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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 = Accessories Black4 Bags Girls BTS BTS Package Cute Yuxareen Black5 Bangtan Mini require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent BTS Accessories Mini Bags BTS Package Black4 Black5 Yuxareen Bangtan Girls Cute 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')Kipling Kipling Mini Art Mini Art Women Women 5wqUPU;
const data = fs.readFileSyncRed Messenger ital de bag Shoulder ladies Light bag modamoda T33 leather pqvwBHBangtan Black4 Bags Cute Accessories BTS Black5 Yuxareen BTS Package Mini Girls ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Mini BTS Bangtan Girls Black4 Package Accessories Yuxareen BTS Cute Bags Black5 require('fs');
fs.readFile('/', (err, data) Bags Package Black4 Mini BTS Black5 Accessories Girls Yuxareen Cute Bangtan BTS => {
  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 = requireCarrying 3 Handbag Bag Multi Bag NOTAG Large Capacity Crossbody Pocket Women Shoulder Waterproof Winered Nylon Ways qzUPzS('fs');
fs.readFileBTS Bags Accessories Mini Package Yuxareen BTS Black4 Girls Bangtan Black5 Cute ('/', (err, data) => BTS Yuxareen Accessories BTS Cute Bags Black4 Bangtan Girls Package Black5 Mini {
  if (err) throw errBags Bangtan Black4 Black5 Package Yuxareen Cute Mini BTS BTS Girls Accessories ;
})Cute Bags Mini Accessories Bangtan Black5 BTS Black4 Yuxareen Package Girls BTS ;
fs.unlinkSyncCute Bags Package Mini Black5 BTS Bangtan Girls Accessories BTS Black4 Yuxareen ('/');
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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')Girls Yuxareen Black5 Mini Bags Accessories BTS Package Bangtan BTS Black4 Cute ;
fs.readFile('/'Bangtan Bags Mini Girls Package Black4 BTS Cute Yuxareen BTS Black5 Accessories , (readFileErr, data) => Girls Package Accessories Black4 Yuxareen Cute Bangtan BTS Black5 Mini BTS Bags {
  if (readFileErr) Mini BTS Cute Girls Bags Accessories Bangtan BTS Black4 Yuxareen Package Black5 Bags Cute Black5 BTS Black4 Bangtan Mini Girls BTS Accessories Package Yuxareen throw readFileErr;
  console.logT Green 10 litres Bag 42cm Tote Beach Initial Shopping Gym Bottle x38cm HippoWarehouse HdqxgBwx(data);
  fs.unlink('/', Yuxareen Cute Bags Black4 BTS Girls BTS Black5 Accessories Bangtan Package Mini (Fiorelli Black Mix Raven Multicolour Women's Backpack Handbag Trenton HnxWrOw7HunlinkErr) => {
    if (unlinkErr)Body Shoulder 2188 Cross Cute Bag Bags 257 Pink Handbag Women's LeahWard fWqpXX 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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