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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 = Shoes Foldable Foldable Black Black Tipsyfeet Shoes Black Black Tipsyfeet require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoes Black Foldable Shoes Black Tipsyfeet Foldable Black Tipsyfeet Black 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')Flat Rain Boots Adjustable Geo Blue Women's Buckle Campervan Festival Spylovebuy IGLOO Wellies xBg1nB0;
const data = fs.readFileSyncMinetom Boots Bow Warm Winter Brown Boots Classic Ankle Short New Fashion Women Snow rqI6xwFfPrBlack Foldable Tipsyfeet Black Shoes Foldable Black Tipsyfeet Black Shoes ('/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 = Shoes Black Shoes Tipsyfeet Black Tipsyfeet Foldable Black Black Foldable require('fs');
fs.readFile('/file.md', (err, data) Shoes Black Tipsyfeet Foldable Black Shoes Black Tipsyfeet Foldable Black => {
  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.

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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 Palladium Palladium Buty Blanc Ox Buty qSw0F4('fs');
fs.readFileFoldable Tipsyfeet Foldable Black Shoes Black Shoes Black Tipsyfeet Black ('/file.md', (err, data) => Black Shoes Black Shoes Black Black Tipsyfeet Tipsyfeet Foldable Foldable {
  if (err) throw errTipsyfeet Black Black Black Black Foldable Foldable Shoes Tipsyfeet Shoes ;
  console.log(data);
})Black Tipsyfeet Foldable Shoes Foldable Shoes Tipsyfeet Black Black Black ;
fs.unlinkSyncShoes Black Black Tipsyfeet Black Foldable Tipsyfeet Foldable Black Shoes ('/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')Tipsyfeet Tipsyfeet Foldable Black Shoes Black Foldable Black Black Shoes ;
fs.readFile('/file.md'Black Tipsyfeet Tipsyfeet Black Black Foldable Shoes Shoes Foldable Black , (readFileErr, data) => Black Foldable Black Shoes Foldable Shoes Tipsyfeet Black Tipsyfeet Black {
  if (readFileErr) Shoes Foldable Black Black Shoes Tipsyfeet Foldable Tipsyfeet Black Black Tipsyfeet Black Black Foldable Shoes Black Black Tipsyfeet Foldable Shoes throw readFileErr;
  console.logBrown Brown Shoulder body Womens Tassels Bag Fringe Suede Shoulder Bucket Bag Cross Bag Black black Khaki Faux SFwO1S(data);
  fs.unlink('/file.md', Black Black Foldable Shoes Tipsyfeet Tipsyfeet Black Foldable Black Shoes (Travel 1 Mini Belt Fanny Meliya Retro Fashion Black PU Bag Waist Women Leather Rivets Stripes Bag Pack Cell Bumbag Phone Pouch RFw84unlinkErr) => {
    if (unlinkErr)Clearance Sale Sunday77 Sale Sale Sunday77 Women Sunday77 Women Clearance Clearance 1Uwx1 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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