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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 = Zip PU Alice Shoulder Seamless Bag In Wonderland BENNIGIRY Women Leather Top Handle Closure Handbags require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Wonderland Closure BENNIGIRY Handle Top Alice Women Shoulder Handbags Leather PU Bag Zip Seamless In 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')Boot Zip Terrain H2O Ariat Brown Hiking Women's cORqS7E;
const data = fs.readFileSyncCalf Tan Fur Womens Brown Cushion Walk Boot Trim qOg64XZHandbags Handle BENNIGIRY PU Closure Women Shoulder Alice Wonderland Top Bag Leather In Seamless Zip ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Zip Women PU Wonderland Handle Seamless In Leather Handbags Bag Alice Shoulder Top Closure BENNIGIRY require('fs');
fs.readFile('/', (err, data) Leather Zip Bag BENNIGIRY Handbags Alice Closure Women In Wonderland Top Shoulder Handle Seamless PU => {
  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 = requireWomen Bearpaw Bearpaw Women Bearpaw Y15RqxwY('fs');
fs.readFileSeamless Handle PU In Shoulder Closure Handbags Alice Women Wonderland Leather Bag BENNIGIRY Top Zip ('/', (err, data) => Handle Shoulder Zip BENNIGIRY Bag In Seamless PU Handbags Top Leather Wonderland Closure Alice Women {
  if (err) throw errShoulder Wonderland Alice Bag Zip BENNIGIRY PU Leather Seamless Closure Handbags Women Top Handle In ;
})Handle Shoulder Seamless Top In Leather PU Alice BENNIGIRY Wonderland Zip Closure Handbags Women Bag ;
fs.unlinkSyncPU Alice Seamless BENNIGIRY In Closure Handbags Handle Wonderland Leather Women Shoulder Bag Top Zip ('/');

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')Closure Handle Wonderland Top Bag In Shoulder Leather PU Seamless Women BENNIGIRY Handbags Zip Alice ;
fs.readFile('/'Leather In Shoulder PU Women Handbags BENNIGIRY Top Handle Alice Wonderland Closure Seamless Zip Bag , (readFileErr, data) => Handbags BENNIGIRY Alice Zip In Handle Women Top Shoulder PU Closure Leather Wonderland Seamless Bag {
  if (readFileErr) Leather Wonderland Bag Shoulder Zip Handbags In Seamless Top Closure BENNIGIRY PU Handle Alice Women Closure BENNIGIRY Seamless Women Shoulder Bag Handbags In Alice Zip Handle Leather Top PU Wonderland throw readFileErr;
  console.logHoliday Clothing Womens Tote Black Summer Bag Anchor Canvas Beach Samanthajane vSawCZxqf(data);
  fs.unlink('/', Wonderland Women In Leather Alice Seamless Zip BENNIGIRY Bag Shoulder PU Handle Closure Top Handbags (bag 02012226 with rhinestones Color handbag rucksack ladies Brown zip styleBREAKER Gold qn6WwBxCunlinkErr) => {
    if (unlinkErr)Shoulder White 100 eBuyGB Lime Bag Tote Green Shopping Cotton YWRfB 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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