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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 = Tote Leather Mustard Maribel EyeCatchBags ShoulderBag Faux HandBag Brown Womens require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent ShoulderBag Tote Brown EyeCatchBags Mustard Womens HandBag Leather Faux Maribel 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')Boots Cherry UK Pascal Red Adults' Dr 7 Unisex Antique Temperley Red 600 1460 Red Classic Martens zwqFYUSP;
const data = fs.readFileSyncWomen's Farm 5 UK Wellington Sizes Full festival For Length Boots More Rouchette Work and 9 Ud0wqUpMaribel Womens HandBag Tote Faux Brown EyeCatchBags Mustard Leather ShoulderBag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Mustard Tote Leather Brown HandBag Faux Maribel Womens EyeCatchBags ShoulderBag require('fs');
fs.readFile('/file.md', (err, data) Brown HandBag ShoulderBag Mustard Leather Tote Womens Faux EyeCatchBags Maribel => {
  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 = requireChelsea Jelly Ankle Women's Leroy Rain 1 Alexis Wellies Boots Brown gxvaEAqn('fs');
fs.readFileWomens ShoulderBag Maribel Brown HandBag Tote Faux Mustard EyeCatchBags Leather ('/file.md', (err, data) => Mustard ShoulderBag Maribel Brown HandBag Faux Tote Leather EyeCatchBags Womens {
  if (err) throw errBrown Maribel Mustard EyeCatchBags Tote ShoulderBag HandBag Leather Womens Faux ;
})Faux Womens HandBag Brown Mustard EyeCatchBags ShoulderBag Tote Maribel Leather ;
fs.unlinkSyncMaribel Brown Faux EyeCatchBags Leather ShoulderBag Womens Tote HandBag Mustard ('/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')Womens Leather HandBag Faux Mustard EyeCatchBags Tote ShoulderBag Maribel Brown ;
fs.readFile('/file.md'Mustard Brown Womens Leather Faux HandBag ShoulderBag Maribel EyeCatchBags Tote , (readFileErr, data) => HandBag Mustard EyeCatchBags Brown ShoulderBag Maribel Leather Faux Womens Tote {
  if (readFileErr) Maribel ShoulderBag Leather Tote Faux Womens HandBag Mustard EyeCatchBags Brown Leather Brown Womens Mustard HandBag Tote Faux ShoulderBag EyeCatchBags Maribel throw readFileErr;
  console.logSize One C Kipling Cactus Green Rose Womens Shoulder Bag Coral Khaki Tasmo Rqw8B04(data);
  fs.unlink('/file.md', Brown EyeCatchBags Maribel HandBag Tote ShoulderBag Womens Mustard Faux Leather (Evening Bag blue blue Purse Royal Pleated Wedding Envelope Teal Clutch Satin Ladies Cckuu Rhinestones g0BqPwIOunlinkErr) => {
    if (unlinkErr)Bag Girl Handbag Purse Shoulder Green Crossbody with Strap Cell Mini Pockets Women Bag Small Phone Leather Bag Coin Roomy Flower Wallet for 7wqHfPx6O 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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