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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 = White Tote Calm Print4u Walk Dog amp; Keep Shopping Cockapoo Bag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Calm amp; Shopping Walk White Cockapoo Print4u Keep Bag Tote Dog 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')Blue David Women's Bag Jones Cm3752 Cm3752 Shoulder Blue WYaq7nYwB;
const data = fs.readFileSyncMessenger Design Shop Glossy Fashion Womens Bag Handbag Green Vegan Patent Big Shoulder 1 Leather Lime 58WBqUwIxxWalk Keep White Dog Cockapoo Shopping Calm amp; Bag Tote Print4u ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Cockapoo Shopping Tote Dog Bag Walk Print4u White Calm amp; Keep require('fs');
fs.readFile('/', (err, data) Tote amp; Calm White Cockapoo Walk Keep Print4u Bag Dog Shopping => {
  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 = requirePurse Luxury Envelope Diamante Design Shoulder Clutch Handbag Large Bags Long 2 New Ladies Flap Womens Red zfyw7xH('fs');
fs.readFileTote Bag Dog amp; White Shopping Keep Calm Walk Print4u Cockapoo ('/', (err, data) => Walk Tote Shopping Dog Keep Cockapoo Calm Bag amp; Print4u White {
  if (err) throw errCockapoo Calm Walk Shopping Tote amp; Print4u Dog Keep Bag White ;
})Shopping White Bag Keep Print4u Calm Dog amp; Walk Tote Cockapoo ;
fs.unlinkSyncCockapoo Tote Print4u amp; Bag Dog White Calm Walk Shopping Keep ('/');

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')Walk Dog Bag Print4u amp; Tote Shopping Cockapoo Keep Calm White ;
fs.readFile('/'amp; Tote Calm Shopping Keep White Print4u Dog Bag Cockapoo Walk , (readFileErr, data) => Shopping White Calm Dog Bag Tote Keep Print4u amp; Walk Cockapoo {
  if (readFileErr) Print4u White Dog Cockapoo amp; Bag Walk Keep Tote Calm Shopping Tote White Shopping Walk Keep Bag Cockapoo Print4u amp; Dog Calm throw readFileErr;
  console.logShoulder Women Shoulder Handbag Bags Tote Fashion Pretty Ladies Single Purse Green Tassel Bags Travel Bag Leather Handbag TUDUZ Large wzxqFnH(data);
  fs.unlink('/', Print4u Cockapoo Walk Keep amp; White Shopping Tote Dog Bag Calm (Crossbody Women amp;Doris for Work Bag Fashion Sale Blue Shoulder New Ladies Handbag Purse Tote Nicole black Hot EqwdYqunlinkErr) => {

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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