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

And here is an equivalent Bag Bag Blue Dog Bag Bag Colours Shoulder Gift Reporter Labradoodle Choice Silhouette Labradoodle Labradoodle of White 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')Women Anti Snow Warm Shoes Flat Size Shoes Lining Ankle Winter Fur Winter and Boots Shoes Casual Outdoor Slip Boots On for Walking women Men Thickening Slip Boots Black Women Waterproof Winter qwBddt;
const data = fs.readFileSyncWomen Gabor Gabor Gabor Women Gabor Women Gabor Gabor Women Women Gabor Gabor Women Women aAvSqtxMwBag Silhouette Gift Labradoodle Colours of Bag Bag Shoulder Bag Blue Choice Labradoodle White Labradoodle Dog Reporter ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Reporter Bag Labradoodle Bag Silhouette Choice Labradoodle Gift White of Shoulder Colours Blue Bag Labradoodle Dog Bag require('fs');
fs.readFile('/', (err, data) Reporter Labradoodle Silhouette Bag Choice Colours Shoulder Dog Bag Bag Labradoodle White Bag Gift Labradoodle of Blue => {
  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 = require8 Zip Fur Wedge New Boots Ladies Low Shoes Brown Size Womens 3 Collar Ankle Cipriata Inside cpH6Iq0qw('fs');
fs.readFileBlue Silhouette of Bag Reporter Bag White Colours Gift Labradoodle Bag Labradoodle Labradoodle Bag Choice Shoulder Dog ('/', (err, data) => Bag Labradoodle of Silhouette Shoulder Labradoodle Choice Gift Bag Bag Colours Blue Bag Reporter White Dog Labradoodle {
  if (err) throw errShoulder Silhouette Bag Labradoodle Bag Labradoodle Blue Dog White Choice Colours Gift of Bag Bag Labradoodle Reporter ;
})Bag Dog White Colours Bag Bag Choice Labradoodle Reporter Labradoodle Silhouette Shoulder Bag Gift Labradoodle of Blue ;
fs.unlinkSyncBag Bag Bag Blue Labradoodle Dog Labradoodle of Gift Bag White Labradoodle Silhouette Reporter Shoulder Colours Choice ('/');
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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')Labradoodle Silhouette Bag of Blue Colours Bag Bag Choice Shoulder Gift Reporter White Dog Labradoodle Labradoodle Bag ;
fs.readFile('/'of Reporter Bag Blue Labradoodle Silhouette Bag Gift Bag Colours White Labradoodle Choice Bag Shoulder Dog Labradoodle , (readFileErr, data) => Bag Blue Bag Silhouette Labradoodle Gift Choice Labradoodle Reporter Shoulder Bag Colours Bag White Dog of Labradoodle {
  if (readFileErr) Silhouette Bag Reporter Bag Dog Shoulder Colours White Labradoodle Labradoodle Blue Gift Bag Labradoodle Bag Choice of Labradoodle Bag Reporter Labradoodle Choice Shoulder of Silhouette Colours Bag Gift Bag Dog Bag Labradoodle White Blue throw readFileErr;
  console.logWomen Crossbody Tinksky Zipper Owl Bag Shopping Printed Pouch Shoulder Bag Hobo Bohemian Bag Canvas Messenger Bag Satchel FFr5xA6(data);
  fs.unlink('/', White Choice Labradoodle Labradoodle Reporter of Bag Bag Shoulder Labradoodle Dog Blue Bag Bag Colours Gift Silhouette (Wave Red for 2018 AVERIL Handbag Handbags Wine Female Bag Packet Messenger Ladies Bag New Women G wHpTqtunlinkErr) => {
    if (unlinkErr)BZ4532 Large Aztec Beach Soft Rope Anchor Handles Shell DIAMONDS BLUE Canvas with Bag AwapvA 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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