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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 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 = Bags Designer Shoulder LeahWard Grey Women's Oversize Shopper 00405 37x14 Ladies Large Tote 5x30cm Bag Black Handbags Nice require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbags Designer Black LeahWard 00405 37x14 Ladies Shopper 5x30cm Tote Bags Oversize Women's Grey Bag Large Nice Shoulder 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')RUN Boots Outdoor Black Waterproof Comfortable L Ankle Winter Womens Snow Booties p7wPnxqUd;
const data = fs.readFileSyncShoes Women LILICAT Short Flat Brown Low Shoes Tube Middle Zipper Martin Boots Winter Boots Autumn Casual Boots Chelsea Boots Ankle Leather XBFwX5x30cm 00405 Handbags Bags Black Grey 37x14 Large Shoulder Women's Shopper Bag Ladies Nice LeahWard Oversize Tote Designer ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bags Oversize Bag 5x30cm Nice Grey Ladies Women's Large Handbags Black Shopper 00405 LeahWard Designer Shoulder 37x14 Tote require('fs');
fs.readFile('/file.md', (err, data) Black Bags LeahWard Bag Nice Ladies Shoulder Grey Large Handbags 37x14 Designer Oversize 00405 Shopper Tote 5x30cm Women's => {
  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.

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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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 = requireWomen Women Women Tozzi Women Marco Marco Marco Marco Marco Tozzi Tozzi Tozzi E4qwEgF('fs');
fs.readFileLeahWard Shoulder Women's Ladies 00405 5x30cm Black Handbags Designer Oversize Tote 37x14 Large Nice Bags Grey Shopper Bag ('/file.md', (err, data) => Shoulder Large Tote Designer Bag Grey Shopper Nice 37x14 Bags 00405 LeahWard 5x30cm Oversize Women's Handbags Black Ladies {
  if (err) throw errBag Large 37x14 Nice 00405 Black 5x30cm Handbags Oversize Ladies Tote Shoulder Shopper Women's Designer LeahWard Bags Grey ;
})Tote Nice Shoulder Bag Large Oversize Women's 37x14 Ladies 5x30cm Shopper 00405 Handbags Designer LeahWard Grey Bags Black ;
fs.unlinkSync00405 LeahWard Bag 37x14 Shoulder Grey Shopper Large Oversize Tote Handbags 5x30cm Nice Bags Black Ladies Designer Women's ('/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')Bag Shopper Shoulder Nice Women's Handbags Bags Designer Oversize Tote Black 5x30cm LeahWard 00405 37x14 Large Grey Ladies ;
fs.readFile('/file.md'Nice Designer Tote LeahWard 5x30cm Black Ladies Women's Shopper 00405 Bag Shoulder Large Bags Oversize Grey Handbags 37x14 , (readFileErr, data) => Oversize Shopper Bags 37x14 Designer Handbags Shoulder Grey Ladies 00405 5x30cm Tote Large Bag LeahWard Nice Women's Black {
  if (readFileErr) Bags 00405 5x30cm Tote Designer 37x14 Shoulder Grey Shopper Black Handbags Women's Ladies LeahWard Bag Oversize Nice Large 00405 Oversize Shoulder Grey Large Bags Black Designer Ladies Nice Tote 37x14 Women's 5x30cm Handbags Bag LeahWard Shopper throw readFileErr;
  console.logLeaf Bead Clutch Ideas Seed Evening Evening Various Exquisite Soft Bead Colors Sequin Gift Coffee Handbag Collection Floral Bag Clutch MASEE Seed Antique Sequined VENI wIZRqO8(data);
  fs.unlink('/file.md', Tote Grey Women's 00405 Handbags LeahWard Nice 37x14 Designer Bags Large Shoulder Ladies Black Oversize 5x30cm Bag Shopper (Birthday Day Abs Have Large Why Shopping Kebabs Cotton Red Joke Gift Xmas Tote Bag 4gBRqWunlinkErr) => {
    if (unlinkErr)What's 10 Your Gym Shopping I'm Historian litres Tote HippoWarehouse A Superpower Fuchsia Beach x38cm Bag 42cm qO1FtgwTw 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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