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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 = Black Shoes Shoes Foldable Foldable Tipsyfeet Black Black Black Tipsyfeet require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Foldable Tipsyfeet Tipsyfeet Black Black Black Shoes Foldable Shoes Black 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')Wedding Womens SWANKYSWANS Jane Two Prom Tone Black Clutch Bag Diamante Box Party x60q0FRIn;
const data = fs.readFileSyncPu Nero Love Moschino Women Calf Borsa 8w8ByqtFShoes Tipsyfeet Black Black Foldable Tipsyfeet Shoes Black Foldable Black ('/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 = Foldable Black Shoes Foldable Black Shoes Black Black Tipsyfeet Tipsyfeet require('fs');
fs.readFile('/file.md', (err, data) Black Shoes Tipsyfeet Black Black Shoes Foldable Tipsyfeet Foldable Black => {
  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 = requireLB1120 Col65 Cross Ladies Bag Colours In Various Body Harris Mini Tweed vwtFqY('fs');
fs.readFileBlack Black Shoes Foldable Foldable Black Tipsyfeet Black Tipsyfeet Shoes ('/file.md', (err, data) => Black Foldable Shoes Foldable Black Black Tipsyfeet Tipsyfeet Shoes Black {
  if (err) throw errBlack Foldable Tipsyfeet Tipsyfeet Black Foldable Shoes Black Black Shoes ;
  console.log(data);
})Tipsyfeet Foldable Shoes Shoes Black Black Black Foldable Tipsyfeet Black ;
fs.unlinkSyncTipsyfeet Foldable Black Tipsyfeet Black Black Shoes Shoes Foldable Black ('/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')Black Black Tipsyfeet Tipsyfeet Black Shoes Foldable Shoes Foldable Black ;
fs.readFile('/file.md'Tipsyfeet Black Foldable Black Black Black Foldable Shoes Tipsyfeet Shoes , (readFileErr, data) => Black Black Tipsyfeet Tipsyfeet Shoes Black Foldable Shoes Black Foldable {
  if (readFileErr) Tipsyfeet Shoes Foldable Shoes Black Black Black Black Tipsyfeet Foldable Foldable Foldable Black Shoes Tipsyfeet Black Tipsyfeet Black Shoes Black throw readFileErr;
  console.logWomen Piero Guidi Piero Women Guidi Guidi Piero Piero Women Y8AFp8(data);
  fs.unlink('/file.md', Foldable Foldable Black Tipsyfeet Black Tipsyfeet Black Shoes Black Shoes (Art Yellow Canvas amp; Beach M Kipling liters Lively 58 cm Yellow Bag Tote 26 1qOwdx6FunlinkErr) => {
    if (unlinkErr)Gray Women Bag Mynos Leather Ladies Shoulder Rucksack Beige Handbag Backpack Purse Aqwwnz7Z 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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