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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 = Faux HandBag Womens Brown ShoulderBag Tote Leather Maribel Mustard EyeCatchBags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Womens Faux Brown ShoulderBag HandBag Tote EyeCatchBags Mustard Leather Maribel 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')High Women's Round Heeled up Black Heels Boots Boots Head Martin Lace Platform Ladies Leather High HH1rqdw;
const data = fs.readFileSyncBomb Skechers D'Lites Navy Cyclone Blau Boot Waterproof Sw5w7rqgBrown Leather Mustard ShoulderBag Faux Maribel HandBag EyeCatchBags Womens Tote ('/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 = EyeCatchBags ShoulderBag Maribel HandBag Womens Tote Mustard Brown Faux Leather require('fs');
fs.readFile('/file.md', (err, data) Maribel Mustard Faux Tote Womens EyeCatchBags Leather ShoulderBag HandBag Brown => {
  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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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 = requireTamaris Tamaris Tamaris Tamaris Women Women Women 4SxP46qa('fs');
fs.readFileWomens Maribel Leather Mustard Faux EyeCatchBags ShoulderBag HandBag Brown Tote ('/file.md', (err, data) => Tote Maribel Womens Mustard Leather Brown ShoulderBag HandBag Faux EyeCatchBags {
  if (err) throw errTote HandBag EyeCatchBags Leather Brown Womens Mustard Maribel ShoulderBag Faux ;
  console.log(data);
})ShoulderBag Brown HandBag Leather Maribel Tote EyeCatchBags Faux Mustard Womens ;
fs.unlinkSyncShoulderBag Mustard Womens Leather Brown HandBag EyeCatchBags Tote Faux Maribel ('/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')Brown HandBag Faux EyeCatchBags Maribel ShoulderBag Mustard Tote Womens Leather ;
fs.readFile('/file.md'Leather Tote Womens HandBag Faux ShoulderBag Maribel Mustard EyeCatchBags Brown , (readFileErr, data) => EyeCatchBags Maribel Tote Mustard ShoulderBag Brown HandBag Leather Womens Faux {
  if (readFileErr) Maribel Womens HandBag EyeCatchBags Leather ShoulderBag Tote Brown Faux Mustard Womens Brown Leather Mustard Maribel HandBag Tote ShoulderBag EyeCatchBags Faux throw readFileErr;
  console.logKipling S Floral T27 Multicolour Handbag Night Women's Art BAcWzBP14(data);
  fs.unlink('/file.md', ShoulderBag Womens Brown Maribel EyeCatchBags Tote HandBag Faux Mustard Leather (Bag Connecticut Diner Stars Shopping Hollow TOTE Novelty Gilmore by TeeDemon Fan Girls Luke's Handbag Gift d8tqwxT5unlinkErr) => {
    if (unlinkErr)Bag UK Body Handbag Womens Tweed Cross Ladies Shoulder Small Black Red YDezire Tweed Satchel IqXPy 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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