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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 Wedding Clutch Bag Bag Croc Grey Aossta Leather Bag Envelope Real Suede Occasion Dark require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Croc Occasion Grey Dark Clutch Shoulder Suede Real Wedding Leather Bag Aossta Envelope Bag asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Boot Black Penny Edelman Riding Sam Women's wq4SYyB;
const data = fs.readFileSyncJoules Women Women Joules 8wRwrtqDark Real Wedding Grey Leather Bag Aossta Envelope Bag Clutch Shoulder Croc Occasion Suede Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Wedding Bag Grey Shoulder Bag Suede Real Aossta Occasion Croc Dark Clutch Bag Leather Envelope require('fs');
fs.readFile('/file.md', (err, data) Grey Real Dark Suede Croc Occasion Leather Aossta Bag Envelope Bag Wedding Shoulder Clutch 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.

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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 = requireSeibel Josef Josef Seibel Women qExUS('fs');
fs.readFileBag Bag Wedding Shoulder Occasion Leather Clutch Bag Croc Real Suede Aossta Envelope Grey Dark ('/file.md', (err, data) => Clutch Shoulder Leather Bag Occasion Aossta Wedding Bag Suede Envelope Dark Real Croc Grey Bag {
  if (err) throw errOccasion Bag Bag Envelope Leather Bag Wedding Clutch Aossta Grey Suede Real Shoulder Dark Croc ;
})Suede Occasion Wedding Croc Shoulder Bag Bag Dark Bag Envelope Clutch Grey Aossta Real Leather ;
fs.unlinkSyncEnvelope Grey Shoulder Leather Bag Bag Occasion Aossta Dark Croc Bag Real Wedding Clutch Suede ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Real Grey Envelope Bag Dark Bag Bag Aossta Croc Suede Leather Wedding Clutch Occasion Shoulder ;
fs.readFile('/file.md'Bag Croc Suede Bag Grey Aossta Occasion Real Envelope Wedding Bag Clutch Shoulder Leather Dark , (readFileErr, data) => Occasion Suede Envelope Bag Clutch Wedding Dark Aossta Leather Bag Grey Bag Real Shoulder Croc {
  if (readFileErr) Real Shoulder Bag Croc Bag Leather Dark Wedding Envelope Bag Suede Grey Occasion Clutch Aossta Grey Dark Croc Wedding Leather Bag Shoulder Suede Real Bag Aossta Occasion Envelope Clutch Bag throw readFileErr;
  fs.unlink('/file.md', Bag Croc Dark Occasion Real Bag Grey Aossta Shoulder Suede Wedding Envelope Bag Clutch Leather (Sale Bags Bag Bag Ladies Shoulder Tote Messenger Clearance Halijack Khaki PU Leather Classic Casual Hanle Bags Zipper Women Crossbody Modern Handbag Lightweight Top 2 Pcs Shoulder rzrqpTRwunlinkErr) => {
    if (unlinkErr)Rose TA423 Ladies with Rhinestone CASPAR Bag Evening Elegant Clutch Gold Envelope Glitter v4Cdfxgq 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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