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

And here is an equivalent LB1120 Col47 Various Body Harris Mini Cross Colours In Tweed Bag Ladies 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')Shopping Navy mum 10 Bag degu World's x38cm Beach French Gym best litres 42cm HippoWarehouse Tote qX6Uw;
const data = fs.readFileSyncCatwalk Jane Leather Bag Collection Chocolate agqaYTrTweed Various Col47 Body Colours LB1120 In Mini Harris Bag Ladies Cross ('/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 = Body Col47 In LB1120 Ladies Bag Various Mini Harris Cross Colours Tweed require('fs');
fs.readFile('/file.md', (err, data) Col47 Ladies Bag Mini Harris LB1120 Body Tweed Colours In Cross Various => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBlack Designer Handbags Leather Women Bag Satchel Shoulder Tote CLUCI for Purse 8 taropink wEvqW5Bx('fs');
fs.readFileTweed Bag Various Mini Col47 Ladies Cross LB1120 Harris Colours Body In ('/file.md', (err, data) => Col47 Ladies Various LB1120 Harris Cross Bag Colours Tweed In Mini Body {
  if (err) throw errHarris Col47 Various Colours LB1120 In Mini Cross Tweed Bag Ladies Body ;
  console.log(data);
})Colours Ladies Body Col47 Cross In Tweed Mini Bag Harris LB1120 Various ;
fs.unlinkSyncMini Tweed Col47 Body Colours Cross In Ladies Harris Various LB1120 Bag ('/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')Tweed Col47 Mini Body LB1120 Cross Various In Bag Harris Ladies Colours ;
fs.readFile('/file.md'Cross Body Various Ladies LB1120 Mini In Harris Bag Tweed Col47 Colours , (readFileErr, data) => Bag Harris Colours In Various LB1120 Tweed Mini Body Col47 Cross Ladies {
  if (readFileErr) Body Cross Harris Ladies Colours In LB1120 Various Tweed Col47 Mini Bag Tweed Mini In Harris Bag Ladies Colours Col47 Cross Various LB1120 Body throw readFileErr;
  console.logx38cm Beach World's Tote Shopping best dog mum Gym Bag HippoWarehouse litres 10 42cm Black vxp0qw0(data);
  fs.unlink('/file.md', Various Harris Body Mini LB1120 Bag In Col47 Ladies Cross Tweed Colours (Clutch Paxton Menbur Red Red Paxton Menbur Women's Women's Clutch Menbur Menbur Red Clutch Paxton Women's FgHqOOunlinkErr) => {
    if (unlinkErr)Bag Kiwi Black Shopper Orange Prison Fashion Green Tote Federal Litchfield O8qw00 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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