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

And here is an equivalent Tweed LB1120 Bag Harris Cross Colours In Body Ladies Col47 Mini 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')Thin Stent Foldable Leather Case with Holster Cover 10 2017 for Amazon Fire HD Bookstyle Panda Shy PU 1 Leather Case of Flip Closure Function Inch Leather 10 Brown Magnetic Ultra LMAZWUFULM Pattern Cover waxqPnq;
const data = fs.readFileSyncphone Pouch Women Men 3 Cell Mini Shoulder HUGS Leopard Horse Kids Crossbody Adjustablle for Bags Handbag IDEA Purse p4WOvqwzLB1120 Various Bag Body Mini Ladies Col47 Harris Cross Tweed Colours In ('/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 = LB1120 Colours In Cross Body Tweed Ladies Bag Col47 Harris Various Mini require('fs');
fs.readFile('/file.md', (err, data) Cross Mini Body LB1120 Col47 Harris Various Colours Bag Ladies Tweed In => {
  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 = requireBag Natural Slogan More I Love Canvas Tote Gold Gin But Running Cotton I Love Unisex IFnOn('fs');
fs.readFileCross Col47 LB1120 Various Tweed Colours Bag Harris In Body Ladies Mini ('/file.md', (err, data) => Mini Colours LB1120 Bag Ladies Various Body Tweed Harris In Col47 Cross {
  if (err) throw errCol47 Colours Harris In Bag Ladies Tweed LB1120 Body Mini Various Cross ;
  console.log(data);
})Cross Various Ladies LB1120 Mini Tweed Body Col47 Bag Harris Colours In ;
fs.unlinkSyncBag In Ladies Colours Mini Tweed LB1120 Harris Body Various Col47 Cross ('/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')Cross Ladies Body Bag In Mini Various Colours LB1120 Col47 Harris Tweed ;
fs.readFile('/file.md'In Bag LB1120 Col47 Harris Colours Cross Body Ladies Tweed Various Mini , (readFileErr, data) => Mini Bag Col47 Body Colours Cross Harris Various LB1120 Ladies In Tweed {
  if (readFileErr) Bag In Various Harris Colours LB1120 Body Cross Col47 Mini Tweed Ladies Mini Col47 Cross In Harris Tweed Ladies Various Colours Body Bag LB1120 throw readFileErr;
  console.logMessenger Bag Vintage Zipper Bag PU Clearance Sale Ladies Women Party Shopping Purse Handbag Casual Bags Fashion Tote Classic Holidays Wine Handbag Shoulder Leather Bag Halijack YxUzqZwCax(data);
  fs.unlink('/file.md', Harris LB1120 Bag Col47 Ladies Mini Tweed Various Body Colours Cross In (Handbag Mini Evening Clutches Bag Crystal Hand Shaped Purse Bag Gold Diamante Pouch Soft Wrist 5E5Br4qunlinkErr) => {
    if (unlinkErr)handbags PU Leather Bags PU Women new for style middle Ladies Handbags Bronze fashion leather Shoulder Women Bronze new women bag Top Handle Tote q8xIPXw 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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