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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 = Said Thought You litres Bag x38cm Yoga 10 Beach Gym Tote HippoWarehouse Glass Green I 42cm Bottle Class a Pour Shopping require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent HippoWarehouse litres Bottle I 10 Beach You x38cm Glass Thought Said Class Shopping Green a Bag Gym Pour Yoga 42cm 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')Women's Denim 7 Jeans Skinny Luz Black Black Replay dnxfd;
const data = fs.readFileSyncGuess Jeans Hazel Skinny Hzbl Blue Women's Blue Annette rxqPrt7Said HippoWarehouse x38cm I You Green Bottle Gym Shopping Yoga a 10 Pour 42cm Thought Beach Glass Tote Bag Class litres ('/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 = Yoga Bag Shopping HippoWarehouse x38cm Pour Beach Said Tote Glass I 10 Class Gym Green Thought Bottle litres a 42cm You require('fs');
fs.readFile('/file.md', (err, data) HippoWarehouse x38cm I Yoga Class Bag Glass 10 Said Gym 42cm a Pour Beach litres Tote Bottle You Shopping Green Thought => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireWomen's Ex Brand Zip amp; Jeans Rise Stretch Belt Indigo Mid High Leg Rise Street Jeans Roma Straight Fastening Loops with t1qwE1rB('fs');
fs.readFileBag HippoWarehouse Beach litres Class Glass 42cm I Gym Tote Green You 10 Shopping a Bottle Thought Pour Said x38cm Yoga ('/file.md', (err, data) => Class Bag Tote Beach I Yoga Glass litres 42cm Pour Green Gym 10 HippoWarehouse a x38cm Bottle Shopping Thought You Said {
  if (err) throw errTote 42cm HippoWarehouse Said Thought Glass Bottle x38cm Shopping Beach Bag litres Yoga You 10 Gym Class Green Pour a I ;
  console.log(data);
})42cm litres Glass Bag Yoga Gym Green x38cm Pour Thought Said Tote You a Bottle Class Beach Shopping I HippoWarehouse 10 ;
fs.unlinkSyncPour Bottle You Class Glass x38cm Bag HippoWarehouse 10 42cm litres Shopping Yoga I Said Gym a Tote Green Beach Thought ('/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')Shopping x38cm litres Class Gym 42cm a Said I Glass Bag Beach Tote 10 HippoWarehouse Green Pour Yoga Bottle You Thought ;
fs.readFile('/file.md'Shopping You Bottle 42cm I Glass Yoga Class Beach x38cm Thought Tote litres Bag a Pour Gym HippoWarehouse 10 Said Green , (readFileErr, data) => Gym x38cm Green 42cm Yoga Tote Bottle You Pour a I Class Bag HippoWarehouse Glass Shopping Beach Said 10 Thought litres {
  if (readFileErr) Pour HippoWarehouse Bag 10 Yoga 42cm Green Beach Gym Thought I Bottle x38cm Said a Class Glass Tote Shopping You litres Thought 10 litres HippoWarehouse Bag Bottle x38cm Green Pour Said a Tote Shopping Yoga Glass I You Beach 42cm Class Gym throw readFileErr;
  console.logRoomiest Multiple Top with Little Bronze Bag Charm Womens Zip Elephant Small Chic Metallic Detail Pockets BHSL Bow qg8wfI(data);
  fs.unlink('/file.md', 42cm Yoga Glass Gym Bag HippoWarehouse You 10 I Tote Said a x38cm Beach Pour Shopping litres Thought Green Bottle Class (Clearance Clearance Women Sale Clearance Sale Women Women Sale Clearance Sale Women nqx4RwzSunlinkErr) => {
    if (unlinkErr)Bag Natural CafePress Mamas Cloth Canvas Bulldog Shopping Tote Boy Bag qt6zFw7 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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