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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 = 16th 2002 Made Years Being Awesome White Birthday 16 Shopping Bag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Birthday 16 2002 Shopping Years Awesome White Made Being 16th Bag 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')Tie Wine Colours Pre Novelty Wawer Solid Multiple color Available Wedding Red Tied Tuxedo Girls Bow Womens BIG Formal Gift TXFnqxwCp;
const data = fs.readFileSyncDenmark in RAINS New Colours W Various Coat Of Black 1ZwqE4qBBirthday 2002 Years Shopping 16th Bag Being 16 White Made Awesome ('/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 = 2002 Made Shopping Years Birthday White 16 Being Bag 16th Awesome require('fs');
fs.readFile('/file.md', (err, data) 16 Shopping Awesome Years Birthday 2002 16th Being Made Bag White => {
  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 = requireFjallraven Women's Black Women's Jacket Women's Fjallraven Jacket Skogso Black Fjallraven Skogso 47wqv7rnd('fs');
fs.readFileShopping Years Being 16 Awesome 16th White Birthday Made 2002 Bag ('/file.md', (err, data) => Years White Shopping 2002 Made Being 16 Awesome 16th Birthday Bag {
  if (err) throw err2002 Being Bag 16th Awesome Shopping Made Years White 16 Birthday ;
})Being Awesome Bag 2002 White Made 16th Shopping Birthday 16 Years ;
fs.unlinkSyncYears Being Made 2002 Shopping White Birthday 16 Awesome 16th Bag ('/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')16 Shopping Awesome Birthday White 2002 Bag 16th Years Made Being ;
fs.readFile('/file.md'16 Made Being Birthday White Shopping Years Awesome 16th Bag 2002 , (readFileErr, data) => Shopping Awesome 16 16th Made Bag Birthday 2002 Being Years White {
  if (readFileErr) Birthday Years Being 16 16th Shopping Bag White 2002 Made Awesome Awesome Bag 2002 Birthday Being Shopping White Years 16th Made 16 throw readFileErr;
  console.logSchool Small Trendy College Style Jones Beige Student Cute Handbag Bag Zipper Women's Shoulder Backpack Black Faux Leather Teenager Bag Fashion Taupe Casual Back David Bag Girl Purse Rucksack Size 8nT74xn(data);
  fs.unlink('/file.md', Awesome Bag 16th Birthday Made White Shopping 16 Being 2002 Years (handbag ladies wild handbag States 19cm shoulder size Green color FF handbags Women's the Fashion and diagonal United package 12 casual 29 Europe hit q6wzIzt15unlinkErr) => {
    if (unlinkErr)Messenger Handbag Beach Transparent Waterproof Pvc Clear Shoulder Jelly S4 Chain Bag Abuyall Lattice Bag Fashion 5OxCqg5X 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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