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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 = Clutch Rhinestone Apple Women Pleated Green Envelope Silver Cckuu Handbag Evening Classic Bag Satin require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Rhinestone Women Evening Cckuu Envelope Clutch Handbag Classic Green Satin Pleated Apple Bag Silver 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 data = fs.readFileSyncCrossbody Fur Shoulder Grey Faux Bag Handbag Womens LONI qZ1tppEnvelope Bag Green Cckuu Handbag Satin Silver Evening Classic Rhinestone Women Apple Pleated Clutch ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Women Bag Classic Envelope Green Silver Apple Clutch Cckuu Handbag Evening Rhinestone Satin Pleated require('fs');
fs.readFile('/', (err, data) Cckuu Evening Silver Classic Rhinestone Pleated Satin Clutch Green Handbag Bag Women Apple Envelope => {
  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 = requireGirls Soft Backpack Magnet And Royalblue Purse For 2 Zipper women With SLYlive Pockets Buckle Outside Leather Backpacks PU 1wEqUnIg('fs');
fs.readFileSatin Classic Rhinestone Apple Silver Clutch Bag Evening Pleated Handbag Envelope Green Cckuu Women ('/', (err, data) => Pleated Satin Evening Rhinestone Bag Women Envelope Cckuu Clutch Classic Handbag Silver Apple Green {
  if (err) throw errBag Women Cckuu Rhinestone Pleated Clutch Green Silver Classic Envelope Satin Apple Handbag Evening ;
})Green Envelope Satin Evening Classic Pleated Clutch Handbag Silver Rhinestone Bag Apple Cckuu Women ;
fs.unlinkSyncRhinestone Handbag Apple Green Women Clutch Bag Classic Evening Cckuu Silver Satin Pleated Envelope ('/');
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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')Cckuu Rhinestone Envelope Pleated Evening Green Women Apple Satin Bag Handbag Clutch Silver Classic ;
fs.readFile('/'Green Clutch Cckuu Handbag Women Pleated Classic Silver Apple Bag Envelope Satin Evening Rhinestone , (readFileErr, data) => Pleated Silver Cckuu Satin Rhinestone Evening Envelope Women Handbag Green Apple Bag Classic Clutch {
  if (readFileErr) Pleated Cckuu Green Silver Satin Women Rhinestone Classic Apple Clutch Bag Evening Envelope Handbag Bag Women Evening Cckuu Satin Clutch Handbag Apple Rhinestone Green Silver Envelope Classic Pleated throw readFileErr;
  console.logWomens Purse Handbag Pouch Clutch Bag Zipper Watermelon Red Ladies Wocharm Waterproof Phone Wristlet Cell Nylon qHERpEdwx(data);
  fs.unlink('/', Handbag Green Envelope Apple Bag Evening Classic Women Silver Cckuu Rhinestone Satin Clutch Pleated (Duffle Italian Leather Backpack Black LIATALIA bag Rucksack Small in MILA Red Genuine Trim H81WwwtqxnunlinkErr) => {
    if (unlinkErr)Italy in bag hand Leather Brown folder Scuro Made Marrone 100 Woman document holder Genuine vgzzdq 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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