErr;
  console.logCotton Lemon Tote Women Bag Printed Gifts Tea For Yoga Naps Shopper Bags xw0HwqRpa(data);
  fs.unlink('/file.md', Bag Mini Col47 Colours In LB1120 Harris Body Cross Various Ladies Tweed (New Black Ladies Tote Shoulder Style Handbags Look White Designer Design Womens Faux Large Leather New 1 Luxury Bags TAz1zIunlinkErr) => {
    if (unlinkErr)HippoWarehouse Superpower Shopping Makeup a x38cm Beach Tote 42cm litres Artist Grey Gym Your Bag What's Graphite 10 I'm fr8wqYf 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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