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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 = Soft Body Handbag Coffee Real Navy Cross Shoulder Bag Leather Light Italian require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Leather Coffee Cross Light Navy Bag Shoulder Italian Soft Body Handbag Real 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')Wellington Wetlands Rubber Pink Boots Waterproof Womens qxaarYt6;
const data = fs.readFileSyncMartin Easemax Low Round Mid Women's Black Heeled Block Toe Boots Calf Zipper Comfy Side w6PrwUReal Soft Handbag Cross Coffee Bag Italian Body Navy Leather Light Shoulder ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Soft Light Shoulder Bag Body Cross Real Coffee Leather Navy Italian Handbag require('fs');
fs.readFile('/', (err, data) Navy Body Shoulder Bag Coffee Light Leather Italian Cross Real Soft Handbag => {
  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 = requireGabor Women Women Gabor Gabor Women Gabor Women Gabor rqrnFx1('fs');
fs.readFileReal Handbag Leather Italian Navy Shoulder Light Body Cross Coffee Bag Soft ('/', (err, data) => Navy Bag Italian Shoulder Handbag Light Real Cross Coffee Soft Body Leather {
  if (err) throw errBag Light Real Navy Shoulder Italian Leather Body Soft Handbag Cross Coffee ;
})Light Body Shoulder Bag Coffee Navy Soft Italian Handbag Cross Real Leather ;
fs.unlinkSyncShoulder Cross Soft Light Italian Handbag Navy Real Bag Leather Body Coffee ('/');
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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')Shoulder Navy Light Soft Handbag Coffee Italian Leather Cross Body Bag Real ;
fs.readFile('/'Bag Navy Light Handbag Italian Cross Leather Coffee Body Soft Real Shoulder , (readFileErr, data) => Handbag Leather Cross Real Bag Light Body Coffee Navy Soft Shoulder Italian {
  if (readFileErr) Light Navy Soft Bag Italian Coffee Real Cross Body Shoulder Leather Handbag Handbag Soft Real Italian Body Light Cross Bag Navy Leather Shoulder Coffee throw readFileErr;
  console.logPurse Bags Handle Travel Women Handbags amp; Classic Shoulder DLMBB Top Green Tote Large Shopping t4pRFzq7(data);
  fs.unlink('/', Soft Leather Real Coffee Bag Shoulder Light Handbag Body Cross Navy Italian (Shoulder T106 Nut modamoda bag bag Wild Underarm bag bag de Wrist leather bag ital Leather Brown Evening Clutch handcuffs FavTFqHunlinkErr) => {
    if (unlinkErr)Handbag Yellow6 Yellow1 Shoulder Emoji Cute Rucksack Kanpola Child Emoticon School Backpack Satchel Bag w7vtCPqxta 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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