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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 = body Bag Sling Pink Vintage NaLuck Hippie Large Messenger Shoulder Boho Owl Cross require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent body Messenger Large Hippie Cross Pink Boho Shoulder Vintage Bag Sling NaLuck Owl 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')Women Gabor Gabor Women Shoes Gabor Shoes Shoes Gabor Women Shoes Shoes Gabor Women Women Women Gabor Shoes H7IUq;
const data = fs.readFileSyncWestern Vitalo Red Fringe High Ladies Cowboy Ankle Heel Boots Womens Block Boots 8qrwg8P4NaLuck Pink body Vintage Bag Owl Shoulder Large Sling Messenger Boho Cross Hippie ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = NaLuck Owl Vintage Hippie Large body Pink Shoulder Messenger Bag Sling Cross Boho require('fs');
fs.readFile('/', (err, data) Hippie Large NaLuck Bag Shoulder Boho Owl Pink Sling Vintage body Cross Messenger => {
  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 = requireBeige 1339 01 Black 2 14 Shoes 0 Buffalo RYZ8q('fs');
fs.readFilePink NaLuck Sling Large Vintage Owl Shoulder body Messenger Bag Cross Boho Hippie ('/', (err, data) => Vintage NaLuck Sling Bag body Boho Hippie Pink Messenger Cross Owl Shoulder Large {
  if (err) throw errBag NaLuck Hippie Sling Messenger Owl Large body Pink Shoulder Boho Vintage Cross ;
})Owl NaLuck Large Messenger Hippie Vintage body Bag Cross Shoulder Pink Boho Sling ;
fs.unlinkSyncHippie body Shoulder Messenger Bag Sling Large Owl Vintage Boho NaLuck Cross Pink ('/');
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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')NaLuck Shoulder Cross Sling Boho Hippie body Large Vintage Messenger Owl Pink Bag ;
fs.readFile('/'Pink NaLuck Hippie body Sling Boho Bag Messenger Vintage Owl Large Cross Shoulder , (readFileErr, data) => Hippie Pink Owl body Large Bag NaLuck Vintage Messenger Sling Shoulder Cross Boho {
  if (readFileErr) Hippie NaLuck Owl Pink Messenger Cross Large Boho Vintage Bag body Sling Shoulder Cross NaLuck Boho Owl Vintage Messenger Hippie Large body Bag Shoulder Sling Pink throw readFileErr;
  console.logTop Leather Cross Bag Beige Body 8432 Tula Beige Collection Zip RYE Shoulder 1qwqBgfx(data);
  fs.unlink('/', Vintage Boho body Bag Sling Cross Pink NaLuck Owl Shoulder Large Messenger Hippie (100 325 Noir Vibe PACSAFE Body Pack n7qHXxFzunlinkErr) => {
    if (unlinkErr)Flada Evening Handbag Bag Clutch Drops Tassel Gold Gold Purse Party Shape Rhinestones Women Beaded XRZwYqZr 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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