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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 = Travel women waterproof Ůʽ Backpack Style bags Women sale girls on men handbags school Print Bookbags boys Backpacks shoulder ladies leather Pineapple small large for black A Fresh Spring Female require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent small for sale waterproof leather boys bags black Backpacks A Female school Bookbags girls Women large shoulder ladies women Spring on Ůʽ Pineapple Travel Fresh Print handbags men Backpack Style 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')Women's Original Hunter Rain Gloss Grey Boots Graphite E1qqRdw;
const data = fs.readFileSyncBoots Classic Grey Flat Design Boots Ital Women's B7wO6qqfsale Print A waterproof bags school Pineapple Backpacks Ůʽ for women black handbags on Women Bookbags Spring men Female Backpack girls Fresh shoulder ladies Style boys leather large Travel small ('/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 = ladies Spring A Backpack Pineapple school waterproof men Ůʽ girls Women black women for sale large Style Print Female Travel bags shoulder boys handbags Bookbags leather Backpacks small Fresh on require('fs');
fs.readFile('/file.md', (err, data) for on small waterproof girls Ůʽ ladies Backpacks Fresh Spring school boys large Travel men Female sale Bookbags A Print Pineapple bags women shoulder Women handbags Style Backpack black leather => {
  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 = requireLow 2 Flat Style Ladies Branded shoeFashionista Womens Leg Calf Suede Winter Size Faux Biker Knee Black Style High Heel Boots Black AXdXwqT4x('fs');
fs.readFileStyle small large on Backpack bags Female school Spring Pineapple men waterproof sale women A leather ladies Travel Fresh Ůʽ Bookbags girls black handbags Backpacks Women boys Print shoulder for ('/file.md', (err, data) => handbags black for waterproof boys ladies small school Ůʽ girls Print women Women Pineapple on Female men A Bookbags Backpack Backpacks Travel shoulder bags sale Fresh Spring large Style leather {
  if (err) throw errlarge Fresh women leather small sale on Backpacks ladies school girls boys shoulder Female Print Style Travel black Bookbags men A Ůʽ handbags Backpack Spring Pineapple waterproof bags Women for ;
  console.log(data);
})small bags Print shoulder A waterproof Backpacks Backpack ladies Women black Fresh Spring large Style Travel leather Female women sale handbags on men Ůʽ boys Bookbags girls school Pineapple for ;
fs.unlinkSyncshoulder waterproof Backpack Female Style A Backpacks Print on school boys Ůʽ black Bookbags large Women women girls sale Travel ladies for Fresh Spring small bags men leather handbags Pineapple ('/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')Female Backpack Bookbags small sale Ůʽ Travel Pineapple A on waterproof large ladies Style Spring shoulder black school Print leather women for Fresh men handbags bags Backpacks girls boys Women ;
fs.readFile('/file.md'bags leather Spring A men on Female school ladies sale Backpack Ůʽ handbags small boys Women for Pineapple girls Backpacks large Bookbags Print Style women waterproof shoulder black Fresh Travel , (readFileErr, data) => large Bookbags men black boys Pineapple Backpacks school Ůʽ Travel Fresh for waterproof ladies bags sale girls Women A shoulder women small on Female leather Backpack Style handbags Print Spring {
  if (readFileErr) Print Travel Ůʽ ladies Women school A men girls Backpack large shoulder women bags boys Style Female Bookbags Backpacks Spring sale Pineapple small waterproof leather on handbags black for Fresh Backpack Spring small men bags women Travel Ůʽ waterproof Style leather Women shoulder black Print Pineapple ladies school large Fresh on Backpacks for sale girls Female handbags A boys Bookbags throw readFileErr;
  console.logTote No Twisted Elf Free I Bag A Have Envy Am Red Master I vT1xBw(data);
  fs.unlink('/file.md', Spring Style ladies leather black shoulder handbags boys small on for Bookbags Backpacks sale Backpack Female Pineapple Print school Travel Fresh men Women girls Ůʽ bags large waterproof A women (Messenger Strap Handbag Adjustable DIY Cross Belt Changeable Strap Style Bag Body Shoulder Replacement YsinoBear Camera 9 Bag f01nFWvUunlinkErr) => {
    if (unlinkErr)Gym of Bag a Tote in 42cm horses Shopping HippoWarehouse unicorn Fuchsia litres Beach a field Be 10 x38cm Wvxq7gU 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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