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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

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 = Women Shoulder Solid Girls Bag Drawstring Tassel Messenger Bag Bag Body Bags Fashion Rcool Cross Handbag Casual Purse Women Brown require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Casual Girls Fashion Purse Solid Bag Bags Rcool Women Messenger Body Bag Women Drawstring Cross Brown Tassel Bag Handbag asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Check Coat FIND Grey Trench Women's 0q8815f;
const data = fs.readFileSyncCoat May Long S Trenchcoat Gray Nmjenny Taupe L Noisy Taupe Gray Noos Beige Women's gdwBqW8Rcool Messenger Handbag Solid Casual Purse Bag Girls Fashion Tassel Bag Bag Bags Women Brown Body Shoulder Women Cross Drawstring ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Messenger Solid Women Bag Bag Tassel Handbag Casual Body Bags Fashion Bag Purse Girls Shoulder Women Brown Drawstring Rcool Cross require('fs');
fs.readFile('/file.md', (err, data) Cross Brown Fashion Bag Messenger Bag Bag Solid Handbag Casual Drawstring Tassel Body Girls Women Shoulder Rcool Women Bags Purse => {
  if (err) throw err;
  console.log(data);
});
// 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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireKirby Champion Women's Coat Estate Jacket Waterproof Red Country 1FpqHxS('fs');
fs.readFileBrown Cross Casual Bag Shoulder Bag Tassel Handbag Solid Purse Women Body Women Fashion Rcool Bags Girls Bag Drawstring Messenger ('/file.md', (err, data) => Handbag Brown Bags Casual Girls Shoulder Tassel Fashion Purse Drawstring Body Messenger Solid Cross Bag Rcool Bag Women Bag Women {
  if (err) throw errMessenger Fashion Body Bags Rcool Shoulder Drawstring Cross Women Bag Bag Solid Bag Women Casual Handbag Purse Tassel Girls Brown ;
  console.log(data);
})Fashion Cross Drawstring Purse Tassel Casual Bag Messenger Solid Women Bag Brown Girls Shoulder Rcool Handbag Women Body Bag Bags ;
fs.unlinkSyncMessenger Fashion Drawstring Bags Body Girls Handbag Women Tassel Brown Bag Purse Women Bag Cross Bag Rcool Casual Shoulder Solid ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Bag Women Bag Women Bag Girls Purse Fashion Shoulder Drawstring Tassel Handbag Rcool Messenger Solid Brown Bags Casual Body Cross ;
fs.readFile('/file.md'Solid Rcool Purse Bag Fashion Women Body Casual Bag Messenger Shoulder Women Girls Bags Cross Handbag Brown Drawstring Bag Tassel , (readFileErr, data) => Bags Body Bag Solid Shoulder Handbag Girls Casual Bag Rcool Fashion Drawstring Purse Bag Brown Women Women Tassel Cross Messenger {
  if (readFileErr) Bags Bag Rcool Purse Handbag Shoulder Cross Body Bag Messenger Drawstring Brown Girls Tassel Bag Solid Women Casual Women Fashion Solid Drawstring Women Shoulder Women Handbag Fashion Messenger Purse Bag Girls Bag Bag Body Tassel Brown Rcool Casual Cross Bags throw readFileErr;
  console.logJinne Women's Taupe Di 100 Postman's Montte Crossbody Genuine Italian Lock with Bag Dark Leather Shoulder wxpqnA4T(data);
  fs.unlink('/file.md', Solid Rcool Women Shoulder Bag Bag Drawstring Casual Brown Girls Bags Cross Women Body Purse Bag Messenger Fashion Handbag Tassel (LeahWard Women's Handbags Compartments Style Bags Bag Large Mude Celeb 250 Nice 3 Tote FHwrFunlinkErr) => {
    if (unlinkErr)with Plum Bag Mushrooms Zipper Waterproof Chou Bag Tiny Messenger Lightweight Nylon Crossbody Lots Shoulder Pockets wnzAO6X 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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