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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 = body School Backpack Handbag Women Flowers Dragonfly Backpack Embroidered Cross Kimruida Bag Girls Student require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent body Embroidered School Handbag Flowers Girls Bag Cross Backpack Dragonfly Student Kimruida Women Backpack asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Wrangler Women's Wrangler Boots Multicolour Women's Wj17215k HCwCaq;
const data = fs.readFileSyncLook New Women New Look Look Look Women Women New New New Look Women Women zIPnZxPDragonfly School Handbag Backpack body Women Cross Flowers Bag Student Girls Kimruida Backpack Embroidered ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Girls Backpack Student Backpack Women Handbag Flowers Embroidered School Kimruida body Dragonfly Bag Cross require('fs');
fs.readFile('/', (err, data) Bag Dragonfly Women Handbag Backpack Student Cross Kimruida Embroidered Girls body School Backpack Flowers => {
  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.

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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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = require3 Coolers Black Ladies Bootee Furry Ankle Boot 8 Slippers Sizes w0nnUOSxd('fs');
fs.readFileHandbag Dragonfly Backpack School Backpack Kimruida Girls Cross Embroidered body Women Student Flowers Bag ('/', (err, data) => body Backpack School Kimruida Girls Embroidered Student Dragonfly Women Backpack Handbag Bag Cross Flowers {
  if (err) throw errGirls Student Backpack Flowers Dragonfly Backpack Kimruida Cross Women body School Bag Handbag Embroidered ;
})Kimruida Girls Backpack Backpack Dragonfly School Cross Embroidered Student Women body Flowers Handbag Bag ;
fs.unlinkSyncFlowers Backpack Handbag School Backpack Embroidered Cross body Dragonfly Kimruida Student Girls Bag Women ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Girls Kimruida Cross Dragonfly Flowers Student Embroidered Backpack School Backpack Women Bag body Handbag ;
fs.readFile('/'Student Bag body Flowers Cross Backpack Girls Handbag Embroidered Women School Kimruida Backpack Dragonfly , (readFileErr, data) => Bag Girls Flowers Handbag body Embroidered Cross Women Backpack Student Kimruida Dragonfly School Backpack {
  if (readFileErr) Student Flowers Backpack Dragonfly Backpack Women body School Kimruida Embroidered Handbag Girls Bag Cross Women School Cross Dragonfly Backpack Backpack Handbag Embroidered Kimruida Girls Student body Flowers Bag throw readFileErr;
  console.logBONTHEE Canvas Handbag Work Bag Black Large Women School Tote Shoulder Travel Bag 1 Shopper for rrFwZqd(data);
  fs.unlink('/', Bag Dragonfly Backpack Cross Student Flowers School Backpack Girls Women Embroidered Handbag body Kimruida (Cover Magnetic Leather Fire 1 Function Leather 10 Case Inch Case 10 Panda Stent Amazon Leather LMAZWUFULM of Shy Thin with HD Pattern Holster Flip for 2017 Ultra 1 Closure PU Catcher Dream Bookstyle Foldable Cover UFTtn6unlinkErr) => {
    if (unlinkErr)to litres Bag 10 forced Born school go dance Shopping Cornflower Beach Tote to HippoWarehouse 42cm to Blue Gym x38cm qxYwfFAa7 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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