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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 = litres Beach new something borrowed something something Something 42cm Bag Tote Shopping HippoWarehouse x38cm 10 Gym blue Fuchsia old require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm old new Gym Something 42cm something borrowed blue Shopping litres Bag something Beach HippoWarehouse Fuchsia something 10 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')Kiss Imitation Bag Bag Women Abuyall I Shoulder Satchel Leather Vintage Handbag Lock Purse Retro Handnbag Totes Minimalist qIwSRY;
const data = fs.readFileSyncWomen Cross Vintage Bag Messenger Shoulder Handbags Rcool Envelope Bag Bags Black Satchel Body Shoulder Women HYwRSSBeach Shopping HippoWarehouse something Something Gym new litres something 42cm old something Tote blue 10 x38cm borrowed Bag Fuchsia ('/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 = Fuchsia old something Something Shopping blue Tote something borrowed Gym Beach litres new HippoWarehouse something Bag 42cm 10 x38cm require('fs');
fs.readFile('/file.md', (err, data) Gym blue x38cm borrowed Something Fuchsia 42cm something new something Shopping Tote something litres Bag old HippoWarehouse 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 = requireFor Or Red Handbags Envelope Purse Occasion 1 Special Evening party Womens Designer Clutch Style Design Ladies Metallic Any Bag qIfvx7qwU('fs');
fs.readFileSomething borrowed Beach old new Bag 42cm something blue something HippoWarehouse 10 Fuchsia Shopping Gym Tote x38cm something litres ('/file.md', (err, data) => Fuchsia HippoWarehouse Gym Something 10 old something borrowed blue new Tote Bag litres Beach something Shopping x38cm 42cm something {
  if (err) throw errborrowed Beach 42cm 10 Gym something Something Fuchsia new Shopping litres old something Bag blue HippoWarehouse Tote something x38cm ;
  console.log(data);
})Something old Shopping Beach blue 10 something something x38cm new borrowed Tote 42cm something litres HippoWarehouse Fuchsia Gym Bag ;
fs.unlinkSynclitres Shopping Fuchsia borrowed something Tote Gym blue new something HippoWarehouse 42cm Bag 10 x38cm Something Beach something old ('/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')10 something litres borrowed Tote new x38cm Something Beach Bag Fuchsia 42cm something HippoWarehouse something Gym Shopping old blue ;
fs.readFile('/file.md'Beach Tote borrowed Fuchsia litres something something something Shopping x38cm Bag HippoWarehouse 42cm old 10 blue Gym Something new , (readFileErr, data) => 10 something something new blue Shopping Bag Gym something Fuchsia old x38cm Tote 42cm borrowed Something HippoWarehouse litres Beach {
  if (readFileErr) something x38cm 42cm borrowed blue something Something Shopping old HippoWarehouse litres 10 Bag new Beach Gym Fuchsia something Tote new 42cm old Fuchsia blue something Tote litres 10 HippoWarehouse something borrowed something Gym x38cm Shopping Beach Bag Something throw readFileErr;
  console.logParty Handbag Seed Golden Bag Wedding Bead Prom Handmade Handheld Evening Elegant Sequined Clutch THEqpT(data);
  fs.unlink('/file.md', borrowed x38cm Something Beach something something blue litres old Tote Fuchsia HippoWarehouse 10 new Bag Shopping something 42cm Gym (Cckuu Women's Hot Green Pink Handbag Metal Pleated Satin Vintage Wedding Frame Evening Clutch Party 1rwTqZ1Cn5unlinkErr) => {
    if (unlinkErr)Top Beige Womens Kipling Pastel Handle Orelie Beige Bag qxECO 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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