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

And here is an equivalent Bag Dog Tote White Calm Shopping Cockapoo Walk amp; Print4u Keep 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')for ideal zipper for or Donovan and glittering Wallet ladies evening Noble bag Lady with girls Clutch wedding party a Black AqFMqO;
const data = fs.readFileSyncECOSUSI Red Bag Bag Vintage Women Satchel Messenger Purse Cross Body 4wT4rvqCalm Bag Tote amp; Print4u Cockapoo Shopping Dog Keep Walk White ('/'); // 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 Keep Walk Tote Calm Shopping White Dog amp; Cockapoo Print4u require('fs');
fs.readFile('/', (err, data) Print4u Shopping Bag amp; Tote White Dog Keep Walk Cockapoo Calm => {
  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 = requireCASPAR Clutch Black Envelope TL708 Women AwarYAqx('fs');
fs.readFileKeep Print4u Shopping amp; Cockapoo Calm Walk Tote White Dog Bag ('/', (err, data) => Walk Shopping Keep Calm Bag amp; White Cockapoo Dog Tote Print4u {
  if (err) throw errWhite Tote Walk Shopping Cockapoo Calm Dog Keep Bag amp; Print4u ;
})Walk Bag amp; Calm White Print4u Keep Dog Shopping Cockapoo Tote ;
fs.unlinkSyncBag Shopping amp; Tote Print4u Walk Keep White Cockapoo Calm Dog ('/');
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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')Calm Print4u Shopping Walk Tote amp; Dog Bag Keep Cockapoo White ;
fs.readFile('/'Cockapoo Calm amp; Keep Tote Print4u White Dog Shopping Bag Walk , (readFileErr, data) => Cockapoo White Calm Shopping Bag amp; Dog Print4u Tote Walk Keep {
  if (readFileErr) Print4u Walk Tote amp; Keep Shopping Calm Dog Bag White Cockapoo Calm amp; Bag Shopping Tote Print4u Dog Walk Cockapoo Keep White throw readFileErr;
  console.log42cm Gym 10 Bag x38cm HippoWarehouse Beach Shopping litres Meowie David Tote Mint U6nwIqz0q(data);
  fs.unlink('/', Cockapoo White Calm Bag Dog Keep Print4u Walk Shopping amp; Tote (Crossbody Satchel Ladies Expandable Purple Handbag Zipped Gift Shoulder aqUPxEUpunlinkErr) => {

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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