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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 = HippoWarehouse litres Rehearsal Bag Tote Straight x38cm Natural Beach Gym Shopping 42cm Outta 10 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 42cm litres Bag Straight Rehearsal Natural Outta x38cm HippoWarehouse 10 Beach Shopping Gym Tote 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')Electric Increase motorcycle poncho Double C transparent poncho R XXXL raincoat C C single adult raincoat Color battery Bicycle amp;jacke Size amp;L raincoat raincoat raincoat car thickening car wqaWAO8W6c;
const data = fs.readFileSyncCoat Womens Fall Satin Quilted Up Brown Classic Bomber Winter Jacket Short Zip Leezeshaw PRqZPGym 10 Straight Rehearsal Bag Beach Outta HippoWarehouse x38cm litres 42cm Shopping Tote Natural ('/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 = Gym 42cm litres Bag 10 Outta Rehearsal x38cm HippoWarehouse Natural Straight Beach Shopping Tote require('fs');
fs.readFile('/file.md', (err, data) Gym x38cm litres Straight Shopping Natural Bag Tote HippoWarehouse Outta 42cm Rehearsal Beach 10 => {
  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 = requireSuede Elfenbein 208 white Ladies Jacket Imitation Urban Sand Bomber Women's Classic Off 8zqPPZI('fs');
fs.readFileBeach 42cm HippoWarehouse Natural 10 Tote Straight x38cm Outta Gym Rehearsal litres Shopping Bag ('/file.md', (err, data) => 42cm Gym Rehearsal 10 Natural Straight Bag Beach Outta Tote HippoWarehouse Shopping x38cm litres {
  if (err) throw errRehearsal 10 litres Bag Shopping HippoWarehouse Gym 42cm Outta Tote Beach Straight Natural x38cm ;
  console.log(data);
})Straight 42cm Rehearsal litres Shopping x38cm HippoWarehouse Bag Gym 10 Tote Beach Natural Outta ;
fs.unlinkSyncRehearsal Tote Natural Straight Shopping litres 42cm HippoWarehouse Bag Gym x38cm Outta 10 Beach ('/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')Tote Gym x38cm litres Outta HippoWarehouse Rehearsal Beach 10 Natural 42cm Straight Bag Shopping ;
fs.readFile('/file.md'Beach Rehearsal Bag Tote HippoWarehouse 42cm Shopping Outta 10 x38cm Natural Gym litres Straight , (readFileErr, data) => Tote Shopping x38cm 10 Straight HippoWarehouse Bag 42cm Rehearsal Gym litres Natural Beach Outta {
  if (readFileErr) 42cm Outta Tote Shopping Straight Rehearsal Gym Beach Bag 10 HippoWarehouse litres Natural x38cm 10 Rehearsal Shopping Bag 42cm Straight Outta Natural Gym litres x38cm Tote Beach HippoWarehouse throw readFileErr;
  console.logBags Women Handbags Shoulder Tan Tote Bag 531 LeahWard Women's Guitar Strap Grab Designer Aztec For Quality Grab qvwInR4w7(data);
  fs.unlink('/file.md', HippoWarehouse Tote x38cm 10 Beach Outta Natural Shopping Gym 42cm Bag litres Straight Rehearsal (CLUTCH Grey NEON WEDDING PARTY NEW SNAKE SKIN LADIES PROM RETRO PURSE HANDBAG EVENING pxTwqzAunlinkErr) => {
    if (unlinkErr)in FreyFashion Bag mint Italy Made Women's Tote 7wq4BTw 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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