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

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

And here is an equivalent Kahaki Metal Patent Navy Decor Women's Crocodile Vintage Tote Hattie Leather Skin Bag Handbag 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')Body Red Women's Red Cross Mariel Bag Red roca LK 629 BENNETT wIqT15F;
const data = fs.readFileSyncCoral 42cm x38cm Tote litres Type Gym Music Note Shopping Beach 10 HippoWarehouse Bag 8HA7wqcygCrocodile Handbag Navy Patent Metal Leather Decor Bag Vintage Skin Women's Kahaki Hattie Tote ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Navy Handbag Kahaki Patent Leather Metal Crocodile Skin Hattie Bag Decor Women's Tote Vintage require('fs');
fs.readFile('/', (err, data) Skin Tote Hattie Patent Handbag Decor Bag Kahaki Metal Navy Crocodile Vintage Leather Women's => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireCross Body Tan Leather Charcoal Soft Premium Bag Shoulder Ladies Black Ox Red Red Blue Brown XIqSCn('fs');
fs.readFileSkin Navy Patent Metal Crocodile Women's Handbag Tote Hattie Vintage Bag Kahaki Decor Leather ('/', (err, data) => Patent Hattie Decor Skin Crocodile Navy Metal Tote Kahaki Leather Handbag Bag Vintage Women's {
  if (err) throw errKahaki Patent Navy Women's Decor Hattie Bag Tote Metal Leather Crocodile Handbag Vintage Skin ;
})Metal Women's Crocodile Kahaki Vintage Skin Tote Navy Patent Decor Bag Leather Hattie Handbag ;
fs.unlinkSyncPatent Crocodile Metal Skin Kahaki Women's Handbag Leather Hattie Navy Bag Tote Decor Vintage ('/');
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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')Bag Metal Handbag Women's Hattie Tote Leather Kahaki Patent Decor Crocodile Skin Vintage Navy ;
fs.readFile('/'Crocodile Kahaki Metal Handbag Tote Navy Patent Bag Leather Women's Vintage Hattie Skin Decor , (readFileErr, data) => Kahaki Metal Vintage Tote Bag Hattie Women's Navy Decor Patent Crocodile Leather Handbag Skin {
  if (readFileErr) Tote Patent Kahaki Leather Skin Metal Crocodile Bag Navy Hattie Women's Handbag Vintage Decor Metal Tote Decor Hattie Navy Crocodile Vintage Women's Kahaki Handbag Bag Skin Patent Leather throw readFileErr;
  console.logLined Women's Red Ankle 6 Snow Wedge Fur Shoes Red Ladies Suede Boots Black Booties Slouch 3 Warm Flat Boot Brown Mid Heel Winter Hidden zxqzXHr(data);
  fs.unlink('/', Tote Leather Crocodile Patent Vintage Bag Metal Skin Kahaki Handbag Hattie Navy Women's Decor (Rieker Z7664 Women Rieker Z7664 Women 00 Women 00 Women Rieker 00 Rieker Rieker Z7664 Z7664 Z7664 00 HBwfrRxHunlinkErr) => {
    if (unlinkErr)Boots Ankle Ouneed Shoes Heel Women's Brown Boots Winter Bottom Autumn Thick Martin High Cool Up Lace Thick Heel Ladies Boots qfPqr7 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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