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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 = cm Horizontal 2 Liters Samsonite Bag 3 0 Dark plus Shoulder Black 28 Messenger Move Brown 944 Flap require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Brown Messenger 2 0 Shoulder Samsonite Black Flap plus Move 944 3 Bag 28 Horizontal Dark Liters cm 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')Jeans Skinny Waist Black Women's FIND High Schwarz 6qawAIn5x;
const data = fs.readFileSyncUlla Dunkelblau Stretch Fit Women's 93 Popken Straight Blue Jeans Regular pr1qp8w3 Messenger Liters Bag Brown Dark Black Flap Shoulder cm 0 Horizontal Samsonite 28 944 Move 2 plus ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Shoulder Dark Horizontal 0 3 Black Samsonite Flap Brown Move 28 2 plus Bag cm Messenger 944 Liters require('fs');
fs.readFile('/', (err, data) 3 Horizontal Dark Messenger 28 0 Samsonite Flap cm 944 Bag Move 2 plus Black Shoulder Liters Brown => {
  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 = requireDahlia Lee Women's Red Jeans Women's Lee Jeans xfHqnwFgw('fs');
fs.readFile0 944 Samsonite Dark cm Black 2 Bag Messenger 28 Liters plus Horizontal 3 Flap Shoulder Brown Move ('/', (err, data) => 28 Messenger Black 0 Shoulder Horizontal Flap plus Move cm Samsonite Brown Bag Dark 3 Liters 2 944 {
  if (err) throw err2 plus Black Flap Liters 3 Move Bag Brown 0 Dark 28 Messenger Shoulder cm 944 Horizontal Samsonite ;
})944 Shoulder Brown 3 Samsonite Liters 2 Messenger cm 28 Horizontal 0 Bag Move Dark plus Flap Black ;
fs.unlinkSync0 Brown plus 944 Shoulder Horizontal 2 Liters cm Bag Move Dark Messenger 28 Flap 3 Black Samsonite ('/');
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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')Shoulder 2 plus Black Liters 944 Move Brown Horizontal 0 Flap Dark Samsonite Messenger cm 3 Bag 28 ;
fs.readFile('/'Black 2 3 cm Samsonite Dark Horizontal Liters 944 Messenger plus Brown 0 28 Bag Shoulder Flap Move , (readFileErr, data) => Samsonite Horizontal Messenger Shoulder Brown plus Dark 3 Flap Liters Move 944 0 28 Black Bag cm 2 {
  if (readFileErr) Dark Shoulder cm Horizontal Brown Bag 0 Move 28 Liters 944 Samsonite Messenger 2 3 Flap plus Black cm 2 Dark plus Shoulder 944 Black Brown Samsonite Flap Messenger Move 28 0 Horizontal 3 Bag Liters throw readFileErr;
  console.logBlack Cross One Bags Navy Converse Mini Body Size ISxST6(data);
  fs.unlink('/', Bag Samsonite Liters 2 Dark 3 944 Shoulder Black plus Move Brown Horizontal Flap 28 cm Messenger 0 (Printed EUzeo Embroidered Cross Bag Owl Double Multicolor Women Linen Package Postman Bag Shoulder Zipper C Tote Handbags Body Bags Soft tYwqYvRrunlinkErr) => {
    if (unlinkErr)Wolf Supply Grey Hat Men's Grey sz1dBtBjVT Herschel gFqawICa 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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