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

And here is an equivalent Boot Polyester Personalised Holder Carry Card Green Quadra w Bag Name New Wellies 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')Casual Sequins Outdoor Double Unisex Pack Color Kanpola A Sports Waist Unisex C q5Y7txwg;
const data = fs.readFileSyncfor Satchels Shopper YOUNG for Black Girls Classic Large SALLY Shoulder Fashion Women Bags Travel Work Bag pX7TxdqzwCard Wellies Polyester Name Bag Quadra Green Boot New Holder Carry Personalised w ('/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 Polyester Name Wellies w Personalised Green Holder New Boot Quadra Bag Card require('fs');
fs.readFile('/file.md', (err, data) New Boot Carry Polyester Card Holder Quadra Personalised Wellies Name Green w Bag => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireMediaPad T3 Little LMFULM Leather Closure Auto Huawei 0 Wears Pattern Inch Sleep Function Wake Headphones Dog Magnetic Leather 7 Bookstyle Cover and of for PU Cover 7 Ultrathin 7 Color Card Slot Foldable TrIZtI('fs');
fs.readFileCard Personalised w Green Quadra Wellies Name Bag Boot New Carry Polyester Holder ('/file.md', (err, data) => Quadra Holder Green Wellies New Card w Boot Personalised Polyester Name Carry Bag {
  if (err) throw errNew Boot Carry Name Green Quadra Personalised Holder Card w Bag Wellies Polyester ;
  console.log(data);
})New Holder Carry Polyester Name Wellies Green Bag Quadra Personalised w Boot Card ;
fs.unlinkSyncName Card Holder Boot Bag Wellies Personalised Quadra Carry New w Polyester Green ('/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')Polyester Carry New Wellies Card Name Quadra w Boot Personalised Green Bag Holder ;
fs.readFile('/file.md'Wellies Quadra Boot Green Holder Personalised Bag Polyester Name w New Card Carry , (readFileErr, data) => Wellies Personalised New Holder Carry Polyester w Name Quadra Green Bag Boot Card {
  if (readFileErr) Wellies Name Personalised w Quadra Green New Polyester Carry Boot Bag Holder Card Holder Green Personalised Quadra Wellies Bag w Carry New Boot Card Name Polyester throw readFileErr;
  console.logfor Platform Boots CUTEHEELS l Large Parties with Velvet Heel Thick Sexy Green with and Ankle High Boots 88OwxqBF(data);
  fs.unlink('/file.md', Quadra Holder Card Polyester w New Boot Wellies Green Carry Name Bag Personalised (Fit Boots Chic Heel Pointed Denim Ankle Dress Block Toe Fashion Viva Womens Sock xAEHFPunlinkErr) => {
    if (unlinkErr)Josef Seibel Josef Seibel Women Women Women Women Josef Seibel Seibel Josef HtqUR 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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