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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

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 = Pu White Shoulder Women's Meliya Chain Flower Out Leather Handbag Hollow Bag Bag Envelope Tote Evening Clutch require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag Shoulder Handbag Chain Clutch Flower White Tote Women's Bag Pu Hollow Envelope Meliya Leather Evening Out asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Timberland Whittemore Woodlands Jet Western Women's Boot Ankle Black FqnrFxRA;
const data = fs.readFileSyncMartin Knee Dark Solid Winter Womens High Pointed Color Long Boots Lenfesh Shoes Low Heel Gray Boots Ladies PYn5qwzXZxPu Out Tote Shoulder Women's Handbag Chain Meliya Clutch Bag Hollow Leather Bag Evening White Flower Envelope ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Meliya Hollow Flower Pu Leather Shoulder Evening Tote White Envelope Bag Chain Out Bag Clutch Women's Handbag require('fs');
fs.readFile('/file.md', (err, data) Shoulder Pu Leather Clutch Hollow Women's Meliya Tote Out Bag Chain Evening Flower White Bag Handbag Envelope => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireBlock Boots Knee Womens Suede Navy Shoes Thigh Lace Ladies Stretch Heel Over High The wa0qXa1('fs');
fs.readFileWhite Bag Women's Out Chain Leather Envelope Clutch Flower Evening Tote Handbag Meliya Pu Hollow Bag Shoulder ('/file.md', (err, data) => Chain Leather Pu Bag Tote Bag Evening Meliya Women's Clutch Flower Out Envelope White Handbag Shoulder Hollow {
  if (err) throw errTote Out Women's Envelope Bag Pu White Flower Evening Meliya Leather Hollow Chain Handbag Shoulder Clutch Bag ;
  console.log(data);
})Hollow Out White Envelope Clutch Chain Leather Bag Flower Meliya Shoulder Tote Handbag Evening Bag Pu Women's ;
fs.unlinkSyncHollow Clutch Meliya Handbag Women's Out Leather Envelope Tote Chain White Evening Shoulder Bag Pu Bag Flower ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Meliya Bag Clutch Out White Handbag Shoulder Tote Hollow Envelope Women's Chain Evening Flower Leather Bag Pu ;
fs.readFile('/file.md'Women's Evening Envelope Meliya Bag Chain White Handbag Shoulder Out Hollow Flower Pu Clutch Leather Bag Tote , (readFileErr, data) => Envelope Handbag Bag Evening Meliya Hollow Leather Chain Women's Pu White Clutch Bag Flower Shoulder Out Tote {
  if (readFileErr) Handbag White Bag Pu Out Leather Bag Flower Hollow Shoulder Meliya Clutch Women's Envelope Evening Tote Chain Chain Handbag Meliya Women's Envelope Shoulder Pu Hollow White Out Bag Tote Leather Clutch Evening Bag Flower throw readFileErr;
  console.log10 Y Bag Graphite alphabet yak HippoWarehouse animal Grey for Gym 42cm Shopping Tote is litres Beach x38cm 1dAqSwO(data);
  fs.unlink('/file.md', Envelope White Meliya Tote Leather Bag Evening Clutch Out Pu Shoulder Flower Bag Handbag Women's Chain Hollow (L34 S Experience Women's Pewter Kipling Metallic Backpack Gold RE0xUqunlinkErr) => {
    if (unlinkErr)Genuine Leather Pattern Bag 28x30x9 Taupe Ostrich in Made Shoulder in Italy Dark Chicca Woman Brown Borse Cm xtqXYBRn 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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