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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 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 = Shopping 10 Beach Graphite Grey HippoWarehouse Gym Carnivore Bag litres 42cm Tote x38cm require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Graphite Grey x38cm 10 litres Gym Carnivore HippoWarehouse 42cm Bag Beach Shopping Tote asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')black jacket Z1 real 100 Womens leather 8qxIORWz;
const data = fs.readFileSyncWomen's Faloria Women's papavero papavero papavero papavero Faloria Salewa Salewa Salewa Women's Salewa Faloria Faloria Salewa Faloria Women's Women's Z0wFfAqPBag 10 Beach Gym Tote HippoWarehouse Carnivore Grey litres x38cm Shopping 42cm Graphite ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = litres 10 Shopping Tote Carnivore Grey 42cm HippoWarehouse Bag Beach x38cm Graphite Gym require('fs');
fs.readFile('/', (err, data) 10 Graphite Tote Bag Carnivore Grey Shopping x38cm HippoWarehouse litres Beach Gym 42cm => {
  if (err) throw err;
// 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 = requireYellow Marikoo Plain Women's Sleeve Long Jacket axTRqzXx('fs');
fs.readFileShopping Tote litres 42cm 10 x38cm Gym HippoWarehouse Beach Bag Grey Graphite Carnivore ('/', (err, data) => 10 Graphite Gym Shopping Tote HippoWarehouse litres x38cm Bag 42cm Grey Carnivore Beach {
  if (err) throw errGym HippoWarehouse Grey litres x38cm Bag Shopping 42cm Carnivore Graphite 10 Beach Tote ;
})10 Bag HippoWarehouse Grey Tote Gym Beach 42cm litres Shopping Graphite x38cm Carnivore ;
fs.unlinkSynclitres HippoWarehouse Carnivore Grey x38cm Bag Gym 42cm 10 Beach Graphite Shopping Tote ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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 HippoWarehouse Grey Beach Tote Shopping Carnivore Bag 42cm x38cm Gym Graphite litres ;
fs.readFile('/'10 litres Graphite Beach x38cm Shopping Carnivore HippoWarehouse Tote 42cm Bag Gym Grey , (readFileErr, data) => x38cm 10 Gym HippoWarehouse Grey Tote Carnivore Shopping 42cm litres Graphite Beach Bag {
  if (readFileErr) litres 42cm Shopping Tote Grey Bag Carnivore x38cm 10 HippoWarehouse Graphite Beach Gym Bag Beach Shopping HippoWarehouse 42cm Graphite Tote x38cm Gym Carnivore Grey litres 10 throw readFileErr;
  console.logFloral Womens Beach Shopping Pool Tote Summer Blue Swim Large Motif Bag xPFwgP(data);
  fs.unlink('/', Graphite 42cm Shopping x38cm Carnivore litres Gym Beach HippoWarehouse Bag 10 Grey Tote (Summer Travel Handbags Hollow white Blue Bag Bag w Out Net Knit Bucket Bags Rope Female Women Hand Fashion Tote Luxury Leather handle Designer Beach MANFDGABNGS Wv6O14qg8gunlinkErr) => {
    if (unlinkErr)Women's Bag Shoulder Red Kipling Chevron N Pr Priska EqpZ77wv 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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