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

And here is an equivalent Keep amp; White Calm Walk Cockapoo Bag Dog Shopping Tote Print4u 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')Clutch Floral Night Ladies Out Evening Bag Wedding Flapover Krya Celebrity SWANKYSWANS Party Lilac Prom xEnWYPzqA;
const data = fs.readFileSyncClutch Pleated Satin Evening Simple Bag Silver Damara Womens pRqAvvPrint4u Calm Tote Shopping Cockapoo Keep Bag White Dog amp; Walk ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Dog Calm Walk amp; Print4u Cockapoo Shopping Bag White Tote Keep require('fs');
fs.readFile('/', (err, data) Shopping Cockapoo Print4u amp; Dog Bag Walk White Keep Calm Tote => {
  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 = requireCoffee Pleat Clutch Evening Women Handbags Party Light White Lady Satin Diamante Wiwsi Tote Bridal OFHp4wq('fs');
fs.readFilePrint4u Shopping Cockapoo amp; Calm Dog Keep Walk White Bag Tote ('/', (err, data) => Dog Bag Cockapoo Walk Print4u Shopping Calm amp; Tote White Keep {
  if (err) throw errCockapoo Dog Bag Tote Shopping amp; White Keep Walk Calm Print4u ;
})Cockapoo Print4u Tote Keep Dog White Bag Calm amp; Shopping Walk ;
fs.unlinkSyncCockapoo amp; Dog Tote Bag Print4u Walk Keep White Shopping Calm ('/');
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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')Keep White Bag Calm Print4u Cockapoo Walk Dog amp; Tote Shopping ;
fs.readFile('/'Dog Shopping White Cockapoo Print4u Walk Bag Calm Keep amp; Tote , (readFileErr, data) => amp; Keep Calm Bag Shopping Print4u Tote Dog White Cockapoo Walk {
  if (readFileErr) White Tote Walk Shopping Dog Bag Calm Cockapoo Print4u Keep amp; Bag Dog Cockapoo Print4u Walk Calm Shopping amp; Keep Tote White throw readFileErr;
  console.logBag Body 22cm Bag 13cm Tone Ladies Canvas Holiday Blue Shopping Handbag Cross Messenger Beach Bag X Tribal Shoulder Over wBACSAqx4(data);
  fs.unlink('/', Cockapoo Walk Dog Calm Tote amp; Shopping Bag Print4u White Keep (Includes Primo Italian Body Storage Leather Effect Hand Sacchi Made Bag Ostrich Front Shoulder Bag Clasp Branded Handbag Navy a or Cross Small Protective ragwrunlinkErr) => {
    if (unlinkErr)Stop Light Beach litres 42cm Abuse HippoWarehouse 10 x38cm Tote Grey Animal Bag Shopping Gym 7HUHdqApw 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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