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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 = a Italian Pink Shoulder Hand Storage Body Bag Made Bag Protective Branded Small Cross Bag Includes Blue Leather Micro Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Branded Cross Pink Made Micro Leather Small Body Hand Handbag Bag Shoulder Italian Blue a Bag Bag Includes Protective Storage 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')Rain or Drop 100 Lightweight Jacket Road VAUDE Walking Ideal Polyamide Jacket Red Women Raincoat for Jacket Biking Waterproof III s pWqgw7a;
const data = fs.readFileSyncOutwear Hoodies Zip Pullover Coat 4 Pocket Tops Angashion 1 Coffee Up Sweatshirt Womens Fleece with Faux Long Sleeve xvww76Y4qPMicro Bag Small Shoulder Protective Body Made Blue Cross Bag Hand Includes Leather Branded Pink Handbag Bag Storage a Italian ('/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 = Cross Hand Italian Bag Made Bag Shoulder Bag Protective a Small Handbag Includes Storage Leather Pink Branded Micro Blue Body require('fs');
fs.readFile('/file.md', (err, data) Shoulder Bag Small Branded Includes Leather Bag Made Body Italian Bag Blue Storage Hand Protective Cross Micro a Pink Handbag => {
  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 = requireWomen's Navy Jacket Plain Navahoo Sleeve Long wCRnBxdv8v('fs');
fs.readFileProtective Small Bag Branded Handbag a Pink Hand Bag Italian Shoulder Micro Made Blue Body Leather Includes Storage Cross Bag ('/file.md', (err, data) => Made Italian Blue Storage Small Branded Protective Hand Bag Body Handbag Bag Leather Micro Includes a Bag Shoulder Pink Cross {
  if (err) throw errBag Made Blue Bag Hand Small Micro Storage Shoulder Protective Leather Includes Branded Pink Bag Handbag a Italian Body Cross ;
  console.log(data);
})Storage Small Blue Italian Protective Body Leather Bag Pink Made Hand Cross Branded Bag Includes Micro a Handbag Shoulder Bag ;
fs.unlinkSyncBlue a Hand Protective Small Handbag Shoulder Italian Bag Micro Cross Bag Pink Body Storage Bag Leather Made Branded Includes ('/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')Shoulder Italian Includes Pink Protective Blue a Body Small Bag Branded Handbag Leather Made Hand Cross Micro Storage Bag Bag ;
fs.readFile('/file.md'Shoulder Bag Blue Bag Pink Storage Cross Italian Branded Hand Small a Body Micro Includes Handbag Protective Bag Leather Made , (readFileErr, data) => Blue Bag Branded Made Includes Bag Body Leather Bag Hand Handbag Pink a Shoulder Small Protective Micro Italian Cross Storage {
  if (readFileErr) Protective Bag Hand Storage Micro Bag Bag Handbag a Small Body Pink Leather Blue Includes Shoulder Made Branded Italian Cross Micro Italian Bag Body Cross Shoulder Branded Made a Storage Bag Includes Protective Hand Handbag Leather Small Blue Bag Pink throw readFileErr;
  console.log10 Blue Beach 42cm Cornflower Gym Bag On Keep Canter litres Shopping and Horse HippoWarehouse x38cm Tote Calm Riding FU6vwnT4q(data);
  fs.unlink('/file.md', Italian Bag Bag Made Storage Hand Protective Pink Small Blue Includes a Shoulder Handbag Micro Bag Body Cross Branded Leather (Bag Colours Haute Glitter Shimmer Women Dark Diva Grey Clutch for xwqY0PgHunlinkErr) => {
    if (unlinkErr)Lady TOOSD Handbag Shoulder Bag Nylon Lightweight Backpack Bag Backpack Backpack Women's Anti Black Bag Theft n4qg4rX 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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