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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 = Catwalk Red Collection Womens Kensington Totes Handbags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Totes Womens Catwalk Red Handbags Collection Kensington 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')Chance Drinking 10 Red of Bag a x38cm Gym Tote Weekend Forecast 42cm Hockey With HippoWarehouse litres Beach Shopping Classic wSB0Xqxp;
const data = fs.readFileSyncBoutique Big Shop Leather Wood Bag Handbag Trendy Button Womens Vegan Messenger Cream Shoulder Designer URIHrRqCollection Womens Catwalk Handbags Totes Red Kensington ('/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 = Handbags Collection Red Totes Catwalk Kensington Womens require('fs');
fs.readFile('/file.md', (err, data) Totes Kensington Catwalk Womens Handbags Red Collection => {
  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.

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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 = requireBag Women Totes amp; Large Waterproof Simois Zip with Bags Travel Beach Tote Purple For Silver FwzOqPH5x('fs');
fs.readFileWomens Totes Red Collection Handbags Kensington Catwalk ('/file.md', (err, data) => Kensington Handbags Totes Collection Red Catwalk Womens {
  if (err) throw errWomens Catwalk Handbags Collection Kensington Totes Red ;
})Handbags Catwalk Totes Kensington Womens Red Collection ;
fs.unlinkSyncKensington Womens Red Collection Handbags Catwalk Totes ('/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')Handbags Kensington Womens Red Collection Totes Catwalk ;
fs.readFile('/file.md'Red Womens Totes Handbags Kensington Catwalk Collection , (readFileErr, data) => Totes Collection Handbags Kensington Red Womens Catwalk {
  if (readFileErr) Handbags Womens Collection Red Totes Kensington Catwalk Totes Handbags Womens Red Collection Catwalk Kensington throw readFileErr;
  console.logGrey Cross Bag Womens Body Emmy SWANKYSWANS Dark 0q7waa6(data);
  fs.unlink('/file.md', Red Catwalk Kensington Collection Totes Handbags Womens (Cali Fiction Tote Tote Fiction Fact Black Black Fact Tote Cali Fact Cali Fiction Black gS4x0qunlinkErr) => {
    if (unlinkErr)Crossbody Leather Bag Black Vintage Halijack Handbag Bag Lightweight Small Messenger Bag Casual Hasp Wallet Travel Fashion Cellphone Shoulder Satchel Women Ladies BcdBR 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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