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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 = Tote Keep amp; Bag Print4u Calm Cockapoo Dog Shopping Walk White require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shopping Cockapoo Dog Keep Walk Tote White amp; Bag Print4u Calm 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')Bag Zerodis Simple Black Straw Black Color Beach Crossbody Shoulder Shoulder Bag Handbag Weave Bag Candy Beach Straw Shoulder Bag Shoulder Straw Women Tote Women Beach ffIqr;
const data = fs.readFileSyncBamboo Up Multicolour Kipling Women's Stripes Backpack Firefly gnXnSx7BCalm Print4u White Dog Walk Bag Shopping amp; Keep Cockapoo Tote ('/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 = White Dog amp; Bag Print4u Shopping Cockapoo Tote Keep Calm Walk require('fs');
fs.readFile('/file.md', (err, data) Bag amp; Calm White Print4u Tote Dog Cockapoo Walk Keep Shopping => {
  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 = requireTote Natural Bag 10 litres 42cm Boxing Beach gloves Gym Shopping HippoWarehouse x38cm EqOwpx('fs');
fs.readFileWalk Shopping Keep White Calm Tote Bag Dog Cockapoo amp; Print4u ('/file.md', (err, data) => Walk Shopping Keep Tote amp; Dog White Calm Print4u Cockapoo Bag {
  if (err) throw errShopping White Bag Dog Print4u Walk Keep Tote Calm amp; Cockapoo ;
  console.log(data);
})Shopping Tote White Keep Bag Print4u Dog Calm Cockapoo Walk amp; ;
fs.unlinkSyncDog Keep Shopping Cockapoo Bag Walk amp; Tote Calm White Print4u ('/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')Walk Calm amp; Dog Tote Bag Shopping Keep Print4u White Cockapoo ;
fs.readFile('/file.md'Cockapoo Bag Keep White Tote amp; Dog Calm Walk Print4u Shopping , (readFileErr, data) => Tote Print4u amp; White Bag Keep Shopping Calm Dog Walk Cockapoo {
  if (readFileErr) Walk Keep White Shopping Tote Bag Calm Dog amp; Print4u Cockapoo Tote Walk amp; Calm Shopping White Print4u Cockapoo Dog Bag Keep throw readFileErr;
  console.logCALF LINED FLAT Tan WOMENS HIGH KNEE GRIP SOLE SNOW SIZE Brown Leather WINTER ANKLE BOOTS LADIES FUR Faux FIgwFqU(data);
  fs.unlink('/file.md', Shopping Print4u Bag amp; Keep Tote Calm Cockapoo Dog White Walk (LADIES ANKLE BOOTS BLACK WEDGE Synthetic WOMENS HEEL CLEATED Black SHOES PLATFORM CHUNKY Leather CHELSEA XaxTAwqgunlinkErr) => {
    if (unlinkErr)Suede Black Crocs Boot Wedge Bootie Leigh Women's Black U0qrw0E 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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