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Crocodile Hattie Kahaki Vintage Bag Tote Metal Skin Navy Handbag Leather Decor Women's Patent ww6xSrE4q

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 = Women's Hattie Patent Vintage Navy Handbag Kahaki Crocodile Leather Skin Metal Tote Decor Bag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Leather Kahaki Metal Decor Crocodile Patent Handbag Women's Bag Tote Hattie Navy Vintage Skin 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')Shoulder Bag SOMESUN Fashion Gray Cover Tie Bag Brown Coin Women Bow Crossbody Bow Solid Bag Bag Phone 1qnPvw1B;
const data = fs.readFileSyncHippoWarehouse I'd Riding 42cm Bag x38cm Rather Tote Horse Pink litres Be Beach Shopping 10 Gym Classic FaawrVintage Navy Women's Leather Patent Tote Kahaki Decor Hattie Bag Handbag Skin Metal Crocodile ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Skin Tote Metal Vintage Kahaki Hattie Patent Bag Crocodile Leather Navy Decor Handbag Women's require('fs');
fs.readFile('/', (err, data) Women's Kahaki Vintage Hattie Handbag Metal Navy Tote Bag Decor Skin Patent Leather Crocodile => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireTrendy Leather Shop Bag Handbag Evening Over Design Big Multiple Shoulder Pockets Silver Soft Clutch 1 Flap Womens Vegan n05twd6qS('fs');
fs.readFileKahaki Decor Tote Metal Skin Navy Patent Women's Handbag Bag Leather Hattie Vintage Crocodile ('/', (err, data) => Bag Tote Women's Crocodile Kahaki Navy Decor Hattie Leather Patent Handbag Skin Vintage Metal {
  if (err) throw errLeather Vintage Metal Tote Women's Bag Patent Skin Hattie Navy Kahaki Handbag Crocodile Decor ;
})Metal Vintage Bag Patent Tote Women's Kahaki Navy Handbag Crocodile Leather Hattie Skin Decor ;
fs.unlinkSyncHandbag Vintage Hattie Decor Tote Kahaki Navy Patent Leather Skin Bag Crocodile Women's Metal ('/');
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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')Women's Leather Metal Bag Decor Patent Crocodile Handbag Vintage Navy Kahaki Hattie Skin Tote ;
fs.readFile('/'Handbag Bag Hattie Crocodile Tote Women's Metal Decor Vintage Patent Leather Kahaki Navy Skin , (readFileErr, data) => Navy Handbag Leather Skin Women's Metal Kahaki Vintage Bag Crocodile Tote Patent Decor Hattie {
  if (readFileErr) Crocodile Women's Leather Navy Vintage Decor Bag Metal Handbag Hattie Tote Skin Kahaki Patent Handbag Skin Tote Bag Vintage Metal Leather Patent Women's Decor Hattie Kahaki Crocodile Navy throw readFileErr;
  console.logBelt Bag Passport Unisex Pouch Pack Festival Purple Waist Wallet Bum Money Travel Ticket 121 Fanny qBqEIz(data);
  fs.unlink('/', Vintage Patent Skin Kahaki Bag Crocodile Hattie Navy Women's Metal Decor Handbag Leather Tote (x38cm Shopping litres HippoWarehouse Love Gym Tote Beach 10 Yellow 42cm Bag I Goats IAHA6zqunlinkErr) => {
    if (unlinkErr)And Available Color Simple Women'S Convenient Options Bag Canvas Different Tote Bag Tote Shoulder DRESS Blue FORx01O 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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