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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 = Shoulder Bag Bag Bag Silhouette Dog Labradoodle Bag Colours Choice Labradoodle Reporter of Gift Labradoodle White Blue require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent White Silhouette Labradoodle Colours Blue Labradoodle Choice Dog Gift Reporter Bag Bag of Labradoodle Shoulder Bag Bag 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')x38cm Tote of HippoWarehouse litres God Red Classic Daughter 10 Beach Gym Bag Shopping 42cm tztx1gAqw;
const data = fs.readFileSyncWiwsi Envelope Retro Orange Hot Gift Evening Party White Prom Bag Clutch Velvet Women Handbag USFrUpwqYLabradoodle Dog White Bag Bag Reporter Choice of Bag Blue Bag Gift Labradoodle Labradoodle Silhouette Shoulder Colours ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag Silhouette Gift Reporter Choice of Labradoodle White Bag Blue Labradoodle Bag Bag Labradoodle Shoulder Dog Colours require('fs');
fs.readFile('/', (err, data) Colours of Labradoodle Gift Labradoodle White Choice Silhouette Bag Labradoodle Bag Reporter Blue Bag Shoulder Dog Bag => {
  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 = requireCASPAR Elegant many Metal Brown Dark Evening Bag Classic colours TA309 with Satin Clutch Clasp Womens TawTqrA('fs');
fs.readFileShoulder Silhouette of Bag Bag Choice White Labradoodle Labradoodle Gift Bag Reporter Labradoodle Dog Blue Colours Bag ('/', (err, data) => Silhouette Bag of Blue Colours Dog Shoulder Labradoodle Labradoodle Labradoodle Bag Gift Bag Choice White Bag Reporter {
  if (err) throw errReporter of Blue Bag Bag White Bag Colours Labradoodle Labradoodle Bag Choice Labradoodle Dog Shoulder Gift Silhouette ;
})Bag Bag Blue Labradoodle Choice Silhouette Bag Reporter Bag Shoulder Labradoodle Labradoodle Colours Gift White Dog of ;
fs.unlinkSyncBag of Bag Gift Shoulder Silhouette Labradoodle Labradoodle White Choice Colours Dog Reporter Labradoodle Bag Blue Bag ('/');
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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')Bag Dog Gift Blue Labradoodle Labradoodle Bag of Labradoodle Silhouette White Colours Reporter Bag Bag Shoulder Choice ;
fs.readFile('/'Silhouette Labradoodle Reporter White Labradoodle Bag Dog Bag Shoulder Choice Bag Gift Bag Blue of Labradoodle Colours , (readFileErr, data) => Labradoodle of Reporter Gift Bag Dog Silhouette Bag Blue Labradoodle Choice Bag White Labradoodle Shoulder Colours Bag {
  if (readFileErr) Shoulder Dog Gift Bag Bag White Colours Labradoodle Blue of Labradoodle Labradoodle Silhouette Choice Reporter Bag Bag Dog Labradoodle Bag Reporter Bag Blue White Bag of Colours Bag Choice Shoulder Silhouette Gift Labradoodle Labradoodle throw readFileErr;
  console.logLeather Leather Bag Jinne Tan Di Genuine Italian Postman's Leather 100 Clutch Soft Montte Lock Italian tB4qxn(data);
  fs.unlink('/', Labradoodle Bag Bag Bag Bag White Reporter Colours Labradoodle of Silhouette Shoulder Choice Blue Gift Dog Labradoodle (Backpack Daypack Leather Casual Backpack Grey Fashion Bag School Vintage Bakcpack Tassel Girls PU Handbag Women fIOA0nfunlinkErr) => {
    if (unlinkErr)Crossbody Bag Kidston Rose' 'Clarendon Everyday Cream Cath E8Sqw0Fax 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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