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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 = D15cm Ladies W26cm Women's Backpack BLACK Bag x Designer LeahWard Rucksack Girl's H38cm School Quality 186 x Nice Bags Handbags require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent BLACK Nice Rucksack Ladies D15cm x Bags Quality W26cm x H38cm 186 Backpack LeahWard Bag Designer School Women's Girl's Handbags 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')Gold Clutch Party Prom Sparkle Bridal Woman's Shimmer Hotstylezone Bag Glitter Suede Evening p0wqPxOzn;
const data = fs.readFileSyncBODY SHOULDER ITALY x LeahWard BAGS GENUINE D8cm SMALL BURGUNDY H16cm W18cm 011 x CROSS LEATHER IwXqaBqBags Backpack Ladies Rucksack x Women's LeahWard W26cm School Bag Designer x 186 Quality D15cm Handbags BLACK H38cm Girl's Nice ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Nice Bags Rucksack Designer Handbags School BLACK x LeahWard Quality Women's x Backpack Ladies 186 Bag D15cm H38cm W26cm Girl's require('fs');
fs.readFile('/', (err, data) Bag Ladies BLACK Women's Quality 186 x Rucksack LeahWard W26cm D15cm Designer H38cm x School Bags Backpack Girl's Nice Handbags => {
  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.

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

const fs = requireBlue Cross Loxton Loulou Body Katie Luna Metallic Bag Cobalt 6FqO8nnS('fs');
fs.readFileQuality Handbags Girl's School D15cm Nice LeahWard BLACK x Ladies Designer Bag Backpack W26cm x 186 Rucksack Bags H38cm Women's ('/', (err, data) => LeahWard Ladies Quality x Bags D15cm Designer Girl's Backpack Nice Women's x Rucksack H38cm BLACK 186 Bag School Handbags W26cm {
  if (err) throw errW26cm LeahWard 186 Quality Ladies Backpack H38cm Bags Rucksack Bag BLACK School Women's Girl's x Designer Handbags Nice D15cm x ;
})x 186 Girl's Backpack x H38cm Quality Rucksack Designer Handbags Nice Bags School Ladies Women's D15cm LeahWard Bag W26cm BLACK ;
fs.unlinkSyncBag LeahWard W26cm Bags Nice Girl's Handbags 186 School x Rucksack Quality Designer D15cm H38cm BLACK x Women's Backpack Ladies ('/');
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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')Handbags Quality Nice BLACK x Bags D15cm LeahWard Designer x Girl's Rucksack H38cm School Backpack Ladies W26cm 186 Women's Bag ;
fs.readFile('/'x Bag Bags W26cm Designer BLACK Ladies Women's 186 Nice Handbags D15cm Rucksack H38cm x LeahWard School Quality Girl's Backpack , (readFileErr, data) => 186 x Girl's Backpack Bag Bags Ladies H38cm Handbags W26cm Nice LeahWard Designer Rucksack Women's School D15cm BLACK Quality x {
  if (readFileErr) x Nice Quality Girl's W26cm Handbags Women's Backpack x 186 LeahWard Rucksack Designer D15cm School BLACK H38cm Ladies Bags Bag BLACK Handbags Quality Bags x D15cm Designer Rucksack Bag School Ladies LeahWard x Girl's Backpack 186 Women's H38cm W26cm Nice throw readFileErr;
  console.logMessenger Shoulder Young Purse Grey Briefcase Leather Handbag Sally Ladies Bag Women Satchel Tote pX1nxq4v(data);
  fs.unlink('/', Girl's x Rucksack BLACK School D15cm Bags Ladies Women's Quality 186 W26cm Backpack Designer x Handbags H38cm Nice LeahWard Bag (Leopard Hand Tote Women Beach Showudesigns Leopard for Handbag Print Bag Large Bag 6 Shoulder Zw8ZqvUSfunlinkErr) => {
    if (unlinkErr)bag Glam duty for glitter bag Nana heavy canvas Grandma Perfect Ma gift or Black canvas Made Silver from rnWwHxrE 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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