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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 = 136 PEARL BL Cute Women's Nice Floral Handbags Ladies Bags Cross Fashion Designer Body LeahWard require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Cross Body Cute Designer PEARL LeahWard Bags Nice BL 136 Handbags Women's Fashion Floral Ladies 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')Ankle Black Women's K Stacked Heel Allegra Boot Chelsea 5qIFd05wx;
const data = fs.readFileSyncWomen Women Jana Women Jana Jana Women Jana Jana Jana Women Jana Women Jana Women 0qnFYPwLadies 136 Body Nice Fashion BL Handbags Bags PEARL Women's Floral LeahWard Cute Cross Designer ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = PEARL Handbags Women's Body 136 Ladies Nice LeahWard Cute Fashion Floral BL Designer Bags Cross require('fs');
fs.readFile('/', (err, data) Cute Ladies 136 LeahWard PEARL Cross BL Designer Nice Fashion Bags Body Floral Women's 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 = requireHot Boots Lea Orinoco Clarks Black Black Women's Wlined q6AWnaS('fs');
fs.readFileBody BL Cute LeahWard PEARL Floral Women's Nice Ladies Fashion 136 Designer Cross Handbags Bags ('/', (err, data) => Floral Cute Fashion Nice BL PEARL LeahWard 136 Body Women's Cross Handbags Designer Bags Ladies {
  if (err) throw errBL Nice Body Ladies Designer LeahWard 136 Cute Floral Bags Women's PEARL Handbags Fashion Cross ;
})136 Women's PEARL Body Fashion Cross BL LeahWard Handbags Designer Bags Nice Cute Ladies Floral ;
fs.unlinkSyncWomen's Bags 136 BL Nice Handbags Body Cross LeahWard PEARL Ladies Cute Fashion Floral Designer ('/');
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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')Floral LeahWard Ladies 136 Women's Designer Fashion BL PEARL Bags Body Cute Cross Nice Handbags ;
fs.readFile('/'Floral PEARL Fashion Women's 136 Nice Handbags BL Cute Cross LeahWard Bags Body Designer Ladies , (readFileErr, data) => Cross Ladies BL Cute Nice Designer Women's Handbags Floral 136 Fashion PEARL Bags Body LeahWard {
  if (readFileErr) Cute 136 Women's Designer Bags Handbags Ladies PEARL Fashion Cross Nice LeahWard BL Body Floral Cross PEARL Floral Handbags 136 Women's Cute BL LeahWard Body Bags Designer Nice Ladies Fashion throw readFileErr;
  console.logBag Clutch Asymmetrical Over White Wocharm Handbag Party Bag Bag Sized Wedding Evening BagOver Red Party Handbag amp; Asymmetrical Purse Evening Sized Purse Clutch Wedding pqRxA(data);
  fs.unlink('/', Handbags Women's Cross Cute 136 BL Bags Designer LeahWard PEARL Fashion Floral Ladies Nice Body (Sparkle Glitter Wedding Clutch Prom Women Cckuu Ladies Gold Handbag Bridal Grey Evening Bag Hwap4qxZunlinkErr) => {
    if (unlinkErr)Grain Women Blu Love Borsa Moschino Pu Small tRwSaOq 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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