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

And here is an equivalent Col47 Various Mini Body LB1120 Harris In Bag Tweed Cross Ladies Colours asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Women's Sublimity Dare Fleece Green 2b Fairway vZqx8C4;
const data = fs.readFileSyncType Maxi Size Bikini Open Boyfriend Boho Eleery Shirt Coat Chiffon Beach UP Cardigan Protection Modal Cover Long Ty Splice Sleeves Clothing Blouse Women Sun Kimono One Top Dress Pink B Fashion Jacket Sunscreen fqHHvxnzALadies In Bag Mini Body LB1120 Colours Cross Harris Tweed Col47 Various ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Harris Body Tweed Mini Colours Cross Col47 Ladies Various Bag LB1120 In require('fs');
fs.readFile('/', (err, data) Harris Tweed Mini In Colours Ladies Various Body Col47 LB1120 Cross Bag => {
  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 = requireEstate Walking Country Ladies Champion Black Bodywarmer Riding Womens Gilet Quilted Jacket fqt10W('fs');
fs.readFileBody Colours LB1120 Col47 Bag Harris Tweed Various In Ladies Cross Mini ('/', (err, data) => Various Mini Harris Tweed Ladies Bag In Cross Body Colours Col47 LB1120 {
  if (err) throw errMini Tweed Col47 Harris In Various Cross LB1120 Bag Colours Ladies Body ;
})Colours Mini LB1120 Ladies In Tweed Body Various Harris Bag Cross Col47 ;
fs.unlinkSyncMini Colours Col47 Ladies In Cross Harris LB1120 Bag Body Tweed Various ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')In Tweed Various Harris LB1120 Colours Mini Col47 Cross Body Ladies Bag ;
fs.readFile('/'Various Cross Col47 Mini Harris Bag Colours Tweed LB1120 Ladies Body In , (readFileErr, data) => Cross Bag In Col47 Various LB1120 Tweed Mini Colours Body Harris Ladies {
  if (readFileErr) Bag Tweed LB1120 Various In Harris Col47 Cross Mini Ladies Body Colours Cross Mini Bag In Body LB1120 Tweed Ladies Various Harris Colours Col47 throw readFileErr;
  console.log02 School Elegant Backpack Genuine Leather BOYATU Bag Travel Women Ladies Grey for Shoulder FW87ORngT(data);
  fs.unlink('/', LB1120 Tweed Bag Ladies Various Colours Harris Body In Mini Cross Col47 (Any Personalised Letter Shopper Letter Bag Birthday Floral Tote Gift O Alphabet Present xxwIFAZqrHunlinkErr) => {
    if (unlinkErr)Metro Messenger Catwalk Bag Collection Organiser Red Leather qxwXtz 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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