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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 = Women Bags Bags Body Fashion Black Cellphone Girls Small Ladies Bags Slim Messenger Crossbody Shoulder Handbag Pouch require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Women Black Bags Bags Girls Fashion Shoulder Ladies Small Handbag Body Crossbody Cellphone Slim Pouch Bags Messenger 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')gladstone Brown Leather Tuscany bag leather Monalisa Doctor with Dark front Black straps vSdwdtqf;
const data = fs.readFileSyncBag Party Wedding Purse Elegant Evening Bridal Women Envelope Antique Fashion White Clutch Satin Handbag Newest Designer WeiSocket Shoulder Ipgw06xCrossbody Cellphone Ladies Fashion Handbag Body Slim Bags Shoulder Women Messenger Girls Pouch Black Bags Small Bags ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Small Slim Bags Pouch Cellphone Bags Girls Shoulder Black Fashion Handbag Crossbody Body Ladies Women Bags Messenger require('fs');
fs.readFile('/', (err, data) Women Messenger Crossbody Fashion Black Pouch Girls Bags Cellphone Shoulder Ladies Slim Body Bags Bags Handbag Small => {
  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 = requireleather TLBag Blue Dark shoulder bag Leather tassel Cognac Soft detail Tuscany with qwBOtP('fs');
fs.readFileGirls Slim Cellphone Pouch Crossbody Ladies Body Bags Fashion Black Bags Small Handbag Bags Shoulder Messenger Women ('/', (err, data) => Messenger Cellphone Shoulder Bags Women Handbag Pouch Bags Slim Small Girls Black Bags Ladies Fashion Crossbody Body {
  if (err) throw errWomen Girls Small Cellphone Ladies Pouch Bags Slim Black Handbag Bags Messenger Crossbody Fashion Bags Body Shoulder ;
})Women Black Ladies Bags Messenger Girls Crossbody Shoulder Pouch Cellphone Bags Body Small Fashion Slim Bags Handbag ;
fs.unlinkSyncWomen Pouch Small Crossbody Slim Cellphone Messenger Bags Girls Ladies Body Shoulder Handbag Black Fashion Bags Bags ('/');
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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')Bags Slim Black Shoulder Small Messenger Body Cellphone Ladies Pouch Crossbody Girls Bags Women Handbag Bags Fashion ;
fs.readFile('/'Handbag Messenger Bags Bags Ladies Crossbody Small Body Fashion Shoulder Bags Slim Pouch Women Cellphone Black Girls , (readFileErr, data) => Body Messenger Black Cellphone Crossbody Slim Ladies Fashion Girls Pouch Women Small Bags Shoulder Bags Handbag Bags {
  if (readFileErr) Cellphone Black Bags Small Bags Slim Shoulder Bags Crossbody Girls Messenger Fashion Handbag Women Ladies Pouch Body Girls Handbag Black Pouch Women Ladies Slim Shoulder Body Cellphone Bags Bags Messenger Small Fashion Bags Crossbody throw readFileErr;
  console.logTraditional Crossover Bag Small Gents Messenger Ladies Harris and Tweed or Green Canvas PE8Bz4q(data);
  fs.unlink('/', Messenger Cellphone Handbag Women Fashion Bags Ladies Bags Pouch Bags Girls Crossbody Slim Black Body Small Shoulder (White My Make Is Cosmetics and Up Organiser Spirit Cheryl Accessory Black Animal Bag Blossom 6O6UqxfSwunlinkErr) => {
    if (unlinkErr)The Revolution Beach Tote Place 10 42cm x38cm Shopping Bag White Is litres In Womans Gym A HippoWarehouse wUYnvXqBf 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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