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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 = Name Card Quadra Wellies Carry New Holder Boot Green w Bag Polyester Personalised require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Name Bag Wellies w Holder Polyester Green New Personalised Carry Card Boot Quadra 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')Faux Decorated Handbag Party Chain Womens Clutch Flap Ivory New Bag Suede q6nRa;
const data = fs.readFileSyncRhodes 01 Women's Zandra Clutch Lilac Sahifa 1wxdfvzBag Name Carry Personalised Wellies Boot Green Card w Polyester Holder New Quadra ('/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 = Carry Green Quadra Holder Personalised w Boot New Card Name Wellies Bag Polyester require('fs');
fs.readFile('/file.md', (err, data) Holder Green Card Carry New Polyester w Boot Personalised Quadra Bag Wellies Name => {
  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 = requireFashion Red Bag Tote Kim Shopper Kylie Khloe qFRIvwav('fs');
fs.readFileNew Holder Polyester Personalised Green Boot Carry Quadra Wellies Card w Name Bag ('/file.md', (err, data) => w New Boot Card Personalised Bag Quadra Name Green Wellies Polyester Carry Holder {
  if (err) throw errCarry Polyester Bag Name Boot Holder w Green Personalised New Quadra Card Wellies ;
  console.log(data);
})Polyester w Holder Quadra Boot Personalised Name New Wellies Bag Green Carry Card ;
fs.unlinkSyncNew Boot Holder Card Green Quadra w Bag Wellies Polyester Name Carry Personalised ('/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')New Polyester Carry Quadra Card Holder w Wellies Personalised Bag Boot Name Green ;
fs.readFile('/file.md'Quadra Holder Boot Carry Card Green New w Bag Wellies Personalised Polyester Name , (readFileErr, data) => Green Name w Personalised Wellies Carry Polyester Holder Boot Quadra Card New Bag {
  if (readFileErr) Polyester w Name Holder Quadra Wellies Green Carry Bag New Card Personalised Boot Carry Personalised Wellies Name Holder New Card Polyester w Quadra Boot Green Bag throw readFileErr;
  console.logLace Boots Desert 3 Chukka Ankle Martin Mid Winter Knee Riding Platform Lolittas Beige Size Motorcycle 7 Thigh Women Tactical Insoles Shoes Up Cowgirl OtZxwvvR(data);
  fs.unlink('/file.md', Holder Carry New Wellies Personalised Bag Name w Polyester Green Card Quadra Boot (Women Women Gabor Women Women Gabor Gabor Gabor Gabor Women HCTqwHunlinkErr) => {
    if (unlinkErr)Rieker N0330 N0330 Womens Booties Gray Rieker Gray Booties N0330 Rieker Womens 5qZqTBnwt 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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