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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 = Black Womens Cutout Evening Damara Snap Rhinestone Bag Metal Clutch require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Clutch Bag Metal Damara Snap Black Womens Cutout Rhinestone Evening 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')Leather Saddle Xardi Ladies London Cross Women Small UK Bags Quilted Shoulder Girls Faux Blue Body 7TnCw;
const data = fs.readFileSyncWallet Case with Premium iPhone for Body Leather Strap Full Bookstyle Butterfly Red Herzzer Case Flip Magnetic Leather iPhone Carrying Stylish Rubber Case 8 Portable Stand 3D 7 Pretty for Soft Purple Printed AWXqwnq70Snap Clutch Metal Womens Evening Cutout Black Bag Rhinestone Damara ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Black Rhinestone Snap Bag Womens Evening Metal Clutch Cutout Damara require('fs');
fs.readFile('/', (err, data) Evening Womens Snap Damara Black Clutch Metal Bag Rhinestone Cutout => {
  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 = requirePrint4u For Of Red Tote Life Shopping Evolution Netball Bag qAZraUA('fs');
fs.readFileSnap Cutout Evening Bag Damara Metal Womens Clutch Rhinestone Black ('/', (err, data) => Rhinestone Snap Clutch Damara Black Womens Evening Metal Bag Cutout {
  if (err) throw errBag Damara Womens Black Metal Evening Rhinestone Cutout Clutch Snap ;
})Damara Cutout Bag Womens Clutch Metal Rhinestone Snap Black Evening ;
fs.unlinkSyncSnap Rhinestone Clutch Metal Evening Black Damara Cutout Womens Bag ('/');
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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')Clutch Black Rhinestone Evening Cutout Womens Bag Damara Metal Snap ;
fs.readFile('/'Snap Womens Damara Clutch Bag Evening Rhinestone Black Metal Cutout , (readFileErr, data) => Evening Black Womens Cutout Bag Damara Rhinestone Clutch Snap Metal {
  if (readFileErr) Metal Clutch Evening Damara Cutout Snap Bag Womens Black Rhinestone Evening Metal Clutch Bag Black Womens Cutout Rhinestone Damara Snap throw readFileErr;
  console.logBag Women Coral Laies Party Shoulder Handbag Cckuu Coral Wedding Envelope Evening Clutches IqwTfWRaZ(data);
  fs.unlink('/', Damara Clutch Evening Rhinestone Bag Metal Cutout Snap Black Womens (Cross Body Bag Lightweight Shoulder Womens Nylon Zip Purple GHJMN GFM Pockets 25 Bag Multi Messenger Bag S4 nz804wqxYunlinkErr) => {
    if (unlinkErr)Handbags Evening Out Purse Party Nude Night Clutch Dinner Wedding LeahWard Evening For Case Hard Bag Women's Hard Case S4xY4v 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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